Symposium Organizers
Julia R. Greer California Institute of Technology
Daniel S. Gianola University of Pennsylvania
Blythe G. Clark Sandia National Laboratories
Ting Zhu Georgia Institute of Technology
Alfonso H. W. Ngan The University of Hong Kong
P3: Poster Session I
Session Chairs
Tuesday AM, November 30, 2010
Exhibition Hall D (Hynes)
P1: Twinning at the Nanoscale
Session Chairs
Monday PM, November 29, 2010
Room 210 (Hynes)
9:00 AM - **P1.1
Dislocation Nucleation Governed Softening and Maximum Strength in Nanotwinned Metals.
Xiaoyan Li 1 , Yujie Wei 3 , Lei Lu 2 , Ke Lu 2 , Huajian Gao 1
1 , Brown University, Providence, Rhode Island, United States, 3 , Institute of Mechanics, Beijing China, 2 , Institute of Metals Research, Shenyang China
Show AbstractIn conventional metals, there is plenty of space for dislocations to multiply, so that the metal strengths are controlled by dislocation interactions with grain boundaries and other obstacles. For nanostructured materials, in contrast, dislocation multiplication isseverely confined by the nanometre-scale geometries so that continued plasticity can be expected to be source-controlled. Nanograined polycrystalline materials were found to be strong but brittle, because both nucleation and motion of dislocations are effectively suppressed by the nanoscale crystallites. Here we report a dislocation-nucleation-controlled mechanism in nano-twinnedmetals in which there are plenty of dislocation nucleation sites but dislocation motion is not confined. We show that dislocation nucleation governs the strength of such materials, resulting in their softening below a critical twin thickness. Large-scale molecular dynamics simulations and a kinetic theory of dislocation nucleation in nano-twinned metals show that there exists a transition in deformation mechanism, occurring at a critical twin-boundary spacing for which strength is maximized. At this point, the classical Hall–Petch type of strengthening due to dislocation pile-up and cutting through twin planes switches to a dislocation-nucleation controlled softening mechanism with twin-boundary migration resulting from nucleation and motion of partial dislocations parallel to the twin planes. Most previous studies did not consider a sufficient range of twin thickness and therefore missedthis strength-softening regime. The simulations indicate that the critical twin-boundary spacing for the onset of softening in nano-twinned copper and the maximum strength depend on the grain size: the smaller the grain size, the smaller the critical twinboundary spacing, and the higher the maximum strength of the material.
9:30 AM - P1.2
Strong Crystal/Grain Size Effects on Deformation Twinning.
Evan Ma 2 1 , Jinyu Zhang 1 , Qian Yu 1 , Zhiwei Shan 1 , Gang Liu 1 , Ju Li 3 1 , Jun Sun 1
2 Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, Maryland, United States, 1 School of Material Science and Engineering, Xi’an Jiaotong University, Xi'an, Shaanxi, China, 3 Department of Materials Science and Engineering, University of Pennsylvani, Philadelphia, Pennsylvania, United States
Show AbstractWhile a Hall-Petch type dependence is known for deformation twinning (DT) in metals of conventional grain sizes (D > 1μm), the effects of sample (crystal) size has not been explored. We report here that crystal size strongly influence the propensity for DT in a Ti alloy (Q. Yu et al., Nature 2010), with a large Hall-Petch slope until the crystal size falls below 1μm. Also, with D decreasing in the nanocrystalline (D < 200 nm) regime the propensity for DT turns around to increase, and then decrease again. This behavior is demonstrated here for nanocrystalline Cu films (J.Y. Zhang et al., submitted, 2010). The inverse D-dependence of DT is explained by modeling the competing D-effects on the emission of the first partial dislocation, and the plane-to-plane promotion of partial dislocation slip afterwards.
9:45 AM - P1.3
TEM Characterization of Twinned Nanocrystalline Palladium Thin Films with High Ductility and Work Hardening.
Idrissi Hosni 1 , Wang Binjie 1 , Colla Marie Stephane 2 , Raskin Jean-Pierre 2 , Schryvers Dominique 1 , Pardoen Thomas 2
1 , EMAT. University of Antwerp, Antwerp Belgium, 2 , Université catholique de Louvain, Louvain la neuve Belgium
Show AbstractNowadays, thin films attract much attention because of their applications in flexible electronics, microelectromechanical systems (MEMS) devices, as well as thin functional coatings. Mechanical properties, especially ductility, play an important role for many applications of thin films. However, due to submicron grain sizes, most metallic thin films suffer from a lack of ductility. In the present work, nanocrystalline Pd thin films deposited by electron-beam evaporation have been deformed using a new lab-on-chip tensile testing stage based on the MEMS concept. The basic idea is to use internal stresses introduced in a long actuating beam to deform another material attached to it by removing the underneath sacrificial layer separating the two materials from a substrate. Using this method, uniaxial tensile tests have been performed on Pd thin films with different thicknesses (80, 160 and 310nm). The obtained results reveal an increase of the yield stress with decreasing film thicknesses. A ductility of typically 3-4 percent is reached, as well as an average strain hardening capacity close to 0.4. These values are extremely large compared to what is currently reported in the literature for thin films, especially considering the nanoscopic grain size.Transmission electron microscopy (TEM) shows columnar grains parallel to the growth direction. No significant changes in grain morphology and size were observed in the samples after plastic deformation. A high density of fine coherent {111} growth twins in the as-deposited films with an average twin thickness of 5.0±0.3 nm was observed. From high-resolution TEM (HRTEM) images, it was demonstrated that the coherency of the twin boundaries decreases considerably after deformation, due to the interactions of dislocations generated during the plastic deformation with the existing growth twins. Coherent twin boundaries offer multiple barriers to dislocation motion, as well as an additional volume for dislocation storage and multiplication via specific reactions between dislocations gliding from the matrix and the twin boundaries, thus elevating ductility and work hardening.The mechanism(s) controlling the dislocation/twin boundary interactions are investigated using HRETM, and compared to recent results in order to relate the defect analysis to the macroscopic behaviour of Pd thin films.
10:00 AM - P1.4
Effects of Nanoscale Twin Boundaries on Mechanical Properties in Nanostructured Metals.
Haofei Zhou 1 , Shaoxing Qu 1 , Wei Yang 2
1 Department of Engineering Mechanics, Zhejiang University, Hangzhou, Zhejiang, China, 2 University Office, Zhejiang University, Hangzhou, Zhejiang, China
Show AbstractConventional strengthening methods typically lead to suppressed ductility and fracture toughness. Recent experimental results demonstrate that introducing nanoscale twin boundaries into ultrafine-grained polycrystals leads to joint enhancement on strength and toughness. Large-scale atomistic simulation is an effective method to reveal the underlying atomic-scale strengthening and toughening mechanisms dominated by nanoscale twin boundaries and their effects on mechanical properties of materials. Both experimental and computational investigations observe a softening process as the twin boundary spacing deceases below a critical value. It is indicated by our simulations that, in columnar-grained nano-twinned polycrystals, softening is deterred and the strength continues to increase at the smallest twin boundary spacing. Moreover, four toughening mechanisms by nanoscale twin boundaries are identified: (i) crack blunting through dislocation accommodation along nano-scaled twin boundaries; (ii) crack deflection in a manner of intragranular propagation; (iii) daughter crack formation along the nano-scaled twin boundaries that further enhances the toughness; and (iv) curved TB planes owing to an excessive pileup of geometrically necessary dislocations. These toughening mechanisms jointly dictate the mechanical behavior of nanostructured materials.
10:15 AM - P1.5
Phase-field Modeling of Twin Boundary Motion in MagneticShape Memory Alloys.
Christian Mennerich 1 , Frank Wendler 1 , Marcus Jainta 1 , Britta Nestler 1
1 Institute of Materials and Processes (IMP), Karlsruhe University of Applied Sciences, Karlsruhe, Baden-Württemberg, Germany
Show AbstractMagnetic shape memory (MSM) alloys have attained major interest in the last decade. They offer fast response times by low energy costs in operation. The MSM effect is settled completely in the martensitic state of a magnetic hard material and bases on the magnetically induced microstructure rearrangement by moving low energetic twin boundaries. The MSM effect is accompanied by large strains (up to 10% in Ni2MnGa specimens). Because the MSM effect is often spoiled in polycrystalline samples, current research focusses on the optimization of microstructure in polycrystalline and thin film applications. Our attempt is the modeling and simulation of microstructure evolution in polycrystalline samples composed of different martensitic twin variants under the application of external magnetic fields. A combined continuum approach, basing on the complex interplay of phase-field dynamics, linear elasticity and micromagnetics, is used. In the adopted phase-field model (pfm) an order parameter of N phase-field variables Φ=(Φα)α=1, ..., N (with N the number of twin variants belonging to different grains having individual orientations) is introduced that varies smoothly on the computation domain Ω. The constituting Ginzburg-Landau type free energy functional depends also on the the displacement field u and the spontaneous magnetization m:∫Ω [ (εa(Φ, grad Φ) + 1/ε w(Φ)) + g(Φ,u,m) ] dΩ.Gradient energy density a and potential w come from the pfm (see [1]), whereas the (interpolated) bulk free energy densityg(Φ,u,m) = eext(m) + edemag(m) + eexch(m) + ∑α[ ( eαaniso(Φ,m) + eα mel(Φ,u,m) ) h(Φα) ]drives the phase transition and includes contributions from the external applied magnetic field (Zeeman energy), demagnetization energy, exchange energy, anisotropy energy (stating the local dependence on directions of preferred magnetization, the so called easy axes) and the magnetoelastic energy (including stress-free strains originating from the presumed martensitic transformation). See [2] for a detailed description of the energies . The evolution for the phase fields is derived from variational principles, for the micromagnetic evolution the Landau-Lifshitz-Gilbert equation is adapted. The complete dynamics for the displacement field is resolved by solving a wave equation. In different simulation set-ups the general applicability of the model to single crystals and polycrystalline settings in 2D and 3D is shown.[1] B. Nestler, H. Garcke, B. Stinner, Multicomponent alloy solidification: Phase-field modelling and simulations, Phys. Rev. E 71, (2005) 041609.[2] J. E. Miltat, M. J. Donahue, Numerical micromagnetics: Finite difference methods, in: H. Kronmüller and S. Parkin (Editors), Handbook of Magnetism and Advanced Magnetic Materials, John Wiley and Sons Ltd. (2007).
11:00 AM - **P1.6
Strengthening Mechanism in Cu with Nano-scale Twins.
Lei Lu 1
1 SYNL, Institute of Metal Research, Shenyang China
Show AbstractThe strength and ductility of the nanostructured metals are strongly influenced by its internal boundaries and defects. Whereas the strengthening mechanisms of the conventional high-angle grain boundaries have been well studies in the naocrystalline metals, in this presentation, we will focus on the recent systematic studies of the strengthening mechanism relative to the coherent twin boundaries. It is demonstrated that the increasing the twin density, while keeping the grain size fixed, results in a significant increase in strength but without compromising its ductility. In particular, a maximum strength was found in the nanotwinned Cu at a twin thickness of 15 nm, which followed by a rapid softening and a monotonic enhancement in both the strain-hardening and ductility at smaller twin thicknesses. On the other hand, increasing the grain size while keeping the twin thickness constant, ductility and work hardening of nt-Cu are effectively promoted, but strength is not sacrificed obviously. Detailed MD simulation of the role of nano-twins in influencing deformation and ductility will also be reported along with the experimental results. These findings provide insights into the possible promising routes for optimizing the mechanical properties of nanostructured metals by tailoring internal interfaces.
11:30 AM - P1.7
Microstructural Changes Produced by Severe Shear and Compressive Stresses in Cu Consisting of Highly Aligned Columns of Nanotwins.
Carla Shute 1 , Benjamin Myers 1 , Shu-You Li 1 , Andrea Hodge 2 , Troy Barbee 3 , Y. Zhu 4 , Julia Weertman 1
1 , Northwestern University, Evanston, Illinois, United States, 2 , University of Southern California, Los Angeles, California, United States, 3 , Lawrence Livermore National Laboratory, Livermore, California, United States, 4 , North Carolina State University, Raleigh, North Carolina, United States
Show AbstractNanotwinned metals have been found to have a number of advantages over their nanocrystalline counterparts. The present paper examines the effect of severe deformation on the microstructural stability and strength of nanotwinned Cu. Samples of high purity Cu consisting of highly aligned columns of nanotwins were subjected to severe compression (1800 MPa) or high pressure torsion (HPT). Compression was found to roughly double the spacing between twin boundaries (TBs) from an initial median value of 30 nm to 60 nm while the average column spacing remained unchanged, despite the dimensional changes in the sample. (This behavior differs from that reported in similar material but with a much finer TB spacing that was grown epitaxially on its substrate [1].) A nominal shear strain of 21 from HPT was found to be concentrated near sample surfaces, dropping to a small fraction of this value in the sample interior. The high stresses near the surfaces tilt the twins and break them into equiaxed grains. The transition from equiaxed grains to the original nanotwin structure permits calculation of the stability limit of the nanotwins under shear strain. TEM images illustrate the processes leading to structural changes in both the compression and HPT. [1] Anderoglu, O., et al., Int. J. Plasticity (2009)
11:45 AM - P1.8
FIB-less Fabrication and Mechanical Properties of Nano-twinned Cu Nano-pillars through In-situ Mechanical Testing.
Dongchan Jang 1 , Julia Greer 1
1 Division of Engineering and Applied Science, California Institute of Technology, Pasadena, California, United States
Show AbstractNano-twinned materials have attracted a lot of scientific interests because of their simultaneous attainment of high strength and ductility. We developed a fabrication technique that does not rely on the use of focused ion beam (FIB) to create vertically aligned cylindrical Cu nano-pillars with 50nm-250nm diameters containing orthogonal penny-like nano-twins with uniform ~3nm thickness. Uniaxial mechanical testing of these structures provides a deeper understanding of plastic deformation mechanisms in nano-twinned metals, mainly benefiting from the well-defined stress and strain distributions compared with commonly used in-grain nano-twins. By precise control of the microstructure, i.e. lamella thickness and twin boundary orientation, as well as pillar diameter, we are able to define the parameter space for attainment of optimal mechanical properties. These nano-pillars were fabricated via electrodeposition through templated nanometer-sized cylindrical holes, and uniaxial tension and compression tests at a range of strain rates (1e-2 ~ 1e-4 sec-1) were conducted via in-situ testing. Microstructural changes due to plastic deformation were investigated via site-specific transmission electron microscopy (TEM) of the same pillars before and after deformation under the same diffraction conditions.
12:00 PM - P1.9
On the Sequence of Events during Pop-ins in Spherical Nanoindentation.
Siddhartha Pathak 1 , Jessica Riesterer 1 , Shraddha Vachhani 2 , Kilian Wasmer 3 , Surya Kalidindi 4 2 , Johann Michler 1
1 Mechanics of Materials and Nanostructures Laboratory, EMPA - Swiss Federal Laboratories for Materials Science and Technology, Thun Switzerland, 2 Department of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania, United States, 3 Advanced Materials Processing Laboratory, EMPA - Swiss Federal Laboratory for Materials Science and Technology, Thun Switzerland, 4 Department of Mechanical Engineering and Mechanics, Drexel University, Philadelphia, Pennsylvania, United States
Show AbstractDuring nanoindentation experiments under spherical indenters (or sharp indenters with a residual radius at their tip) crystalline materials have been observed to yield extremely high stresses – the magnitude of these stresses sometimes approaching the theoretical shear strength of the material. Commonly referred to as ‘pop-in’ events, they generally result in sudden excursions in depth at low loads (for a load controlled experiment) which acts as a trigger for the onset of plastic deformation. In this work we analyze these pop-ins and their underlying mechanisms in significant detail using the combined capabilities of spherical nanoindentation stress-strain curves and atomic force microscopy (AFM). As shown in this work, this combination can capture the details of the pop-in events much more accurately than what is possible from the nanoindentation load-displacement data alone. For example, the indentation stress-strain curves depict a region of almost elastic unloading following the pop-in, only at the end of which the contact stresses reach a level unaffected by the pop-in – a rather perplexing observation if one were to rely only on the raw load-displacement measurement. In order to investigate this effect further, hybrid AFM/scanning electron microscopy (SEM) was conducted on indents which were interrupted at the point of pop-in. By comparing these AFM images with those of indents at larger depths, we review the sequence of events occurring immediately after a pop-in in spherical nanoindentation, and suggest a possible mechanism for the resulting phenomena.
12:15 PM - P1.10
Epitaxial Nanotwinned Silver Films.
Daniel Bufford 1 2 , Haiyan Wang 3 , Xinghang Zhang 2
1 Materials Science and Engineering, Texas A&M University, College Station, Texas, United States, 2 Mechanical Engineering, Texas A&M University, College Station, Texas, United States, 3 Department of Electrical and Computer Engineering, Texas A&M University, College Station, Texas, United States
Show AbstractSilver films are grown on (111) and (110) silicon substrates by magnetron sputtering, resulting in two different epitaxial variants: Ag(111)/Si(111) and Ag(110)/Si(110). Examinations of the Ag (111) microstructure reveal dense twin boundaries with Σ3{111} coherent twin boundaries oriented normal to the growth direction and Σ3{112} incoherent twin boundaries parallel to the growth direction. Examinations of the Ag (110) film reveal much lower densities of twin boundaries. The films are compared to similar microstructures seen in copper, nickel, and stainless steel films, and the influence of the twinned microstructures on mechanical strength and resistivity are discussed.
12:30 PM - P1.11
Crack-twin Interactions in Ultrafine-grained Copper Thin Films.
Seong-Woong Kim 1 , Xiaoyan Li 1 , Huajian Gao 1 , Sharvan Kumar 1
1 Division of Engineering, Brown University, Providence, Rhode Island, United States
Show AbstractThe interactions of an advancing crack with nano-twins in submicron grained Cu films were examined by in-situ experiments in the transmission electron microscope (TEM). Free-standing, electron-transparent, Cu films with a uniform thickness in the range 40 ~ 100 nm and grain sizes of 30~70 nm were produced by vapor deposition and subsequently heat-treated in the TEM to enable grain growth and generate annealing twins. In parallel, large-scale atomistic simulations were used to investigate this interplay between a moving crack and multiple twins in a free standing copper thin film. The embedded atom method potential is used to describe the atomic force, and a Nose-Hoover thermostat is used to maintain a NVT ensemble at 300K. A multiple time-step algorithm is adopted with shorter and longer time steps of 2 femtoseconds and 6 femtoseconds, respectively. The sample is first relaxed and equilibrated at room temperature for 100 picoseconds. Then, the tensile loading (Mode I) or mixed loading (Mode I & II) is applied under a constant strain rate of 1×108/s. Experiments and computations show that upon applying a strain, the pre-existing crack initially became blunt as numerous dislocations were emitted from the crack tip. As the imposed strain reached a critical value scaled by the film thickness, a daughter crack was nucleated, as a result of localized deformation, at the intersection between the persistent slip band and the twin plane, about one matrix domain away from the main crack. Such crack hopping appears to be unique in copper thin films with nanoscale growth twins and is thought to arise from complicated interactions between dislocations and the coherent twin boundaries in front of the crack tip. Thus, the coherent twin plane appears to play a key role in determining the crack propagation path and enhancing the toughness of the film.
12:45 PM - P1.12
Strong Size-dependent Twinning Behaviors in Single Crystal Ti Alloy at Micron/Submicron Scale.
Qian Yu 1 2 , Zhiwei Shan 2 , Ju Li 3 , Xiaoxu Huang 4 , Jun Sun 2 , Evan Ma 5
1 Materials Science and Engineering, University of California at Berkeley, Berkeley, California, United States, 2 , State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an, Shannxi, China, 3 Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States, 4 , Danish-Chinese Center for Nanometals, Materials Research Division, Roskilde Denmark, 5 , Department of Materials Science and Engineering, Baltimore, Maryland, United States
Show AbstractDeformation twinning is an important plastic deformation mode that controls the mechanical behavior of many materials. Specifically for materials with HCP structure, twinning deformation plays significant role in compensating the plastic deformation due to the insufficiency of slip systems. However, for the study of size effect on materials mechanical properties, the response of twinning deformation to the decrease of sample size is relatively lack of study though there have been a lot of work done for size-dependent dislocation slip behavior. Meanwhile the origin and spatio-temporal features of twinning are still shrouded in mystery. Using micro-compression and in situ nano-compression experiments, we find that the stress required for deformation twinning shows strong size-dependence. The formation of twin becomes harder when the sample size reduce from several millimeters to around one micron. The maximum yield stress is approached when the sample size is reduced to one micrometer, below which the deformation twinning is entirely replaced by less ordinary dislocation plasticity. Accompanying with this deformation mode transition continues plastic deformation appears instead of the sudden strain burst in the larger samples with twinning control plasticity. The saturation of the maximum flow stress appears and shows size-independence in the samples with critical size below one micron. A ‘stimulated slip’ model which is based on Pole mechanism for the formation of twin is developed to explain the strong size dependence of deformation twinning. The relatively large size for this plastic deformation modes transition makes our understanding highly relating to application.
P2: Nanocrystalline Metals & Deformation Mechanisms
Session Chairs
Jeff de Hosson
Julia Greer
Monday PM, November 29, 2010
Room 210 (Hynes)
2:30 PM - **P2.1
Nanometer Scale Mechanical Behavior of Grain Boundaries Characterized by Hybrid Scientific Computation/Experiment.
Chien-Kai Wang 1 , Huck Beng Chew 1 , Kyung-Suk Kim 1
1 Engineering, Brown University, Providence, Rhode Island, United States
Show AbstractIn recent years scientific computation hybridized by experiment is extensively developed to characterize mechanical behavior of nanostructures for emergying technological applications. As an example, nanometer scale mechanical properties of grain boundaries in FCC metals are analyzed by a hybrid method of MD simulations/experiment. For the analysis, a nonlinear field projection method has been developed and used to characterize the processes of dislocation nucleation and crack trapping along nanocrystalline grain boundaries. The nonlinear field projection is based on the principle of virtual work, for virtual variations of atomic positions in equilibrium through nonlocal interatomic interactions such as EAM potetial interaction, to get field-projected subatomic-resolution traction distributions on various grain boundaries. The hybrid analyses show that the field projected traction produces periodic concentrated compression sites on the grain boundary, which act as crack trapping or dislocation necleation sites. The computational field projection is hybridized by HRTEM experimental as well as AFM interferometric field projections. These field projections have identified configurational defect sites on grain boundarys where external stress loading can generate configurational forces that can control nanometer scale failure processes of the grain boundaries.
3:00 PM - P2.2
New Insight into Acoustoplastic Softening Through Nanoscopically Localized Electron Microscopy Examination and Dislocation Dynamics Analysis.
Kai Wing Siu 1 , Alfonso Ngan 1
1 Mechanical Engineering, University of Hong Kong, Hong Kong Hong Kong
Show AbstractUsing focussed-ion-beam (FIB) milling to prepare nanoscopically localized samples for electron microscopy examination is a powerful technique for the study of a wide range of metallurgical phenomena old and new, especially when interpretation of results is augmented by dislocation dynamics simulations. In this work, we used such a combination of state-of-the-art techniques to gain new insight into acoustoplastic softening, or the so-called Blaha effect. Such an effect involves using ultrasonic vibration to soften metals, and finds important applications in metal forming and welding. Although this effect was discovered in the 1950’s, its mechanism is still poorly understood. The existing understanding of acoustoplastic softening builds on the assumption that the ultrasonic irradiation either imposes additional stress waves to augment the quasi-static applied load, or causes heating of the metal, whereas the metal’s intrinsic deformation resistance or mechanism is unaltered by the ultrasound. In this study, we used a combination of cross-sectional microscopic techniques involving FIB, transmission electron microscopy and electron backscattered diffraction to examine aluminum samples subjected to indentation deformation with and without the simultaneous application of ultrasound. We discovered that the softening effect of the ultrasound is due to recovery associated with extensive subgrain formation during deformation. By comparing the microstructures of samples deformed with and without simultaneous application of ultrasound, and those subsequently excited by ultrasound after deformation, the subgrain formation is proved to be one due to the combined application of the quasi-static loading and the ultrasound, but not a simple addition of the two. Similarly, by comparing with samples deformed while being simultaneously or subsequently heated up, the subgrain formation by the ultrasound is proved to be a lot greater than that due to the heat that it generates within the metal. Dislocation dynamics simulations show that when an oscillatory stress is superimposed on a static stress, the dislocations travel faster in a jerky manner, allowing them to more efficiently explore and adjust to a suitable environment for dipole annihilation. The present findings represent a new phenomenon of dislocation group behaviour which deserves further in-depth investigation.
3:15 PM - P2.3
Dislocation Kinetics in Fe and Fe Alloys Investigated by In-situ TEM Straining Experiments.
Daniel Caillard 1
1 CEMES, CNRS, Toulouse France
Show AbstractIn situ straining in the TEM remains up to now the most efficient experimental method to analyse the mechanisms controlling the mechanical properties of crystals at the nano-scale. This technique has been continuously improved, and it can now yield detailed quantitative information on the kinetics of individual dislocations, as a function of local stress and temperature. In situ experiments have been carried out in pure Fe, in order to check the validity of the most recent ab-initio atomistic calculations of the core structure of screw dislocations, and to study the change of mechanism which is expected to occur at around 250K [1, 2]. Microsamples have been strained in a JEOL 2010HC transmission microscope, using room-temperature and nitrogen-cooled GATAN devices. These experiments benefit from the facilities offered by the most recent digital video cameras. For instance, image differences allow one to increase the contrast of the moving dislocations, and to measure their displacement with the pixel resolution. Results will be illustrated by dynamic sequences.At room temperature [3], dislocation loops exhibit straight screw portions moving slowly and steadily, and curved non-screw ones with a much higher mobility. Dislocation sources allowed us to show that the velocity of screw parts is proportional to their length, in agreement with the kink-pair model. The shape of the non-screw parts can be analysed with a fairly high accuracy, by using difference-images. For instance, the cusp predicted by anisotropic elasticity can be observed, and the average radius of curvature can yield the local stress. Under such conditions, a relaxation experiment performed in situ has allowed us to determine for the first time the velocity-stress dependence of a single dislocation.At low temperatures [4], the motion of screw dislocations becomes jerky. The frame-by-frame analysis of this jerky motion shows that it cannot result from the same kink-pair mechanism as at room temperature. This change of kinetics has been interpreted by a change in the mechanism of motion of screw dislocations across the Peierls potential, and correlated to the change in the corresponding macroscopic activation parameters.In the whole temperature range, the elementary slip planes are shown to be of the {110} type, in spite of extensive cross-slip. This confirms the results of ab-initio calculations, showing that the core of screw dislocations is non-degenerated in pure Fe.The softening effect of carbon, and the hardening effect of silicon and chromium, are shown to result from the shift of the transition between the two mechanisms to respectively lower and higher temperatures.References[1] Y. Aono, K. Kitajima, E. Kuramoto, Scripta Met. 15 (1981) 275.[2] D. Brunner, J. Diehl, Phys. Stat. Sol. (a) 160 (1997) 355.[3] D. Caillard, Acta Mater. 58 (2010) 3493.[4] D. Caillard, Acta Mater. 58 (2010) 3504.
3:30 PM - P2.4
Dislocation Reactions and Multiplication in α-Fe at High Temperature Modelled with Anisotropic Dislocation Dynamics (AniDiS).
Steve Fitzgerald 1 , Sylvie Aubry 2 , Sergei Dudarev 1 , Wei Cai 2
1 Theory and Modelling, Culham Centre for Fusion Energy, Abingdon United Kingdom, 2 Mechanical Engineering, Stanford University, Stanford, California, United States
Show AbstractAs the α-γ transition at 912°C is approached, the crystal lattice of α-Fe becomes extremely elastically anisotropic. This phenomenon, resulting from the interplay between magnetic fluctuations and atomic vibrations at elevated temperature, has profound consequences for the nanoscale physics of dislocations, the elementary carriers of plasticity, and hence on the deformation behaviour and mechanical properties of macroscale material samples [1,2]. For example, the strength of steels treated as a function of temperature, exhibits a characteristic transition near ~450°C, which is likely associated with a “phase transition” in the dislocation microstructure of the material. We report the latest results from the recently developed anisotropic discrete dislocation dynamics package AniDiS, which we developed from the massively-parallel isotropic code ParaDiS [3]. Particular attention is paid to Frank-Read sources, one of the most important dislocation multiplication mechanisms, and junction formation, which plays a central role in work hardening. The full numerical simulation predictions are compared with the more qualitative results available from idealized models such as the line tension approximation (where interactions are neglected) and a fully analytical variational procedure which will be discussed in detail. The common tangent construction on the inverse Wulff plot of dislocation energy vs. orientation is fully explained in terms of necessary and sufficient stability conditions on the variational solution for the dislocation shape [4]. Finally a connection is made with data from micromechanical testing experiments, and TEM characterization of irradiated α-Fe, for a temperature range over which the lattice anisotropy varies considerably. Key features of dislocations in highly anisotropic crystals, in particular the sharp-cornered configurations which arise as a result of thermodynamically unstable dislocation orientations, and the precipitous fall in strength at high temperatures, are observed in good agreement with the theoretical and numerical predictions. [1]SPF and SLD, Proc R Soc A464 (2098) 2549 (2008)[2]SLD et al, Phys Rev Lett 100 (13) 125503 (2008)[3]VV Bulatov et al, Nature 440 (7088) 1174 (2006)[4]SPF, Phil Mag Lett 90 (3) 209 (2010)
3:45 PM - P2: NanoXtal
Break
4:00 PM - P2.5
Deformation Mechanics of Nanocrystalline Metals.
Steven Van Petegem 1 , Julien Zimmermann 1 , Helena Van Swygenhoven 1
1 NUM/ASQ, Paul Scherrer Institut, Villigen PSI Switzerland
Show AbstractIt is well-known that nanocrystalline metals exhibit increased hardness and strength compared to their coarse-grained counterparts. The mechanisms responsible for these enhanced properties are still under debate, especially for the smallest grain sizes where deviations from the Hall-Petch relationship are frequently reported. Experimental observations suggest that dislocation activity may still be present, even at very small grain sizes. However it is generally accepted that when the grain size is reduced under 100nm dislocation sources inside the grains become less effective and dislocation pile-ups are largely reduced. On the other hand, grain boundaries start playing a more prominent role; acting as both sources and sinks for dislocations in a single dislocation regime.In situ x-ray diffraction is a well-established technique to study the evolving microstructure during deformation. With respect to nanocrystalline metals it has been successfully applied by multiple groups to investigate the evolution of grain size, residual strains and defect densities. However, there are still many unresolved issues and even conflicting results have been reported.In this presentation we report on the evolution of both the elastic lattice strains and elastic inhomogeneous strains during deformation of nanocrystalline Ni and NiFe at different temperatures. In particular we focus on the transition between micro- and macroplasticity, strain recovery after unloading and the yield point phenomenon in NiFe.
4:15 PM - P2.6
Strain Rate Sensitivity of Nanocrystalline Au Films at Room and High Temperatures.
Ioannis Chasiotis 1 , Nikhil Karanjgaokar 2 , Chung-Seog Oh 3
1 Aerospace Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 2 Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 3 School of Mechanical Engineering, Kumoh National Institute of Technology, Gumi Korea (the Republic of)
Show AbstractThe effect of strain rate on the inelastic properties of nanocrystalline Au films was quantified with 0.85-μm and 1.76-μm freestanding microscale tension specimens tested over eight decades of strain rate, between 6x10^-6 - 20 s^-1. The elastic modulus was independent of the strain rate, 66±4.5 GPa, but the inelastic mechanical response was clearly rate sensitive. The yield strength and the ultimate tensile strength increased with the strain rate in the ranges 625-825 MPa and 750-850 MPa, respectively, with the yield strength reaching the tensile strength at rates faster than 10^-1 s^-1. The activation volumes for the two film thicknesses were 4.5b3 and 8.1b3, at strain rates smaller than 10^-4 s^-1 and 12.5b^3 and 14.6b^3 at strain rates larger than 10^-4 s^-1, while the strain rate sensitivity factor and the ultimate tensile strain increased below 10^-4 s^-1. The latter trends indicated that the strain rate regime 10^-5 - 10^-4 s^-1 is pivotal in the mechanical response of the particular nanocrystalline Au films. The increased rate sensitivity and the reduced activation volume at slow strain rates were attributed to grain boundary processes that also led to prolonged (5-6 hr) and significant primary creep with initial rates of the order of 10^-7 s^-1 or faster. The strain rate experiments were extended to temperatures 55C, 85C and 110C resulting in even more dramatic changes in strain rate sensitivity.
4:30 PM - P2.7
Strain Rate Sensitivity and Deformation Mechanisms of Nanocrystalline Pd Alloys.
Ruth Schwaiger 1 , Insuk Choi 2 3 , Thomas Neithardt 1 , Oliver Kraft 1 2
1 IZBS, Karlsruhe Institute of Technology, Karlsruhe Germany, 2 IMF II, Karlsruhe Institute of Technology, Karlsruhe Germany, 3 High Temperature Energy Materials Center, Korea Institute of Science and Technology, Seoul Korea (the Republic of)
Show AbstractObservations of size-dependent deformation are becoming more common as materials and structures are engineered to smaller and smaller dimensions. In general, a reduction in grain size leads to higher strength in metallic materials, with different mechanisms dominating deformation on smaller length scales. In the grain size regime below 30 nm, dislocation activity is reduced and it is well accepted that the motion of full dislocations is not the predominant deformation mechanism. Suggested mechanisms include nucleation and motion of partial dislocations, grain boundary sliding as well as grain rotation and growth. This mechanism change is supported by the observation that nanocrystalline metals exhibit much stronger strain rate sensitivity at low temperatures, with small activation volumes, compared to their coarse-grained counterparts. In this presentation, the mechanical behavior of nanocrystalline Pd, Pd-Ag and Pd-Au with grain sizes between a few and about 150 nm will be discussed. The activation volume at very small grain size has been observed to be of the order of a few atomic volumes and increases with increasing grain size. First results indicate that the addition of Au or Ag reduces the strain rate sensitivity at room temperature. Our findings will be discussed in the light of suggested models for the deformation of nanocrystalline materials.
4:45 PM - P2.8
Elasto-plastic Transition in Nanostructured Materials: Definition and Effect of Internal Stress on Microplastic Regime.
Ludovic Thilly 1 , Florence Lecouturier 2 , Pierre-Olivier Renault 1 , Steven Van Petegem 3 , Helena Van Swygenhoven 3
1 Pprime Institute, University of Poitiers, Futuroscope France, 2 , LNCMI, Toulouse France, 3 , Paul Scherrer Institute, Villigen Switzerland
Show AbstractNanocomposite wires composed of a multi-scale Cu matrix embedding Nb nanotubes are in situ cyclically deformed in tension under synchrotron radiation in order to follow the x-ray peak profiles (position and width) during mechanical testing. The evolution of elastic strains vs. applied stress suggests the presence of phase-specific elasto-plastic regimes in direct relation with size characteristics. The nature of the elasto-plastic transition is uncovered by the ‘‘tangent modulus” analysis and correlated to the microstructure of the Cu channels and the Nb nanotubes. Finally, a new criterion for the determination of the macroyield stress is given as the stress to which the macroscopic work hardening, θa = dσa/dε0, becomes smaller than one third of the macroscopic elastic modulus. This criterion appears to be valid to determine the transition from elasto-microplastic to macroplastic regimes in several nanocrystalline materials in contradiction to the traditional 0.2%-strain offset criterion [Acta Materialia 57 (2009) 3157–3169]. Finally, different thermal treatments are applied to heavily cold drawn Cu/Nb nanocomposite wires to study their effect on the occurrence/extension of the microplastic regime.
5:00 PM - P2.9
Strain Induced Changes in Nanocrystalline Metals Characterized by Automated Acquisition and Indexing of Diffraction Patterns in the TEM.
Andreas Kulovits 1 , Jorg Wiezorek 1 , Kai Zweiacker 1
1 Mechanical Engineering and Materials Science, Swanson School of Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania, United States
Show AbstractNano-crystalline Ni with an average grain size of 40nm has been cold deformed at room temperature by rolling to thickness reductions of up to 85%. In this grain size regime plastic deformation is facilitated by dislocation activity. However, in contrast to micro-crystalline Ni here dislocation–grain boundary interactions instead of dislocation-dislocation interactions dominate during plastic flow after yielding. Grain boundaries act as dislocation sources and sinks. Every time a grain boundary segment emits or absorbs a dislocation the character of the grain boundary changes. As deformation progresses grain boundaries initially high angle in nature are transformed into low angle boundaries, which subsequently dissolve as they continue to interact with dislocations, thereby inducing grain coalescence. At large strains dislocation–dislocation activity is observed in the coalesced larger grains. We used automated acquisition and indexing of precession diffraction patterns in the transmission electron microscope TEM (ASTAR/DigiSTAR from NanoMEGAS)) to monitor the evolution of grain orientations and in grain boundary character with increasing cold deformation strain. Hollow cone precession dark field imaging was used to determine changes in average grain size and grain size population. The high spatial resolution of the TEM allowed us to monitor these changes on the nanoscale, which was previously not possible with SEM based approaches. We acknowledge use of the facilities of the Materials Micro-Characterization Laboratory of the Department of Mechanical Engineering and Materials Science, University of Pittsburgh, and support by a grant from the National Science foundation NSF-CMS 0140317.
5:15 PM - P2.10
Plastic Deformation of Nanocrystalline CuAu Alloys: On the Interplay of Composition, Ordering and Grain Boundary Relaxation.
Jonathan Schaefer 1 , Alexander Stukowski 1 , Karsten Albe 1
1 Materials Modeling, TU Darmstadt, Darmstadt Germany
Show AbstractPlastic deformation of nanocrystalline Cu–Au is studied by means of atomic scale computer simulations. The main goal of this work is to elucidate the relationship between the intrinsic properties of a nanocrystalline, ordering alloy and the observed macroscopic mechanical behavior. The distribution of solutes is equilibrated in nanocrystalline model structures of different grain sizes using a combination of Monte-Carlo and molecular dynamics methods. The resulting samples are deformed under uniaxial load. The role of grain boundary relaxation is analyzed in detail by comparing chemically and structurally relaxed samples with model structures that were only structurally relaxed. The role of ordering within the grains is studied by an additional comparison with model structures, that were equilibrated above the order-disorder transition temperature.By analyzing the local chemical ordering and the evolution of defects within the microsturcture, we make a connection between the atomistic configuration and the observed stress-strain behavior. The simulations reveal that the relaxation state of the GBs is of great importance for the maximum strength of a nanocrystalline alloy. The activation barrier for GB sliding, which is an important mode of deformation at small grain sizes, is raised by a chemical equilibration of the GBs.In contrast, the evolution of the flow stress and the capability for work hardening at small strains is influenced by intragranular deformation mechanisms. Here, the nucleation of the major carriers of intragranular deformation, namely dislocations and superdislocations is mainly affected by the chemical ordering within the grains.
5:30 PM - P2.11
In-situ Observations of Stress Induced Microstructural Evolution in Thin Films.
John Sharon 1 , Brian Schuster 2 , Claire Chisholm 3 4 , Andrew Minor 3 4 , Kevin Hemker 1
1 Mechanical Engineering Dept, Johns Hopkins University, Baltimore, Maryland, United States, 2 , US Army Research Laboratory, Aberdeen Proving Ground, Maryland, United States, 3 National Center for Electron Microscopy , Lawrence Berkeley National Laboratory , Berkeley, California, United States, 4 Dept. of Materials Science & Engineering, University of California, Berkeley, Berkeley, California, United States
Show Abstract In-situ tensile experiments coupled with electron backscatter diffraction (EBSD) have been employed to observe stress induced microstructural evolution in nanocrystalline thin films. It is well known that nanocrystalline materials can differ from their bulk counterparts not only in terms of properties but also in the modes of deformation. These nano-structured materials, which are common in micro and nano devices, can have unstable microstructures which trigger novel deformation mechanisms. One mechanism of particular interest is stress driven grain boundary migration which has been reported in several nanocrystalline metals including Al, Cu, and Ni. With this mechanism, deformation is accommodated when shear stresses couple to a grain boundary causing it to move. To observe such microstructural evolution, we have conducted experiments on metallic thin films using a custom built SEM in-situ tensile stage designed to allow for EBSD scans during the mechanical deformation. With this approach, the tensile straining activates the grain boundary migration while the EBSD, which can elucidate changes in grain size, orientation, and boundary character, is used to track the boundary motion thus allowing connections to be made between the stress state in the film and its microstructural development. Results from experiments with Al and Au free standing thin films will be presented.
5:45 PM - P2.12
Surface Effects and the Limits of Strength in FIB-less Single Crystalline Copper Nanopillars.
Andrew Jennings 1 , Julia Greer 1
1 , California Institute of Technology, Pasadena, California, United States
Show AbstractUniaxial compression and tension tests on single crystalline micro and nanopillars have revealed a strong size effect. For face-centered cubic metals, this size effect is characterized by a power-law: where n is between .5 - .7. The majority of these micro-mechanical tests have been performed on pillars produced by the focused-ion-beam (FIB), a process known to introduce surface damage into the material and to limit the smallest attained pillar diameter to ~150nm while maintaining its shape integrity. In order to overcome these detriments, we developed a new technique combining electroplating and electron beam lithography to create single crystalline Cu nano-pillars with diameters down to 50 nm. We find the mechanical response of these samples to exhibit the same power-law strengthening behavior as other fcc metals down to the diameter of 100nm, as revealed by in-situ uniaxial compression and tension tests conducted in a custom-built in-situ mechanical deformation instrument, SEMentor and nanoindenter. TEM investigations of the microstructure of pillars produced by the FIB and by electroplating show similar initial dislocation densities of ~1014 m-2 implying that size-dependent strength at the nano-scale is a strong function of initial microstructure and not of fabrication method. We examine the limits of this power-law trend down to diameters of 50nm, as at these small sizes, deformation behavior has been theoretically predicted to change due to the activation of surface dislocation sources and the increasing influence of the surface stress. Furthermore, we find that these single crystalline Cu nano-pillars show a remarkable strain-rate dependence that increases with decreasing diameter further revealing the thermally activated nature of dislocation sources and corresponding changes in activation volume. HRTEM investigations of post-mortem structures will be presented in the context of dislocation-based phenomenological modeling.
P3: Poster Session I
Session Chairs
Tuesday AM, November 30, 2010
Exhibition Hall D (Hynes)
9:00 PM - P3.10
Mechanisms of Stress-driven Motion of Asymmetrical Tilt Grain Boundaries.
Zachary Trautt 1 , Yuri Mishin 1
1 Physics and Astronomy, George Mason University, Fairfax, Virginia, United States
Show AbstractIn their seminal paper of 1950, Read and Shockley presented a dislocation model of grain boundary (GB) migration accompanied by shear deformation of the volume swept by the boundary. For symmetrical tilt GBs, this coupled GB motion was later observed in experiments and reproduced in many atomistic computer simulations. In the same paper, Read and Shockley asserted that coupled motion of asymmetrical tilt GBs, which are composed of different types of dislocations, would be impossible as the different dislocations would be pulled apart or squeezed together. Contrary to this, we present results of molecular dynamics simulations showing that asymmetrical low-angle tilt GBs in Cu and Al are in fact moved by applied shear stresses and demonstrate all features of the coupling effect. The simulations, performed over a range of inclination angles, temperatures and GB velocities, uncover the possible mechanisms of collective glide of dislocation arrays composed of different dislocations. Consequences for stress-driven GB motion in nano-crystalline materials are discussed.
9:00 PM - P3.13
Wear Mechanisms in Nanocrystalline Al-Si.
Ian Baker 1 , F. Kennedy 1 , M. Gwaze 1 , P. Munroe 2
1 Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire, United States, 2 Electron Microscope Unit, University of New South Wales, Sydney, New South Wales, Australia
Show AbstractIn this presentation the wear behavior and wear mechanisms of both nanocrystalline eutectic Al-Si alloys and unalloyed Al nanocrystalline will be compared with that of conventional material. Nanocrystalline powders were prepared through mechanical alloying of elemental powders and consolidated using equal channel angular extrusion. The microstructure of the as-extruded materials was characterized using transmission electron microscopy (TEM) including X-ray microanalysis. Wear behavior was examined using pin-on-(zirconia) disk wear tests and the worn pins were characterized using both scanning electron microscopy and TEM. The consolidated materials were found to exhibit lower strengths than the as-cast materials. Wear tests on both as-cast and nanocrystalline pins showed wear characterized by an oxide-rich tribolayer. Debris from broken zirconium oxide asperities appeared to be the main abrasive medium.
9:00 PM - P3.14
Bottom-up modeling of the Elastic Properties of Organosilicate Glasses and Their Relation to Composition and Network Defects.
Jan Knaup 1 2 , Li Han 3 , Joost Vlassak 3 , Efthimios Kaxiras 1 2 3
1 Laboratory for Multiscale Modeling of Materials, EPFL, Lausanne Switzerland, 2 Department of Physics, Harvard University, Cambridge, Massachusetts, United States, 3 School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, United States
Show AbstractOrganosilicate glasses (OSG) are used as low-k inter metal dielectrics for advanced integrated circuits. In this application, the material must fulfill two conflicting requirements, on the one hand it has to have a low density to reduce the dielectric constant, on the other hand it has to be mechanically stable enough to withstand the thermally induced stress arising during subsequent steps of integrated circuit manufacture. Recent experimental advances in improving the mechanical and electrical properties of these materials have, however, not yet been systematically studied theoretically on an ab initio level. In this work, we employ the very efficient density-functional based tight-binding (DFTB) method to achieve an accurate description of OSG of different compositions with perfect and defective network coordination. Our results show, that a transition between different mechanisms of elastic deformation between silica glass and silicon hydrocarbide leads to different behaviors of the density, stiffness and sensitivity to network defects depending on the hydrocarbon content.
9:00 PM - P3.16
Deformation Mechanisms of Ultra-nanocrystalline Diamond.
Yifei Mo 1 , Donald Stone 2 1 , Izabela Szlufarska 2 1
1 Materials Science Program, University of Wisconsin, Madison, Wisconsin, United States, 2 Department of Materials Science and Engineering, University of Wisconsin, Madison, Wisconsin, United States
Show AbstractUltrananocrystalline diamond (UNCD) is known to have excellent mechanical properties, such as wear resistance and hardness. Because of the exceptional hardness of UNCD, its deformation mechanisms have been challenging to investigate. Additionally, since dislocation activity is entirely suppressed in UNCD, it is a model material for studying of the role that grain boundaries (GB) play in deformation. We performed large-scale molecular dynamics simulations of nanoindentation of UNCD. We found that the primary mechanism of plastic deformation in UNCD is GB sliding, which is accommodated elastically. We demonstrate the analogy between GB sliding and dislocation plasticity. Specifically, the yield stress and hardness of UNCD are controlled by the shear strength of GBs in analogy to the Peierls stress that controls dislocation dynamics. Similarly to dislocations, GB sliding events can interact and pile-up leading to hardening of the material. We discover that phenomenogical theories relating hardness and flow stress to the critical shear stress that controls plasticity apply in the case of UNCD, despite the fact that these theories have been developed for dislocation-based plasticity in metals. The above conclusions are expected to apply to other nanocrystalline materials in the regime where GB sliding is the primary deformation mechanism. Combined understanding of GB sliding and dislocation plasticity will enable tailoring mechanical properties of nc materials by GB engineering.
9:00 PM - P3.17
Self-healing and Fracture Mechanisms for Star Polymer Networks.
Isaac Salib 1 , German Kolmakov 1 , Krzysztof Matyjaszewski 2 , Anna Balazs 1
1 Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania, United States, 2 Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States
Show AbstractThe understanding of the underlying mechanism of self healing and rupture of the polymeric networks is important in the designing and manufacturing of smart polymers and gels. These materials possess a decisive role in shaping the future of electronic packaging, tissue manufacturing, protective coatings, etc. In this work, the mechanical properties of the self-healing polymeric networks, which are constructed from star polymer units, are investigated. The star polymers can form two kinds of intra- and inter-bonds, one of which is much stronger than the other. The weaker bonds are referred to as the labile bonds. The percentage of the labile bonding in the polymeric network defines its mechanical behavior and the way it reacts to exterior mechanical load, as well as its self-healing ability. These supramolecular structures are modeled by Dissipative Particle Dynamics (DPD), a coarse grained simulation method. In our simulations, we apply tensional deformation to the network in an ambient solution until fracture occurs. We calculate the optimal percentage of labile bonds needed to obtain the star polymer mesh with the largest yielding strength. We suggest a hypothesis for the molecular mechanism of the self-healing procedure of such materials. We show that the conformational uniqueness of the star polymers as the building blocks of these networks and the kinetic nature of the intermolecular bonding allow the design of an efficient healing polymeric structure once the star polymers' parameters are chosen judicially. We compare the results of these investigations to those drawn for nanogels from previous studies.
9:00 PM - P3.18
Formation of an Intermediate Ordered Stage by Forced Mixing of Nano-grained Immiscible Alloys.
Yinon Ashkenazy 1 2 , Nhon Vo 2 , Robert Averback 2 , Pascal Bellon 2
1 Racah Institute of Physics , The Hebrew University of Jerusalem, Jerusalem Israel, 2 Department of Materials Science and Engineering, University of Illinois at Urbana Champaign, Urbana, Illinois, United States
Show AbstractImmiscible alloys can often be formed by mechanical alloying of pure metal powders. We studied the response to severe plastic deformation of phases with dissimilar crystalline structures using a nano-grained sample of Cu-Nb as a model system. We show using molecular dynamics simulations that forced mixing at low temperatures in this system results in unique structures.While for strains at strains of the order of 100 and above full mixing is attained, we show that at intermediate strains of order of 10, Nb particles embedded in a Cu matrix form elongated faceted shapes with (110) Nb being parallel to (111) Cu at the interfaces. This is a consequence of most dislocation activity being limited to the copper, and that the Nb particles reorient with the Cu matrix. Thus while the matrix deforms by dislocation activity, the embedded particle becomes faceted and changes shape. The observation that for this system dislocation activity is limited to the ductile Copper, agrees with previous observations in layered structures. Due to this compliance mechanism, particles reshape into elongated structures, and a preferred direction is spontaneously chosen. This stage is followed by a formation of a supersaturated region where Nb atoms are mixed into the Copper matrix at relatively high concentration. This leads to local amorphization, which agrees with experimental observations in wire drawn Cu-Nb [X. Sauvage, L. Reanud, B. Deconihout, D. Blavette, D. H. Ping and K. Hono, Acta mater. 49, 389 (2001) ]. We show that in our case amorphization is non-uniform as it is localized in regions restricted by glide planes of the matrix. We demonstrate the effects of loading conditions, strain path, and temperature on the formation of these unique structures and discuss possible experimental scenarios for their formation.
9:00 PM - P3.19
MD Simulation of the Impact of Burnup on Thermal Properties of Uranium Dioxide.
Li Liu 1 , Bin Wu 1 , Xiangyu Wang 1
1 Mechanical, Aerospace, and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States
Show AbstractMolecular dynamics is capable of delivering knowledge of micro-structural evolution of materials at atomic scale. This microscopic view could help us better interpret macroscopic behavior of materials and predict their characteristics under severe conditions that experiments could not readily reach. Uranium dioxide is of great research value owing to its significant role in nuclear reactor. Due to intensive exposure to harsh environment, UO2 suffers from radiation induced damage that could be aggravated by higher burnup which is preferred in view of economics for next generation reactors. This paper studies the influence of burn-up on radiation induced microstructure evolution in uranium dioxide through MD simulation. As is widely known, the reliability of MD simulation lies in the accuracy of inter-atomic potential in describing the interaction between atoms. Numerous empirical potentials have been developed in the literature. But each of them has respective applicability and limitations. For present study, the potential proposed by Morelon [1] is utilized since this potential is proven to be able to best reproduce the properties of uranium dioxide regarding to radiation caused defects. However, this potential shows unrealistic features when the inter-atomic distance is considerably smaller than the one at equilibrium, which could take place during ballistic displacement cascade. Hence the universal Ziegler-Biersack-Littmark (ZBL) potential [2] is employed for small inter-atomic separations. The smooth transition between ZBL potential and Morelon potential is achieved via a fifth-degree polynomial function which guarantees the continuities of potential energy, the forces and the first derivative of the forces. Representative simulation results are selected to compare with available experiment data in order to validate the credibility of this study. Satisfactory agreements are observed.This work was supported in part by US Department of Energy, under NERI-C Award No. DE-FG07-07ID14889, and US Nuclear Regulatory Commission, under Award No. NRC-38-08-950.References:[1] N. –D. Morelon, D. Ghaleb, J.-M. Delaye, L.V. Brutzel, Phil. Mag. 83 (2003) 1533.[2] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Stopping and Ranges of Ions in Matter, vol. 1 , Pergamon Press, New York, 1984.
9:00 PM - P3.2
Multi-scale Examination of the Effect of Σ3n CSL Boundaries on Radiation-induced Degradation in Stainless Steels.
Christopher Barr 1 , Greg Vetterick 1 , Daniel Scotto D’Antuono 1 , Chris Winkler 1 , Steven Spurgeon 1 , Marquis Kirk 2 , Collin Knight 3 , Mitra Taheri 1
1 Material Science and Engineering, Drexel University, Philadelphia, Pennsylvania, United States, 2 Materials Science Division, Argonne National Laboratory, Argonne , Illinois, United States, 3 , Idaho National Laboratory, Idaho Falls, Idaho, United States
Show AbstractAustenitic stainless steels are extensively used throughout the vessel core in light water nuclear reactors. These steels, exposed to severe operating conditions, must operate at high temperature and neutron irradiation dosages in a corrosive environment for long service times. These operating conditions lead to multiple radiation damage processes that can ultimately lead to a failure phenomenon such as stress corrosion cracking. A method to improve a materials response to radiation damage could help extend the overall service time. One solution proposed is to increase the percentage of coincidence site lattices at boundaries by a grain boundary engineering (GBE) approach. This approach has shown successful in studies of non-irradiated corrosive environments with the ability reduced sensitization, improved resistance to intergranular stress corrosion cracking and increased ductility.A strain-annealed thermomechanical GBE process was examined at multiple scales on austenitic stainless steels as a potential method to improve resistance to irradiation induced material degradation. The twin-induced grain boundary engineering process resulted in a modest increase in the length and number fraction of Σ3n (n=1,2,3) coincidence site lattice (CSL) grain boundaries. Base, as received, 304L and 316L steels were compared to GBE 304L and 316L steels by elevated temperature mechanical testing including micro-tensile. Mechanical testing was supported by pre and post electron backscattered diffraction (EBSD) to help understand the relationship between the deformation failure mechanisms and the effect of different grain boundary character and orientation. The mechanical testing will be used in a comparison with post-neutron irradiated 304 and 316 stainless steels from the advanced test reactor at Idaho National Laboratory. In addition to the macro-scale mechanical testing, elevated temperature heavy ion-irradiation followed by straining was completed by in-situ TEM for both GBE and non-GBE stainless steels. The results of the in-situ TEM work were aided by ex-situ analytical TEM and EBSD analysis to help understand the effects of specific CSL grain boundary orientations on radiation-induced mechanisms such as segregation and defect clustering. Portions of this research were supported by the U.S. DOE, Office of NE under DOE Idaho Operations Office Contract DE-AC07-051D14517, through the ATR NSUF and at the EMC for Materials Research at ANL, a U.S. DOE Office of Science Laboratory operated under Contract No. DE-AC02-06CH11357 by UChicago Argonne, LLC.
9:00 PM - P3.20
Evaluation of Crack Growth Retardation Effect Due to Nano-scale Voids Based on Molecular Dynamics Method.
Shin Taniguchi 1 , Toshihiro Kameda 1
1 Engineerging Mechanics & Energy, University of Tsukuba, Tsukuba, Ibaraki, Japan
Show AbstractIt is known that there exist various types of lattice defects in the reactor and/or aerospace material due to long-term irradiation process. It is generally considered that these lattice defects cause the deterioration of materials. Especially, Stacking Fault Tetrahedra (SFT) is generated due to crystalline structural changes under the irradiated environment. In the initial deformation, the strength of irradiated materials is higher than unirradiated ones because SFT restrains dislocations activity. However, lattice defects grow gradually with progress of deformation, and void and/or crack is generated in the irradiated material. There is often the case that the brittle fracture is caused by crack growth in the long-term operation. On the other hand, some atomistic simulations show that the nano-scale void acts as dislocation absorption and formation during deformation. Therefore, the nano-scale void enhances dislocation activity and plastic deformation behavior becomes smooth, which means a crack growth may be deterred by the purposely introduced nano-scale voids in the vicinity of crack. However, experimental trial for the optimum nano-scale void arrangement is not realistic from a convenience and safety standpoint. We investigate this retardation effect by using a large scale parallel molecular dynamics (MD) method. In particular, focusing on the interaction between a nano-scale void and dislocations emitted from crack tip, we evaluate the relationship of crystal size, crack length and/or location of void. MD simulation model is uniaxial tensile loading of a single crystal model having center crack and nano-scale void put on the primary slip direction from the crack tip. Embedded atom method is adopted to describe the interatomic potential, and the temperature control system based on the velocity scaling method is applied. MD simulation shows the following results; (1) The crystal with nano-scale void exhibits almost the same strength as a perfect crystal. (2) A nano-scale void in the vicinity of a crack has a crack growth retardation effect due to a smoother plastic deformation mechanism. (3) When the crystal size is less than or equal to 15nm, the optimum location of a nano-scale void exists for the crack growth retardation, and the distance between the void and the crack tip shows a proportional relationship to the crystal size.
9:00 PM - P3.21
Temperature and Strain Rate Effect on Plasticity and Yield Stress in Nanocrystalline Copper under Compression.
Virginie Dupont 1 , Timothy Germann 1
1 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractShock compression of materials is a complex process involving high strain rates and elevated temperatures, which both affect material properties. Molecular Dynamics (MD) simulations allow both the visualization of defects in the sample and the control of parameters such as temperature and strain rate. We are using MD simulations to study the effect of the strain rate and temperature on the properties of Cu during shockless compression. A half-million-atom nanocrystalline Cu sample with an average grain size of 5 nm is subjected to strain rates ranging from 107 s-1 to 1012 s-1 at temperatures ranging from 50 K to 1500 K. Plasticity mechanisms as well as the evolution of the micro- and macro- yield stress are observed. We find that the yield stress increases with decreasing temperature and increasing strain rate.
9:00 PM - P3.22
Ideal Tensile and Shear Strengths of hcp Transition Metals Re, Ru, Os: A First Principles Study.
Yunjiang Wang 1 2 , Shigenobu Ogata 2
1 Department of Physics, Tsinghua University, Beijing China, 2 Department of Mechanical Science and Bioengineering, Osaka University, Osaka Japan
Show AbstractThe application of materials in high stresses/pressures acquires great advancement in ultrastrength, superhardness, and also high temperature creep strength. Recently, the large atomic number hcp transition metals with high valence electron density were successful used to synthesized superhard materials with light covalent elements, such as B, O, C and N. Especially the hcp metal Re, Ru, Os are added as important alloying elements to nickel-based superalloys, which greatly improves the high-temperature mechanical properties. However, there are no report on their intrinsic strengths and ideal deformation processes related to atomic bonding behaviors. The study on the ideal mechanical properties of these hcp metal will provide basic information on the future application of these important strong metals. In this simulation work, we systematically study the mechanical properties of hexagonal close packed transition metal Re, Ru, and Os using density functional theory in the framework of generalized gradient approximation. A complete set of elastic constants, ideal stress-strain relations with under both tension and shear are presented. The elastic constants and moduli of these three metal are calculated to be in the order of Os>Re>Ru. They all have large elastic moduli, and our calculated values are in good agreement with experimental observations. The ideal tensile directions in the calculations include [0001], [1-100], and [11-20], as well as slip systems (0001)[1-100], (0001)[11-20], and (1-100)[11-20]. They all have the largest ideal tensile strengths along [0001], and smallest in [11-20]. According to the calculated ideal shear strengths, the main slip planes of these hcp metal are consistently (0001)[11-20]. The magnitude of ideal strengths of these three metal is in the similar order of elastic moduli. However, two directions are excepted. The ideal tensile strength of Ru in [11-20] direction is 23.9 GPa, which is larger that 18.4 GPa of Re. Similarly, the shear strength of Ru in (0001)[11-20] is 17.6 GPa, whereas the couterpart in Re is 11.0 GPa. Their critical shear strain are 0.14 and 0.10, respectively. The related deformation processes and the electron origin of the ideal tensile and shear behaviors of the metals are analyzed by providing the electronic structures varying with different strain magnitudes.
9:00 PM - P3.25
Fabrication and Mechanical Properties of Si3N4/SiC Nano-nano Composite.
JungHoo Shin 1 , Seoung-Hyeon Hong 1
1 , Seoul national university, Seoul Korea (the Republic of)
Show AbstractSilicon nitride (Si3N4)-based ceramics have been extensively studied due to their excellent mechanical properties and high temperature performance. Especially, Si3N4/SiC composites are under active developments because of their superior properties to monolithic Si3N4 ceramics. Niihara et al reported the drastic increase of toughness and strength in micro/nano Al2O3/SiC composite system and many other types of composite have also been studied due to their potential properties and applications. Among them, nano/nano types of composite are expected to exhibit the improved mechanical properties based on Hall-Petch relations. However, it was rarely studied due to the difficulty in fabrication. The present research is focused on fabrication of crystalline, second phase-free nano/nano Si3N4/SiC composite through carbothermal reduction treatment (CRT) and spark plasma sintering (SPS), and investigation of the mechanical, especially tribological properties. The initial Si3N4 and SiC powders were mixed and ball milled in ethanol for 24 h and mixed powders were subjected to carbothermal reduction treatment in order to remove the Si2N2O glassy phase by using phenolic resin containing a 50 wt% carbon with flowing N2 gas at 1450 celsius degree for 12 h. SPS was conducted to fabricate fully densified nano-grains at 1550 celsius ctjdegree for 5 min. The influence of SiC on the mechanical properties was thoroughly investigated. The tribological performance was assessed when subjected to self-mated sliding wear in dry unlubricated conditions. The dominant material removal mechanisms were elucidated with respect to grain refinement and improvement of mechanical properties.The addition of SiC resulted in the improvement of elastic modulus, hardness, and fracture toughness. The hardness increased from 17 GPa to 19 GPa as SiC added from 0 wt% to 30 wt%. The indentation fracture toughness was increased with increasing the amount of SiC up to 20 wt% and inverse trend was observed. Sliding wear tests revealed at least an order magnitude improvement in wear resistance with SiC addition. A maximum wear rate of 2.0E-6 was recorded for the samples of Si3N4/SiC 0 wt%, while a minimum wear rate of 1.14E-7 was observed for the Si3N4/SiC 20 wt%. The improved mechanical properties including elastic modulus, hardness and fracture toughness might be main reasons for improvement of wear resistance. SEM analysis indicated that the main mechanism of wear in Si3N4/SiC 0 wt% specimens is debris compaction and microcracking, while that of Si3N4/SiC 20 wt% specimens is plastic deformations and micro cracking.
9:00 PM - P3.26
Modify the Elastic Properties of Nanowires through Altering Their Surface Structures.
Guofeng Wang 1
1 Department of Mechanical Engineering, Indiana University Purdue University Indianapolis, Indianapolis, Indiana, United States
Show AbstractNano-devices employ nanowires as their active components to generate, transmit, and convert powers and motions. Hence, the dependence of their mechanical properties on their geometric size is a very important factor in determining the performance of nanowires in those devices. So far, several different fashions of the size dependence of the elastic properties of nanowires have been revealed: (1) elastic modulus increases with the decreasing size, for examples, in Ag and Pd nanowires; (2) elastic modulus decreases with the decreasing size, for examples, in ZnO and Si nanowires; and (3) elastic modulus shows little dependence of the size of the nanomaterials such as Au nanowires. In this work, we combined first-principles density functional theory calculations with linear elasticity theory. Using the concept of surface stress, we developed a model that is able to predict the elastic modulus of the nanowires as a function of their diameters based on the calculated properties of its surface and bulk materials. Furthermore, we applied this computation approach to Ag, Au, and ZnO nanowires. For both Ag and Au nanowires, our prediction results agree excellently with the experimental data in the literature. For ZnO nanowires, our predictions are qualitatively consistent with some of experimental data for ZnO nanobelts. Therefore, we found that surface stress plays a very important role in determining the elastic modulus of nanowires. Our results suggest that the elastic properties of nanowires could be engineered by altering the surface stress through rational control of the adsorptions, charges, structure, and impurities in the surfaces.
9:00 PM - P3.27
Achieving the Ideal Strength in Focused Ion Beam Machined Molybdenum Nanopillars by In Situ Annealing.
Matthew Lowry 1 2 , Daniel Kiener 3 , Mary LeBlanc 2 , Claire Chisholm 1 , Jeffrey Florando 2 , J. Morris 1 , Andrew Minor 1 3
1 Materials Science and Engineering, UC Berkeley, Berkeley, California, United States, 2 Engineering Directorate, Lawrence Livermore National Laboratory, Livermore, California, United States, 3 National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractThe focused ion beam (FIB) allows for the machining of mechanical test specimens with submicron diameters, thereby enabling the study of mechanical properties in limited volumes. However, the Ga+ ion bombardment from the FIB induces considerable damage in the test specimens, resulting in a high density of dislocations along with Ga implantation. The induced defect density alters the mechanical response of the specimens, complicating investigations into the role of size on strength. Here we show that through in situ annealing in a transmission electron microscope (TEM), it is possible to at least partially remove ion bombardment damage from FIB-machined molybdenum nanopillars. Starting with well-annealed, decarburized single crystal molybdenum, pillars are milled by FIB along both single slip and multiple slip orientations. The pillars are then annealed in situ, allowing for the observation of damage removal and any associated shape changes of the pillars. The result is a reduction in defect density that significantly improves the visualization of deformation behavior during in situ compressions. Moreover, fully annealed pillars containing either zero or one isolated dislocation are found to deform at stresses approaching the ideal strength of molybdenum, or the stress at which a defect-free sample of molybdenum will become elastically unstable. Through a Hertzian contact analysis, it is shown that fully annealed pillars deform at stresses between 84% and 95% of the calculated theoretical ideal strength.
9:00 PM - P3.28
Enhancing Performance of Multi-walled Carbon Nanotubes via Thermally Activated Interlayer Bonding.
Chun Tang 1 2 , Wanlin Guo 2 , Changfeng Chen 1
1 Department of Physics and High Pressure Science and Engineering Center, University of Nevada Las Vegas, Las Vegas, Nevada, United States, 2 Institue of Nano Science, Nanjing University of Aeronautics and Astronautics, Nanjing, Jiangsu, China
Show AbstractWe report a new routine to generate interlayer bonds in multi-walled carbon nanotubes (MWCNTs) via high temperature annealing using molecular dynamics simulations. We find that with increasing temperature, the ability to form interlayer sp3 bonding enhances in MWCNTs and remains robust when annealed to room temperature. The thermally activated interlayer bonds play important roles in improving the performance of MWCNTs; the load carrying capability is significantly improved via interlayer bonding, in agreement with recent experiments; we also observe enhanced tensile ductility at high temperatures. First-principles calculations show that interlayer bonding can also convert metallic MWCNTs into semiconducting MWCNTs, suggesting a new route to producing all-semiconducting MWCNTs.*This work is supported by DOE Cooperative Agreement DE-FC52-06NA26274.
9:00 PM - P3.29
Predicting the Mechanical Properties and Structure of Silica Nanowires via Atomistic Simulations.
Lilian Davila 1 , Valerie Leppert 1 , Eduardo Bringa 2
1 School of Engineering, University of California Merced, Merced, California, United States, 2 CONICET & Instituto de Ciencias Basicas, Universidad Nacional de Cuyo Mendoza, Mendoza Argentina
Show AbstractInorganic nanostructures such as nanosprings, nanowires and nanorings are important morphologies of great scientific interest for future technological progress. We have focused our work on the nature and properties of silica nanowires. Nanowires have useful mechanical, electrical and optical properties that could make them useful in small-scale sensing and micro-system applications. We have performed large-scale molecular dynamics (MD) simulations to study the nature and mechanical properties of amorphous silica nanowires. The behavior of non-crystalline silica nanowires is studied using empirical interatomic potentials developed by Feuston and Garofalini. We have applied MD simulations to study the response of the silica nanowires to elevated compressive loads. We have centered our studies on the nanostructural changes occurring in the material and the correlation between the medium-range order (~10 nm), through the characteristic ring distribution of this material. Several glassy nanowires ranging in diameter from 1.4 nm to 20 nm are investigated. We also derived the elastic modulus of the nanowires from the stress-strain curves and found a distinctive dependence on nanowire diameter. For a nanowire length of ~14 nm and a diameter of ~4 nm, we do not observe any change in the amorphous structure through 25% uniaxial compression because the nanowires expand laterally to accommodate uniaxial stresses. A longer nanowire, with length of ~57 nm and diameter of ~4 nm, shows a buckling instability and reduced strength at similar strain conditions. In both cases, the ring size distribution reveals the glassy nanostructures remain essentially unaffected at elevated compressions. The ring structure and Young’s modulus for thicker nanowires, with diameters above ~6 nm and lengths of ~14 nm, increasingly resemble those typical of the bulk glass. Results are compared with recent experimental findings and theoretical predictions. Thin silica nanowire (diameter 4.3 nm) results are consistent with prior theoretical calculations using elasticity theory. This investigation contributes to an understanding of the nature of silica nanowires and their mechanical properties, influencing structure-dependent applications and design of nanoscale devices, with implications in nanotechnology, materials science, photonics and medicine.
9:00 PM - P3.3
Computer Simulation on Structure Modification in Metallic Nanoparticles due to Hydrogenation.
Hiroshi Ogawa 1 , Thi Viet Bac Phung 1
1 NRI, AIST, Tsukuba, Ibaraki, Japan
Show AbstractStructure variations during hydrogenation of metallic nanoparticles with b.c.c, f.c.c. and icosahedral structures were investigated by classical MD and first-principles calculation. Classical MD simulation with model interatomic potentials revealed that radial distribution of hydrogen atoms inside nanoparticles varies depending on both length and energy parameters of metal-hydrogen interaction. [1] In cases of weak M-H bonds, hydrogen atoms were homogeneously absorbed in the particle. In cases of strong M-H bonds, on the other hand, hydrogen-rich surface layers were generated. Origin of such surface layers is considered to be lattice deformation due to partial hydrogenation at the surface which results decreasing diffusivity of atomic hydrogen. Generation of grain boundaries was also observed during hydrogenation. [2] Small nanoparticles less than 2 nm in diameter showed complex variation in atomic arrangements between cubic and five-fold symmetries. The authors will report also on the results of first-princples calculation on hydrogenation of cubic and icosahedral nanoparticles. This work has been supported by New Energy and Industrial Technology Development Organization (NEDO) under "Advanced Fundamental Research Project on Hydrogen Storage Materials".[1] H. Ogawa et al., Mater. Trans., 49 (2008) 1983. [2] H. Ogawa et al., Int. J. Nanosci., 8 (2009) 39.
9:00 PM - P3.30
Stability of BCC Iron Screw Dislocation Core Structure: A High-precision DFT Calculation.
Mitsuhiro Itakura 1 , Hideo Kaburaki 1 , Masatake Yamaguchi 1
1 , Japan Atomic Energy Agency, Tokyo Japan
Show AbstractWe report high-precision DFT calculation of screw dislocation core structure in BCC iron, focusing on the stability of non-degenerate six-fold structure. This work has been motivated by a fact that the widely-used Mendelev potential reproduces six-fold core structure at zero temperature but the symmetry is broken as soon as temperature exceeds 100K. Thus it is necessary to evaluate the energy barrier to break the symmetry by high-precision quantum calculation to see if six-fold structure is still stable at room temperature. We also report the effect of spin polarization on the structure.
9:00 PM - P3.31
Defects Evolution in a Helical Nanowire under Stretch.
Xiangyang Kong 1
1 School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai China
Show AbstractIt is well known that nanowires exhibit unusual physical and mechanical properties due to the large surface area to volume ratio. However, there is little attention to the geometrical curvature effects on nanowires. Here, we focus on defects evolution in a helical metallic nanowire under stretch by molecular dynamics (MD) simulation. Helical nanowire remains elastic until dislocations are nucleated from the inner surface. A group of dislocations may interact to each other, giving rise to dislocation locks. It is worthy to note that the geometrical curvature of the helical nanowire plays a key role to hinder dislocations motion within a helical nanowire due to the torsion energy barriers. As a result, Eshelby twist with asymmetrical force field may be formed. In other case, deformation twinning also occurs from dislocations motion in a helical nanowire. When twinning occurs, it will trigger lattice re-orientations and helps to the release strain energy, responsible for the limiting strain hardening mechanism in a helical nanowire.
9:00 PM - P3.32
Cracks are Slower in Nanocrystals.
Fouad Atrash 1 , Dov Sherman 1
1 Materials Engineering, Technion-Israel Institute of Technology, Haifa, -, Israel
Show AbstractFreund equation of motion correlates the dynamic crack speed to the energies in the body. Being continuum mechanics based theory, the involved energies in the equation of motion are the external applied energy, the internal quasi static strain energy and the energy consumed by the motion of the elastic wave. The energy dissipated by the atomistic vibrations (heat or phonon emission) is not considered. We used molecular dynamics simulations to measure the thermal phonon energy release rate in brittle nano scale single crystal silicon specimen resulted atomistic vibrations, and added it to Freund equation of motion. We show that the additional energy is responsible for major phenomena associated with dynamic crack propagation in the nano scale: the thermal phonon energy consumes a considerable fraction (~30%) of the total strain energy inserted by the external loads, and, therefore, the terminal crack velocity is reduced by nearly 40%. In addition, our results revealed energetically preferred cleavage directions and crack deflection phenomena at the atomic scale.
9:00 PM - P3.34
Modeling Nano-indentation of a Self-healing Material.
Solomon Duki 1 , German Kolmakov 1 , Victor Yashin 1 , Krzysztof Matyjaszewski 2 , Anna Balazs 1
1 Department of Chemical Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania, United States, 2 Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States
Show AbstractOur previous computational studies showed that introduction of a small fraction of labile bonds to dually-crosslinked nanogels can improve the tensile strength of the nanogel-based material. The tensile strength increases due to the nanogel rearrangements, which are facilitated by the reversible breakage and reformation of the labile inter-particle bonds and, therefore, lead to the stress relaxation. Recently, nano-indentation experiments demonstrated that the nanogel-based materials exhibit a self-healing behavior. In this presentation, we report the results of computer modeling of the self-healing process. We consider a 2D system encompassing the nanogel particles linked together by both permanent and labile bonds. The nanogel particles are simulated by using the lattice spring model. The Bell model is utilized to simulate the material fracture, which is made by a hard indenter moving across the sample with some velocity. The simulations reveal that the labile bonds reform behind the indenter as the latter object moves find through the gel and in this manner, the gel undergoes self-healing. We discuss how the fraction of labile bonds, the nanogel stiffness, and the indenter size and velocity affect self-healing of the material.
9:00 PM - P3.35
Measurement of Internal Friction in Ultra-thin Films of Aluminum, Silver, and Gold.
Guru Sosale 1 , Srikar Vengallatore 1
1 Mechanical Engineering, McGill University, Montreal, Quebec, Canada
Show AbstractAccurate measurement of internal friction in ultra-thin films (with thickness ranging from 10 to 100 nm) can provide valuable insight into the effects of scale and confinement on the mechanisms of anelastic relaxation, and guide the design of high-Q nanomechanical resonators used for sensing. An effective approach to measurement is to deposit these films on a relatively thick, low-loss substrate, and then measure the damping of the composite bilayered resonator. The measured damping, however, contains contributions from several sources: clamping losses, support losses, thermoelastic damping in the bilayer, sliding at the interface between film and substrate, and internal friction in the film. Identifying the magnitude and frequency dependence of internal friction from the measured damping has been a long-standing problem in this field. To solve this problem, we have developed and implemented a vacuum-operated microcantilever platform that enables accurate measurement of internal friction by using thermoelastic damping for calibration. First, single-crystal silicon microcantilevers with thickness ranging from 70 to 150 μm, and fundamental frequencies ranging from 100 Hz to 1.5 kHz, were fabricated and demonstrated to operate at the fundamental limit of dissipation specified by thermoelastic damping. The logarithmic decrement due to all other mechanisms of dissipation, including support losses, is shown to be less than 10-5. Subsequently, the beams were coated with thin adhesion layers (~15 nm thick films of Cr and Ti), and then with the films of interest (Al, Au and Ag with thickness ranging from 50 to 100 nm). All films were deposited using sputtering and care was taken to control the intrinsic residual stress by tuning the argon gas pressure during deposition. The contribution of the films to the thermoelastic damping in the composite was calculated using an exact theory that does not require any free parameters, and was shown to be negligible. Finally, the internal friction in the film is obtained as the difference between the measured damping in the bilayer and the thermoelastic damping of the substrate. Even though the films are a thousand times thinner than the silicon cantilever, the damping in the bilayer is dominated by internal friction in the films. The internal friction changed monotonically with film thickness but was only a weak function of frequency over the range of 0.1–1.5 kHz. Aluminum and silver resulted in consistently higher damping than gold for all thickness and frequencies measured in this study. The contribution of grain boundary sliding and dislocation damping to internal friction in these polycrystalline thin films will be discussed.
9:00 PM - P3.36
Crack Propagation in Diamond-CNT Nanocomposites.
Fabio Pavia 1 , William Curtin 1
1 , Brown University, Providence, Rhode Island, United States
Show AbstractNanofibers, and in particular carbon nanotubes, are attractive as high performance nano-reinforcements for composite materials. In this work we study diamond - CNT nanocomposites, for which the exceptional properties of both the matrix and fibers make these materials attractive for a broad range of applications. Using molecular dynamics on CNT-diamond unit cells, we study cracks propagating through the diamond matrix and impinging on the interface with the nanotube under an applied quasistatic traction, as a function of the degree of interfacial bonding between the matrix and the external shell of the nanotubes. The original REBO potential for C-C bond formation and breaking can show inappropriate fracture mechanisms and overestimation of the stress for bond breaking, so we use a modified potential studied by Pastewka and coworkers [1] that employs an environmental screening coefficient to better capture bond breaking and reforming. Through our studies, we develop insights to guide design of the CNT/matrix interface to enhance the strength and toughness of these composites. [1] Lars Pastewka et al., "Describing bond-breaking processes by reactive potentials: importance of an environment-dependent interaction range". Phys. Rev. B 78. 161402(R)(2008)
9:00 PM - P3.37
Morphological Instabilities in Thin Films: Evolution Maps.
Mohsen Asle Zaeem 1 , Sinisa Mesarovic 1
1 School of Mechanical and Materials Engineering, Washington State University, Pullman, Washington, United States
Show AbstractWe consider morphological instabilities in binary multilayers and the post-instability evolution of the system. The alloys with and without intermediate phase are considered, as well as the cases with stable and meta-stable intermediate phase. Using the Galerkin finite element formulation for coupled Cahn-Hilliard – elasticity problem, maps of different evolution paths are developed in the parameter space of relative thicknesses of initial phases. We consider the relative importance of elastic and chemical energy of the system and develop maps for different cases. The systems exhibit rich evolution behavior. Depending on the initial configuration (which determines the mass conservation condition), the final equilibrium varies, but even greater variety is observed in evolution paths. The paths may consist of multiple evolution steps, which may proceed at different rates. Except for few special circumstances, the instabilities are to perturbations non-homogeneous in the film plane. Post-instability evolution is essentially two-dimensional, and cannot be reduced to the one-dimensional model.
9:00 PM - P3.39
Evolution of Nano- to Micro-scale Surface Structures in Hydroxyl-rich Fused Silica through Localized CO2 Laser Heating.
Nan Shen 1 , Manyalibo Matthews 1 , James Fair 1 , Jerry Britten 1 , Hoang Nguyen 1 , Juliet Cooke 1 , Selim Elhadj 1 , Steven Yang 1
1 , Lawrence Livermore National Laboratory, Livermore, California, United States
Show AbstractNano-roughness and thermal stability of glass surfaces has recently drawn interest through the development of active surface devices for chemical and biological sensing applications. Understanding the optical properties of small scale irregularities on glass surfaces is also important for high power laser applications such as inertial confinement fusion due to nearfield intensification. One approach to modify and control glass nano-structure morphology is through the use of localized CO2 laser heating. However, the fundamental mass transport processes and surface chemistry that drives surface dynamics at high temperatures and at the nanoscale is not well understood. Our goal in the present study is to combine computational fluid dynamics with in situ microthermography to elucidate the physics involved in nanoscale surface relaxation phenomena on hydroxyl-rich fused silica surfaces at elevated temperatures. In order to better understand the mass transport and the effect of hydroxyl, we examine the morphological evolution of 1D and 2D 0.6-1.5μm sized dry-etched periodic surface structures on type II and type III SiO2 substrates under 10.6μm CO2 laser irradiation using atomic force microscopy (AFM). In-situ microthermography was used to map the transient temperature field T(r,t) across the heated region, allowing assessment of the T-dependent mass transport mechanisms. Computational fluid dynamics simulations of the viscoelastic relaxation of glass were carried out and correlated with experimental results. By comparing fused silica samples with different hydroxyl (OH) content and measuring near-surface OH depletion using confocal Raman microscopy, we explore the additional T-dependent nonlinearity in surface visocity caused by OH diffusion.Although previous theoretical predictions of nanoscale surface relaxation suggests that surface diffusion should dominate mass transport, we observe that capillary flow dominates at least down to the resolution of our surface measurements. We believe this is the result of surface defects in glass which can act to frustrate surface diffusion. The hydroxyl content of the glass plays an important role in influencing its property such as viscosity, which in turn determines the thermomechanical characteristics of glass. We showed that the local, dynamic surface viscosity of OH-rich SiO2 glass under the high temperature CO2 laser heating is significantly different from bulk values. By including this additional chemistry-driven nonlinearity in surface viscosity and neglecting surface diffusion, a more consistent model of nanoscale fused silica is thus presented.
9:00 PM - P3.4
A New Approach to Obtain Effective Indenter Shape Function in Nanoindentation Data Analysis.
Guanghui Fu 1 , Tiesheng Cao 2 , Ling Cao 1
1 , LC Dental, Fremont, California, United States, 2 , Fourth Military Medical University, Xi'an, Shaanxi, China
Show AbstractThe classic approach to obtain effective indenter shape function from a given pressure distribution is through the superposition of Boussinesq solution of point loading of an elastic half-space. This paper presents an alternative approach. It is based on the analytical solution of normal indentation of an elastic half space with a rigid frictionless axisymmetric punch. A first order ordinary differential equation and Abel’s integral equation are solved to obtain the solution. The presented method is simpler and more straightforward than the classical approach. The limitations of the new approach are also discussed.
9:00 PM - P3.42
Investigation of the Bauschinger Effect in Pure and Alloyed Materials with the Help of Dislocation Dynamics.
Sylvain Queyreau 1 , Benoit Devincre 2 , Ladislas Kubin 2
1 Nuclear Engineering, UC Berkeley, Berkeley, California, United States, 2 , Laboratoire d’Etudes des Microstructures, Onera, Chatillon France
Show AbstractThe Bauschinger Effect (BE), defined as a reduced reverse flow stress if a forward loading is first applied, is a fundamental question of material plasticity. The BE has been observed in a large variety of materials, but its causes remain unclear. We performed three-dimensional simulations of Dislocation Dynamics (DD) to improve the understanding of this effect. DD is probably the most appropriate tool to tackle this problem in both pure materials and alloys for deformations of up to few percent.In the case of precipitation-strengthened materials, the BE is related to the nature of the particles, a fact which is in agreement with experiments. Indeed, the loops stored around incoherent particles during the first loading will help dislocations during their backward motion, whereas sheared precipitates play only a small role in the BE. An original model is proposed to describe the simulation results. This new mechanism is the main cause of the BE for small to intermediate forward deformations. For larger deformations (more than 10 %), additional effects must be accounted for as described in [L.M. Brown, W.M. Stobbs, Phil. Mag. 23 (1971) p.1185.].At high temperature, the plastic behavior of pure materials is controlled by dislocation-dislocation interactions (reactions). Our DD simulations are probably the first ones devoted to the understanding of the BE in pure single-crystals. Preliminary results show that a BE can be observed even at small strains (< 1 %) if more than one slip system is activated and a dislocation organization exists. Surprisingly, the stress calculation along the dislocation lines clearly demonstrates that no long-range internal stress is present even for strains of a few percent. The BE is in fact controlled by the polarization of junctions; this polarization assists the junction unzipping process during the backward loading. Here, a model is proposed to describe this mechanism. Two parameters are found to be of particular importance: (i) the strain reached during the first loading and (ii) the orientation of the loading axis; these findings are in agreement with experiments. These effects are explained within the framework of the proposed model as a consequence of the number of interacting systems and the nature of the reactions.
9:00 PM - P3.43
Brittle to Ductile Transition in the Fracture Process of an α-iron Single Crystal Containing a Grain Boundary – Molecular dynamics Simulations.
Hideo Kaburaki 1 , Tomoko Kadoyoshi 1 , Mitsuhiro Itakura 1 , Masatake Yamaguchi 1
1 Center for Computational Science and e-Systems, Japan Atomic Energy Agency, Tokai, Ibaraki, Japan
Show AbstractMany single crystal metals and intermetallics intrinsically exhibit brittle-to-ductile transition (BDT) as a function of temperature and strain rate. These materials are generally brittle at low temperatures or high strain rates, and become ductile as the temperature rises or the strain rate decreases. Experiments on the brittle boundary in BCC metals, such as Fe, Mo, and W, revealed an empirical Arrhenius relation between the BDT temperature and the strain rate or the time to fracture. Moreover, in the case of a single crystal, it is predicted that the presence of defects, such as dislocations, grain boundaries, and microcracks, influences greatly the transition state. The whole atomistic picture of the BDT is hindered by a very high strain rate usually used by the molecular dynamics method. Here we employ a clean single crystal system consisting of half a million iron atoms to detect the brittle boundary using the molecular dynamics method with a wide range of temperatures up to 1100K and strain rates of 107 - 109s-1. Since the BDT temperature is presumed to be pushed to the higher temperature region due to the high strain rate, we introduce a grain boundary and a microcrack in the system. With this system, we have successfully found that the brittle boundary is in the range around 1000K depending on the strain rate. We have evaluated dislocation density, surface energy, stress-strain curve and energy dissipation in this region to elucidate the atomistic picture of the BDT.
9:00 PM - P3.44
Interphase Energies and Nonequilibrium Growth of γ-precipitates in Al-Ag: a DFT Study.
D. Watts 1 , D. Johnson 2 , Daniel Finkenstadt 1
1 Physics, U.S. Naval Academy, Annapolis, Maryland, United States, 2 , Ames Laboratory, Ames, Iowa, United States
Show AbstractDensity-functional theory (DFT) calculations of interphase boundary energies provide useful input for many precipitate growth models in alloy systems [1]. One example is Al-Ag, where a rich variety of precipitate types exist, and the sizes and shapes are determined roughly by a Wulff construction, namely, minimizing surface free energies with respect to geometry. This is only a first approximation, however, as kinetic-considerations and crystallography do no allow for a uniform, isotropic growth. Consequently, a nonequilibrium growth model is developed for γ-plates [2], which attempts to connect semi-coherent (ledge) and incoherent (edge) interface growth rates in a way that incorporates shape and interface energies. Through this connection, we make a DFT model with approximate unit cells that mirror experimental conditions, which gives accurate predictions for precipitate aspect ratios and time-development of nonequilibrium shapes. Starting from an explicit calculation of Suzuki segregation of solute to stacking-faults, we find a mechanism for nucleation of nanoscale γ-plates on quenched defects, identify a bulk structure from a calculated phase diagram that gives the relevant hcp equilibrium precipitate structure occurring at 50at.% Ag and calculate critical nucleation parameters for γ-precipitate formation. Applications to island-coarsening and lath morphology will also be discussed. [1] D. Finkenstadt and D.D. Johnson, Phys. Rev. B 73, 024101 (2006); ibid. 81, 014113 (2010). [2] D. Finkenstadt and D. D Johnson, Mater. Sci. and Eng. A 525, 174 (2009).
9:00 PM - P3.45
Effect of Mixed Naturally-occurring Filler Materials on the Compression and Flexural Properties of Polyester Composites.
Babaniyi Babatope 1 , Ezinne Igbokwe 1 , Morenike Bolarinwa 1 , Olasupo Ogundare 1 , Ebenezer Adeoya 1 , Partrick Imoisili 1
1 R&D, Engineering Materials Development Institute, Akure, Ondo, Nigeria
Show AbstractStudies of materials from non-petroleum sources have gained greater momentum because of their renewability. In this study two naturally occurring filler materials have been investigated. Banana short fiber has been mixed with finely milled cocoa-pod husk powder as fillers in the fabrication of polyester based composites. The effect of the mixed fillers and surface treatment on the compression and flexural properties of the composite was studied. The result showed that naturally-occurring filler materials could innovatively be mixed for composite structures applicable as panels for roofing, flooring and partitioning.
9:00 PM - P3.47
Influence of Nanoindenters Tip Radius on the Estimation of the Elastic Modulus.
Karim Gadelrab 1 , Matteo Chiesa 1
1 Material Science and Engineering, Masdar Institute, Abu Dhabi United Arab Emirates
Show AbstractNanoindentation results are very sensitive to tip rounding and neglecting the value of the tip radius produces erroneous estimation of the material elastic properties. In this study we investigate the effect of tip radius on the estimation of the Elastic modulus by means of finite element analysis of Berkovich and conical tips with different tip radii. Our numerical results are supported by an experimental study on fused silica with Berkovich tips with different tip radii. The use of the classical Oliver Pharr equation overestimates the Elastic modulus. A new analytical model that modifies the Oliver Pharr equation to consider the value of the tip radius is employed to derive the Elastic modulus from load-displacement curves yielding improved results compared to the classical Oliver Pharr equation. The model relies on accurate measurement of the tip area function and tip radius by means of AFM scanning.
9:00 PM - P3.48
Molecular Dynamics Simulation of Surface Morphology and Defect Distribution in Metallic Nanoparticles.
Yoshiaki Kogure 1
1 Physical Therapy, Teikyo University of Science, Uenohara, Yamanashi, Japan
Show AbstractMetallic nanoparticles produced by gas-evaporation technique has extensively been investigated by means of electron microscope observations. Those particles are 10-1000 nm in diameter and have highly symmetric external shape. The static morphology of these particles is strongly affected from surface energy and compared with the Wulff polyhedron. A theoretical calculation of Wulff polyhedron based on the Morse potentials has been reported. Recently, the embedded atom method (EAM) potentials have been developed, which can realize the many body nature of the atomic interaction in metals. It has successfully been applied to the problems of surface and defects in crystals. Molecular dynamics simulations using the EAM potential for copper are performed in the present study, and the process of formation and defect distribution in the nanoparticles are investigated. The nanoparticle is a suitable system for the molecular dynamics simulation because it consists of manageable number of atoms for MD simulation. Polycrystalline copper nanoparticles were realized by slowly cooling and annealing the molten state of model system of copper, in which 1,000-40,000 atoms were involved. The size dependence of the surface morphology and the defect distribution were investigated through the 3-d graphics and the distribution of potential energy. External stresses were also applied and the displacements of individual atoms were analyzed to investigate the structure and dynamics of defects.
9:00 PM - P3.49
On Determining Physical Length Scales for Material Behavior by Lower Bound Geometrically Necessary Dislocation (GND) Densities.
Muin Oztop 1 , Jeffrey Kysar 1 , Brent Adams 2 , Christian Niordson 3
1 Mechanical Engineering , Columbia University, New York, New York, United States, 2 Mechanical Engineering, Brigham Young University, Provo, Utah, United States, 3 Mechanical Engineering, Technical University of Denmark, Kongens Lyngby Denmark
Show AbstractThe size dependence of the mechanical behavior of elastic-plastic materials at the meso-and micro-scale is studied by investigating the deformation field associated with a wedge indentation in a face-centered single crystal; the resulting deformation field is two-dimensional in the sense that the only non-zero lattice rotation occurs in the plane. The lattice rotation is measured by high resolution electron backscatter diffraction (EBSD) analysis, which has an angular uncertainty in lattice orientation as small as 0.005o . The measurements were performed with 2500 nm, 500 nm, 100 nm, and 50 nm spatial resolution over different length scales. The primary observation was the development of high lattice rotation gradients in a region beneath the indenter tip. This region was depicted as a discontinuity line in previous studies, however the high resolution measurements show the structure of the lower bound of the GND fields in this region with great clarity. It is observed that there is distribution of characteristic length scales as a result of the plastic deformation. These results are investigated in the framework of a computational model with a non-work conjugate formulation in which dislocation densities are incorporated as internal variables.
9:00 PM - P3.5
A Study on Kaolin in the Application of High-strength and Electronic-grade Glass Fibres 01/06/2009-20/03/2010.
Yong Li 1 , Zhengen You 1
1 , China Kaolin Clay Company, Suzhou, Jiangsu Province, China
Show AbstractThe kaolin clay, which is concentrated by China Kaolin Clay Company, has been applied in high-strength and electronic-grade glass fibres. Due to its fewer impurities, even ingredients, low temperature in preparation, fast melting speed and higher yields, the kaolin clay has been a new and ideal type of raw material in glass fibre industry.
9:00 PM - P3.50
Role and Modification of Crystalline Defects at Nano-scale Size in Y2O3 Thin Films Grown by Ion Beam Sputtering: Modelling of the Internal Residual Stress and Influence on Phase Transitions.
Bertrand Lacroix 2 , Fabien Paumier 1 , Rolly Gaboriaud 1
2 CIMAP, ENSICAEN, CAEN France, 1 Institut Pprime, University of Poitiers, Chasseneuil Futuroscope France
Show AbstractThin films deposited by ion beam sputtering (IBS) could exhibit a very high concentration of in grain crystalline defects. This highly perturbed structure constrained by a substrate (thin films of few manometers) leads to introduce a specific description of the residual stress with a triaxial strain field approach. In this context, oxides with fluorite-related structure as rare earth oxide with bixbyite structure (Ia3) Re2O3 have been recently an active area of researches because of their properties for multiple application domains. Among them, yttrium oxide, Y2O3, exhibits several physical properties particularly relevant for MOS devices and optical applications. However, the physical properties of these materials deposited as a thin film strongly depend on their microstructure and thus on the elaboration conditions. In this work, Y2O3 thin films were grown by IBS: this deposition technique promotes a very particular structure with different and controlled nonstoichiometry and defects concentration due to a strong perturbation of the oxygen vacancy network induced by argon bombardment during the growth (atomic peening effect). The primary ion beam parameters can be modified to control and then to study the influence of the atomic defects on the mechanical properties in thin films.By combining RBS, XRD and HRTEM it is shown that energetic ions involved during the IBS process induce a strong disorder on the oxygen network: oxygen stoichiometry can be accommodated either by a local disorder of the ‘intrinsic’ oxygen vacancy network of the cubic C-type structure (a disordered fluorite-like structure) or, in a strong nonstoichiometric case by the formation of extended defects (oxygen vacancies clustering into a disc across which the crystal collapses to create prismatic dislocation loops). This work shows two major results leading to establish a significant relationship between internal stress and phase transformations. Both types of defects (oxygen vacancy network disorder and oxygen vacancy clustering) induce an important elastic strain field and leads to a high compressive stress in thin films due to the finite size of the system. The use of a modified atomic inclusion Eshelby’s approach leads to a complete and realistic description of the triaxial stress field of the constrained thin film.Calculation of the Gibbs free energy, taking into account the stored elastic energy in the thin film, gives a possible explanation about the origin of the stabilization of a high temperature non-equilibrium phase (fluorite-like structure) and finally point out the particular influence of atomic defects. Then, the stress relaxation and/or phase transformation under thermal annealing was studied in-situ by quantitative XRD (KJMA’s theory). It is clearly shown that the process occurs by complete recrystallization via nucleation and growth of a new cubic-C structure with a lower density of defect with activation energies depending of the initial stress.
9:00 PM - P3.51
Atomistic Simulation of Dislocation Interactions in Solute Strengthened Al/Mg Alloys.
Yi Dong 1 , William Curtin 1
1 Division of Engineering, Brown University, Providence, Rhode Island, United States
Show AbstractSubstitutional solutes and forest dislocations are two important sources of strengthening in fcc metal alloys. Individual strengthening due to solute atoms or forest dislocations has been widely investigated but solute effects on the dislocation-dislocation interactions remain unclear. Here, the mechanism of dislocation interactions in fcc alloy crystals are examined with emphasis on the formation and strength of dislocation junctions in Al/Mg alloys. By simulating a mobile dislocation moving through a field with randomly placed solute atoms and forest dislocations, the atomic equilibrium structures of dislocation junctions as a function of solute atom concentration are examined. Then, under the influence of an applied stress, the mobile dislocation strengthening by combined junction and solute effects is studied extensively. The simulation results are then compared against simple and more advanced can be used to check theories regarding the operation of combined strengthening mechanisms.
9:00 PM - P3.52
Crack Tip Stress Fields Predicted by Strain Gradient Crystal Plasticity Models.
Prateek Nath 1 , William Curtin 1 , Alan Needleman 2
1 , Brown University, Providence, Rhode Island, United States, 2 Department of Materials Science and Engineering, University of North Texas, Denton, Texas, United States
Show AbstractClassical plasticity ignores the role of stress enhancement due to accumulation of net Burgers vector of dislocations associated with plastic strain gradient. However such stress enhancements give rise to size effects in plasticity at the micron scale, and can necessary to predict fracture toughness of, for example, coatings and bi-materials.Discrete dislocation simulations capture these effects of dislocation accumulation but are computationally intensive. We discuss two alternative approach which can be viewed as extension of conventional crystal plasticity. The enhanced crystal plasticity approach accounts for long range stress fields due to net burgers vector in the body undergoing plastic deformation and its influences the plastic flow.Gurtin's strain gradient crystal plasticity which also accounts for the net burgers vector in the viscoplastic flow rule. We present the results of crack tip fields using two different approach.
9:00 PM - P3.53
A Universal Analytical Solution for the Stress Field around any Elastoplastic Indentation/Contact.
Gang Feng 1
1 Mechanical Engineering, Villanova University, Villanova, Pennsylvania, United States
Show AbstractThe closed-form stress fields for many elastic contacts, such as spherical and point contacts, have been solved, being widely used for analyzing contact-induced phenomena, such as indentation, scratch, as well as contact-induced fracture, fatigue, and thin-film delamination. However, due to the strong stress concentration under the contact, the materials there may deform plastically, inducing a stress field significantly deviated from that for an elastic contact, and the rigorous closed-form solution is not available. In our previous paper [Feng et al., Acta Materialia, 2007, p2929], it was found that, to estimate the stress field outside the plastic zone around a conical contact, the effect of the plastic zone is equivalent to an embedded center of dilatation (ECD). In this study, the same equivalence is found to be satisfied for many other elastoplastic contacts, e.g. spherical, wedge, and cylindrical contacts. Consequently, at the fully loaded state, the elastoplastic contact stress field can be estimated by the superposition of an elastic contact field and an associated ECD field, while the residual stress can be estimated by the ECD field alone. Here, the elastic contact field can be estimated by a corresponding cylindrical or spherical Hertzian field. The analytical solution based on the ECD model matches the finite element analysis nearly perfectly for various contact geometries, indicating that the equivalency between an contact-induced plastic zone and an ECD is valid and universal, which is a great simplification and gives the physical insight for understanding any elastoplastic contacts.
9:00 PM - P3.54
Mechanical Property Measurement of Nanocrystalline Thin Films: Part I. Elastic Modulus.
Seong-Woong Kim 1 , Jin-Woo Yi 1 , Hyun-Gyu Kim 1 2 , Kyung-Suk Kim 1 , Sharvan Kumar 1
1 Division of Engineering, Brown University, Providence, Rhode Island, United States, 2 Department of Mechanical Engineering, Seoul National University of Technology, Seoul Korea (the Republic of)
Show AbstractA hybrid method of experiment and finite element analyses is developed to measure in-plane elastic modulus of free standing nanocrystalline thin films. With this method we have measured elastic moduli of polycrystalline thin copper films of ~50 nm grain size and ~100 nm film thickness, with and without ~ 500 nm diameter through-thickness chromium “pillars”. A novel technique using electron beam evaporation onto NaCl single crystals, coupled with lithography and patterning as necessary, has been used to produce these uniform thickness, free-standing film specimens of nanocrystalline and ultra-fine-grained Cu as well as the Cu-Cr pillar/fiber composites. The spatial arrangement of pillars and pillar diameter can be controlled in this technique. Unlike the ~1 μm-thick films, thin films of ~ 100 nm thickness exhibit substantial warping undulations caused by relaxation of growth induced stresses during removal from the NaCl substrate. A white light topography interferometer with 10 nm height resolution measures the undulation of approximately within the free standing film area bounded by the copper grid to which the film is attached. The film is deflected approximately ~100 nm near the center of the free-standing area by an AFM bead tip of ~15 μm diameter. The compliance of the deflection is recorded with the AFM which was calibrated with a newly developed diamagnetic normal force calibrator (D-NFC). This calibrator is capable of measuring normal forces exerted on an AFM tip with three digit accuracy in the range from to 1 μN. The blunt bead tip loading minimizes effects of contact compliance on evaluating the thin film deflection compliance. Since the film has substantial warping undulations, the deflection compliance is attributed to more from in-plane membrane compliance than from bending compliance. Our finite element analysis shows that the warped geometry is approximately three times stiffer than flat plate. The finite element analysis uses shell elements. Accuracy of the test is comparable to that of a blister test; however, this testing method is more versatile and simple. The in-plane elastic modulus of the thin film is measured to be 124 GPa. In addition, the effective elastic modulus of the polycrystalline (nanocrystalline) chromium pillar/fiber is evaluated to be 215 GPa, which is a lower bound of the modulus as it includes compliance of the interface cohesive compliance.
9:00 PM - P3.55
A Phase-field Model for Deformation Twinning.
Tae Wook Heo 1 , Yi Wang 1 , Saswata Bhattacharya 1 , Xin Sun 2 , Shenyang Hu 2 , Long-Qing Chen 1
1 Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania, United States, 2 , Pacific Northwest National Laboratory, Richland, Washington, United States
Show AbstractWe propose a phase-field model for modeling microstructure evolution during deformation twinning. We employ order parameters that are proportional to the shear strains defined in terms of twin plane orientations and twinning directions. Using a face-centered cubic aluminum as an example, the deformation energy as a function of shear strain is obtained using first-principle calculations. The elastic strain energy of a twinned structure is included using the Khachaturyan’s elastic theory. We simulated the twinning process and microstructure evolution under a number of fixed deformation magnitudes and predicted the twinning plane orientations and microstructures. It is shown that twinning may take place through either nucleation and growth or spinodal mechanism, and the relative volume fractions of twin variants has approximate linear dependence on the magnitude of deformation strain.
9:00 PM - P3.56
Forced Atomic Mixing of Immiscible Ternary Alloys during Severe Plastic Deformation.
Nhon Vo 1 , Robert Averback 1 , Michael Campion 1 , Thuy Nguyen 1 , Brad Stumphy 1 , Shankar Sivaramakrishnan 1 , Pascal Bellon 1
1 Materials Science and Engineering, University of Illinois, Urbana Champaign, Urbana, Illinois, United States
Show AbstractNi and Ag67Cu33 homogeneous alloy powders were ball milled together at room temperature (RT) with average Ni atomic concentrations of 4, 9, 15, and 25%. X-ray diffraction and Molecular Dynamics (MD) simulations indicate formation of Cu-Ni alloy particles, where Cu atoms leave the Ag matrix and join the Ni phase. High Resolution Transmission Electron Microscopy (HRTEM) and Atom Probe Tomography (APT) showed that the grain size of the Ag matrix and the particle size of Cu-Ni particles were as small as ~6 nm and ~4 nm respectively. The alloy particle structure is discussed in terms of shear-induced atomic mobility at particle interfaces, even at RT. The ultrafine particle sizes, in comparison to those observed in the corresponding binary alloy systems, is discussed in terms of a core (Ni)-shell (Cu) structure. The very small Ag grain size is attributed to grain-boundary pinning at triple junctions. The self-organization observed in this model ternary alloy system under SPD suggests a flexible method for designing and fabricating alloys with nano-particle dispersions.
9:00 PM - P3.57
Calibrated Monte Carlo Simulation of UO2 Nuclear Fuel Texture Evolution under Temperature Field.
Liangzhe Zhang 1
1 Physics, Colorado School of Mines, Golden, Colorado, United States
Show AbstractUranium dioxide nuclear fuel experiences microstructural evolution under thermo-mechanical environment during operation. Different types of grains emerge due to the non-uniform distribution of temperature field and pores migration. As results, fine grains appear at the colder side, equiaxed grains present in the middle, while column grains show at the hotter side along with a center pore. The population and distribution of these grains dramatically affect the thermo-mechanical properties of nuclear fuels during operation. The current research then intends to quantify this type of microstructural evolution with Monte Carlo models.To facilitate this simulation, a calibrated Monte Carlo (CMC) approach is developed and implemented. Different from the conventional Monte Carlo (MC) models, the CMC algorithm is endowed with physical time, length, and energy scales. This is accomplished by the established parametric links between Sharp-interface (SI) and MC models. Therefore, the temperature and misorientation dependent grain boundary energy and mobility can be easily considered.The CMC model is first applied to study the disorientation-dependent grain boundary energy effects on texture evolution. It is found the effects of anisotropic grain boundary energy are negligible unless pre-texture exists and low angle grain boundaries dominate. This is consistent with experiments and others simulation results. On the other hand, temperature field effects are modeled with a temperature-dependent mobility function, which is determined based on experimental values. As expected, fine and equiaxed grains appear at the low and high mobility sides, respectively. Furthermore, column grains are evolved near the hotter side because of the pores migration towards fuel rod center. This is in good agreement with experimental results.
9:00 PM - P3.58
Interface Character, Recrystallization and Texture Development in Cu-Nb Roll-bonded Composites.
Anthony Rollett 1 , Sukbin Lee 1 , Samuel Lim 2 , Jonathan Ledonne 1
1 Materials Sci & Eng, Carnegie Mellon Univ., Pittsburgh, Pennsylvania, United States, 2 , Singapore Institute of Manufacturing Technology, Singapore Singapore
Show AbstractThe extraordinary strength values of composites with nano-scale layers or phases have inspired much investigation into the strengthening mechanisms of laminated composites such as Cu-Nb. The annealed microstructure and texture of any material govern its mechanical properties in composites just as much as in single-phase materials yet studies on the annealing of such deformed layered composites are still very limited as compared to strengthening mechanisms. In order to investigate the annealing response in such materials, recrystallization textures of monolithic pure Cu and alloyed Cu - C19210, with and without Nb reinforcement were investigated. Metal-metal composites of Cu and Nb were made using accumulated roll-bonding with layer thicknesses down to 200 nm. Samples were annealed at temperatures of 300oC to 800oC and characterized with high resolution EBSD scans. The elongated grain structures observed in the as-deformed state were found to evolve into “bamboo-like” structures with increasing annealing temperature. No evolution was evident in the Nb layers, although recovery may well take place. The growth of the grains in the thin layers was also found to be restricted by the phase boundaries. The hardness of the composite decreased with increasing annealing temperature but softening was negligible in the composite with the thinnest layers. The full five-parameter character of the Cu-Nb interface was investigated for various conditions of the composite materials. Well developed orientation relationships (OR) were found that had the same lattice misorientation across the interface as for Kurdjumov-Sachs but the crystal planes were reversed from the usual close packed planes for each phase. The reason for this reversal is that the orientations are dominated by the rolling deformation used in the ARB process that induces textures with {111} parallel to the rolling plane in the bcc Nb phase, and {110} in the fcc Cu phase.
9:00 PM - P3.59
Humidity and Topography Effects on the Fracture Toughness of Copper-silica Interfaces.
Dandapani Vijayashankar 1 , Ranganath Teki 1 , Saurabh Garg 1 , Michael Lane 2 , Ganpati Ramanath 1
1 Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Chemistry, Emory and Henry College, Emory, Virginia, United States
Show AbstractThe toughness of a metal/dielectric interface is strongly influenced by the local chemical environment at the crack tip as well as the physical topography of the crack path. The role of moisture in the toughness of Cu/SiO2 interfaces is of particular relevance to nanoelectronics device wiring. Here we show that the interfacial toughness is greatly influenced by the strength of water-sensitive Cu-O-Si bonding and water diffusion at the interface, by studying moisture-induced delamination along patterned trenches of different aspect ratios. Silicon wafers were lithographically patterned with 5 μm-wide parallel lines, etched to different depths using tetramethyl ammonium hydroxide and then thermally oxidized to obtain a smooth conformal 100-nm-thick silica layer over the trenches. Thin film sandwich structures with crack path orthogonal to the above trenches were made by sputtering 50 nm Cu on the patterned wafers and tested in a four-point bend test system at 50 °C in a humidity- controlled environment. Our results reveal an 80 % decrease in the subcritical fracture toughness ΓFT from a value of 5.38 J/m2 on increasing the ambient humidity from 5 – 80 % for the planar Cu/SiO2 interface. The orthogonally patterned structures with a trench aspect ratio (depth/width) of 0.4 exhibit a far less pronounced drop of 15 % in ΓFT from a value of 27.68 J/m2, likely from the sterically hindered transport of water molecules to the crack tip. Thermodynamic calculations indicate Cu-O-Si bond rupture due to the spontaneous hydroxylation of the O-Si bond, a behavior reminiscent of stress corrosion cracking seen in bulk silica glasses. Our results suggest that increased susceptibility of Cu-O-Si bridges to cleavage in the presence of moisture is responsible for lowering of interfacial toughening, with the extent of bond cleavage being affected by restricted water transport through the tortuous crack path at a corrugated interface.
9:00 PM - P3.6
Zinc Oxide Nanostructures Synthesized by Oxidization of Zinc.
Nasimul alam Syed 1 , Shakti Swarup Sahu 1 , Manish Kumar 1 , Animesh Kumar Singh 1 , Madhukar Poloju 1
1 Metallurgical and Materials Engineering, National Institute of Technology Rourkela, Rourkela, Orissa, India
Show AbstractZinc oxide (ZnO) is a unique material that exhibits semiconducting as well as piezoelectric properties. Oxidation of pure metallic Zn at low temperatures of 400-600oC for 2 h has resulted in a range of nanostructures of ZnO on the surface of metallic Zn. The large variety of nanostructures formed on the surface of Zn clearly demonstrates that ZnO has a very rich family of nanostructures. These nanostructures could have applications in a wide range of areas like optoelectronics, sensors, transducers and biomedical sciences. Here we have reported how the morphology of ZnO formed on Zn surface changes with oxidation temperature. The structure and morphology of ZnO formed by oxidation of high purity metallic Zn at temperatures ranging from 300-1000oC for 2 h were analyzed. Oxidation of metallic Zn at higher temperatures of 700-1000oC for 2 h has resulted in ZnO having a completely different needle like morphology as compared to the nanostructures that could be formed at lower temperatures of 400-600oC. The change in the morphology of ZnO formed on the surface of Zn as it was oxidized at different temperatures is reported here. Energy dispersive x-ray spectroscopy and x-ray diffraction analysis of the zinc oxide structures were also performed.
9:00 PM - P3.60
Inception of Plasticity in the Presence of Vacancies in FCC Single Crystals.
Iman Salehinia 1 , Veronica Perez 1 , Marc Weber 1 , David Bahr 1
1 School of Mechanical and Materials Engineering, Washington State University, Pullman, Washington, United States
Show AbstractThe transition from elastic to plastic deformation during instrumented indentation loading in relatively defect free single crystalline materials is often ascribed to the nucleation of dislocations. Most available atomistic simulations of nanoindentation tests on perfect FCC single crystals have shown homogeneous nucleation of partial dislocations. Some recent studies have discovered that the first displacement burst has a heterogeneous nature. Several mechanisms have been suggested including nucleation of partial dislocations from a vacancy or formation of dislocation rings from pre-existing vacancies condensed into a plate. Some processes, such as quenching, severe plastic deformation and radiation damage, increase the vacancy concentration to a significant degree. The purpose of this work is to study the effect of lattice vacancies on the nucleation of dislocations and consequently on the onset of plastic deformation in metals using both atomistic simulations and complementary experimental measurements. Simulations have been carried out using a spherical tip of radius 4 nm to demonstrate the effect of position relative to an indenter of a single vacancy on the onset of plasticity of a (111) Ni single crystal. The region beneath the indenter in which a single vacancy can significantly affect (i.e. a 10% change in load) the onset of plasticity is up to 12 layers under the surface of the Ni single crystal. This is an effective vacancy fraction of 3x10^-6. Since in experiments, the specimen and indenter are much larger than those used in this research, we expect that the effective distance of a single vacancy from the surface of the material is much larger than that found in this study. In the presence of a single vacancy, the recorded lowest yield load is 127 eV/Å which indicates 34% weakening when it is compared to the yield load of a perfect crystal (191 eV/Å). This shows that even in absence of other defects such as grain boundaries or surface steps, a single vacancy can significantly reduce the load required to initiate plasticity during indentation. These results are compared with experimental indentations on three electropolished (111) Ni single crystals using three different indenter tips. The samples were measured using positron annihilation spectroscopy to identify point defect concentration in the samples. The probability functions for yield are shown to be dependent on point defect concentration; higher defect concentrations lead to lower applied stresses at yield. This suggests that point defects (both vacancies and possible incorporation of H during electropolishing) cause lower observed yield points in previously published experiments, and the ability to probe the theoretical shear stress in metals using indentation techniques will be ultimately limited by defect concentration.
9:00 PM - P3.61
NanoLAB Triboprobe: Characterising Dynamic Wear, Friction and Fatigue at the Nanoscale.
Aiden Lockwood 1 , Jan Wedekind 2 , Ralph Gay 1 , M. Bobji 3 , Bala Amavasai 2 , Martin Howarth 2 , Guenter Moebus 1 , Beverley Inkson 1
1 Engineering Materials, University of Sheffield, Sheffield, South Yorkshire, United Kingdom, 2 MMVL, Sheffield Hallam University, Sheffield, South Yorkshire, United Kingdom, 3 Mechanical Engineering, Indian Institute of Science, Bangalore, Karnataka, India
Show AbstractIn-situ TEM microscopy has developed rapidly over the last decade. In particular, the inclusion of scanning probes in TEM holders, allows both mechanical and electrical testing to be performed whilst simultaneously imaging the microstructure at high resolution. In-situ TEM nanoindentation and tensile experiments require only an axial displacement perpendicular to the test surface. However, here, through the development of a novel in-situ TEM triboprobe, other surface characterisation experiments are now possible, with the introduction of a fully programmable 3D positioning system.Lateral displacement control allows scratch tests to be performed at high resolution with simultaneous imaging of the changing microstructure. With the addition of repeated cyclic movements, both nanoscale fatigue and friction experiments can also now be performed. We demonstrate a range of movement profiles for a variety of applications, in particular, lateral sliding wear.The developed NanoLAB TEM triboprobe also includes a new closed loop vision control system for intuitive control during positioning and alignment. It includes an automated online calibration to ensure that the fine piezotube is controlled accurately throughout any type of test. Both the 3D programmability and the closed loop vision feedback system are demonstrated here.
9:00 PM - P3.62
Morphology Control and Enhanced Mechanical Performance of γ-alumina Based Catalyst Support.
Magnus Rotan 1 , Erling Rytter 2 , Mari-Ann Einarsrud 1 , Tor Grande 1
1 Department of Materials Science and Engineering, Norwegian University of Science and Technology, Trondheim Norway, 2 , Statoil, Research Centre, Trondheim Norway
Show AbstractCo-based catalyst on γ-alumina support is very attractive in Fischer-Tropsch processes for conversion of natural gas to liquid products such as diesel. A major challenge is to improve the resistance to attrition of the catalyst support used in Fischer-Tropsch slurry reactors. Superior mechanical performance has been obtained by chemical impregnation of γ-alumina with oxide precursors and thermal treatment. Here we report on in-situ high-temperature X-ray diffraction (HTXRD) of γ-alumina infiltrated by metal oxide (MgO, ZnO, NiO, MnO) precursors. During thermal treatment the oxide precursor reacts with γ-alumina to form a spinel phase in addition to α-alumina. A relatively complex phase evolution is observed including formation of the spinel phase superimposed on the well known phase transitions from γ-alumina to the final α-alumina at high temperature. We show that the morphology and phase composition of the catalyst support is strongly dependent on the overall composition of the support and the type and the oxidation state of the metal oxide infiltrated in the support. The mechanical performance of the support is characterized by measurement of attrition resistance and Vickers hardness, showing the dramatic increase in mechanical strength without dramatic reduction in surface area.
9:00 PM - P3.64
Nondeterministic Multiscale Modelling of Biomimetic Crack Self-healing in Nanocrystalline Solids under Mechanical Loading.
Eduard Karpov 1 , Mykhailo Grankin 1
1 , University of Illinois at Chicago, Chicago, Illinois, United States
Show AbstractCoupled multiscale approaches for the analysis and simulation of complex multiphysics phenomena in solids have been of great interest during the last decade [1,2]. These include concurrent MD/FEM models, atomistic-to-continua homogenization techniques and deterministic multiple time scale approaches. In this paper we first discuss a generic approach to concurrently couple the Monte-Carlo master equation of the microscopic kinetic processes, not accessible by the deterministic particle dynamics, with the continuum elastomechanics (FEM) formulation. This approach can be adequate for the modeling and validation of advanced contemporary materials with dynamic internal structure, as well as evolutionary and degradation processes in materials. The approach is illustrated in application to the model of biomimetic material precipitation and successive crack self-healing in nanocrystalline materials. Effect of static and dynamic loading patterns on the crack healing rates are investigated. Refs: [1] Liu WK, Karpov EG, Park HS. Nano Mechanics and Materials: Theory, Multiscale Methods and Applications. Wiley, 2006. [2] Liu WK, Karpov EG, Liu Y. Computational Nanomechanics. In Handbook of Nanophysics. Editor: Klaus D. Sattler, Taylor & Francis Publisher, 2010.
9:00 PM - P3.65
Indentation Measurement of Poroelastic Material Properties.
Michelle Oyen 1 , Matteo Galli 2 1
1 Engineering Dept. , Cambridge University, Cambridge, 0, United Kingdom, 2 , EPFL, Lausanne Switzerland
Show AbstractThere is a clear need for the development of characterization techniques that allow for the measurement of multiple facets of material behavior. Hydrated biological tissues and hydrogels are characterized as poroelastic: the mechanical response can be described with a continuum two-phase model that incorporates fluid flow through a porous elastic solid. Although poroelasticity has been studied for many decades, the coupled nature of poroelastic constitutive equations results in a problem that is typically solved only with numerical or computational models. Thus, poroelastic analyses have not been popular for routine data analysis compared with simpler time-dependent models based on empirical viscoelasticity. In the past decade, however, automated nanoindentation testing has allowed for the generation of large volumes of mechanical data, and the need for high-throughput data analysis for time-dependent materials has been established. In the current study, a new method is developed for high- throughput data analysis based on a master library of indentation creep curves for spherical indentation of a poroelastic material. Thin poroelastic layers on rigid substrates are considered after the framework for bulk poroelastic solids is established. This study demonstrates the potential for gaining greater understanding of mechanical responses in hydrated materials while remaining in a simple experimental indentation testing framework conducive to high-throughput materials characterization.
9:00 PM - P3.66
Discrete Dislocation Models of Fracture in Plastically Anisotropic Metals.
Sutee Olarnrithinun 1 , Srinath Chakravarthy 1 , William Curtin 1
1 Division of Engineering, Brown University, Providence, Rhode Island, United States
Show AbstractThe Peierls stress represents the intrinsic lattice resistance to dislocation motion and depends on the core structure of the dislocation. This lattice resistance is one essential feature of plastic anisotropy that arises in materials such as HCP Zn, Mg, and Ti. Here, we implement an anisotropic Peierls model as a friction stress within a 2D discrete dislocation (DD) plasticity model and then investigate the role of plastic anisotropy on the crack tip stress fields, crack growth, and toughening. First, tension tests for a pure single crystal with no other obstacles to dislocation motion are carried out to capture the general flow behavior in HCP-like materials having slip on basal and pyramidal planes. Then Mode I crack growth in a such a single crystal of HCP material is analyzed using the modified 2D DD model. As the Peierls stress on the pyramidal planes is increased, there is a decreasing resistance to crack growth. In the isotropic limit, comparisons are also made to materials where dislocation glide is inhibited by discrete obstacles, which provides insight into the role of internal material length scales in controlling fracture.
9:00 PM - P3.67
Mapping the Chemical Composition Variation in Metal-loaded Diblock Copolymer Micelle Monolayers via Tapping Mode Atomic Force Microscope.
Alim Solmaz 1 , Taner Aytun 1 , Cleva Ow-Yang 1
1 Materials Science & Engineering, Sabanci University, Istanbul Turkey
Show AbstractIn the last decade, polymeric micelles have been used extensively as nanoscale reactor vessels for nanoparticle synthesis, since these systems provide enhance stability, monodisperse size distribution and control over arrangement in 2-D. The initial step of nanoparticle formation requires the first reactant, such as a metal salt, to segregate to the core of the reverse diblock copolymer micelle and to form a complex with the polymer block constituting the core. The formation of such a nanocomposite system with an embedded inorganic phase should be verifiable without removal of the polymer. Conveniently, the core of the micelle becomes more rigid when loaded, in contrast to its corona. Thus the chemical composition variation at the nanoscale could be mapped, by tracking the variation in compliance of the core and the corona of the micelles, using the phase angle shift of tapping mode atomic force microscope (TM-AFM) under well-defined tip-sample interaction parameters. In this study we present the details of the relationship between phase angle shift and composition variation in an embedded system. We determine the stiffness variation across the loaded micelles via ‘true’ nanoindentation with TM-AFM. Indentation values as low as 5 nm are found to be sufficient for determining the elastic modulus of metal salt loaded diblock copolymer micellar nanocomposite system. To extract the elastic modulus from the measured curves, the parabolic Hertz model for elastic deformation is applied.
9:00 PM - P3.69
Dependence of Mechanical Properties of Nanocrystalline Materials on Grain Size.
Georgios Kopidakis 1 , Nikos Galanis 1 , Ioannis Remediakis 1
1 Materials Science and Technology, University of Crete, Heraklion, Crete, Greece
Show AbstractUnderstanding and controlling nanoscale phenomena is fundamentally interesting and technologically important for the design of materials with tailored properties. The dependence of the mechanical properties of polycrystalline materials on the grain size, when this size is reduced to a few nanometers, is of great interest in this context. We perform atomistic simulations for the structure and elastic properties of several nanocrystalline materials. We observe softening at small grain sizes, in analogy with the reverse Hall-Petch effect for plastic deformations in nanocrystalline metals. The decrease of elastic constants is explained by the increase of the fraction of grain boundary atoms for smaller grains. By decomposing the energy into contributions from atoms in the bulk of grains and at interfaces, we derive simple scaling relations for various properties as a function of the average grain size. These theoretical predictions fit very well the simulation data. Our results are for different materials that range from nanocrystalline metals to nanocrystalline diamond. This suggests that softening at small grain sizes is a general nanoscale effect.
9:00 PM - P3.7
Correlation Between Adhesion Strength and Thin Film/Substrate Mechanical Properties Using the Nano-scratch Technique.
Bo Zhou 1 , Nicholas Randall 1 , Barton Prorok 2
1 , CSM Intruments Inc., Needham, Massachusetts, United States, 2 Materials Engineering, Auburn University, Auburn, Alabama, United States
Show AbstractScratch testing, as a mature technique for coating adhesion quantification, has been widely adopted by both industrial and academic fields in recent years. Following the urgent needs of very small materials characterization, nano-scratch testing has gradually replaced the traditional pull-off test for the study of ultra-thin film properties. In this research, the relationship between the adhesion strength and film/substrate mechanical properties was investigated to provide fundamental but crucial knowledge of the scratch mechanism. Five thin films were deposited using sputtering onto different polished substrate materials which span from soft silicon dioxide to hard sapphire. The surface roughness of the bare substrates was measured using Atomic Force Microscopy. Scratch tests were performed using a Nano Scratch Tester with a sphero-conical diamond indenter. A progressive load mode was employed to cause coating failure during scratch on the film surface. The critical values of different failure mechanisms, such as cracking, spallation, and delamination were accurately determined according to the scratch panorama image, penetration and residual depth data. In addition, the Hardness (H) and Modulus (E) values of the thin films and substrates were measured with an Ultra Nanoindentation Tester. The scratch critical failure loads were then plotted versus film/substrate H and E ratios. A unique relationship was expected to be found between these parameters that could help understand the true mechanism behind scratch adhesion and leverage this methodology to a new theoretical level.
9:00 PM - P3.73
Preferred Nucleation Sites of Dislocation at fcc/bcc Metallic Interfaces.
Ruifeng Zhang 1 , Jian Wang 1 , Irene Beyerlein 1 , Timothy C Germann 1
1 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractInterfaces are a common planar defect in materials, and can act as sources, sinks and barriers for both point and line defects. Recent studies have shown that interfaces play a crucial role in determining material strength for nanolayered composites. Taking advantage of the role interfaces play as barriers to slip transmission, nanolayered composites with high strength, good ductility and good thermal stability have been made for a number of material systems. Atomistic simulations offer a powerful tool for providing insight into deformation mechanisms that are dominated by interfaces, such as storage, recovery, nucleation and emission of dislocations at interfaces. Here, we study dislocation nucleation at interfaces, which is an important event during mechanical deformation, particularly severe plastic deformation and shock compression. Using Cu/Nb multilayers as a prototype, dislocation nucleation sites and the corresponding activation energies with respect to each slip system are studied at the Cu/Nb interfaces. Two types of interfaces are adopted in this study that are created, with either Kurdjumov–Sachs (KS) and Nishiyama-Wasserman (NW) orientation relationships. It was found that preferred dislocation nucleation sites are related to the interfacial structure, which in turn is determined by this orientation relationship. In addition, the activation energies for dislocation nucleation at the same site differ from one slip system to another due to the particular local atomic structure. Furthermore, the number of preferred nucleation sites for KS and NW interfaces differs due to the different interface patterns. Such findings will provide insight into understanding the mechanical response of a wider range of fcc/bcc metallic nanocomposites.
9:00 PM - P3.74
Optimizing Load Transfer in Multiwall Nanotubes through Interwall Coupling: Theory and Simulation.
Emmett Byrne 1 2 , Alexis Letertre 3 , Michael McCarthy 1 2 4 , William Curtin 5 , Zhenhai Xia 6
1 Department of Mechanical and Aeronautical Engineering, University of Limerick, Limerick Ireland, 2 Composites Research Centre, University of Limerick, Limerick Ireland, 3 , Institut Français de Mecanique Avancee, Les Cezeaux, Aubière, France, 4 Materials and Surface Science Institute, University of Limerick, Limerick Ireland, 5 Division of Engineering, Brown University, Providence, Rhode Island, United States, 6 Department of Mechanical Engineering, University of Akron, Akron, Ohio, United States
Show AbstractThe outstanding mechanical properties of carbon nanotubes (CNTs) make them highly attractive as potential reinforcing constituents in structural composites. However, development of high performance materials using CNTs requires a fundamental understanding of the mechanics of nanoscale reinforcement. Multi-wall CNTs (MWCNTs) present the possibility of increased load carrying ability per unit area over single-wall CNTs (SWCNTs) if all walls can be made to share the load. Experiments have shown that controlled sputtering and irradiation of pristine MWCNTs can enhance interwall coupling and sliding resistance via the formation of interwall sp3 bonds [1]. Peng et al. [2] induced variable degrees of interwall coupling in MWCNTs and tested the resulting specimens in tension up to failure. Results indicated the failure loads were up to ~11.6 times the load expected if only their outermost wall had been loaded. Previous work by the present authors has also confirmed the benefits of interwall coupling for the mechanical properties of MWCNTs [3,4]. Irradiation thus appears to be a promising route to producing MWCNTs with controlled levels of interwall coupling. In this work, we expand on our previous observations of load transfer in irradiated MWCNTs [3] by developing an analytical model to determine the length scales over which load is transferred from outer to inner walls of MWCNTs as a function of the amount of bonding between walls. The model predicts that the characteristic length for load transfer scales as l~t√(E/μ) , where t is the CNT wall spacing, E is the effective wall Young’s modulus, and μ is the average interwall shear modulus due to interwall coupling. Analogous Molecular Dynamics simulations provide data against which the model is evaluated. Results show excellent agreement between analytic and numerical models when interwall bonding is uniformly distributed in the axial direction, showing that continuum mechanics concepts apply down to the atomic scale in this problem. The simulation models also show, however, that load transfer is sensitive to natural statistical fluctuations in the spatial distribution of the interwall bonding between pairs of walls, and such fluctuations generally increase the net load transfer length needed to fully load an MWCNT. Optimal load transfer is achieved when bonding is uniformly distributed axially, and all interwall regions have the same shear stiffness, and is shown to occur over a length of ~1.5nl. This model can be used to design MWCNTs for structural materials, and to interpret load transfer characteristics deduced from experiments on individual MWCNTs.[1] Pregler SK, Sinnott SB. Phys Rev B 2006;73:224106.[2] Peng B, Locascio M, Zapol P, Li SY, Mielke SL, Schatz GC, et al. Nat Nanotechnol 2008;3:626.[3] Byrne EM, McCarthy MA, Xia Z, Curtin WA. Phys Rev Lett 2009;103:045502.[4] Xia ZH, Guduru PR, Curtin WA. Phys Rev Lett 2007;98:245501.
9:00 PM - P3.75
Instrumentation for Nanoscale Characterization in SEM.
Andrew Smith 1 , K. Schock 1 , S. Kleindiek 1
1 , Kleindiek Nanotechnik GmbH, Reutlingen Germany
Show AbstractPrecise positioning and force detection are required, in order to perform fine scale characterization experiments. Three different approaches to this task using piezo driven manipulation systems installed into scanning electron microscopes are presented.The first approach entails the use of the MM3A-EM micromanipulator. This three-axis manipulator with minimum step sizes of down to 0.25 nm can be fitted with a piezo- esistive force measurement cantilever. Detecting the change in resistivity while the cantilever is deflected during the experiment yields force information.Another approach incorporating the MM3A-EM micromanipulator is the use of a spring table. After mounting the sample to a spring table the sample is deflected, deformed or indented using a tool mounted to the tip of the micromanipulator. A sequence of images is recorded during the experiment.These images are processed with a custom software, yielding force distance curves derived from the displacements measured in the SEM images and the well-known force constant of the spring table.Finally, an in situ AFM is introduced. The SF-AFM consists of a micromanipulator and a super flat piezo scanner mounted to a load-lockable platform. The AFM cantilever is mounted to the micromanipulator for pre-positioning. The three-axis piezo scanner - with a scan range of 15 um, 15 um, 3 um - is used for AFM imaging and force measurement. The entire system can be introduced into the SEM chamber via the load lock for quick and effortless sample and tip exchange.
9:00 PM - P3.76
Phase Field Modeling of Microstructural Evolution in Viscoplastic Polycrystals.
Saswata Bhattacharya 1 , Ricardo Lebensohn 2 , Long-Qing Chen 1
1 Department of Materials Science and Engineering, Penn State University, University Park, Pennsylvania, United States, 2 Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractWe report a novel phase field model in combination with continuum crystal plasticity method to study microstructural evolution in elasto-viscoplastically deformed polycrystals. Our model takes into account elastic and plastic anisotropy and inhomogeneity. The local response of viscoplastic anisotropic polycrystals is computed using a Fast Fourier Transform (FFT) algorithm developed by Lebensohn[1]. Elastic response of the polycrystal is computed using a Fourier spectral iterative perturbation method. Our model for microstructural evolution includes driving forces arising from both elastic and plastic strain fields. Plastic strain is explicitly incorporated in our model. The evolution of concentration and order parameter fields is governed by Cahn-Hilliard and Allen-Cahn equations, respectively. We have studied precipitation of ordered phases and cubic to tetragonal transformation in anisotropic polycrystals using our model. In this study we describe how plastic strain affects the morphological evolution of precipitates in polycrystals. [1] R.A.Lebensohn, Acta Mater., 49(2001) 2723-2737
9:00 PM - P3.8
Optical and Mechanical Properties of Neat and Hollow Silica Nanoparticle Composites.
Markus Retsch 1 , Edwin Thomas 1 , Marcus Schmelzeisen 2 , Andreas Unger 2 , Hans-Juergen Butt 2
1 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 , Max Planck Institute for Polymer Research, Mainz Germany
Show AbstractWe investigate the optical and mechanical properties of films made from well defined hollow silica nanoparticles (HSN) and HSN polymer composites. The hollow spheres are synthesized by coating monodisperse polymer nanoparticles with a silica or titania shell in a solgel process. Combustion of the polymeric core yields the hollow particles. Depending on the particle size, shell thickness, and material type, an intriguing color effect in these non-absorbing, unordered powders is observed. The origin of the color will be discussed in the light of Mie scattering. The mechanical properties of the particles are analyzed by nanoindentation and pulsed-laser shock experiments. The low effective density of hollow nanoparticles (ρ = 0.2 – 0.6 g/qcm) makes them useful filler compounds for light weight polymer nanocomposites. The monodispersity of the hollow particle geometries allows direct structure-property assessment.
Symposium Organizers
Julia R. Greer California Institute of Technology
Daniel S. Gianola University of Pennsylvania
Blythe G. Clark Sandia National Laboratories
Ting Zhu Georgia Institute of Technology
Alfonso H. W. Ngan The University of Hong Kong
P4/U3/T3: Joint Session: Deformation Mechanisms, Microstructure Evolution, and Mechanical Properties of Nanoscale Materials
Session Chairs
Tuesday AM, November 30, 2010
Room 210 (Hynes)
9:00 AM - **P4.1/U3.1/T3.1
Ultra-strength Materials.
Ju Li 1 , Ji Feng 1 , Ting Zhu 2
1 Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States, 2 Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractRecent experiments on nanostructured materials, such as nanoparticles, nanowires, nanotubes, nanopillars, thin films, and nanocrystals have revealed a host of "ultra-strength" phenomena, defined by stresses in a material component generally rising up to a significant fraction (>1/10th) of its ideal strength - the highest achievable stress of a defect-free crystal at zero temperature. While conventional materials deform or fracture at sample-wide stresses far below the ideal strength, rapid development of nanotechnology has brought about a need to understand ultra-strength phenomena, as nanoscale materials apparently have a larger dynamic range of sustainable stress (strength) than conventional materials. Ultra-strength phenomena not only have to do with the shape stability and deformation kinetics of a component, but also the tuning of its physical and chemical properties by stress. Reaching ultrastrength enables elastic strain engineering, where by controlling the elastic strain field one achieves desired electronic, magnetic, optical, phononic, catalytic, etc. properties in the component, imparting a new meaning to Feynman's statement "there's plenty of room at the bottom". [Prog. Mater. Sci. 55 (2010) 710]
9:30 AM - P4.2/U3.2/T3.2
Effect of Habit Plane on the Mechanical Properties and Radiation Response of Cu-Nb Interfaces.
Michael Demkowicz 1 , Ludovic Thilly 2 , David Rodney 3
1 Department of Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States, 2 Department of Materials Physics and Mechanics, University of Poitiers, Poitiers France, 3 SIMAP-GPM2, Grenoble Institute of Technology, Grenoble France
Show AbstractWe compare semicoherent Cu-Nb interfaces found in wire-drawn and sputter deposited nanocomposites. Although both have nearly identical orientation relations, their habit planes are different, resulting in unlike atomic-level structures for each. We investigate the consequences of these differences for the mechanical properties and radiation response of each interface using both experimental methods (He ion implantation and transmission electron microscopy) and atomistic modeling (Molecular Dynamics and Activated Dynamics based on the Activation-Relaxation Technique). The prospect of developing a general framework for predicting properties of semicoherent interfaces from their atomic-level structure and its implications for designing extreme environment-tolerant nanocomposites will be discussed.
9:45 AM - P4.3/U3.3/T3.3
Effects of Size, Orientation, and Loading Path through In-situ Uniaxial Deformation of Body-centered Cubic Metals.
Julia Greer 1 , Ju-Young Kim 1
1 Materials Science, California Institute of Technology, Pasadena, California, United States
Show AbstractMechanical behavior of body-centered cubic (bcc) metals is intriguing as they often exhibit non-Schmid plasticity, a strong dependence on temperature and strain rate, and a range of strengths under different deformation modes. Recent developments in sample preparation techniques and in in-situ mechanical testing have made it possible to extend these studies to the nano-scale. In this talk we will discuss the uniaxial compressive and tensile behavior of single crystalline Mo, W, Ta, and Nb nano-pillars with diameters between 100nm and 1micron, at different strain rates and orientations for the case of Mo. Most compression experiments were conducted in G200 nanoindenter with a custom-made flat punch indenter tip while the tension and remaining compression tests were performed in a custom-built in-situ mechanical deformation instrument, SEMentor, comprised of a SEM and Nanoindenter, and fitted with custom-machined tension/compression grips. Material microstructure was evaluated via site-specific TEM analysis of the same pillars before and after deformation. Our results demonstrate that, when reduced to the nano-scale, plasticity in bcc metals shows pronounced, non-equivalent size effects, a notable size- and orientation-dependent tension-compression asymmetry, and the formation of intricate dislocation networks in post-mortem structures. We find the power law slope for size dependent flow stress in compression of Nb(-0.93) being much higher than those of other three metals, W (-0.44), Mo (-0.44), and Ta (-0.43). In tension, group VB metals: Ta (0.80) and Nb (0.77) are higher than group VIB ones:W (-0.58) and Mo (-0.43). Also, unlike in bulk, the difference in flow stresses in tension vs. compression of [001] and [011] oriented single crystalline Mo nano-pillars depends on the pillar size, as the compressive flow stresses are higher than tensile ones in [001] orientation and visa versa in [011] orientation. The tension-compression asymmetry is also a function of size for smaller (<800nm diameter) pillars, while the strength differential for larger nano-pillars approaches size- independent bulk values. We report strong strain rate dependence with the determined activation volume of ~1b3, a value only marginally smaller than bulk, suggesting that the deformation occurs via thermally activated kink pair nucleation, a typical screw dislocation propagation mechanism in bcc metals. We attribute these findings to the differences in lattice resistance to dislocation motion at room temperature inferred from the Peierls stress and critical temperature. We discuss this size-dependent mechanical response in the context of non-planar cores of a/2<111> screw dislocations governing plasticity of bcc metals.
10:00 AM - P4.4/U3.4/T3.4
A Twist of the Eshelby Twist: Unraveling the Mystery of Twinning.
Ting Zhu 1 , Ju Li 2 , Sankar Narayanan 1
1 School of Mechanical Engineering, Georgia Institute of Technology , Atlanta, Georgia, United States, 2 Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States
Show AbstractThe principal difficulty in understanding the deformation mechanisms of small-volume crystalline solids has been to explain how the plastic deformation carriers such as dislocations and twins are generated and multiplied. The sources of such carriers and associated triggering processes have not been well established. In 1953, Eshelby showed that the central region of a thin rod can act as a trap to stabilize the screw dislocation lying parallel to the axis of the rod, and this axial screw dislocation spontaneously induces the torsional deformation of the rod − the so-called Eshelby twist (JD Eshelby, J. Appl. Phys., 24, pp. 176, 1953). While studying the Eshelby twist effect, we incidentally discovered a mechanism of twinning. We found that the axial screw, in addition to being central to crystal growth, can result in regenerative twinning to sustain the continued plastic flow. This is achieved through a 3D process of dislocation intersection followed by spiral glide of a sweeping dislocation around the pole. Our work is the first atomistic realization of the 3D twinning process, and it opens up the possibilities of studying the experimentally relevant deformation twinning mechanisms for a wide range of materials and applications.
10:15 AM - P4.5/U3.5/T3.5
Dislocation and Twin Boundary Interactions in Hard-sphere Colloidal Crystals.
Maria Persson Gulda 1 , David Weitz 1 2 , Frans Spaepen 1
1 School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, United States, 2 Department of Physics, Harvard University, Cambridge, Massachusetts, United States
Show AbstractInteractions between dislocations and twin boundaries can produce considerable increases in strength and strain hardening. Simulations and theory show that when a perfect dislocation hits the boundary it divides up into two dislocations: one propagates through and the other one stays at the boundary, leading to a climb in twin. To study experimentally what happens on the atomic scale remains difficult even with current instruments. Therefore, we have studied dislocations in a twinned colloidal crystal with a confocal microscope, which allows their interactions to be mapped out in detail on the particle scale.
10:30 AM - P4.6/U3.6/T3.6
Dislocation Drag at the Nanoscale.
Christopher Weinberger 1
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractPlasticity in nanocrystals is significantly altered by the presence of free surfaces, especially when the volume of material is small compared to the inherent dislocation structure size. In this talk, we look at how free surfaces directly affect the mobility of dislocations when they are free to glide. Molecular dynamics simulations show that the drag mechanisms are altered, resulting in higher drag forces, which exhibit a one-over-length scaling characteristic of localized surface forces.
10:45 AM - P4.7/U3.7/T3.7
Compression Tests of <111> FCC Au Single-crystalline Microcrystals on a <0001> Sapphire Substrate.
Seok-Woo Lee 1 , Bjoern Backes 1 , Dan Mordehai 2 , Eugen Rabkin 2 , William Nix 1
1 Materials Science and Engineering, Stanford University, Stanford, California, United States, 2 Materials Engineering, The Technion – Israel Institute of Technology, The Technion City, Haifa 32000, Israel
Show AbstractThe <111> single-crystalline FCC Au microcrystals on a <0001> sapphire substrate were deformed under compression. The Au microcrystal was grown by the solid-dewetting of the sputtered Au thin film near its melting temperature, so the crystal is believed to be almost dislocation-free. The compression tests on microcrystals revealed that most of them yield with a huge strain burst as metallic whiskers do, and there is the strong size dependence of the yield strength. The microcrystals, which have a top diameter smaller than 500 nm top diameter, yield near the theoretical strength. Atomic force microscopy showed different surface morphologies with respect to the sample dimension, which indicates that the size effects might come from the existence of grown-in surface defects. In addition, the weakening effects of focused-ion beam damage and prestraining were studied. The importance of surface structure and dislocation contents is discussed, and the results are compared with those of the recent micropillar compression tests.
11:00 AM - P4/U3/T3: ultra
Break
11:15 AM - **P4.8/U3.8/T3.8
Plastic Flow of Nanoporous Gold.
Hai-Jun Jin 1 3 , Joerg Weissmueller 2 3
1 Institute of Metal Research, Chinese Academy of Sciences, Shenyang China, 3 Institute of Nanotechnology, Karlsruhe Institute of Technology, Karlsruhe Germany, 2 Institute of Materials Physics, Hamburg Technical University, Hamburg Germany
Show AbstractBy controlled chemical or electrochemical corrosion of alloys, samples of nanoporous metal are readily prepared with dimensions at the millimeter or centimeter scale, while at the same time the microstructure is a homogeneous array of interpenetrating solid skeleton phase and pore channels with a characteristic size that can reach down to below 5nm. The interest in nanoporous metals as functional materials derives from their extremely small structure size and from the open porosity, which allows the surfaces to be addressed and manipulated by electrical or chemical signals. Furthermore, nanoporous metals provide convenient model systems for studying the mechanical properties of arrays of nano-objects that are assembled to form a material. This talk will discuss three issues: 1, dealloying procedures yielding macroscopic samples of nanoporous gold that are free of cracks and therefore ductile in compression; 2, the mechanical behavior and possible underlying deformation mechanisms; 3, the response of the plastic flow to changes in the electrochemical environment.
11:45 AM - P4.9/U3.9/T3.9
Anelasticity in Nanocrystalline and Nanoporous Metals.
Nicolas Briot 1 , Jochen Lohmiller 1 2 , Christoph Eberl 2 , Oliver Kraft 2 , T. John Balk 1
1 Chemical and Materials Engineering, University of Kentucky, Lexington, Kentucky, United States, 2 Institut für Zuverlässigkeit von Bauteilen und Systemen, Karlsruhe Institute of Technology, Karlsruhe Germany
Show AbstractAnelasticity denotes the time-dependent behavior of metals under load, during which stress and strain are not in phase. Several mechanisms have been proposed for anelasticity, including interactions between dislocations and point defects, nucleation of kink/antikink pairs and grain boundary diffusion. Damping properties of free-standing cantilever samples were investigated between -125°C and +150°C, using a home-built system consisting of a vacuum chamber and laser vibrometer connected to an acquisition system to monitor cantilever deflection in real time. Internal friction peaks were observed for nanocrystalline (nc) nickel samples and for nanoporous (np) metals obtained by dealloying. The damping peaks for nc-Ni appear to arise from interactions between dislocations and point defects, while different relaxation mechanisms may be involved in np metals, due to these materials’ limited ability for dislocation nucleation to occur in a confined structure and due to the high amount of surface area for diffusion.
12:00 PM - P4.10/U3.10/T3.10
Unified Scaled Behaviour for the Deformation of FCC Micropillars and Nanoporous Metals.
Brian Derby 1
1 School of Materials, University of Manchester, MANCHESTER United Kingdom
Show AbstractDuring compression deformation of FCC micropillars from a number of different metals, the critical resolved shear stress, σcrss, (normalised by the resolved shear modulus, μ) is a function of pillar diameter.d, (normalised by the Burgers' vector, b) and a universal empirical scaling law is observed with σcrss/μ = A(d/b)m, where A is a constant and the exponent, m, is close to -0.6 [1]. We have recently reported a similar scaling law for the limited reports on the strength of nanoporous metals with the scaling now controlled by the mean ligament diameter within these highly porous structures [2]. An important question is whether the reported scaling in these two classes of materials represents a universal scaled behaviour, or whether these similarities are coincidental. The geometry of deformation is very different for these two structures, with micropillars deforming through compression and the nanopillars deforming through localised bending. However, careful consideration of these mechanisms and the stresses acting on the appropriate slip systems reveals that the normalised mechanical behaviour of these two classes of nanostructured materials do indeed show the same size scaled behaviour and that the empirical scaling law is observed for all data in the literature for FCC micropillars and nanoporous metals.(1) R. Dou and B. Derby, Scripta Mater. 61, 524-527 (2009).(2) R. Dou, B. Xu and B. Derby, Scripta Mater. 63, 308–311 (2010).
12:15 PM - P4.11/U3.10/T3.11
Mechanical Behavior of Metal-polymer Nanocomposites Based on Nanoporous Au.
Eike Epler 1 , Kodanda Mangipudi 1 , Lorenz Holzer 2 , Cynthia Volkert 1
1 University of Göttingen, Institute for Materials Physics, Göttingen Germany, 2 , Empa, Dübendorf Switzerland
Show AbstractNanoporous Au has received considerable attention in the basic research literature because of its unusually high strength and hardness, making it an interesting model material for understanding high strength-low weight structural materials. However, nanoporous Au has a low fracture toughness, similar to that of a porous ceramic. Both the high strength and the low toughness are believed to be a consequence of the nanometer length scale. Following the examples found in nature, we attempt to improve the foam toughness, while keeping the strength, by infiltrating it with a polymer. The results of Berkovich indentation, microcompression, and notched microbeam bending tests on uninfiltrated and infiltrated nanoporous Au samples will be summarized. Even in systems where good adhesion between the Au and polymer is not expected, infiltration can greatly improve the mechanical performance. This is interpreted in terms of geometrical constraints and load transfer.
12:30 PM - **P4.12/U3.12/T3.12
Breathing of Nanoporous Metal Foam: Disordered Versus Ordered Structures.
E. Detsi 1 , S. Punzhin 1 , P. Onck 1 , Jeff De Hosson 1
1 Applied Physics, Un. of Groningen, Groningen Netherlands
Show AbstractBulk nanoporous materials with ligament and pore sizes in the nanometer regime form a novel class of engineering material that combines the mechanical properties of solid bulk materials, on the one hand, with the functional properties of individual nanoparticles, nanopillars and nanowires, on the other hand. This combination gives rise to intriguing properties, which could be explored for various applications including sensors and actuators. In this presentation we show that variations in the air relative humidity give rise to reversible macroscopic dimensional changes in bulk disordered nanoporous gold exposed to ambient air. Unlike existing actuation materials the actuation mechanism we report in these disordered nanoporous metal makes use of the energy involved in the liquid to vapor phase transition of water. This concept is attractive for environmentally friendly short-stroke actuator and sensor applications. It is anticipated that even a superior actuation performance can be achieved by using an actuating material with an ordered nanoporosity. New results are presented on the synthesis and performance of highly ordered nanoporous metal foams by means of a three-dimensional templating process.
P5/U4/T4: Joint Session: Deformation in Non-metallic, Amorphous, and Alloyed Materials
Session Chairs
Michael Demkowicz
Julia Greer
Tuesday PM, November 30, 2010
Room 210 (Hynes)
2:30 PM - **P5.1/U4.1/T4.1
Failure Mechanisms in Zinc Oxide Nanowires – An Experimental-Computational Investigation.
Horacio Espinosa 1 , Ravi Agrawal 1 , Bei Peng 1 2
1 Mechanical Engineering, Northwestern University, Evanston, Illinois, United States, 2 Mechanical Engineering, University of Electronic Science and Technology of China, Chengdu China
Show AbstractNanowires made of semiconducting materials have drawn significant research interests due to their interesting electromechanical, optoelectronic and biocompatible properties. For example, nanowire-based miniaturized sensors and nanogenerators (1) are being developed using zinc oxide (ZnO) nanowires (NWs). To address the reliability of such nanodevices, mechanical characterization of their individual building blocks is essential. Here we report the findings of a systematic experimental-computational investigation of failure mechanisms in ZnO NWs, oriented along their polar axis. In situ experiments using a nanoscale-material testing system (2) reveal a brittle failure along (0001) cleavage plane, in agreement with the bulk failure mode (3). The strain to failure, however, is much higher as compared to bulk material and ranges from 2.4% to 6%. The failure strains correlate well with the surface area using Weibull-type probabilistic theory, suggesting that the failure initiates at surface defects.In contrast to the experimental findings, atomistic simulations using a Buckingham-type pairwise potential (BP) reveal a structural phase transformation from original wurtzite (WZ) phase to a body-centered tetragonal (BCT) phase at ~6% strain. The phase transformation is accompanied with stress relaxation and the transformed NW undergoes a secondary loading up to strain as high as 18% prior to brittle failure. Such transformation was predicted even in the presence of the surface defects (3), but not observed experimentally. To resolve this discrepancy, first principles-based density functional theory (DFT) calculations were performed to validate the prediction of BP in the large deformation regime. DFT study reveals that the pristine NWs can be deformed elastically to strains as high as 20% without undergoing any phase transformation, suggesting that the BP prediction might be an artifact of its pairwise nature (4). The energy barriers suggest that the phase transformation is likely but only heterogeneously in the presence of surface defects, which act as stress concentrators. Moreover, unlike BP predictions, the transformed structure does not undergo secondary loading and the transformation acts as a precursor to failure. Therefore, in experiments one should observe the transformed phase only locally in the vicinity of a critical defect. Experimental challenges associated with capturing this phase transformation will be discussed.References:1. R. Yang, Y. Qin, C. Li, G. Zhu, Z. L. Wang, Nano Letters 9, 1201 (2009).2. Y. Zhu, H. D. Espinosa, Proceedings of the National Academy of Sciences of the United States of America 102, 14503 (2005).3. R. Agrawal, B. Peng, H. D. Espinosa, Nano Letters 9, 4177 (2009).4. R. Agrawal, J. T. Paci, H. D. Espinosa, under review in Nano Letters, (2010).
3:00 PM - P5.2/U4.2/T4.2
Nanoductility Induced Brittle Fracture in Shocked High Performance Ceramics.
Paulo Branicio 1 , Rajiv Kalia 2 , Aiichiro Nakano 2 , Priya Vashishta 2
1 Materials Theory and Simulation Laboratory, Institute of High Performance Computing, Singapore Singapore, 2 Collaboratory for Advanced Computing and Simulations, Department of Chemical Engineering and Materials Science, Department of Physics & Astronomy, and Department of Computer Science, University of Southern California, Los Angeles, California, United States
Show AbstractRecent progress in experimental techniques have revealed surprising existence of ductility, such as void formation and necking at the nanometer scale in nominally brittle materials. Such nanoductility has been shown to play a crucial role in the damage initiation in high-strength ceramics under high-velocity impact. We report a 300 million atoms molecular dynamics simulation of shocked silicon carbide which reveals a nanoductility induced crack nucleation mechanism mediated by a single dislocation core. In the simulations a 3C-SiC target is impacted at 15 km/s and the atomic trajectories are calculated as a function of time allowing an accurate description of the spontaneous shock-induced damage evolution. The atomistic damage mechanism involves dynamic transitions between clearly delineated regimes - from shock-induced structural transformation to plastic deformation to brittle fracture. Such atomistic understanding may help in the design of nanocrack suppression strategies and be incorporated into continuum simulations to realize predictive modeling of complex damage processes in high-performance ceramics.
3:15 PM - P5.3/U4.3/T4.3
Deformation Mechanisms, Microstructure and Mechanical Properties of Nanoscale Crystalline and Noncrystallyne Materials in Different Temperature Ranges.
Yuliy Milman 1
1 Physics of High-Strength and Metastable Alloys, Institute for Problems of Materials Science, Kiev Ukraine
Show AbstractThe review of the influence of nanoscale structural elements on the mechanical properties of crystals, quasicrystals and metallic glasses (MG) is given. Temperature ranges of cold, warm and hot deformation are distinguished for crystalline materials [1]. Nancrystalline (NC) structure may be formed in the bulk materials by severe plastic deformation in the temperature range of warm deformation, but in the case of surface deformation such structure may be formed at the temperature range of hot deformation in the process of dynamic recrystallization as well.The formation of nanostructure increases hardness and strength, but elongation to fracture approaches to zero, especially for bcc metals. Plasticity characteristic, obtained by indentation [2], may be used for characterization of low ductile NC materials. The main features of the plastic deformation mechanisms of NC materials, including results obtained by computer simulation, are discussed. Two problems are discussed for MG: the first one is the comparison of yield stress for NC and MG (yield stress of NC may be more than of MG at grain size D < Dc, where Dc=50-100nm) and the second one is the possibility of strengthening MG by disperse crystalline nanoscale particles. Such strengthening was shown for Al-Ce-Sc MG strengthened by nano-particles of Al. Quasicrystals with nanosize grain (NQC) is the separate class of materials. In crystals dislocation energy E~lnD, but in quasicrystals E~D. For this reason mechanical behavior of NQCs differ essentially from quasicrystals and from nanocrystals. NQC have high hardness as quasicrystals and some plasticity at room temperature.Stress-strain curves for quasicrystals have two stages: hardening stage and stage of softening.By the example of the quasicrystalline alloy Al-Cu-Fe it was shown that the hardening stage in NQC is two time more than in quasicrystals and softening is not so intensive. NQC is the best material for dispersion hardening of metals. For example, aluminum alloys of Al-Cr-Fe system, obtained by rapid solidification technique, can contain 30-40% NQC phase. For these alloys UTS=350MPa at 300C and elongation to fracture 10% at room temperature that corresponds to the modern demands of aviation.Dispersion hardening of metals by NC particles was the first application of nanoscale structure for structural materials. In the present time this direction develops intensively and, for example, the most strong Al alloy Al-Zn-Mg-Cu-Sc-Zr has very high for aluminum alloys value of UTS=850MPa and sufficient plasticity. Two ensembles of NC particles are used in this case.References:1.Yu.Milman, Mater.Sci.Forum, v.426-432, 2003, p.4399-4404.2.Yu.Milman, J.of Phys.D: Appl.Phys., 2008, v.41, 074013, (9p.)
3:30 PM - P5.4/U4.4/T4.4
Tribological Response of Nanostructured Metals under High Stresses and Strains.
Aparna Singh 1 , Lei Lu 2 , Ming Dao 1
1 Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States, 2 Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang China
Show AbstractNano-indentation and nano-scratch techniques not only offer precise methods in characterizing mechanical properties of materials but are also capable of imitating real life contact and/or tribological events. In the present study, responses of nanocrystalline metals under the influence of cyclic normal contact and sliding fatigue contact at ambient and elevated temperatures under the action of a diamond or sapphire indenter are examined. The effect of microstructural size scale on the response under extremely high strains beneath the indenter is investigated. Strategies for optimizing strength, damage resistance and microstructural stability in nanostructured materials will be discussed.
3:45 PM - P5.5/U4.5/T4.5
Separating Solid Solution and Grain Size Strengthening in Nanocrystalline Alloys.
Timothy Rupert 1 , Christopher Schuh 1
1 Department of Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States
Show AbstractIn recent years, alloying has been successfully used to increase the thermal stability and control the grain size of nanocrystalline metals. However, grain size is intimately tied to composition in many such alloys, making it difficult to separate the effects of grain size and solute addition on mechanical response. In this work, we describe sputtered Ni-W alloys where microstructure is not controlled purely by composition. Of specific interest is a range of solid solution compositions where a constant, nanocrystalline grain size is observed. Nanoindentation of these alloys is used to investigate the solid solution strengthening of nanocrystalline alloys, at a grain size where deformation is dominated by grain boundary processes. The results are used to provide critical insight into the existing literature on the mechanical properties of nanocrystalline alloys.
4:00 PM - P5/U4/T4:Joint
break
4:15 PM - **P5.6/U4.6/T4.6
Molecular Dynamics Simulation of Failure Modes in Metallic Glass: Shear Bands and Cavitation.
Michael Falk 1 2 3
1 Materials Science and Engineering, Johns Hopkins University, Baltimore, Maryland, United States, 2 Mechanical Engineering, Johns Hopkins University, Baltimore, Maryland, United States, 3 Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland, United States
Show AbstractSimulation provides a useful window into the atomic scale processes that control failure in glasses. Here we discuss the two failure modes we deem of primary importance in these materials: strain localization and cavitation. During strain localization plastic softening results in extremely thin shear bands that carry the majority of the plastic strain. Such shear bands can be reproduced in simulation in a variety of geometries, and have been observed in nano-indentation, uniaxial tension, compression and simple shear. The latter simulations provide a unique opportunity to simultaneously study jammed and flowing material at the same applied stress in order to test theoretical predictions of the causal relations between structure and strain rate. Our results verify that a Boltzmann-like relationship relates the effective-temperature of the material, as measured via the potential energy per atom, to the rate of plastic deformation.While shear bands are critical as a primary failure mode, in order to predict the fracture toughness of amorphous solids such as metallic glasses it is necessary to understand the physics by which surface is created in the fracture process zone. Theories of plastic deformation provide information about response to shear, but on their own these theories provide limited insight into the microscopic mechanisms that mediate the free surface generation critical to crack propagation. Previous molecular dynamics simulations indicate that cavitation likely plays this role. We have undertaken a series of molecular dynamics simulations of cavitation under hydrostatic tension in a binary metallic glass analog using pair-wise potentials. These simulations demonstrate that cavitation is strongly favored in glasses due to the existence of structural heterogeneity. We compare the rate of cavity nucleation directly to homogeneous nucleation theory as applied to a solid undergoing irreversible deformation during cavity growth. Preliminary results indicated that the free energy barrier for cavitation is suppressed by 3 orders of magnitude relative to the homogeneous estimate obtained by directly measuring the surface energy of the glass.
4:45 PM - P5.7/U4.7/T4.7
On the Fracture Toughness of Metallic Glasses.
Marios Demetriou 1 , Maximilien Launey 2 , William Johnson 1 , Robert Ritchie 2 3 , Douglas Hofmann 1 4
1 , California Institute of Technology, Pasadena, California, United States, 2 , Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 , University of California, Berkeley, Berkeley, California, United States, 4 , Jet Propulsion Laboratory, Pasadena, California, United States
Show AbstractUnlike covalently-bonded glasses (e.g. oxide glasses) which tend to fail by fracture at stresses well below the plastic yield strength, metallically-bonded glasses are capable of yielding plastically under stress. Specifically, when an opening stress on the order of the material yield strength is applied, plastic shear sliding ensues confined within nanoscopic bands (shear bands) oriented along planes of maximum resolved shear stress. Shear sliding under negative pressure propagates up to some critical strain, beyond which it evolves into an opening crack. Atomistically, local shearing is accommodated by cooperative inelastic rearrangements of clusters of ~100 atoms, and can be sustained under negative pressure until configurations with critical cavities ultimately emerge. Upon the intervention of cavitation, plastic shearing is terminated and mechanical energy is dissipated via crack extension. Owing to their ability to yield plastically under an opening stress, metallic glasses demonstrate fracture energies substantially higher than Griffith’s brittle limit determined by surface-energy limited fracture (~2 J/m2 for metals). Specifically, reported fracture energies for metallic glasses range from ~100 J/m2 for the most brittle alloys to ~100 kJ/m2 for the tough ones, corresponding to fracture toughnesses that vary from just over 1 MPa.m1/2 (for rare-earth and ferrous metal glasses) to slightly higher than 100 MPa.m1/2 (for noble and early-transition metal glasses). The toughness of metallic glasses, therefore, varies over a broad range that extends from values characteristic of brittle ceramics to values characteristic of moderately tough crystalline metals. In this presentation, a scaling law will be introduced that correlates the fracture resistance of a plastically-yielding glass (e.g., a metallic glass) to the maximum attainable plastic shearing prior to crack opening. The law is based on a simple analysis of the basic features of the glass potential energy landscape, and involves scaling of the activation barriers for shear flow and cavity nucleation. The correlation captures data for multiple systems ranging from brittle rare-earth and ferrous metal glasses to tougher early-transition and noble metal glasses, and adequately describes the fracture toughness of plastically-yielding glasses over two orders of magnitude.
5:00 PM - P5.8/U4.8/T4.8
Mesoscopic Theory of Deformation and Shear Localization in Metallic Glasses.
Guang-Ping Zheng 1 , Mo Li 2
1 Department of Mechanical Engineering, Hong Kong Polytechnic University, Hong Kong China, 2 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractMany solids become unstable under applied external loading to initiate plastic flow. In doing so, they all exhibit a remarkable universal behavior, that is, localization. While crystalline metals starts plastic flow with proliferation of dislocations, brittle ceramics and oxides proceed with cracking. Amorphous solids, in particular, amorphous metals, become unstable through highly localized zones called shear band, which typically ranges from tens of nanometers to submicrons and carries nearly all plastic strains. How do these bands form? What atomic mechanisms are involved? How can we establish a general theoretical framework to describe and understand the localization process? Those and many other questions pertaining to flow behavior in amorphous solids challenge our understanding of plasticity and deformation that is largely built from crystalline materials. In this talk, we will present a mesoscopic theory, which is inspired by the conceptual model established by Argon, to explain localization process in metallic glasses. From atomistic modeling, we observe a strong correlation between deformation and atomic volume dilatation with anisotropy or preferred orientation. A finite size effect is also observed. Moreover, the shear localization looks much like a phase transition driven by external stress. The information is mapped to a mesoscopic model using Ginzburg-Landau theory, which predicts the deformation dynamics, instability, and localization process, all in qualitative agreement with experimental observations.
5:15 PM - P5.9/U4.9/T4.9
Interface Properties of Crystalline-amorphous Metallic Multilayers.
Christian Brandl 1 , Timothy Germann 1 , Amit Misra 2
1 T-1: Physics and Chemistry of Materials, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 MPA-CINT: Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractTraditionally ultrahigh-strength crystalline metals are obtained by reducing the governing microstructural length scale which inhibits dislocation mediated plasticity. The usual penalty to be paid is a lack of ductility as for example in nanocrystalline metals. On the other hand, in bulk metallic glasses plastic deformation – again owning a lack of ductility - is believed to be mediated by the so-called “shear transformation zones”. However, the combination of amorphous layers with crystalline layers showed extraordinary high toughness, i.e. ultra-high strength in conjunction with high elongation-to-failure. The plastic deformation, moreover, is confined by the crystalline-amorphous interface, which additionally has to maintain deformation compatibility to mediate homogeneous plastic flow. In nanocrystalline metals molecular dynamics (MD) simulation showed huge varieties of ordered fine structures in the interface, which can be related to the observed dislocation mediated deformation mechanism in the vicinity of the interface structure. Contrary to crystalline-crystalline interfaces, where crystalline phases exhibit long range order, the amorphous structure is characterized by a lack of long-range order. Using MD methods the compensation mechanism of the lacking long-range order at the interface is studied for metallic systems. The observed structural features are discussed in terms of dislocation-based deformation mechanisms such as dislocation transmission, nucleation, and absorption.
5:30 PM - P5.10/U4.10/T4.10
Designing Liquid Stone: Modulating Cement Mechanics via Computational and Physical Chemistry.
Karen Stewart 1 , Rouzbeh Shahsavari 3 , Meng Qu 1 , Joao Pereira 4 , Sandra Lebreiro 4 , Sidney Yip 2 , Franz-Josef Ulm 3 , Roland Pellenq 3 5 , Krystyn Van Vliet 1
1 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 4 , CIMPOR TEC, Lisbon Portugal, 2 Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 5 Centre Interdisciplinaire des Nanosciences de Marseille, Centre National de la Recherche Scientifique and Marseille Universite, Marseille France
Show AbstractAlthough cement is a ubiquitous material formed from hydration of calcium silicates, until recently the molecular structure of the calcium-silicate-hydrate (CSH) granular phases that comprise cement pastes was uncertain. We have recently reported a structure for CSH that is consistent with the density and C/S ratio of CSH indicated from experiments [1]. Here, we leveraged atomistic modeling to consider whether the mechanical properties of the CSH phase itself can be modulated by altering the ratio between calcium and silica in the CSH phase. Our new atomistic simulations predict a peak elastic modulus of CSH at a calcium/silicon (C/S) ratio of of ~1.5, representing a 60% increase in stiffness from C/S = 1.85. In an effort to achieve this ratio experimentally, we systematically varied both clinker phase chemistry and dissolution conditions, and characterized the composition chemistry through x-ray spectrometry. We found that the composition of the nanoscale CSH phase within actual cement pastes can be altered with existing materials and processes. Further, through nanoindentation maps and semi-analytical relations of packing density within each phase, we observed that the indentation elastic moduli ms of fully dense CSH can increase as much as 20% when the C/S ratio is decreased from 1.88 to 1.52. The hardness hs or resistance to permanent deformation of the CSH can also be varied experimentally, as a function of C/S ratios. Such chemomechanical characterization of these cements, motivated by computational predictions, allows clear identification of the processing variables that most directly alter the distribution of calcium-rich phases and CSH C/S ratios in hardened cement pastes, and also facilitates correlation with mechanical performance of macroscopic volumes. These computational predictions and experimental findings enable the potential design of this so-called “liquid stone” to improve salient physical and mechanical properties while reducing environmental impact. [1] R. J. M. Pellenq, A. Kushima, R. Shahsavari, K. J. Van Vliet, M. J. Buehler, S. Yip, and F. J. Ulm, Proceedings of the National Academy of Sciences of the United States of America 106 (38), 16102 (2009).
5:45 PM - P5.11/U4.11/T4.11
From Atoms to Microstructure of Cement Hydrates.
Rouzbeh Shahsavari 1 , Roland J.-M. Pellenq 1 2 , Franz-Josef Ulm 1
1 Department of Civil and Environmental Engineering, MIT, Cambridge, Massachusetts, United States, 2 Centre Interdisciplinaire des Nanosciences de Marseille, CNRS and Marseille Université, Campus de Luminy, Marseille France
Show AbstractConcrete is the world’s dominating manufacturing material, and as such it contributes to 5-10% of the total anthropogenic CO2 emissions worldwide. On the other hand, there is no bulk material on the horizon that could replace concrete as the backbone of our housing, schools and other built infrastructure. There is thus an urgency to rethink concrete for the age of global warming to make it part of the sustainable development of our societies. Despite this global concern and decades of research on concrete, the interplay between structure, morphology and chemical composition of its smallest building block, Calcium-Silicate-Hydrate (C-S-H), is essentially unexplored. Together these characteristics of this “liquid stone” gel define cement hydrate and enable modulation of its physical and mechanical properties with the ultimate goal of reducing concrete environmental footprint. Here, we propose a bottom-up multi-scale approach developed with the focus of unraveling the hierarchical structure of C-S-H, which is the principal source of strength and durability in all Portland cement concretes. First, using statistical mechanics methods in conjunction with a combinatorial approach, we decode the basic molecular structure of a variety of amorphous C-S-H gels for different chemical compositions. By allowing for short silica chains distributed as monomers, dimers, and pentamers, these C-S-H archetypes of molecular descriptions of interacting CaO, SiO2, and H2O units provide not only realistic values of calcium to silicon (Ca/Si) ratios but the densities computed by a unique combination of grand canonical Monte Carlo simulation of water adsorption and NPT molecular dynamics at 300 K. We found that the C-S-H gel structure includes glass-like short-range order features at large Ca/Si, and crystalline features of the mineral tobermorite at low Ca/Si. Second, we show that upon applying strain-controlled tension to the decoded C-S-H polymorphs, rupture occurs mostly around the silica-rich and defected regions. These weak regions lead to indentifying particle boundaries for C-S-H polymorphs. Next, by recourse to experimental X-ray energy dispersive analyses, these particles form the C-S-H microstructure, which is modeled using meso-scale Monte-Carlo simulations with inter-particle interactions based on parameters directly obtained from partial atomic charges computed by ab-initio calculations. Finally, we probe the matrix morphology and mechanical properties of our microstructural model, as compared to experimentally measured properties of C-S-H. This bottom-up approach, motivated by combinatorial atomistic modeling, introduces innovative paradigms to revolutionize the cement chemistry at the molecular level to answer the global needs for greening construction materials.
Symposium Organizers
Julia R. Greer California Institute of Technology
Daniel S. Gianola University of Pennsylvania
Blythe G. Clark Sandia National Laboratories
Ting Zhu Georgia Institute of Technology
Alfonso H. W. Ngan The University of Hong Kong
P6: Mechanical Behavior of Small Dimensional Materials: Nanowires, Whiskers, Fibers, and Pillars
Session Chairs
Kyung-Suk Kim
Alfonso Ngan
Wednesday AM, December 01, 2010
Room 210 (Hynes)
9:00 AM - **P6.1
Mechanical Strength and Deformation Modes of Sub-micron Al Fibers.
Marc Legros 1 , Frédéric Mompiou 1 , Daniel Gianola 2 , Andreas Sedlmayr 3 , Oliver Kraft 3 , Daniel Caillard 1
1 , CEMES-CNRS, Toulouse France, 2 Dept of Mater. Sci and Eng, University of Pennsylvania, Philadelphia, Pennsylvania, United States, 3 Institut für Materialforschung, Karlsruhe Institute für Technologie , Eggenstein-Leopoldshafen Germany
Show AbstractMetals and alloys become stronger when the motion or multiplication of their dislocations is impeded. In whiskers or micro-pillars, dislocations can escape more easily through free surfaces before interacting with each other, which explains why these small crystals do not work-harden. Their initial resistance to deformation (yield strength) is however much larger than their bulk counterpart, and the physical explanations for this phenomenon is still highly debated. Micro-pillars fabricated through Focused Ion Beam (FIB) are weaker than whiskers because their initial dislocation density is higher.The nucleation or the multiplication of fresh dislocations therefore appears like a key process to understand the yield or flow stress in sub-micron size crystals. The classical source, represented by the Franck-Read scheme, is unlikely to operate at very small scale despite its well spread use in computer-based models. Overall, the experimental observation of nucleation and multiplication process in small structures is poorly documented, partly because of the stochastic nature of sources and the difficulty to observe them while operating. In the present work, we relate an extensive in situ transmission and scanning electron microscope (TEM and SEM) study of dislocation multiplication and shearing processes in sub-micrometre Al ligaments and fibers that were prepared without FIB. Operating sources, often single armed and open loops produced by double cross-slip, were among multiplication modes observed. They have been monitored in real time, analyzed and compared to the crystal dimensions. The stress measurements withdrawn from mobile dislocation curvatures will be compared to micro-tensile tests. Possible strengthening mechanisms will be discussed in the light of these results and confronted with recent results obtained in similar experiments [1] and simulations [2]. These experiments clearly underline the prominent role of dislocation nucleation and multiplication in setting the stress resistance of small crystals.References[1]D. Kiener, et al., Acta Materialia, 56 (2008), 580.[2]C. Motz, et al., Acta Materialia, 57 (2009), 1744-1754.[3]S.H. Oh, et al., Nature Materials, 8 (2009), 95-100.
9:30 AM - P6.2
Incipient Plasticity in Single Crystalline fcc Nanowhiskers.
Daniel Gianola 1 , Andreas Sedlmayr 2 , Lisa Chen 1 , Gunther Richter 3 , Reiner Moenig 2 , Oliver Kraft 2
1 Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States, 2 Institute for Materials Research II, Karlsruhe Institute of Technology, Karlsruhe Germany, 3 , Max-Planck Institute for Metals Research, Stuttgart Germany
Show AbstractThe mechanical behavior of single crystalline metals has been known to be size dependent for decades, but a detailed understanding of the underlying deformation mechanisms has seen only recent attention. The emerging picture in small-scale fcc metals suggests that the relationships between density and character of defects, discrete source statistics, and free surface/defect interactions, governs the mechanical response. Theoretical and simulation results also predict a shift in the rate-controlling mechanisms at small specimen size towards deformation modes that require thermal activation. We present results from quantitative in situ experiments on near defect-free fcc nanowhiskers in electron microscopes. Cu and Au nanowhiskers endure stresses near the limit of their ideal strength during tensile straining and the resulting deformation behavior suggests a lack of conventional dislocation-dislocation interactions. Transient and non-ambient temperature nanomechanical experiments are employed to extract activation parameters for plastic deformation. Three-point bending experiments on nanowhiskers are also performed and results are discussed in the context of the availability of strain energy and the probability of finding dislocation sources in small volumes.
9:45 AM - P6.3
Deformation Mechanism in Metallic Nanowires.
Scott Mao 1 , Junhang Luo 1 , Jianyu Huang 2 , He Zheng 1
1 Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania, United States, 2 , Center for Integrated Nanotechnologies, Sandia National Laboratories,, Albuquerque, New Mexico, United States
Show AbstractThe mechanical behavior of bulk metals is usually characterized as smooth continuous plastic flow following by yielding. Here we show, by using in-situ TEM and molecular dynamics simulations, that the mechanical deformation behaviors of single-crystalline Ag and Ni nanowires are quite different from their bulk counterparts. Correlation between the obtained stress-strain curves and the visualized defect evolution during deformation processes clearly demonstrates that a sequence of complex dislocation slip processes results in dislocation starvation, involving dislocation nucleation, propagation and finally escaping from the wire system, so that the wires deformed elastically until new dislocation generated. This alternating starvation of dislocations is unique in small-scale structures. Furthermore, the magnitude of yield stress of these nanowires is strongly dependent of the wire size.
10:00 AM - P6.4
Atomistic Deformation Mechanism of Perfect Crystal Au Nanowires.
Seo Jong-Hyun 1 2 , Youngdong Yoo 4 , Sang Won Yoon 1 2 , Tae-Yeon Seong 2 , In-Suk Choi 3 , Kon Bae Lee 5 , Bongsoo Kim 4 , Jae-Pyoung Ahn 1
1 Nano-Materials Analysis Center, Korea institute of Science and Technology, Seoul Korea (the Republic of), 2 Department of Materials Science and Engineering, Korea University, Seoul Korea (the Republic of), 4 Department of Chemistry, KAIST, Daejeon Korea (the Republic of), 3 High-Temperature Energy Materials Center, Korea Institute of Science and Technology, Seoul Korea (the Republic of), 5 School of Advanced Materials Engineering, School of Advanced Materials Engineering, Seoul Korea (the Republic of)
Show AbstractMetal nanowires are great of technological importance due to their potential applications in miniaturized electrical, thermal, and mechanical system because their structure and properties can be quite different than bulk materials. The recent molecular dynamics works predicted the existence of strong but ductile behavior of metal nanowires achieved by deformation twinning. However it has been not experimentally proven by the tensile test of metallic nanowires. Unfortunately, previous experiment only observed high strength but brittle fracture in nanowires. Thus, the presence of ductile deformation in nanowires by structural transformation is in question and the deformation mechanism of nanowire proposed by atomic scale simulations is still controversial. In this study, we report ultra-strong but ductile deformation behavior of gold nanowires by structural transformation through deformation twinning through the tensile test using Nanomanipulator (MM3A, Kleidiek) equipped in Focused Ion Beam (Quanta 3D, FEI) as well as the observation of the each deforming steps and fracture region using transmission electron microscope (Titan80-300, FEI). The single crystalline Au nanowires were synthesized with the growth direction of <110> on a sapphire substrate. For the uniaxial tensile test, the both sides of single Au nanowire were attached to the W nanomanipulator tip and TEM copper half grid by the Pt deposition of FIB. We observed real time twin nucleation and twin migration leading to the transformation of the entire shape and orientation of gold nanowires using our in-situ tensile test. The Au nanowire was governed by the multiple twin deformation of {111}<112> at initial stress state and by the growth of one twin boundary among them. As the result, the twin growth spontaneously leads to the orientation rotation from <110> to <100>. In the stress-strain curves measured from Au nanowires, the elongation of Au nanowire by twin deformation was about 41%, including the elastic deformation of 3.2%. After the orientation rotation, the <100> Au nanowires were deformed up to the strain of 2.5% and finally were fractured under slip deformation. In this presentation, we are going to visualize the fracture behavior of single crystalline Au nanowire as well as MD simulations.
10:15 AM - **P6.5
Nanopulley System for ElectroMechanical Characterization of Nanowires.
John Boland 1 , Dorothee Almecija 1 , Eoin McCarthy 1
1 School of Chemistry & CRANN, Trinity College Dublin, Dublin Ireland
Show AbstractNanowires are potentially important components in nanoscale electromechanical (NEMs) devices. Successful NEM device operation hinges on a good understanding of the mechanical and electromechanical performance of the nanowire components, in addition to any friction and dissipation elements. Here, in this presentation we introduce a new AFM based scheme that enables the measurement of the mechanical response of nanowires systems in both pure bending and stretching modes, while at the same time allowing the friction between the nanowire and substrate to be measured. The experimental setup comprises a single nanowire that is placed on a trenched SiO2 substrate the top surface of which is metallized with Au. An array of metal pillars is also deposited onto the top Au electrodes and serve as pulleys around which the nanowires can slide following mechanical deformation by the AFM tip. Consequently, is it possible to predict the pure mechanical tension that is produced following local bending of the nanowire induced by the AFM tip. By applying a bias across the electrodes it is also to measure the electromechanical response of the nanowire system. Examples will be discussed of the performance of Ge and Ag nanowire systems.
11:00 AM - **P6.6
Thermo-mechanical Behavior at Nano Scale and Size Effects in Shape Memory Alloys.
Jose San Juan 1 2 , Maria No 3 , Christopher Schuh 2
1 Fisica Materia Condensada, Universidad del Pais Vasco, Bilbao Spain, 2 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 Fisica Aplicada II, Universidad del Pais Vasco, Bilbao Spain
Show AbstractShape Memory Alloys (SMA) undergo reversible martensitic transformation in response to changes in temperature or applied stress, exhibiting specific properties of superelasticity and shape memory. At present there is a high scientific and technological interest to develop these properties at small scale, to apply SMA as sensors and actuators in MEMS technologies.In order to study the thermo-mechanical properties of SMA at micro and nano scale, instrumented nano indentation is being widely used for nano compression tests. By using this technique, superelasticity and shape memory at the nano-scale has been demonstrated [1] in micro and nano pillars, milled by Focused Ion Beam. However the martensitic transformation seems to exhibit a different behavior at small scale than in bulk materials and a size effect on superelasticity has been recently reported [2].In the present talk we will overview the thermo-mechanical properties of SMA at the nano-scale, with special emphasis on size effects. In the second part of the talk, new results on superelastic cycling at nano-scale will be presented. We may advance that a size effect on the stress-induced martensitic transformation is responsible of an anomalous transition from pure superelastic to pseudoelastic behavior.Finally, the above commented size effects will be discussed on the light of the microscopic mechanisms controlling the martensitic transformation at nano scale.[1] J. San Juan, M.L. Nó, C.A. Schuh. Advanced Materials 20 (2008) 272.[2] J. San Juan, M.L. Nó, C.A. Schuh. Nature Nanotechnology 4 (2009) 415.
11:30 AM - P6.7
In-situ Deformation of Semiconductor Micro-pillars: The Case of Indium Antimonide InSb.
Ludovic Thilly 1 , Christophe Swistak 2 , Rudy Ghisleni 2 , Johann Michler 2
1 Pprime Institute, University of Poitiers, Futuroscope France, 2 , EMPA, Thun Switzerland
Show AbstractAt ambient temperature and pressure, most of the semiconductor (SC) materials are brittle: this is the case of the III-V compound SC indium antimonide, InSb. In general, the brittle-to-ductile-transition (BDT) temperature is situated around 0.6Tm where Tm is the absolute melting temperature: for InSb, TBDT is around 150°C. The evolution, with temperature, of the elementary plasticity mechanisms (dislocations) has been studied in InSb by compression of macroscopic samples under hydrostatic pressure and subsequent transmission electron microscopy (TEM) analysis: in the ductile regime (above TBDT), perfect dislocations are observed while at low temperature only partial dislocations are observed. This change of deformation mechanism may explain the occurrence of the BDT: after the emission of the leading partial dislocation, the sources are shut off and crystal plasticity is restricted [Acta Mat, 58, 2010, 1418-1440].To study the role of dislocation nucleation, InSb micro-pillars have been fabricated by FIB and in-situ compressed at room temperature in a scanning electron microscope, in order to correlate the observation of slip traces at the pillars surface and features of the stress-strain curve. TEM thin foils have been cut out of the pillars to study the deformation microstructure.Surprisingly, InSb pillars can be plastically deformed up to strains of 20% for diameters up to ten micrometers. At larger diameters, the pillars exhibit little plasticity followed by crack formation. Moreover, the yield stress increases when reducing the pillar diameter. The study of dislocations and their slip traces shows that increasing the surface-to-volume ratio of the pillars modifies the dislocation nucleation conditions and favours plasticity even at room temperature.
11:45 AM - P6.8
Micro-scale Single Crystal Bauschinger Effect and Reversible Plasticity in Copper During Bending.
Dierk Raabe 1 , Eralp Demir 1
1 , Max-Planck Institut, Duesseldorf Germany
Show AbstractWe study the Bauschinger effect on a bent and straightened micro-sized single crystal copper beam (width: 8.64 micrometers; thickness: 7.05 micrometers) in three subsequent cycles. The reverse yield strengths (straightening step) are much smaller than those in forward loading (bending step). An upper bound estimate shows a load drop of 73 % (1st cycle), 76 % (2nd cycle), and 83 % (3rd cycle) relative to the forward yield stress. EBSD reveals a dramatic reduction in the bending-induced local misorientations upon load reversal (straightening) documenting an unexpected form of microstructure reversibility. The observed Bauschinger softening is interpreted in terms of two effects. The first one are internal backstresses that support load reversal. They are created by polarized dislocation arrays that are accumulated during forward bending. The second one is the reduced requirement to activate dislocation sources during reverse loading as the dislocations that were stored during bending did not participate much in cross hardening and, hence, serve as mobile dislocations upon reverse loading. The Bauschinger effect gradually diminishes during further bending-straightening cycles due to forest hardening that gradually accumulates during cyclic bending and straightening.
12:00 PM - P6.9
Effect of Pre-straining on the Size Effect in Molybdenum Pillars.
Andreas Schneider 3 , Blythe Clark 1 , Carl Frick 2 , Patric Gruber 4 , Eduard Arzt 3 5
3 , INM - Leibniz Institute for New Materials, Saarbrücken Germany, 1 , Sandia National Labs, Albuquerque, New Mexico, United States, 2 Mechanical Engineering Department, University of Wyoming, Laramie, Wyoming, United States, 4 Institut für Zuverlässigkeit von Bauteilen und Systemen, Universität Karlsruhe, Karlsruhe Germany, 5 , Saarland University, Saarbrücken Germany
Show AbstractMicron and sub-micron pillar compression experiments have provided significant insight into the mechanisms responsible for size dependent deformation behavior. Experiments with focus ion beam (FIB) machined pillars of face-centered cubic (fcc) metals have consistently shown an increase in strength with decreasing diameter. Flow stresses were generally found to scale with an inverse power of specimen diameter, with n ≈ 0.6. In contrast to the majority of pillar studies on fcc metals, pillars cut from body-centered cubic (bcc) molybdenum (Mo) have exhibited a less pronounced size effect, with n values between 0.21 and 0.48. In order to gain fundamental insight into the size dependent deformation behavior of bcc metals, this study reports on compression tests of pre-strained Mo pillars. The FIB was used to manufacture pillars on the surface of an electropolished single crystal grown via the Czochralski method. This method will produce a material with a dislocation density on the same order as a well-annealed specimen. Crystallographic orientations of high and low symmetry ([001] and [235], respectively) were chosen to gauge the behavior for different slip systems. In total, 57 columnar pillars with diameters between 300 and 3000 nm were pre-compressed to strains ranging from 10 to nearly 40%. The deformed pillars were then re-cut using the FIB to again obtain a cylindrical shape while removing all prior surface steps. Re-cut, pre-strained FIB pillars were then compressed a second time to approximately 10% additional strain. Unlike in bulk bcc materials, pre-straining had a negligible effect on the stress-strain behavior of the pillars, suggesting that dislocation storage does not occur in small-scale bcc specimens. The prevailing mechanism behind the size effect is attributed to dislocation nucleation mechanisms.
12:15 PM - P6.10
Effect of Crystal Orientation on the Active Slip Systems of BCC Titanium Alloy.
Boopathy Kombaiah 1 , Rudy Ghisleni 1 , Christoph Niederberger 1 , Kai Nowag 1 , Johann Michler 1
1 Laboratory for Mechanics of Materials and Nanostructures, EMPA - Swiss Federal Laboratory for Materials Testing and Research, Thun Switzerland
Show Abstract The violation of Schmid law in BCC crystal as suggested in [W. Pichl, phys.stat. sol. (a) 189 (2002) 5–25] is here experimentally investigated in a β single phase Ti alloy. Single crystal micropillars with 1 µm diameter and 3 µm height were machined inside the grains oriented close to [1 1 0], [0 0 1], and [1 3 5] crystallographic directions and compressed uniaxially inside a SEM primarily aiming to observe the deformation during compression. Localized single slip was observed in the pillars of all the orientations. The active slip systems for all the grain orientations were determined from combined EBSD analysis and the SEM images of the compressed pillars. It was observed that the active slip system in the BCC crystal violates the Schmid`s law of resolved shear stress. In contrary, the active slip system in all the grain orientations is the one which has the largest Schmid factor among favourable slip planes {1 10}, {1 1 2}, and {1 2 3} in the <1 1 1> zone axis. Moreover, the resolved shear stress on the secondary slip system {1 1 2} <1 1 1> at yield is measured to be lower than that of the primary slip system {1 10} <1 1 1>.
12:30 PM - P6.11
Size Independent Shape Memory Behavior of Nickel-Titanium.
Blythe Clark 2 , Daniel Gianola 3 , Oliver Kraft 4 , Carl Frick 1
2 Department of Radiation-Solid Interactions, Sandia National Laboratories, Albuquerque, New Mexico, United States, 3 Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States, 4 Karlsruhe Institute of Technology, Institute for Materials Research II, Karlsruhe Germany, 1 Mechanical Engineering, University of Wyoming, Laramie, Wyoming, United States
Show AbstractShape memory alloys represent a class of so-called “smart” materials that can be returned to their original shape after deformation, either spontaneously or through the application of heat. While several alloys are capable of shape memory behavior, nickel-titanium (NiTi) is the most extensively researched due primarily to its relatively large deformation recoverability, as well as its high strength, corrosion resistance, biocompatibility, and high intrinsic damping. The shape memory effect for NiTi results from a reversible martensitic phase transformation, in which the crystal structure shifts from a B2 (austenite) to a B19’ (martensite) phase in a shear-like manner. Depending upon composition and processing history, the stress-induced martensitic phase transformation is stable at the testing temperature, requiring heat to revert to austenite and recover the strain associated with the phase transformation. However the strains associated with the phase transformation are finite, typically 3-12% depending largely on crystallographic orientation. Displacement larger than this range is known to induce plastic flow in the martensitic phase. Because the actuation mechanism is inherent to the material, NiTi is of particular interest for small-scale applications and is often proposed as the active material in functional devices. This feature has been exploited in micro-electrical-mechanical systems (MEMS), as NiTi has been shown to have a higher work output per unit volume than any conventional actuator. Although understanding the mechanical behavior of NiTi at small-scales is crucial to such applications, it remains unclear how size scale influences the shape memory effect; to date, investigations into the size dependency of the NiTi martensitic phase transformation have yielded contradictory results. Similarly, many unanswered questions remain concerning the influence of sample size on martensite plasticity. To shed light on these areas, this study investigates the size effect on shape memory behavior and plasticity in focused ion beam machined NiTi pillars that were aged to elicit shape memory behavior at room temperature. To study the influence of size on shape memory effect, pillars approximately 1 μm and 200 nm in diameter were subjected to bending via angled application of load during in situ scanning electron microscope observation. After subsequent ex situ heating, all pillars showed partial to full recovery, clearly demonstrating deformation recovery regardless of sample size. To study the size effect on plasticity, uniaxial compression experiments on similar size pillars were conducted using a nanoindenter equipped with a flat punch. The results indicate that plasticity of martensite is independent of sample size or precipitate structure, which corroborates with previous work by the same authors.
12:45 PM - P6.12
Influence of Proton Irradiation on Mechanical Properties in Small Dimensions.
Daniel Kiener 1 2 4 , Peter Hosemann 3 , Andrew Minor 2 4
1 Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Leoben Austria, 2 National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 4 Department of Materials Science and Engineering, University of California, Berkeley, California, United States, 3 Department of Nuclear Engineering, University of California, Berkeley, California, United States
Show AbstractThe size effects encountered when probing the mechanical properties of micro- and nano-sized single crystal samples have attracted considerable interest in recent years [1]. However, less effort was directed towards the effect of specific microstructural defects on the mechanical behavior in these small dimensions.Following up on a recent publication [2], in the current study we investigated multiple slip oriented single crystal Cu(100). Samples were irradiated by either a 0.2 MeV or a 1.2 MeV proton beam to a total dose of 1 dpa in order to introduce a controlled defect population throughout the entire sample volume. This was confirmed by inspection of the samples in the transmission electron microscope (TEM) before and after irradiation. Focused ion beam (FIB) machining was applied to shape nanoscale compression and tensile samples with critical dimensions between 100 nm and 200 nm and high aspect ratios. A nanoindentation holder in the TEM was used to perform quantitative in situ tests of the samples. Besides common compression testing, a new approach was developed to perform in situ tensile testing in a TEM. These complementary tensile tests allow for studying the influence of the sample volume and the surface to volume ratio for the same minimal sample dimension.During in situ testing, distinct differences were observed between un-irradiated and irradiated material in terms of slip morphology and hardening characteristics. The irradiated material showed less pronounced flow intermittency during early deformation and final failure by slip localization. Mechanical annealing of pre-existing dislocations, as observed for pure FIB fabricated single crystals [3], was not pronounced in the irradiated material during testing. Rather, the dislocations were pinned by the proton irradiation induced defect clusters. Most striking, a transition from size-dependent yield stresses at small diameters to size-independent yield stresses for the larger samples was observed for the proton irradiated material.These results will be discussed with respect to the defect structures in the samples governing the observed dislocation plasticity and related to common nanoindentation measurements and recent computational studies on such irradiated material.[1]Uchic MD, Shade PA, Dimiduk D. Ann. Rev. Mater. Res. 2009;39.[2]Hosemann P, Swadener JG, Kiener D, Was GS, Maloy SA, Li N. J. Nuc. Mater. 2008;375:135.[3]Shan ZW, Mishra RK, Asif SAS, Warren OL, Minor AM. Nat. Mater. 2008;73:115.
P7: In situ TEM/SEM Characterization of Nanoscale Deformation and Friction
Session Chairs
Wednesday PM, December 01, 2010
Room 210 (Hynes)
2:30 PM - **P7.1
In-situ Nanomechanical Testing Methods to Explore Fracture and Plastic Deformation Mechanisms of Semiconductors and Metals.
Johann Michler 1
1 , EMPA - Swiss Federal Laboratories for Materials Science and Technology, Thun Switzerland
Show AbstractThe creation of miniaturised mechanical structures and devices calls for an understanding of the mechanical properties of the semiconductor and metallic materials used at these small length scales. Investigation of different types of loading, such as compression or bending, at these length scales requires the development of appropriate miniaturised instrumentation and corresponding handling strategies for micro- and nanosized specimen. The small overall size of the instrumentation creates opportunities for integrating the mechanical testing equipment into different micro-analysis instruments (SPM, SEM, EBSD, optical probes etc.) allowing for in-situ sample surface imaging and in-situ microstructure analysis during mechanical testing. This talk will illustrate three case studies displaying the advantages of this combined approach:1)In-situ SEM, in-situ Raman and in-situ EBSD compression experiments have been carried out to study how the compressive failure of Silicon and GaAs micropillars at room temperature is influenced by their diameter. Slip was observed in most micropillars, often on intersecting slip planes. Cracks could nucleate at these intersections and then grow axially in the sample, with bursts of crack growth. In-situ Raman and in-situ EBSD allowed determination of the onset of yielding/cracking and the observation of changes in crystal orientation during deformation, respectively. GaAs and Si pillars of <100> axis with diameters less than 1 μm and 300nm, respectively, did not displaying splitting behavior. A compression splitting theory is developed that predicts a ductile-brittle transition at a micropillar diameter of approximately 1 μm for GaAs, which is consistent with experimental observations. 2) “Thinness," or the smallest dimension of a structure, has been shown to be the determining factor of the yield strength of crystalline metals. In order to distinguish between the various proposed plasticity size effect theories and to determine the dimensional dependence of uniaxial plastic deformation of small pillars, wall-like structures of different length to width ratio were compressed. It was observed that the yield stress is determined by wall width; the length of the wall having little or no influence. Amorphous metals were seen to exhibit no size dependence down to our measurement limit of diameter of 300nm.3)Single crystal gold nanodots were compressed inside a SEM and found to yield in a stochastic manner. Yielding was immediately followed by displacement bursts equivalent to 1–50% of the initial height. A first order estimate of the apparent energy release rate, in terms of fracture mechanics concepts, for these bursts is on the order of 10–100 J/m2. Fracture strains of monocrystalline Si and ZnO nanowires, in both tensile and bending experiments, were found to be close to theoretical limit of 10%. In contrast, nanocrystalline metal nanowires were found to exhibit lower fracture strains.
3:00 PM - P7.2
In-situ SEM Tension and Compression Behavior of Nanocrystalline and Ultra Fine Grained bcc Metals.
Jonathan Ligda 1 , Brian Schuster 2 , William Sharpe 3 , Zenji Horita 4 , Qiuming Wei 5 1
1 Nanoscale Science, Univeristy of North Carolina at Charlotte, Charlotte, North Carolina, United States, 2 Weapons and Materials Research Directorate, Army Research Laboratory, Aberdeen Proving Ground, Maryland, United States, 3 Mechanical Engineering, Johns Hopkins University, Baltimore, Maryland, United States, 4 Materials Science and Engineering, Faculty of Engineering, Kyushu University, Fukuoka Japan, 5 Mechanical Engineering, University of North Carolina at Charlotte, Charlotte, North Carolina, United States
Show AbstractWe report on recent investigations into the mechanical properties of nanocrystalline (NC) and ultra-fine-grained (UFG) body-centered cubic (bcc) micro-specimens deformed in tension and compression. We have developed a custom test apparatus for in-situ scanning electron microscope (SEM) mechanical testing. Our system utilizes a 5-axis piezoelectric positioning system that enables precise placement and alignment of the micro-specimens, either within the tensile grip or near the compression platen. Loads are applied using a high-resolution linear actuator with a reported resolution of ~ 1 nm and measured with a strain gage based S-beam load cell. The load cells have a capacity of 10 or 100 g with a resolution of ~0.01 g. Specimen load/stress and crosshead displacements are measured using a customized data acquisition program, while the specimen strain is calculated from SEM micrographs using an open source digital image correlation script developed by Eberl and coworkers for Matlab®. We use focused ion beam (FIB) machining to fabricate microcompression and tension samples with minimum dimensions of 1-10 µm. Compression specimens typically have a uniform cross-section with a 2:1 aspect ratio while the tension samples are higher aspect ratio “dog-bone” specimens.Our present work is focused on NC and UFG bcc metals from Group V and VI (the vanadium and chromium families, respectively) processed using high pressure torsion (HPT). HPT samples have an inherent gradient in structure and properties along a radius of a bulk platen; grains are UFG near the center and NC near the edge. Prior work by Wei et al. has shown that some NC-bcc metals are prone to localized plastic deformation during quasi-static and dynamic compression. Using site-specific FIB we have examined the properties of specimens with grain sizes ranging from tens to hundreds of nanometers and have probed the balance between specimen strength, toughness, strain rate sensitivity and the deformation mode (e.g. homogenous or localized plastic deformation). Observations of the proposed mechanism will be supported through microstructural examinations with transmission electron microscopy. This study will aid in the understanding of how the deformation occurs in bcc metals when the grain size is reduced to the nanoscale.
3:15 PM - P7.3
In-situ TEM Observation of Mechanical Annealing in Mo Single Crystal Pillars.
Ling Huang 1 , Zhiwei Shan 1 , Ju Li 1 2 , Jun Sun 1 , Evan Ma 1 3
1 Center for Advancing Materials Performance from the Nanoscale (CAMP-Nano ) & Hysitron Applied Research Center in China (HARCC), State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an China, 2 Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States, 3 Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, Maryland, United States
Show AbstractIn situ TEM compression tests were carried out to investigate the mechanical behavior of single crystalline Mo nano-pillars with diameters from 75 to 300 nm. We observed that the flow stress obviously increased as the size decreased, showing a strong size dependence. Although these pillars are fabricated via FIB with high initial dislocation density, a high critical stress more than 8 GPa can still be observed as the size is reduced to 75 nm. The power-law exponent of strength vs. nanopillar diameter is -0.92, about 2 times higher than that reported in previous works for larger pillars. This suggested a different deformation regime in these reduced pillar sizes. Mechanical annealing is observed during the compression test of pillars with their size less than 200 nm. This striking finding sheds new light on the plastic deformation behavior in sub-micron BCC crystals.
3:30 PM - P7.4
In-situ TEM Tensile Testing of Au Nanowires.
Burkhard Roos 1 , Bahne Kapelle 1 , Gunther Richter 2 , Cynthia Volkert 1
1 , Institut für Materialphysik, Göttingen Germany, 2 , Max-Planck-Institut für Metallforschung, Stuttgart Germany
Show AbstractIncreasing strength with decreasing size is a common phenomenon in metals, and is often explained in terms of dislocation pile-ups and interactions. However, for free standing samples with dimensions below 150 nm, dislocation storage is hard to envision and a convincing explanation for the size-dependent strength is still missing. The goal of this study is to directly observe dislocations in small volumes, using in situ TEM during deformation. Single crystal Au nanowires with diameters between 40 and 250 nm have been used for this study. In wires with diameters below 180 nm, stacking faults appear during deformation as a result of the nucleation and motion of partial dislocations. The stacking faults form homogenously along the wire length and appear and disappear in less than 50 ms. The stacking faults do not move as the wire is further deformed, but may eventually thicken into nanotwins through the sequential activation of partial dislocations on neighbouring (111) planes. Post-deformation TEM studies show that fracture often occurs at a nanotwin. In wires with diameters above 180 nm, a transition to full dislocation based deformation and dislocation storage is observed. Possible explanations for the dependence of the deformation mode on wire diameter and stress will be discussed.
3:45 PM - P7.5
In situ Dislocation Nucleation and Velocity Measurements at Room Temperature in Silicon Nanopillars.
Douglas Stauffer 1 , Andrew Wagner 1 , K. Andre Mkhoyan 1 , William Gerberich 1
1 Chemical Engineering & Materials Science, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractIn spite of decades of research to the contrary, silicon is still considered the classic “brittle material.” Here, uniaxial compression performed in situ with a transmission electron microscope shows both dislocation nucleation and propagation in vapor-liquid-solid grown [111] and [100] silicon nanopillars. At these reduced length scales, Si shows significant plasticity. The relatively high Peierls barrier, compared to the FCC metals, prevents the extremely rapid movement of dislocations. Thus, groups of dislocations were tracked by standard video capture in the transmission electron microscope. Load-displacement data, and post situ observation of the dislocations are also presented. Having tested both [100] and [111] grown pillars with diameters ranging from 300 and 100 nm allows the development of both a size and directional relationship for the nucleation and propagation of dislocations on the {111}<110> slip systems at room temperature in Si nanopillars.
4:00 PM - P7: insitu
break
4:15 PM - **P7.6
In-situ Electrical, Mechanical, and Electrochemical Property Measurements by Using a TEM-SPM Platform.
Jianyu Huang 1
1 , Sandia National Lab., Albuquerque, New Mexico, United States
Show AbstractIn this talk, I will review our recent progress in in-situ studies by using a combined TEM-SPM platform, which integrates a fully functional SPM into a TEM. Integrating the advantage of both the SPM and the TEM capabilities, the TEM-SPM platform provides unprecedented opportunities to probe the structural, mechanical, electrical, and electrochemical properties of materials in-situ down to a nanometer scale. This allows for direct correlation of the physical properties to the atomic-scale microstructure. Several case studies of using the TEM-SPM platform are described. First, superplastic deformation of carbon nanotube, salt nanowire, and nanosized metallic glass is observed. This provided new insight into the deformation mechanisms of nanostructured materials. Second, fractal sublimation and multilayer edge reconstructions in graphene were discovered. Third, ultrahigh strength, cold-welding and surface mediated plasticity were revealed in ultrathin Au nanowires. Emerging directions of using the TEM-SPM platform to conduct cutting edge research in battery studies will be highlighted.
4:45 PM - P7.7
Mechanical Deformation of Nanoscale Metal Rods: When Size and Shape Matters.
Maureen Lagos 1 2 , Fernando Sato 3 , Douglas Galvao 1 , Daniel Ugarte 1
1 Applied Physics, State University of Campinas, Campinas, Sao Paulo, Brazil, 2 , Laboratorio Nacional de Luz Sincrotron-LNLS, Campinas, Sao Paulo, Brazil, 3 Instituto de Ciencias Exatas, Federal University of Juiz de Fora, Juiz de Fora, Minas Gerais, Brazil
Show AbstractThe mechanical properties of a strained nanoscale volume of matter represent a fundamental issue for understanding phenomena such as friction, fracture, adhesion, etc. The structural behavior of atomic-size metal nanowires represents a very interesting case for this study, becausein spite of many years of investigations unexpected results are still being produced, such as, the unusual behavior of nanoalloys [1] and the more recent discovery of the smallest possible silver nanotube with asquare cross-section, spontaneously formed during the mechanical elongation of silver nanocontacts [2].Here, we have used in-situ electron microscopy to reveal for the first time drastic changes of structural behavior during deformation of 1-nm-wide metal rods as a function of temperature. At 300 K, stretched nanowires stay defect-free, while at 150 K, elongation is associated with planar defects.As size is reduced, energy barriers become so small that ambient thermal energy is sufficient to overcome them. Nanorods display an elastic regime until a mechanism with high enough blocking barrier can be nucleated. Ab-initio calculations revealed that contributions from surface steps overrule stacking fault energetics in nanorods, in such a way that system size and shape determines preferred fault gliding directions. This induces anisotropic behavior and, even large differences in elastic or plastic response for elongation or compression. These results provide a new framework to improve theoretical models andatomic potentials to describe the mechanical properties at nanoscale.[1] J. Bettini, F. Sato, P. Z. Coura, S. O. Dantas, D. S. Galvao, and D. Ugarte, Nature Nanotechnology v1, 182 (2006).[2] M. J. Lagos, F. Sato, J. Bettini, V. Rodrigues, D. S. Galvao, and D. Ugarte, Nature Nanotechnology v4, 149 (2009).
5:00 PM - P7.8
Mechanics of Nanotubes/Nanowires.
Reza Shahbazian Yassar 1 , Hessam Ghassemi 1 , Anjana Asthana 1 , Kasra Momeni 1 , Yoke Yap 2
1 Mechanical Engineering, Michigan Technological University, Houghton, Michigan, United States, 2 Physics Department, Michigan Technological University, Houghton, Michigan, United States
Show AbstractThe correct estimation of mechanical properties in nanotubes and nanowires has been a challenging task due to complexity of stress state and end-support boundary conditions. Here we utilized simple beam formulations and buckling theories to explain the deformation and mechanics of boron nitride (BN) nanotubes and zinc oxide (ZnO) nanowires. Size scale effects were observed in ZnO nanowires and were explained by the modification of atomic structure at the nanowire surface. In addition, the rippling and bifurcation of multiwalled BN nanotubes were observed upon buckling and were quantified in terms of number of walls and nanotube’s diameter.
5:15 PM - P7.9
Nanoscale Deformation of MEMS Materials.
Aiden Lockwood 1 , Adarsh Padmanabhan 1 , John Bunyan 2 , Beverley Inkson 1
1 Engineering Materials, University of Sheffield, Sheffield, South Yorkshire, United Kingdom, 2 MEMS Division, QinetiQ, Malvern, Worcestershire, United Kingdom
Show AbstractUsing a novel in-situ TEM triboprobe holder, nanoscale structures formed from polysilicon MEMS materials have been loaded to characterise the mechanisms of failure in reduced dimension structures. Nanobridges with dimensions much less than 1µm, and of a range of geometries, have been deformed using both single, high displacement indentation and cyclic fatigue deformation.For uniaxial indentation loading, we see significant residual plastic deformation accumulating in the polysilicon. This can be seen as a gradual curvature along the entire crossbeam upon unloading. Where the radius of curvature is very high, fracture of the beams at the centre point was generally also seen.When loading at much lower displacement but under fatigue conditions, plasticity is still observed and a gradual curvature of the crossbeam is visible upon unloading. In addition, localised heating around the contact point initiates carbon migration, forming a very strong probe-nanostructure bond. A high tensile force was needed to severe the contact during unload.Such in-situ testing of MEMS materials demonstrates a range of time dependant failure modes which can be overlooked using traditional post-mortem analysis. In particular, it is demonstrated here that the combined effect of localised frictional heating and contamination migration at a cyclically loaded nanocontact changes the microstructure and behaviour of the nanocontact. Such behaviour of nanocontacts may alter the reliability of components that repeatedly come into contact with one another.
5:30 PM - **P7.10
From Molecular Adsorbates to Atomic Membranes: How Interfacial Atomic Structure Influences Nanoscale Friction in Ultrastrong Carbon-based Systems.
Robert Carpick 1 , Qunyang Li 1 , Andrew Konicek 2
1 Mechanical Engineering and Applied Mechanics, University of Pennsylvania, Philadelphia, Pennsylvania, United States, 2 Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania, United States
Show AbstractMany carbon-based materials, including diamond, carbon nanotubes, diamond-like carbon (DLC), graphite, and graphene, exhibit unusual and extreme mechanical and tribological properties which render them highly interesting for a wide range of applications, including for nanoelectromechanical systems (NEMS). I will focus on the atomistic origins of two particularly interesting tribological effects: (1) the dependence of nanoscale friction and adhesion on the atomic structure of diamond interfaces [1, 2]; and (2) the unusual stick-slip friction behavior seen in thin graphene and other atomically-thin sheets [3].1.P. Piotrowski, R.J. Cannara, G. Gao, J. Urban, R.W. Carpick & J.A. Harrison, Atomistic Origins of Adhesion for Diamond and Nanocomposite Diamond Surfaces. J. Adhes. Sci. Technol., in press (2010).2.A.V. Sumant, D.S. Grierson, J.E. Gerbi, J.A. Carlisle, O. Auciello & R.W. Carpick, Surface Chemistry and Bonding Configuration of Ultrananocrystalline Diamond Surfaces and Their Effects on Nanotribological Properties. Phys. Rev. B, 76, 235429/1-11 (2007).3.C. Lee, Q. Li, W. Kalb, X.-Z. Liu, H. Berger, R.W. Carpick & J. Hone, Frictional Characteristics of Atomically-Thin Sheets. Science, 328, 76-80 (2010).
Symposium Organizers
Julia R. Greer California Institute of Technology
Daniel S. Gianola University of Pennsylvania
Blythe G. Clark Sandia National Laboratories
Ting Zhu Georgia Institute of Technology
Alfonso H. W. Ngan The University of Hong Kong
P10: Poster Session II
Session Chairs
Thursday PM, December 02, 2010
Exhibition Hall D (Hynes)
P8: Deformation of Novel Nanosystems and Advanced X-ray Scattering Methods
Session Chairs
Dan Gianola
Johann Michler
Thursday PM, December 02, 2010
Room 210 (Hynes)
9:00 AM - **P8.1
Non-destructive Strain Imaging at the Nanoscale : Coherent X-ray Diffraction and Microbeam X-ray Scattering.
Olivier Thomas 1 , Nicolas Vaxelaire 1 , Stephane Labat 1 , Olivier Perroud 1 , Marie-Ingrid Richard 1
1 IM2NP UMR CNRS 6242, Universite Paul Cezanne, Marseille France
Show AbstractVery high stresses may arise in thin films and in nano-sized structures (lines, dots, etc) because of the constraint of the substrate to which they are attached. The mechanical behavior of these small structures can deviate significantly from scaling laws developed for bulk materials. Moreover, the origins and mag¬nitudes of these stresses are of great interest in technology as many fabrication and reliability problems are stress related. X-ray diffraction is an ideal tool to measure non-destructively displacement fields at very local scales. Thanks to third generation synchrotron radiation sources and the development of focusing x-ray optics it is possible to image strain fields with a spatial resolution as small as 10 nm. I will discuss two strategies that may be used for strain imaging: (i) sub-micrometer x-ray beams, which yield direct space resolution of the order of the beam size; (ii) coherent x-ray diffraction, where direct space resolution arises from high resolution in reciprocal space. These methods are fully compatible with in situ mechanical testing as I will show on a few examples (thermo-elastic straining of single grains in polycrystalline films, annealing of nanowires, AFM indenting of nanocrystals).
9:30 AM - P8.2
In-situ µLaue Tensile Tests on Micron Sized Cu Single Crystals.
Christoph Kirchlechner 1 , Jozef Keckes 2 , Peter Imrich 1 , Wolfgang Grosinger 1 , Christian Motz 1 , Jean Sebastien Micha 3 , Gerhard Dehm 1 2
1 Erich Schmid Institut, Austrian Academy of Sciences, Leoben Austria, 2 Material Physics, University of Leoben, Leoben Austria, 3 CRG-IF BM32 , European Synchrotron Radiation Facility, Grenoble France
Show AbstractSeveral efforts have been made during the past years to characterize the mechanical response of micron and submicron sized specimens. Even though the mechanical loading and sample preparation were improved considerably, the structural investigation of deformed volumes is still limited due to the lack of appropriate characterization methods. Besides transmission electron microscopy [1] and electron backscatter diffraction [2] – which both have essential drawbacks (for instance sample preparation) – µLaue diffraction [3,4] provides some unique statistical information of deformed volumes with a spatial resolution typically in the micrometer regime.We have performed tensile and compression tests at BM32 of the ESRF synchrotron source (Grenoble, France) on focused ion beam (FIB) milled samples. 2-12µm large single slip oriented copper specimens were strained up to 20% engineering strain, using two loading sequences. During the experiment, mechanical data and diffraction patterns obtained by a 1µm sized white X-ray beam were recorded simultaneously. The stress strain curves of the tensile specimens show a pronounced stress plateau with a constant flow stress. In contrast, the compression samples exhibit hardening and therefore no stress plateau is observed. The Laue data were used to analyze the sample rotation and for diffraction peak analysis (width, streaking direction and peak splitting). The sample rotation of tensile samples is as predicted for constrained tensile tests. Furthermore, we observed the storage of geometrically necessary dislocations but not the formation of sub grains. In contrast, compression samples do not rotate as predicted and continuous formation of sub-grains can be observed.In the talk, the mechanical data and the micro structural evolution during the deformation will be correlated and discussed.1Norfleet, D. M., Dimiduk D.M. Polasik S.J. Uchic M.D. Mills M.J. Dislocation structures and their relationship to strength in deformed nickel microcrystals. Acta Materialia 56, 2988-3001 (2008).2Niederberger, C., Mook W.M. Maeder X. Michler J. In situ electron backscatter diffraction (EBSD) during the compression of micropillars. Materials Science and Engineering A (2010).3Ice, G. E., Pang J.W.L. Tutorial on x-ray microLaue diffraction. Materials Characterization 60, 1191-1201 (2009).4Maass, R., Van Petegem S. Van Swygenhoven H. Derlet P.M. Volkert C.A. Grolimund D. Time-resolved laue diffraction of deforming micropillars. Physical Review Letters 99 (2007).
9:45 AM - P8.3
In-situ Laue Diffraction from bcc Mo-alloy Pillars.
Julien Zimmermann 1 , Helena Van Swygenhoven 1 , Steven Van Petegem 1 , Cecile Marichal 1 , Daniel Grolimund 2 , Hongbin Bei 3 , Easo George 3
1 NUM/ASQ, Paul Scherrer Institut, Villigen Switzerland, 2 SYN, Paul Scherrer Institut, Villigen Switzerland, 3 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractHow dislocation networks evolve during plastic deformation of a single crystal bcc metal is less well understood then it is for fcc crystals, and this is a major problem in the development of mesoscopic plasticity models for bcc metals. Although the general behavior of bcc metals can be related to the non-planar structure of the screw dislocation some details, for instance, the mechanisms responsible for the non-Schmid behavior of slip on (110) planes are still under debate. In-situ Laue diffraction during micro-compression was developed to explore the smaller is stronger effect in single crystals. In the case of single crystal fcc pillars, it has been demonstrated that the use of a focused ion beam (FIB) method to synthesize the pillars can induce a strain gradient [Scripta Mat. 62 (2010) 746] and that a pre-existing strain gradient encourages non-Schmid behavior [Acta Mat 57(2009) 5996]. In this talk it will be shown that in-situ Laue diffraction can provide detailed information on yielding and selection of slip systems in bcc metals, information that is invaluable for the further development of mesoscopic plasticity models.In-situ Laue diffraction was performed during compression of 1-µm bcc single-crystal pillars obtained from a directionally solidified (DS) NiAl-Mo eutectic (Bei and George, Acta Mat. 53 (2005) 69). These Mo pillars were investigated in the as-grown (0% pre-strained) and 11% pre-strained conditions, as well after FIB milling. To obtain free standing single pillars for Laue analysis, a special specimen preparation procedure was developed [Scripta Mat. 62 (2010) 746 ]Load-unload experiments under load and displacement control were performed with the aim of investigating the macroscopic yield point and the single slip-multiple slip regions as a function of stress and strain.Among the results presented will be (1) the macroscopic yield stress in as-prepared, pre-deformed and FIBed pillars and how this relates to the mobility of screw dislocations and bulk behaviour, (2) the presence of stage 0 in pillars with a very low defect density, (3) the character of the activated slip planes i.e. (112) and (110) planes, (4) the difference in initial dislocation structure between pillars obtained from the pre-deformed NiAl-Mo composite and pillars compressed as free standing pillars, (5) the formation of substructures in pillars compressed to high strains and (6) the influence of FIB on the deformation behavior.The importance of the current results is two-fold: they provide invaluable data on the evolution of a dislocation network in bcc metals while also shedding light on the smaller is stronger phenomenon observed in single crystals. HB and EPG were supported by the Materials Sciences and Engineering Division, U.S. Department of Energy.
10:00 AM - P8.4
Plasticity in the Nanoscale Cu/Nb Single Crystal Multilayers as Revealed by Ex Situ Synchrotron X-ray Microdiffraction.
Arief Budiman 1 , Seung-Min Han 2 3 , Patricia Dickerson 1 , Nobumichi Tamura 4 , Martin Kunz 4 , John Hirth 1 , Amit Misra 1
1 Center for Integrated Nanotechnologies (CINT), Los Alamos National Laboratory (LANL), Los Alamos, New Mexico, United States, 2 Materials Science & Engineering, Stanford University, Stanford, California, United States, 3 Materials Science & Engineering, Korea Advanced Institute of Science & Technology (KAIST), Daejeon Korea (the Republic of), 4 Advanced Light Source (ALS), Lawrence Berkeley National Laboratory (LBNL), Berkeley, California, United States
Show AbstractThere is much interest in the recent years in the nanoscale metallic multilayered composite materials due to their unusual mechanical properties such as very high flow strength and stable plastic flow to large strains (as well as their extreme radiation damage tolerance). These unique mechanical properties have been proposed to result from the interface-dominated plasticity mechanisms in nanoscale composite materials. Studying how the dislocation configurations and densities evolve during deformation will be crucial in understanding the yield, work hardening and recovery mechanisms in the nanolayered materials. In an effort to shed light on these topics, uniaxial compression experiments on nanoscale Cu/Nb single crystal multilayer pillars using ex situ synchrotron-based Laue X-ray microdiffraction technique were conducted. Using this approach, we studied the nanoscale Cu/Nb multilayer pillars before and after uniaxial compression and found significant Laue peak broadening in the Cu phase which indicates storage of statistically stored dislocations (SSD), while no significant Laue peak broadening was observed in the Nb phase in the nanoscale multilayers. These observations, coupled with other examination results using electron microscopic techniques, suggest that the plasticity here is achieved by multiple storages as well as recoveries of dislocations by the many interfaces in the nanoscale multilayers during the course of the deformation.
10:15 AM - P8.5
The Size Effect in Single Crystals: A Synergy Among In-situ Laue Diffraction, TEM and Computation.
Cecile Marichal 1 , Julien Zimmermann 1 , Jorge Martinez Garcia 1 , Camelia Borca 1 , Emad Oveisi 2 , Cecile Hebert 2 , Steven Van Petegem 1 , Helena Van Swygenhoven 1
1 , Paul Scherrer Institut, Villigen PSI Switzerland, 2 , Ecole Polytechnique Federal de Lausanne , Lausanne Switzerland
Show AbstractThe ongoing miniaturization of micro-electronics and micro-electromechanical systems is facing intriguing mechanical problems at the smaller length scales. Upon miniaturization, the length scales of microstructures (e.g. grain size) are no longer negligible with respect to the component size. Such microstructural effects as well as macroscopically triggered strain gradient effects and surface effects result in the observation of various size effects. Recent measurements on micron-sized single crystals revealed the possible existence of another size effect, i.e. an enhanced strengthening with decreasing sample size. The origin of this size effect is still a matter of debate, partly due to a lack of proper microstructural characterization tools. Recently we have developed a new in situ micro-compression setup, which links macroscopic stress/strain data with the evolving microstructure. It is based on x-ray Laue micro-diffraction in combination with conventional transducer technology. In this contribution we report on in-situ experiments performed on single crystal fcc micropillars. We discuss the influence of the initial microstructure and boundary conditions of the micro-compression testing technique on the evolution of the microstructure. In order to link the shapes of the diffraction spots with the underlying defect structure the pillars are characterized before and after deformation using electron microscopy techniques. The experimental measurements are complemented with a new computational approach consisting of the calculation of Laue diffraction patterns from 3-dimensional dislocation dynamics simulations obtained from the group of P. Gumbsch (Karlsruhe Institute of Technology).
10:30 AM - P8.6
Reversible X-ray Peak Broadening in Supported Metal Films with Mixed Texture during Thermal Cycling.
Shefford Baker 1 , Aaron Vodnick 1
1 Materials Science and Engineering, Cornell University, Ithaca, New York, United States
Show AbstractThin metal films on substrates typically show hysteretic stress-strain behavior during thermal cycling. However, perfectly reversible x-ray peak widths, with no hysteresis, have been observed during thermal cycling of thin Cu films on Si substrates. While diffraction peak widths are commonly interpreted in terms of dislocation strain fields, the lack of hysteresis cannot be justified based on dislocation mechanisms. It is demonstrated that a broadening from dislocation strain fields and elastic grain interactions work in concert to eliminate hysteresis. The effects of inhomogeneous strains due to dislocations and due to grain interactions on x-ray peak width data and interpretations will be discussed.
11:00 AM - **P8.7
Mechanical Behavior of Free-standing Graphene Films: Experiments and Theory.
Jeffrey Kysar 1 , Changgu Lee 1 , Xiaoding Wei 3 , Ryan Cooper 1 , Christopher Marianetti 2 , James Hone 1
1 Department of Mechanical Engineering, Columbia University, New York, New York, United States, 3 Department of Mechanical Engineering, Northwestern University, Evanston, Illinois, United States, 2 Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York, United States
Show AbstractGraphene is a two-dimensional molecular crystal that consists of a close-packed array of carbon atoms of arbitrary in-plane extent. It has many potential applications due to its unique electronic, optical and mechanical properties. In this talk we will discuss a series of experiments that probe the mechanical properties of graphene at very large strains; both monolayer and oligolayer graphene films will be considered. Free-standing graphene films suspended over a circular well in a silicon substrate are mechanically loaded via a diamond indenter with a tip that has a radius of about 20 nm. The force-displacement response and the breaking force of the graphene are recorded. Another set of experiments probes the magnitude of energy associated with the van der Waals force interactions between the graphene and the substrate. The elastic properties of the graphene are interpreted within the context of a fifth-order Taylor series expansion of the strain energy density in order to determine the non-linear elastic properties. In addition, we derive the relationship between the van der Waals energy and the geometry and residual strain of a graphene film that is partially adhered to a substrate.
11:30 AM - P8.8
Failure Mechanisms of Pure Graphene under Tension.
Chris Marianetti 1
1 , Columbia University, New York, New York, United States
Show AbstractRecent experiments established pure graphene as the strongest material known to mankind, further invigorating the question of how graphene fails. Using density functional theory, we reveal the mechanisms of mechanical failure of pure graphene under a generic state of tension. One failure mechanism is a novel soft-mode phonon instability of the K1-mode, whereby the graphene sheet undergoes a phase transition and is driven towards isolated hexagonal rings resulting in a reduction of strength. The other is the usual elastic instability corresponding to a maximum in the stress-strain curve. Our results indicate that finite wave vector soft modes can be the key factor in limiting the strength of monolayer materials.
11:45 AM - P8.9
Nanoscale Characterization of Interfacial Binding Interactions in Carbon Nanotube-based Material Systems.
Changhong Ke 1
1 Mechanical Engineering, State University of New York at Binghamton, Binghamton, New York, United States
Show AbstractUnderstanding and quantifying the long-range interfacial binding interaction in one-dimensional (1D) building blocks (i.e. nanotubes and nanowires) are of great importance for the development of 1D nanostructure-based multi-material, multi-scale, multi-functional material systems. In this talk, I will present our recent work on studying the binding interaction in bundled single-walled carbon nanotubes and its effect on the mechanical behavior of nanostructures. Our state of the art in-situ electron microscopy mechanical characterization techniques enable direct measurements of the mechanical deformation of carbon nanotube structures under a variety of mechanical loading modes, including mechanical peeling, buckling, and pushing/pulling processes, while our nonlinear elastica model-based theoretical approaches allow quantification of the adhesion strength between bundled nanotubes, and its effect on the mechanical strength and elastic behavior of bundled nanotube nanostructures. Our work significantly advances the nanoscale mechanical characterization techniques and our fundamental understanding of the adhesion behavior in carbon nanotube-based material systems.
12:00 PM - P8.10
Collective Behavior of Carbon Nanotubes in Turf Structures.
Harish Radhakrishnan 1 , Sinisa Mesarovic 1
1 School of Mechanical and Materials Engineering, Washington State University, Pullman, Washington, United States
Show AbstractWe present a computational model to analyze the collective behavior of a large number of carbon nanotubes (CNTs). When grown from a substrate, CNTs form a turf structure – a complex collection of intertwined CNTs, bent and interacting through van der Waals forces. The basic mechanism deformation mechanism is explained in [1]. However, under certain circumstances a peculiar collective reorientation and layer buckling may occur [2]. The relation is sought between the nanostructural parameters: density, connectivity and tortuosity, and the macroscopic behavior.To model a large number of interacting CNTs in a turf, we represent the segments of CNTs forming the turf, as elastica finite elements. During compression, the predominant deformation of CNTs is due to bending and buckling. Axial forces and torsion moments naturally couple with the bending moments. Continuum properties of a CNT (bending and torsional stiffness) are based on results from molecular dynamic simulation. The van der Waals forces of interaction between adjacent tubes are modeled as distributed loads. An explicit time integration technique is used to integrate the equations of motion. The resulting computational model is robust and is capable of modeling the collective behavior of CNTs.The generation of the computational model of a turf is accomplished by means of the restricted random walk and subsequent relaxation. [1] Mesarovic, S.Dj., McCarter, C.M., Bahr, D.F., Radhakrishnan, H., Richards, R.F., Richards C.D., McClain, D. & Jiao, J. 2007 Mechanical behavior of a carbon nanotube turf. Scripta mat. 56, 157-160. [2] Zbib, A.A., Mesarovic, S.Dj., Lilleodden, E., McClain, D., Jiao, J. & Bahr, D. 2008 Coordinated buckling of carbon nanotube turf under uniform compression. Nanotechnology 19, 175704.
12:15 PM - P8.11
Atomic-Scale-Deformation-Dynamics (ASDS) of Nanowires and Nanofilms.
Xiaodong Han 1 , Ze Zhang 2
1 , Beijing University of Tech, Beijing China, 2 Department of Mater Sci., Zhejiang University , HangZhou China
Show AbstractNanowires and nanofilms are fundamental building blocks of micro and nano-electronics for both of bottom-up and top-down technologies. Monitoring and recording the mechanical property dynamics at atomic scale are important to understand the atomic mechanism of new and surprising nano-phenomena and design new applications. Through years’ endeavors, we developed tensile and/or bending in-situ atomic-lattice resolution electron microscopy methods and equipments for nanowires and successfully conducted atomic-lattice resolution mechanical tests on individual nano-objects. With this, we observed the brittle materials SiC and Si even SixO1-x-nanowires (NWs) become highly ductile at room temperature. The metallic nanowires show unusual deformation mechanisms for large strain elasticity and plasticity. The crystalline structural evolution processes corresponding to the occurrence of unusual plasticity includes the dislocation initiation, dislocation accumulation and interaction as well as the necking of the one dimensional nanowires were fully recorded at atomic scale and in real time. We also expand the experimental methods and equipments to two-dimensional nanofilms. Examples of tensile experiments on Au and Pt films are presented. The deformation and failure mechanisms of nano-crystalline gold films were observed at the atomic scale and real-time. Cross-over of partial and full dislocations was observed in Pt thin film nano grains with the grain population being scaled down to 10 nm. Dislocation reactions by forming locks strain hardening were in situ captured.Reference[1] Patent: 200710122092.0, Patent: 200610144031.x[2] XiaoDong Han, Kun Zheng, YueFei Zhang, XiaoNa Zhang, and Ze Zhang, Advanced materials, 2007,19,2112[3] Kun Zheng, Xiaodong Han, Lihua Wang, Yuefei Zhang, Yonghai Yue, Yan Qin, Nano Letters 2009, 9, 2471[4] Kun Zheng et al, Nature Communications, in press.
12:30 PM - P8.12
Mechanical Relaxation of Buried Quantum Dots.
Vladimir Chaldyshev 1 , Nikolay Bert 1 , Anna Kolesnikova 2 , Vladimir Nevedomsky 1 , Valerii Preobrazhenskii 3 , Mihail Putyato 3 , Alexey Romanov 1 , Boris Semyagin 3
1 , Ioffe Institute, St.Petersburg Russian Federation, 2 , Institute of Problems in Mechanical Engineering, St.Petersburg Russian Federation, 3 , Institute of Semiconductor Physics, Novosibirsk Russian Federation
Show AbstractSelf-assembled nanoislands and nanoinclusions, often referred to as quantum dots (QDs), have attracted a lot of attention owing a wide variety of their applications in electronics and a number of new physical phenomena related to their atomic structure, electronic and optical properties. The QDs, being self-assembled either on the surface or in the bulk, are usually elastically strained due to a mismatch in the lattice parameters of the QD and the adjacent host material. These elastic strains considerably influence the electronic and optical properties of the QD. They are also responsible for a number of self-organization and coupling phenomena. Relaxation of the mechanical fields, which usually proceeds by formation of dislocations, drastically impacts the properties, performance and reliability of the corresponding devices.In this paper we consider a QD, which was either self-organized in the bulk of a crystalline matrix, or buried by overgrowth after self-organization on the growth surface. We focus on the very first event of the stress relaxation, which manifests itself by formation of mismatch dislocation loops at the QD/matrix interface and satellite dislocation loops nearby the QD. Theoretical consideration shows that the formation of the dislocation loops releases the mechanical energy with the threshold determined by a balance of the self-energies of the strained QD and dislocation loops, and the energies of interactions among them. The theoretical calculations were verified by transmission electron microscopy examination of the coherent and relaxed states for two different types of QDs in the GaAs films grown by molecular-beam epitaxy (MBE). The QDs of type 1 were InAs nanoislands self-organized on the growth surface of GaAs in the Stranski-Krastanow mode and buried by subsequent overgrowth with GaAs. At certain nominal thickness of InAs the InAs QDs appeared to be coherent on the surface but after the overgrowth they relax via formation of specific local defects near QDs. This relaxation scenario was predicted by theory.The QDs of type 2 were AsSb nanoinclusions self-organized in the bulk of Sb-doped epitaxial GaAs films grown by MBE at low temperature and subsequently annealed. We observe a threshold behavior of the relaxation process, which manifests itself by formation of a prismatic satellite dislocation loops. The onset for this process corresponded to a certain amount of elastic energy accumulated during Ostwald ripening of the QDs. The correlation between sizes of the QDs and satellite dislocation loops appeared to be in a quantitative agreement with our theoretical calculations.
12:45 PM - P8.13
Nanoscale Dislocation Patterning for Site Control Nucleation of Nanostrucutres.
Rodrigo Menezes 1 , Paula Caldas 1 , Clara Almeida 2 , Fernando Ponce 3
1 Physics, Pontificia Universidade Catolica, Rio de Janeiro, Rio de Janeiro, Brazil, 2 Materials Division, INMETRO, Rio de Janeiro, Rio de Janeiro, Brazil, 3 Physics, Arizona State University, Tempe, Arizona, United States
Show AbstractThe atomic force microscope allows the application of a measurable force on areas with dimensions close to the atomic scale. By monitoring the force exerted by the tip and the resulting displacement, dislocations can be generated by slip in nanometer-scale volumes. This opens the possibility of nanoscale lithography, where the nature of a crystalline substrate is modified by the controlled introduction of dislocations. It is important to study the nature of such dislocations and the conditions under which they are stable nucleation sites. Recent studies have shownthat nanoindentation can be used to grow dot patterns and that growth nucleation is due to the presence of screw dislocations rather than to the morphology of the indentation pit.A variant of nanoindentation is nanoscratching, where a constant normal force is applied by a stationary tip while the crystal is moved with constant speed in a specific direction. The dislocations thus introduced have been found to be confined in a well-defined volume below the scratch. It has recently been shown that the crystalline defects produced by nanoindentation and scratching are different in nature. Twinning was found to be the main deformation mechanism in nanoindentation, while slip bandsand perfect dislocations have been observed during scratching.The nature of nanoscratching as a lithographic technique for site-selective generation of dislocations is investigated for use in the growth of nanostructures. Linear arrays of dislocations have been selectively introduced into (100) indium phosphide crystals by dragging a diamond tip in an atomic force microscope. The nature of plastic deformation is found to depend on the scratch direction. For <110> directions, anisotropic butterfly-like structures with mostly screw dislocations indicate rotational motion in the vicinity of the advancingtip. For <100> directions, the dislocations do not propagate far from the surface, possibly due to interlocking between dislocations on different slip planes, with a surface morphology suggesting melting of the near surface region by frictional heat. These results indicate that growth of nanostructures should be highly dependent on the direction of the nanoscratch.To test this hypothesis, linear arrays of InAs nanocrystals have been produced by metalorganic vapor phase epitaxy on scratches performed with an AFM tip along specific crystallographic directions of the InP wafer. Inboth cases, the growth of nanocrystals was observed only on the scratched areas. Random nucleation of nanocrystals is observed along <110> scratches, while linearly ordered growth occur along <100> scratches. We attribute these observations to the delocalized nature of the dislocationsin the <110> case, giving the appearance of random nucleation, while highly localized crystal defects along the <100> scratch lines act as nucleation sites for the growth of linear arrays of nanocrystals.
P9: Atomistic & Multiscale Modeling
Session Chairs
Chris Weinberger
Joerg Weissmuller
Thursday PM, December 02, 2010
Room 210 (Hynes)
2:30 PM - **P9.1
Atomistic Modeling of Mechanical Response of Grain Boundaries.
Yuri Mishin 1
1 Department of Physics and Astronomy, George Mason University, Fairfax, Virginia, United States
Show AbstractWe review recent results of atomistic computer simulations of grain boundaries (GBs) under applied mechanical stresses, including stress-induced GB motion (coupling effect), sliding and dislocation emission. Molecular-dynamics studies of coupled GB motion reveal interesting new aspects of the misorientation dependence of the coupling factor, its relation to crystal symmetry, the effect of temperature and strain rate, and transitions from coupling to sliding to slip initiation. Possible implications for grain growth, grain rotation, deformation of polycrystalline materials and other processes are discussed. The results of atomistic simulations are compared with recent experimental observations of deformation behavior in bicrystals and nanocrystalline materials.
3:00 PM - P9.2
Grain Boundary Energies: Comparison of Experimental and Computed Values, and Scaling Between FCC Elements.
David Olmsted 1 , Gregory Rohrer 2 , Stephen Foiles 3 , Elizabeth Holm 3 , Anthony Rollett 2 , Jia Li 2
1 Department of Materials Science and Engineering, University of California, Berkeley, Berkley, California, United States, 2 Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States, 3 Computational Materials Science and Engineering Department, Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractGrain boundaries in polycrystalline materials are key players in mechanical behavior, particularly for nanoscale materials. Fortunately, current advances in experimental and computational techniques and resources are beginning to allow the study of grain boundary properties in their full five-dimensional macroscopic crystallographic space. Here we report on comparisons between experimental and computational studiesof grain boundary energy in fcc Ni for a large variety of boundaries. We have also studied how computed energies for several hundred boundaries vary across four fcc materials and find that, for general boundaries, the lattice constant times the shear modulus C44 is a good predictor of relative grain boundary energy between materials.We acknowledge support from the Department of Energy, Office of Basic Energy Sciences through the core program, through the Computational Materials Science Network program, and through the Scientific User Facilities Division (under Contract No. DE-AC0205CH11231) and from theMRSEC program of the National Science Foundation under Award Number DMR-0520425, Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the US Department of Energy's National Nuclear Security Administration under contractDE-AC0494AL85000.
3:15 PM - P9.3
Lattice Strain Evolution of Nanocrystalline Ni: Insight Based on Quantized Crystal Plasticity Simulations.
Lin Li 1 , Peter Anderson 1 , Steven Van Petegem 2 , Helena Van Swygenhoven 2
1 , The Ohio State University, Columbus, Ohio, United States, 2 , Paul Scherrer Institute, Villigen-PSI Switzerland
Show AbstractThis work reports on new insight to the underlying deformation phenomena in nanocrystalline (NC) metals with grain size ~ 30 nm. This is achieved by interpreting recent measurements of residual lattice strain in NC metals in terms of quantized crystal plasticity simulations of NC material. These recent measurements show that residual lattice strains change rather modestly after imposing uniaxial plastic strains up to ~2%. Also, residual peak widths from x-ray diffraction measurements decrease over this plastic strain regime. These observations are in sharp contrast to conventional grain size metals, for which residual strains and peak widths increase with imposed plastic strain. This has motivated hypotheses related to grain boundary sliding or deformation of a distinct grain boundary phase, separate from the grain interior. This paper presents an alternate hypothesis—namely, that these effects are due to a process termed quantized crystal plasticity. In particular, single slip events across nm scale grains impart large (~1%) increments in grain-average plastic strain. Thus, plasticity does not proceed in a smooth, continuous fashion but rather via large jumps, imparting violent grain-to-grain redistributions in stress. Finite element simulations employing this approach predict the experimental trends in residual strain and peak width mentioned, but only under certain conditions. First, the distribution of critical stress for slip activation is very different from that of conventional grain material—namely, no events occur below a rather large critical stress ~1/grain size. Above this stress, there is a very asymmetric distribution, with a relatively large number of easier-to-slip grains balanced by a minority of harder-to-slip grains. Thus, an alternate hypothesis involving crystal slip rather than grain boundary sliding appears to be quite viable. Indeed, it also provides insight to experimental observations of large reverse plasticity and large internal stress in NC metals.
3:30 PM - P9.4
Diffusive Molecular Dynamics.
Sanket Sarkar 1 , Ju Li 2 , Yunzhi Wang 1
1 Department of Materials Science and Engineering, The Ohio State University, Columbus, Ohio, United States, 2 Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States
Show AbstractThe coupling between diffusional and displacive processes is crucial in understanding phase transformation, deformation mechanisms and microstructure evolution in solids at finite temperatures. Prediction of such processes at fundamental defect level requires atomistic model to reach diffusional time scale. We present a novel method, called Diffusive Molecular Dynamics (DMD), which can capture diffusion time scale while retaining atomic spatial resolution by coarse graining over atomic vibration. DMD solves master equation on a moving atomic grid. It combines long-range elastic effects and short- range atomic interactions simultaneously with gradient chemical thermodynamics. Applications of DMD to hot isostatic pressing of nanoparticles, nanoindentation, diffusional void growth and climb of dislocation dipole will be discussed. In hot isostatic pressing, DMD captures the evolution of multiple nanoparticle compact to theoretical density revealing significance of rigid-body motion and diffusional and displacive processes in obtaining the final microstructure. The effects of particle size distribution and strain rate in the degree of homogeneity in structure as well as in chemical potential in final microstructures will be discussed. In nanoindentation, the simulations demonstrate that displacive plasticity depends sensitively on the remnant debris of prior diffusional plasticity. This is evident from dislocation structure, reduction in yield load and stiffness due to surface step formed by surface diffusion at low indentation rates and/or at high temperatures. Dislocation nucleation triggered by diffusive void growth is also a coupled diffusive-displacive phenomenon captured by DMD. This plays an important role in ductile failure where near a crack tip the intensive dislocation interactions produce vacancies well in excess of their equilibrium concentration. Finally, we present a possible mechanism for dislocation climb in FCC single crystal and energetics associated with it.
3:45 PM - P9.5
Development of Order-N Real-Space DFT and Its Concurrent Hybridization with Classical MD: Accuracy, Performance, and Its Applications.
Shuji Ogata 1 2 , Nobuko Ohba 3 1 2 , Tokoyuki Tamura 1 2 , Ryo Kobayashi 1 2
1 , Nagoya Institute of Technology, Nagoya Japan, 2 , JST-CREST, Saitama Japan, 3 , Toyota Central R&D Labs, Inc., Nagakute Japan
Show AbstractThere has been increasing interest to develop a hybrid simulation code that combines DFT and classical MD, to treat a large-scale atomistic system with chemical accuracy. The possible applications of the code include the nano-contact of MEMS, the diffusion of trace atoms in materials, and the stress-corrosion of materials. Previously we have developed the hybrid DFT-MD code [1,2], in which the order-N-cube DFT is implemented in real-space (RS) with the finite difference approximation for the derivative of the eigen-functions. The hybrid RSDFT-MD code is applied to various processes: the initial stage of the water-induced cracking of Si and alumia [3], the diffusion of oxygen atom in Si system [4], and so on. In this paper, we firstly report a novel order-N real-space DFT (ONRSDFT) that is realized by embedding an each divided region in the total system. The interaction between the electron-density in the divided region and the rest of the density is treated properly based on the idea of the density-functional theory. The physical accuracy and computation speed of the ONRSDFT code are investigated for various kinds of the target systems including the ceramics, semiconductors, and metals, through comparison with that of the RSDFT. Secondary, we combine the ONRSDFT with the classical description of interacting atoms by using the buffered-cluster method [5], to realize the hybrid ONRSDFT-MD code. Thirdly we apply the hybrid ONRSDFT-MD code for various interesting problems: diffusion of multiple Li atoms in graphite, the atomic scale friction at nano-contact with adsorbed water molecules, etc. Refs:[1] S. Ogata et al., Comp. Phys. Comm. 138, 143 (2001);[2] S. Ogata et al., Comp. Phys. Comm. 149, 30 (2002);[3] S. Ogata et al., J. Appl. Phys. 95, 5316 (2004);[4] T. Kouno and S. Ogata, J. Phys. Soc. Jpn. 77, 54708 (2008);[5] S. Ogata, Phys. Rev. B. 72 045348 (2005).
4:15 PM - P9.6
On the Formation of Suspended Atomic Chains from Mechanical Stretching: Differences and Similarities Between Graphene and Metals.
Marcelo Flores 1 , Gustavo Brunetto 1 , Pedro Autreto 1 , Douglas Galvao 1
1 Applied Physics, State University of Campinas, Campinas, Sao Paulo, Brazil
Show AbstractAtomic-size nanowires (NWs) generated by mechanical stretching can provide wealth of information on mechanical properties of nanosystems [1,2]. Metal NWs show ballistic transport and quantized conductance (QC) even at room temperature, behavior that will certainly influence implementation of nanodevices. Finally, the elongation of NWs allow the fabrication of the ultimate self-supported one-dimensional(1D) systems, the suspended linear atomic chains (LACs).Recently, different experimental groups [3,4] have reported the formation of carbon atomic chains (CLACs) from graphene nanoconstrictions. Interestingly, CLAC formation exhibits many features also observed in the metallic LACs [1]. In this work we report a comparative study for the structural and dynamics formation of carbon and metalic LACs (Au, Ag, Pt, and Cu). The simulations for graphene stretching were carried out using a reactive force field based on bond energy bond order (BEBO) methodology, as implemented in ReaxFF [5]. In contrast with more standard molecular force fields ReaxFF can handle making/breaking chemical bonds and the atomic hybridizations are allowed to change during the simulations. For the metallic structures we used classical molecular dynamics simulations with a hamiltonian based on the second moment approximation (SMA) within a code especially designed to handle metallic nanowires [6]. Structures of different sizes, stretching regimes and temperatures were considered for both cases. The stretching is simulated increasing the structures sizes along specific directions. For metallic structures different crystallographic directions produce LAC with different probabilities, similarly to graphene where nanoribbons with zigzag edges are likely to produce more CLACs than armchair ones. The role played by light atom contaminants, such as H, is also very similar to metallic and carbon LACs. From the simulations it was possible to obtain detailed information about the atomistic mechanisms of mechanical rupture, in particular the associated with the formation of atomic chains.[1] J. Bettini, F. Sato, P. Z. Coura, S. O. Dantas, D. S. Galvao, and D. Ugarte, Nature Nanotechnology v1, 182 (2006). [2] M. J. Lagos, F. Sato, J. Bettini, V. Rodrigues, D. S. Galvao, and D. Ugarte, Nature Nanotechnology v4, 149 (2009).[3] C. Jin, H. Lan, L. Peng, K. Suenaga, and S. Iijima, Phys. Rev. Lett. v102, 205501 (2009).[4] A. Chuvilin, J. C. Meyer, G. Algara-Siller, and U. Kaiser, New J. Phys. v11, 083019 (2009).[5] A. C. T. van Duin, S. Dasgupta, F. Lorant, and W. A. Goddard III, J. Phys. Chem A, v105, 9399 (2001).[6] P. Z. Coura, S. B. Legoas, A. S. Moreira, F. Sato, V. Rodrigues, S. O. Dantas, D. Ugarte, and D. S. Galvao, Nano Letters v4, 1187 (2004).
4:30 PM - P9.7
Adaptive Strain-boost Hyperdynamics Simulations of Stress-driven Atomic Processes.
Shotaro Hara 1 , Ju Li 2
1 , The University of Tokyo, Tokyo Japan, 2 , University of Pennsylvania, Philadelphia, Pennsylvania, United States
Show AbstractThe deformation and failure of materials are the results of stress-driven, thermally activated processes at the atomic scale. Molecular dynamics (MD) simulations can only span a very limited time which hinders one from gaining full view of the mechanics. Inspired by the Eshelby transformation formalism, we present here a transformation strain-boost method for accelerating atomistic simulations, which is found to be more efficient and robust than the bond-boost hyperdynamics for exploring collective stress-driven processes like dislocation nucleation, that have characteristic activation volumes larger than single atomic volume. By introducing an adaptive algorithm that safely maximizes the boost factor, we directly access the finite-temperature dynamical process of dislocation nucleation in compressed Cu nano-pillar over timescale comparable to laboratory experiements. Our method provides stress- and temperature-dependent activation enthalpy, activation entropy and activation volume for surface dislocation nucleation with no prior guidance about crystallography or deformation physics. Remarkably, the accelerated MD results indicate that harmonic transition-state theory and the empirical Meyer-Neldel compensation rule give reasonable approximations of the dislocation nucleation rate.
4:45 PM - P9.8
Dislocation Nucleation at Finite Temperature.
Seunghwa Ryu 1 , Keonwook Kang 2 , Wei Cai 2
1 Physics, Stanford University, Stanford, California, United States, 2 Mechanical Engineering, Stanford University, Stanford, California, United States
Show AbstractDislocation nucleation becomes more important for plasticity of metals at smaller scales, because mobile dislocations can easily escape the sample, creating a dislocation-starved state. For the first time, we are able to accurately compute the dislocation nucleation rate by calculating the free energy barrier at finite temperature. The data exhibit an anomalously large entropy, which translates to a pre-exponential factor of many orders of magnitude for the nucleation rate. The physical origin of this large entropy is explained.
5:00 PM - P9.9
Atomistic Prediction of Dislocation Nucleation at Experimental Loads and Temperatures.
Derek Warner 1 , Linh Nguyen 2 , Kristopher Baker 1
1 Civil & Environmental Engineering, Cornell University, Ithaca, New York, United States, 2 Applied & Engineering Physics, Cornell University, Ithaca, New York, United States
Show AbstractThe limited time domain directly accessible to atomistic simulations of plasticity has long hindered their predictive capability. For nanostructured and/or nanodimensioned metals, this shortcoming is exacerbated due to the high strain rate sensitivity of the controlling deformation mechanisms, such as dislocation nucleation. One viable means of overcoming this challenge is to view plasticity within a thermally activated reaction rate context. In this work, we use the finite temperature string method to calculate the most probable path in configurations space for dislocation nucleation from an aluminum nano-void at room temperature. By parallelizing the system’s trajectory along this path we can calculate directly the rate of dislocation nucleation at experimental loads and temperatures. Sampling along the reaction path yields the free energy of activation, and thus, allows for a systematic assessment of the transition state theory prediction and the validity of other simplifying approximations common to recent efforts.
5:15 PM - P9.10
A Nanoscale Mechanism of Hydrogen Embrittlement in Metals.
Jun Song 1 , William Curtin 1
1 Division of Engineering, Brown University, Providence, Rhode Island, United States
Show Abstract The embrittlement of metallic systems by hydrogen is a wide spread phenomenon but the precise role of hydrogen in this process is not well understood and predictive mechanisms are not available. Here, a new model is proposed wherein hydrogen accumulation around a microcrack tip prevents crack-tip dislocation emission/absorption, and thus suppresses crack tip blunting and ductile fracture while promoting cleavage fracture. The conceptual model is demonstrated via atomistic simulations of the evolution of equilibrium hydrogen distributions around a crack tip in Ni under increasing applied load, followed by measurement of dislocation emission and/or cleavage. These analyses are performed in single crystal Ni and for several tilt grain boundaries, and for several initial crack notch radii. A kinetic analysis is used to calculate the size of the crack-tip nanohydride as a function of hydrogen chemical potential, temperature, H diffusion rate, load level, and loading rate. Combining the kinetic analysis with the deformation/fracture analysis generates a mechanism map that predicts a ductile to brittle transition as a function of material and loading parameters. The mechanism map is applied to predict H embrittlement of unnotched tensile specimens of Ni, with the predictions and experiments matching well for material parameter values expected to be pertinent in these materials. The mechanism proposed and validated here directly identifies the role of H in driving a change in fracture mode and toughness as a function of material and loading parameters.
5:30 PM - P9.11
Invariant Characterizing Thermally Activated Plastic Flow in Pure and Alloyed Metals.
Catalin Picu 1 , Gabriela Vincze 2
1 , Rensselaer Polytechnic Institute, Troy, New York, United States, 2 , University of Aveiro, Aveiro Portugal
Show AbstractThermal activation leads to a reduction of the flow stress below the athermal value that would be measured at zero Kelvin. The ratio between the flow stress at a given temperature and the athermal stress (at given material structure) is known as the Cottrell-Stokes ratio – a fundamental parameter describing thermal activation. This ratio is independent of strain in pure metal single crystals. We show experimentally that the ratio is also independent of the annealing state of the material, i.e. an invariant of the deformation and microstructure in Al alloys in solid solution, under-aged, peak-aged and over-aged states. A model of thermally activated dislocation motion across fields of obstacles is used to suggest an explanation for the observed invariance. This observation is important both for the basic understanding of the effect of thermal activation on plastic deformation and for the development of constitutive relations
P10: Poster Session II
Session Chairs
Friday AM, December 03, 2010
Exhibition Hall D (Hynes)
9:00 PM - P10.11
Micro-devices Based on Reversible Deformation of Thin-films by Surface-chemical Modification.
Jatinder Randhawa 1 , Michael Keung 1 , Pawan Tyagi 1 , David Gracias 1
1 Department of Chemical and Biomolecular Engineering, Johns Hopkins University, Baltimore, Maryland, United States
Show AbstractElectrical actuation has enabled high functionality and control of devices across various length scales. However, these devices require electrical power which necessitates wiring or limits miniaturization of tetherless devices. We describe an approach to actuate a thin film multilayer via a chemo-mechanical response. The chemical modification involves a stress altering oxide growth in a chromium (Cr)/copper (Cu) bilayer. As a result, a controlled deformation resulting in bilayer bending is induced in the structure. The deformation can be reversed by removing the oxide layer in a reductive environment, therefore restoring the original shape of the microstructure. For our experiments, we used a highly stressed Cr layer (50 nm) with a stress-neutral Cu layer (200 nm). Subjecting the bilayer to an oxidative environment results in oxide growth of approximately 20 nm onto the Cu, thus creating a highly stressed copper (I) oxide layer. This leads to an overall reversal of the curvature of the bilayer. However, exposure to a reductive environment reduces the oxide thereby restoring the bilayer to its original curvature. The mechanism of deformation was determined by verifying the composition (through Auger spectroscopy) of the thin film bilayer in both oxidative and reductive environments. Curvature could be predicted using thin film mechanical modeling. We will also describe the use of Cr/Cu/polymer based trilayer hinges wherein bending can be controlled by both temperature and chemical modification. Microtools were constructed using these hinges as well as ferromagnetic segments allowing them to be guided from distances as far away as several centimeters. Hence, we were able to develop microgrippers that could be used to pick up and place objects in hard to reach places, without the need for any external or internal electrical power. This chemical-induced mechanical actuation provides a proof of concept towards the development of functional Micro Chemo-Mechanical Systems (MCMS), which are actuated by chemistry as opposed to electricity [as in Micro Electro-Mechanical Systems (MEMS)].[1] J. S. Randhawa, M. D. Keung, P.Tyagi and D. H. Gracias. “Reversible actuation of microstructures by surface chemical modification of thin film bilayers” Advanced Materials 22, 407 (2010).[2] J. S. Randhawa, T. G. Leong, N. Bassik, B. R. Benson, M. T. Jochmans and D. H. Gracias, “Pick-and-Place using Chemically Actuated Microgrippers”, Journal of the American Chemical Society 130, 17238 (2008).
9:00 PM - P10.14
Shell Adhesion in the Presence of Long-range Attraction I: Spherical Cap.
Jiayi Shi 1 , Sinan Muftu 1 , Kai-tak Wan 1
1 Mechanical and Industral Engineering, Northeastern University, Boston, Massachusetts, United States
Show AbstractAdhesion between two solid spheres is extensively investigated in many branches of science and technology, especially in terms of the celebrated models by Johnson-Kendall-Roberts (JKR) and Derjaguin-Muller-Toporov (DMT). These models are, however, invalid in membranous spherical capsules as in biological cells or drug delivery microcapsules and spherical cap of contact lenses. A linear elastic model is derived for a spherical shell deformed by compression between two parallel plates in the presence of intersurface attraction at the shell-substrate (or adhesion) interface. The constitutive relations are derived based on a fixed set of materials parameters (elastic modulus, flexible rigidity, Poisson’s ratio), interface properties (adhesion energy, magnitude and range of intersurface forces), geometry (radius of intrinsic curvature, shell thickness, depth and radius of the spherical cap), and mixed deformation mode of plate-bending and membrane-stretching. Interrelationships between external load, approach distance, deformed shell profile, and contact radius are derived for experimentation. The new shell model captures many distinct features vastly different from the solid-solid adhesion counterpart. First, typical Hertz compression with the contact is absent in shell adhesion, but a strong repulsive annulus at the contact edge. Second, similar to the classical JKR theory, once the external tensile load exceeds a certain threshold, spontaneous delamination, or “pull-off” occurs, but the critical load depends on the shell thickness and radius of curvature (comparable to the contact radius), and the contact radius always shrinks to zero at “pull-off” independent of the intersurface force range. Another interesting consequence is the central buckling at the contact center once the external compressive load exceeds a certain threshold. A reverse buckling is predicted at “pull-off”. The transition from short range (JKR) to long range (DMT) surface forces is also investigated, and a new Tabor’s parameter is derived that is remarkably different from the solid-solid counterpart. The new model will be demonstrated by spherical shape bacteria aggregation and cell adhesion.
9:00 PM - P10.15
Geometrically-controlled Mechanomutability.
Lin Han 1 , Lifeng Wang 2 , Khek-Khiang Chia 3 , Robert Cohen 3 4 , Michael Rubner 1 4 , Mary Boyce 2 , Christine Ortiz 1
1 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 4 Center for Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractIn recent decades, there has been extensive work carried out on stimulus-responsive materials that undergo swelling, actuation, and variable wettability. However, the design of “mechanomutable” materials, which undergo on-demand changes in mechanical behavior and properties, is still in early stages. In this study, geometrically-anisotropic microstructures were fabricated from mechanomutable polyelectrolyte multilayers (PEMs) and new emergent mechanical phenomena were explored due to the coupling between “inherent” responsive material properties and geometry (shape). The model material chosen was a PEM of poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA), which undergoes reversible pH-responsive transition from a condensed, ionically crosslinked state (pH 5.5) to a hydrated, ionized state (pH 2.0). End-attached PAH/PAA planar films and cylindrical “tube forests” were prepared using layer-by-layer assembly in conjunction with a templating approach. AFM-based nanomechanical experiments and microstructurally-based finite element analysis (FEA) demonstrated mechanomutability of both the planar films and tube forests in indentation, force relaxation, dynamic oscillatory loading, and in shear. The tube forests exhibited an order-of-magnitude increase in effective indentation stiffness with pH due to a transition from multiaxial compression (pH 2.0, E* ~ 60 kPa) to the addition of bending/buckling deformation modes (pH 5.5, E* ~ 2 MPa). Parametric FEA studies of the tube forest geometry (dimensions and inter-tube spacing) revealed that mechanomutability could be tuned over 3-4 orders of magnitude.
9:00 PM - P10.16
Atomic Scale Plasticity in Magnesium and Mg-Al Alloys.
Thomas Nogaret 1 , Louis Hector 2 , William Curtin 1
1 Mechanical Engineering, Brown University, Providence, Rhode Island, United States, 2 GM Technical Center, General Motors, Warren, Michigan, United States
Show AbstractMagnesium alloys are increasingly used because of their good strength to weight ratio. Pure Magnesium exhibits a poor deformability at room temperature, but its ductility can be improved thanks to the addition of alloying elements. We used MEAM and EAM potentials to study plasticity in Magnesium and Mg-Al alloys. We calculated at 0K and 300K the Critical Resolved Shear Stresses of < a >, < c + a > and twinning dislocations in Mg and Mg-Al.
9:00 PM - P10.17
Reaction Pathway Analysis of Homogeneous Dislocation Nucleation in a Perfect Molybdenum Crystal.
Hasan Saeed 1 , Satoshi Izumi 1 , Shotaro Hara 1 , Shinsuke Sakai 1
1 Department of Mechanical Engineering, The University of Tokyo, Tokyo Japan
Show AbstractDislocation nucleation is a classical example of stress mediated, thermally activated transitions and is well-defined within the framework of the Transition State Theory (TST). We employ reaction pathway sampling techniques to examine homogeneous dislocation nucleation in a perfect Molybdenum crystal and report the stress dependence of activation energy for a range of stresses including those inaccessible to dynamic atomistic simulation techniques. We have opted for a defect-free crystal for two reasons: One, the athermal stress for homogeneous dislocation nucleation in a given material is effectively the ideal strength of that material, and is therefore of considerable academic interest. Two, the homogeneous case allows us to examine the problem at the most fundamental level because complex surface and stress concentration effects can be kept out of the picture. We show that for a large range of resolved shear stress values the dislocation embryo is far from perfect and the critical nucleate transitions to an in-plane shear perturbation, the shear displacement of most atoms being considerably less than the Burgers vector. We conclude by presenting discussion on our results for homogeneous dislocation nucleation in a perfect Molybdenum crystal including comparison with corresponding results for Cu and Si.
9:00 PM - P10.18
Nanoparticle Induced Ductility in Zirconium Oxide.
Deeder Aurongzeb 1
1 Reliability Engineering, The University of Maryland, College park, Maryland, United States
Show AbstractOxidation kinetics of Zr is of great interest due to fundamental and technological point of view. As the oxide scale grows due to lattice mismatch the oxide cracks. Here we study oxidation kinetics of Zr powder with 2-5um grain size. We picked 100°C-700°C ranges where most of the previous study is available due to typical operating temperature of nuclear reactor. We used XRD and SEM to analyze the data. When dispersed with 10% alumina nanoparticle we find that the Zr oxide became ductile and cracking is minimal in the oxide scale. XRD data shows weak amorphiization in the system, which we attribute to alumina nanoparticle that helps with the ductility. Dispersed nanoparticle might be a way to reduce cracking in Zr alloy.
9:00 PM - P10.19
Mechanical Properties of Thermoplastic Starch (TPS), Polycaprolactone (PCL) and Sisal Fibers Biocomposites Reinforced with Surface Modified Sisal Fibers.
Adriana Campos 1 , Eliangela Teixeira 1 , Rodrigo Tonelli 1 , Ana Carolina Correa 1 , Jose Manoel Marconcini 1 , Sandra Mara Martins-Franchetti 2 , Luiz Henrique Capparelli Mattoso 1
1 Laboratório Nacional de Nanotecnologia para o Agronegócio, Embrapa Instrumentação Agropecuária, São Carlos, São Paulo, Brazil, 2 Biochemistry and Microbiology Department , Universidade Estadual Paulista Júlio de Mesquita Filho - UNESP, Rio Claro, São Paulo, Brazil
Show AbstractNatural fibers as sisal have received considerable attention as an environmentally friendly because of their light weight, non toxic, low cost and biodegradable properties. However, lack of good interfacial adhesion, low melting point, and poor resistance towards moisture make the use of natural fiber reinforced composites less attractive. Pretreatments of the natural fiber can clean the fiber surface, chemically modify the surface, stop the moisture absorption process, and increase the surface roughness.Among the various pretreatment techniques, chemical treatments is one of the methods for surface modifies the fiber.In the present work, fibers were treated with alkaline peroxide (bleaching). These fibers with surface modification were used in polymer matrix of thermoplastic starch (TPS) and polycaprolactone (PCL), both biodegradable polymers. Sisal fibers, in different compositions: 5, 10 and 20% were extruded in a twin-screw extruder with TPS/PCL (80:20 wt) and analysed by scanning electron microscopy (SEM), mechanical test, dynamic mechanical thermal analysis (DMTA), thermogravimetry (TGA) and differential scanning calorimetry (DSC). Composites with 20% sisal fiber showed increase of 41% of elastic modulus. It is observed by Tan delta curves the increase of Tg when increase the fiber percentage, indicating good adhesion and good dispersion of fiber in matrix. Thermal stability was increased by addition of sisal fibers in matrix. This study showed that sisal fibers treated with alkaline peroxide (bleaching) have good interaction/adhesion with TPS/PCL matrix.
9:00 PM - P10.2
Reactive Modeling of the Mechanics of Disulfide Bonds in Protein Materials.
Sinan Keten 1 , Markus Buehler 1
1 Department of Civil and Environmental Engineering, MIT, Cambridge, Massachusetts, United States
Show AbstractDisulfide bonds are relatively weak covalent links that play a key role in folding of extracellular proteins with structural and mechanical functions. Mechanical and thermal properties of polymer materials such as vulcanized rubber are associated with the formation and cleavage of disulfide bonds under a wide range of chemical environments, where key reactions are controlled by the introduction of reducing and oxidizing agents. Similar pathways are utilized in biology in the formation of hierarchical protein structures that give rise to the diverse features of biological materials such as hair, however, exact molecular mechanisms of the forward (formation) and reverse (fracture) reactions are largely unknown. Here we report reactive, full-atomistic molecular dynamics studies on disulfide bonds, focusing on fracture mechanics of protein materials. Long-time scale simulations employing advanced sampling methods indicate that rupture strength of proteins with disulfide bonds can exceed that of proteins with only weak interactions. Deformation and failure mechanisms of proteins employing disulfide bonds are largely controlled by the chemical environment, where the energy barrier to rupture can be modulated with changes in solvent conditions. The results explain findings from single molecule experiments, and illustrate potential strategies for improving elasticity, degradation and fracture properties of polymer materials and structural proteins.
9:00 PM - P10.20
Atomistic Study of the Mechanical Properties of Cu-Zr Metallic-glass Nanowires.
K. Koshiyama 1 , K. Shintani 1
1 Department of Mechanical Engineering and Intelligent Systems, University of Electro-Communications, Chofu, Tokyo, Japan
Show AbstractMetallic glass is an amorphous metal which shows clear glass transition. Compared with other crystalline metals, metallic glass has superior mechanical properties such as high strength, high elasticity, high hardness, etc. Recent researches have revealed that metallic-glass nanowires are created during the breaking of bulk metallic glass by viscous flow. Metallic-glass nanowires can be bent elastically, and they also exhibit a high strength. Furthermore, metallic glass of various alloy compositions can be processed to become nanowires. Such nanowires will be applied to highly sensitive sensors for hydrogen and magnetism. In this paper, melt-quench simulations based on the molecular-dynamics method for Cu-Zr crystalline nanowires of B2 structure are performed to produce metallic-glass nanowires of amorphous structure. Next, elongation of the nanowires is simulated at various temperatures. For the sake of comparison, Cu-Zr crystalline nanowires of B2 structure are also be elongated. It is revealed that the Young's modulus of Cu-Zr metallic-glass nanowires is half of the Young's modulus of Cu-Zr crystalline nanowires, and that the tensile strength of the former is fifth of the tensile strength of the latter. As tensile strain is increased, a crystalline nanowire of B2 structure changes its structure twice, whereas a metallic-glass nanowire only becomes narrow in the middle.
9:00 PM - P10.21
Atomistic Study of the Mechanical Stability of Multi-layered Graphene Nanobridges.
T. Nakajima 1 , K. Shintani 1
1 Department of Mechanical Engineering and Intelligent Systems, University of Electro-Communications, Chofu, Tokyo, Japan
Show AbstractGraphene has attracted much interest of researchers because of its extraordinary mechanical and electronic properties. Graphene nanoribbons (GNRs), which mean rectangular graphene flakes of high aspect ratios produced from graphene, are expected to be applied to field effect transistors (FETs) and catalytic composites. It is worth noting that 1/f noises of FETs using double-layered GNRs are much lower than those of FETs using single-layered GNRs. On the other hand, ripples, wavy structures, inevitably occur for GNRs, and elucidation and control of such wavy structures are necessary to fabricate high performance GNR devices. In this paper, the stabilities of single-layered and multi-layered GNRs are investigated by molecular-dynamics simulation. In order that GNRs are to be modelled as nanobridges connecting two terminals of electronic devices, the short edges of the GNRs are constrained. The distances between the two constrained edges are gradually increased, and the GNRs are uniaxially strained. The energies and out-of-plane deformations of such uniaxially strained GNRs are examined. The energy of a multi-layered GNR will be lower than the total energy of the isolated GNRs because the surface area of a multi-layered GNR is smaller than the total area of the isolated GNRs. The out-of-plane deformations which are caused by ripples will become small with the increase of strain. Understanding the relationship between the out-of-plane deformations and strain will lead to the control of the ripple structures of GNRs.
9:00 PM - P10.22
In-Situ Spectroscopy and Modeling Approaches for Deformation Behavior of Nanoscale Interface Materials.
Takakazu Suzuki 2 , W. Suetaka 1 , A. Suzuki 1 , T. Sato 1 , T. Suzuki 1
2 Nano-System Research Institute, AIST, Tsukuba Japan, 1 , TSS Research Laboratory, Tsukuba Japan
Show AbstractIn-situ techniques that enable observation of contacting surfaces are ideal, and have the potential to confirm commonly accepted classical models, as well as to allow close comparison with molecular simulation results. Our study is based on the hypothesis that boundary lubrication films possess mechanical properties that differ from those of bulk fluids, because of those molecular interaction forces that tend to maintain molecular orientation. Thin films on solid surfaces can be observed if a transparent solid is used in high-sensitivity reflection infrared spectroscopy. This technique allows thin films on metal surfaces to be studied in air, and the behavior of the lubricant molecules can be observed in situ. The pressure ranged from 0.2 to 2 MPa. The substrates were composed of Fe, Cu, or Au. The temperature ranged from room temperature to 400 K. The thickness of the films ranged from 20 to 100 nm.The absorption intensity increased sharply with temperature when the polarization of the beam was parallel to the plane of incidence, but decreased gradually when the polarization was normal to the plane. The dipole moment of asymmetric methylene group stretch was found to be nearly parallel to the surface. Therefore, the methylene groups of n-octadecane were oriented nearly parallel to the metal surface. The absorption intensity decreased with temperature when the polarization of the beam was parallel and increased with temperature when the polarization of the beam was perpendicular. These results were nearly reversible in molecular orientation of the film. At room temperature, the addition of C18 fatty acid increased the intensity of the methylene group stretching bands when the polarization of the beam was perpendicular and decreased the intensity of the methylene group stretching bands when the polarization of the beam was parallel. These results show that the addition of C18 fatty acid increases the anisotropic molecular orientation of straight-chain hydrocarbons. Chain-length matching in the hydrocarbon lubricant film that contained C18 fatty acid resulted in higher molecular orientation than chain-length mismatching in hydrocarbon lubricant films that contained fatty acids with straight-chain lengths ranging from C10 to C16. In the case of the gold plate, no increase in the orientation of the hydrocarbon molecules was observed. The addition of bulky iso-stearic acid resulted in a nearly isotropic orientation, regardless of temperature.We have run confined shear stress MD simulation in the NPT ensemble with temperatures ranging from 300K to 800K, under pressure of 0.3-1 GPa to study the thermal stability of the interface in 30nm thick and influence of the temperature and the pressure in film and the mechanical properties of films.
9:00 PM - P10.23
Dislocation Junctions and Jogs in a Free-standing FCC Thin Film.
Seok-Woo Lee 1 , Sylvie Aubry 2 , Wei Cai 2 , William Nix 1
1 Materials Science and Engineering, Stanford University, Stanford, California, United States, 2 Mechanical Engineering, Stanford University, Stanford, California, United States
Show AbstractThin film-based techniques have been used to produce device structures in emerging technologies, and the reliability of these devices is directly related to mechanical properties of their components. These devices contain various metallic thin film components, whose mechanical properties are governed primarily by dislocation mechanisms. Recently, the technology calls for building more complex 3D structures, such as Micro-Electro Mechanical Systems (MEMS). Therefore understanding the intrinsic mechanical properties associated with dislocation mechanisms in a free-standing thin film is important for designing reliable mechanical devices at the small scale. In this poster, the dislocation junctions and jogs in a free-standing FCC thin film were studied using 3-dimensional discrete dislocation dynamics simulations. Dislocation junctions and jogs have been hypothesized as the primary dislocation structures to produce a dislocation source. Due to the unconstrained motion of surface nodes and dislocation annihilation at the free surface, junctions and jogs do not maintain their structures except under special conditions. If the film thickness is small relative to the dislocation spacing, a significant part of the dislocation network is terminated at the free surface, with the consequence that junctions and jogs can exist only for a finite time during deformation. Therefore, the creation or operation of junction/jog-related dislocation sources is more limited as the film thickness decreases. This effect could lead to insufficient dislocation multiplication to compensate for dislocation annihilation at the free surface.
9:00 PM - P10.24
Micro/Nano Structure and Morphology of Multi-phase Polymer/Oxide Composites Prepared by Powder Melt Processing.
Giorgiana Giancola 1 , Richard Lehman 1
1 Materials Science and Engineering, Rutgers University, Piscataway, New Jersey, United States
Show AbstractPowder melt processing is an advantageous approach to forming thermoplastic composites in micro and nano scale systems. By using powder precursors, a high level of mixedness and homogeneity can exist prior to the melt. Achieving homogeneity prior to the melt assists in lowering processing temperatures thus reducing the degradation of the polymer components in the system and allows for a broader array of polymer blends to be explored for processing. An aspect of polymer blending is aimed at producing a novel material with enhanced mechanical properties such as high strain to failure behavior, ductility and specialized load transfer. This approach has relatively low costs associated with it. Pellet melt processing is more commonly used. This process relies heavily on high temperatures to assure homogeneity and suffers the previously discussed drawbacks which are minimized through the powder processing approach.PMMA/HDPE blends were prepared from two physically different precursors. The first system was prepared using micron-size PMMA and HDPE prepared through emulsion polymerization. The intimate blending of these fine powders is anticipated to produce fine domain size composites using compression molding and without the high shear processing associated with extrusion. Sheets were formed by compression molding and samples were cut from these sheets for subsequent characterization. The preliminary results from this system were reported previously. The second system was prepared from commercial pellet PMMA and HDPE using traditional extrusion to generate mixing and fine domain sizes. Extruded bars were collected and sampled for characterization. In both cases ten volume percent SiO2 particles (5 micron spheres) were added to provide a means to evaluate mixing and homogeneity in the blends. In this segment of the work we characterized blends made by the two routes using electron microscopy, image analysis, and Raman and FTIR spectroscopy. The electron microcopy and image analysis was used to access the morphology of the composites and to assess the fineness of the domain size, the nature of the immiscible blends, and other artifacts of the structure. Image analysis was used to assess the mixedness of the blend as measure principally by the distribution of the inorganic particles within the composites. Raman and FTIR were used to determine if polymer conformational differences occur at the interface and the degree to which these effects depend on the processing method.
9:00 PM - P10.25
A New Method to Evaluate the Interfacial Friction between Carbon-nanotubes and Matrix.
Quan Xu 1 , Yuqin Yao 2 , Jianyu Liang 2 , Zhenhai Xia 1
1 Department of Mechanical Engineering, The University of Akron, Akron, Ohio, United States, 2 Department of Mechanical Engineering, Worcester Polytechnic Institute, Worcester , Massachusetts, United States
Show AbstractInterface plays a key role in many materials systems and processings such as nanocomposites and nanoimprinting. A fundamental understanding of the interfacial phenomena at the nanoscale is critical to the development of a new class of composites and imprinting technology. In this study, a new method has been developed to measure the interfacial friction between carbon nanotubes (CNTs) and polymer matrix. A PMMA stripe was imprinted on an ordered CNT array so that the nanotubes are embedded into the PMMA with a given depth. The stripe was then peeled off from the CNT array. By measuring the maximum load in the decohesion process, the interfacial fracture energy was calculated based on analytical formula. To extract quantitative results of interfacial friction, a 3D finite element model for the decohesion test was developed and a cohesive zone model was used to predict the complex crack growth. The model predicts that the interfacial energy is a function of maximum separation force, and the predictions are consistent with the experiment for interfacial crack growth. The interfacial friction of CNT/PMMA was finally determined based on the experiment, and a shear-lag model for nanotube pullout.
9:00 PM - P10.26
Mechanical Properties and Size Effect of <111>-oriented Si Nanowires.
Yong-Jae Kim 1 , In-Chul Choi 1 , Kwangsoo Son 1 , Won Il Park 1 , Jae-il Jang 1
1 Materials sicence and engineering, Hanyang univeresity, Seoul Korea (the Republic of)
Show AbstractNanowires are of tremendous interest due to their unique and excellent characteristics and, in this regard, are considered as advanced 1-dimensional nano-structures for the next generation electronics. The mechanical stability of the nanowires is essential for their practical application into the nano-systems. However, compared with the well-established information of electrical/optical properties and synthesis techniques, their mechanical properties are still lacking in literature and showing large discrepancy. In the present work, we systematically analyzed the mechanical properties of Si nanowire which is known as one of the most important key building blocks in the nano-devices. Si nanowires were grown along <111> direction by vapor-liquid-solid (VLS) methods with gold nano-particles as catalysts and their diameter is from 30 to 100 nm with few tens of length. Two most common nanomechanical techniques (bending and nanoindentation) were performed with atomic force microscopy (AFM) and commercial nanoindentation equipment. Elastic modulus and yield strength of Si nanowires were measured from the force-displacement responses and we compared directly the results from each testing method. And then, focus was made on the size effect on the mechanical properties which is still controversial. The influence of nanowire diameter on their elastic modulus and nano-strength were discussed in detail. * This work was supported by Mid-career Researcher Program through NRF grant funded by the MEST (No. R01-2008-000-20778-0)
9:00 PM - P10.27
Fracture Mechanism of Copper Micro-crystals by Diamond Single Crystal.
Seisuke Kano 1 , Atsushi Korenaga 1
1 AMRI, AIST, Tsukuba, Ibaraki, Japan
Show AbstractUltra precision machining is usually controlled by fracture process of the work surface by the cutting tool. In this machining the micro-crystals consisted of the work surface are cut the atomic bonds by the mechanical force, therefore, the fracture behavior usually depending on the slip surface and dislocation in the crystal, or the grain boundaries. These crystal fracture mechanisms related to the cutting precision and total compliance of the machining system including the work, cutting tool, and cutting machine on the nano-scale view point.In this study, the copper micro-crystal fracture mechanisms were discussed with the machining precisions under the several cutting conditions, such as cutting speed, cutting depth and width, and line and space of grove formation by the diamond single crystal cutting tool at the scoop face of (100) or (110) crystal face.For the cutting test, the oxygen free copper block at the size of 50x50x30mm3 using as the test piece which cut by single crystal diamond cutting tool with silicon oil on the shaper type ultra-precision cutting machine.Before groves cutting, the specimen surface was cut as flat by cutting-off tool (corner diameter; 50 mm, cutting width; 3.0 mm, scooping angle; 0 degree, and escape angle; 7.0 degree) at the work speed as 4000 mm/min and cutting depth of 5 micrometer for roughing machining and at the work speed as 4000 mm/min and cutting depth of 2 micrometer for finishing machining. For the V-shape grove cutting, the flat copper surface was cut with the diamond-point cutting tool (V angle; 60, 90, and 120 degree, scooping angle; 0 degree, and escape angle; 7.0 degree) at the work speed as 1-8 m/min and cutting depth of 1-20 micrometer for finishing machining. The cut machined surface was observed by optical microscope or SEM comparing the initial grove shape with the final grove shape. The diamond-point tool was also observed by optical microscope or SEM comparing the initial tool shape with the final tool shape.As results of the cutting test of copper micro-crystals, the machining precision was better for the (100) face of diamond-point tool than the (110) face under the deeper and higher speed conditions. The mechanisms of this fracture results considered that the (100) face close to the (111) face which was cleavage face. The mechanical stress loaded to the (110) face and chipped along to the cleavage face. And also the other possibility was considered that the micro-crystals crushed and deformed to nano-crystals by the mechanical force. In this process might be induced lower precision on the cut planes of the groves.
9:00 PM - P10.28
Influence of Processing Conditions on Mechanical and Structural Properties of Micrometer-order DLC Structures Produced by FIB-CVD Method.
Naomichi Sakamoto 1 , Yusai Akita 2 , Hiroyuki Harada 2 , Takuya Yasuno 1 , Yasuo Kogo 2
1 Science and Technology, Iwaki Meisei University, Iwaki Japan, 2 Material Science and Technology, Tokyo University of Science, Noda Japan
Show AbstractFocused ion-beam chemical vapor deposition (FIB-CVD) method is one of the techniques to realize production of three-dimensional micro/nanometer-order structures with complex shapes. Using this process, diamond-like carbon (DLC) can be fabricated with phenathrene gas as source material. Since DLC is widely used as coating material because of its superior mechanical and tribological properties, a combination of FIB-CVD method and DLC must have potential for producing nano-machine parts for MEMS/NEMS. To fabricate the micro/nanometer-order DLC structures with expected mechanical properties, it is necessary to demonstrate the influence of processing conditions on the mechanical properties. The influence of the processing conditions, however, remains to be fully elucidated. In this study, therefore, relationship between the processing conditions and the mechanical properties for production of micrometer-order DLC structures by FIB-CVD method was investigated. In addition, the influence of the processing conditions on structural properties was also studied.
DLC structures were produced by a FIB system with Ga+ ion-beam irradiation equipment. Source gas was supplied by heating phenanthrene powder. In the FIB-CVD method, the phenanthrene molecules were adsorbed on the substrate. The adsorbed phenanthrene molecules were dissociated by the Ga+ FIB irradiation, and DLC was deposited. In our DLC deposition experiments, then, the heating temperature of phenanthrene powder and probe current of FIB were set in the range of 303-383 K and 1-63 pA, respectively. To evaluate the mechanical properties of the DLC structures, indentation hardness and Young's modulus were measured by a nano-indentation tester. Bonding state of carbon atoms was analyzed by high-resolution transmission electron microscope (HRTEM) equipped with electron energy loss spectroscopy (EELS).
From results of nano-indentation experiments, we found correlations between the mechanical properties and ratio of etching rate to corrected deposition rate. In the DLC formation process by FIB-CVD, etching process occurs simultaneously with deposition process. The corrected deposition rate was estimated by sum of etching and deposition rates of the DLC structures under the same beam condition. It was considered that the ratio of etching rate/corrected deposition rate represents qualitatively contribution ratio of etching process to DLC formation process. The hardness was sensitive to the processing condition, which decreased from 14 GPa to 8 GPa with increasing the ratio of the etching rate/corrected deposition rate. The Young's modulus was also decreased from 140 GPa to 120 GPa with increasing the ratio. EELS analysis showed that decrease of carbon sp3 fraction caused the indentation hardness and the Young's modulus to be decrease. Therefore, it was suggested that mechanical properties of DLC structures were influenced by the sp3 fraction varied by the processing condition.
9:00 PM - P10.29
Effect of Ag Content on Electrical Conductivity and Tensile Properties of Cu-Ti-Ag Alloys.
Taek-Kyun Jung 1 , Hyuk-Chon Kwon 1 , Hyo-Soo Lee 1
1 , Korea Institute of Industial Technology, Incheon Korea (the Republic of)
Show AbstractEffects of Ag content on electrical conductivity and tensile properties of Cu-Ti-Ag alloys were investigated. Cu-25wt%Ti master alloy was prepared by plasma arc melting method at Ar atmosphere. 3Kg melt of Cu-Ti-Ag alloys was produced by induction melting with oxygen free copper, pure Ag, and Cu-25wtTi master alloy. The ingots (slab type) were homogenized at 1123K for 24h, solution treated at 1173K for 2, and then quenched into water. After solution treatment, the slabs were cold rolled with reduction ratio of 90% and aged at different temperature. Microstructures were identified by optical microscopy (OM), x-ray diffraction (XRD), and transmission electron microscopy (TEM). Electrical conductivity was measured at room temperature using direct current four-probe technique. Hardness test was carried out using a micro-Vickers hardness tester under 0.5kg. Tensile test was performed using a universal tensile testing machine at room temperature with a strain rate of 1mm/min. Electrical conductivity of both cold rolled and solution treated was decreased with increasing Ti content. However, Ag addition in Cu-Ti alloys didn’t decrease electrical conductivity and enhanced hardness and tensile properties. For as-rolled specimens, the electrical conductivity rapidly increased by aging at and above 623K. It was attributed to precipitate of solute atoms from Cu matrix.
9:00 PM - P10.3
Size Effect on Crack and Dislocation Nucleation in Si NWs under Tension.
Keonwook Kang 1 , Wei Cai 1
1 , Stanford University, Stanford, California, United States
Show AbstractWe performed molecular dynamics (MD) study of tension simulations of small diameter silicon nanowires (D < 10 nm) grown along the [110] direction, and found that the nanowires’ brittle or sliding failure behavior depends on their size and temperature. The MD simulations also showed that the failure behaviors of Si NWs are controlled by the nucleation events: Crack nucleation at NW surface initiates cleavage fracture and dislocation nucleation from NW surface initiates sliding fracture.The nucleation energy barriers of a crack and a dislocation are calculated using the modified string method in Si NWs of different diameters, and compared to see the size effect on preference of the nucleation events. Interestingly, the crack energy barrier is less affected by the NW size than the dislocation energy barrier. For smaller NWs (D < 4 nm), the dislocation nucleation energy reduces so much that dislocation nucleation requires less energy than crack nucleation for all strain range applied. This trend is consistent with the MD simulations favoring sliding fracture in smaller NWs regardless of temperature.
9:00 PM - P10.30
Size Effect on Bending Properties of DLC Nanopillar Produced by FIB-CVD.
Yasuo Kogo 1 , Hiroyuki Harada 1 , Yoji Shibutani 2 , Naomichi Sakamoto 3 , Takuya Yasuno 3
1 Department of Material Science and Technology, Tokyo University of Science, Noda Japan, 2 Department of Mechanical Engineering, Osaka University, Suita Japan, 3 Department of Electronics and Computer Science, Iwaki Meisei University, Iwaki Japan
Show AbstractA Focused ion-beam chemical vapor deposition (FIB-CVD) method has been attracted much attention as one of production methods for three-dimensional structures with complex shape in micro-/nanometer scale. By use of phenanthrene (C14H10) gas as source material, FIB-CVD generates diamond-like carbon (DLC), which has diamond and graphite structures. DLC is generally known as material with superior mechanical properties such as high Young's modulus and high wear resistance. In micro-/nanometer-order DLC structures produced by FIB-CVD method, however, their mechanical properties are not clarified sufficiently. In this study, therefore, bending properties of DLC nanopillar produced by FIB-CVD was evaluated, and microstructure was analyzed by high-resolution transmission electron microscope (HRTEM).
DLC nanopillars were produced on silicon substrates by a FIB irradiation apparatus equipped with Ga+ liquid metal ion source. During the DLC nanopillar production, probe current and accelerating voltage of FIB were fixed at 1.0 pA and 30 keV, respectively. The nanopillars were grown using spot irradiation without FIB scanning. The DLC nanopillars with 160-700 nm in diameter were obtained by defocused FIB irradiation. The bending tests were performed using a commercially available cantilever for atomic force microscope (AFM) as a loading source in a scanning electron microscope (SEM). Young's moduli of the pillars were calculated from load-displacement curves in the bending tests. For the HRTEM analysis, the pillars were etched less than 100nm in thickness by FIB system. Elemental distribution and sp2/ sp3 rate of the nanopillars were analyzed using energy dispersive X-ray spectroscopy (EDS) and electron energy-loss spectroscopy (EELS) built into the HRTEM.
From results of the bending tests, Young's modulus of DLC nanopillars produced by FIB-CVD was estimated in the range of 50-130 GPa. And it was suggested that the Young's modulus was decreased with the diameter of the pillar. In our previous study, bending tests of micrometer-order DLC specimens produced by FIB-CVD showed that Young's modulus of the specimens was in the range of 100-130 GPa. From this comparison between nanometer- and micrometer-order bending tests, the Young's modulus of DLC may be decreased by scale-down of specimen size. In observation of the bending tests for the nanopillars less than 370 nm in diameter, the large deformations without brittle fracture were confirmed, and the pillars didn't showed plastic deformation even though deformed largely. On the other hand, brittle fracture by bending load was observed in the bending tests of the specimens more than 570nm in diameter.
9:00 PM - P10.31
Density Functional Theory Calculations of Properties of the Grain Boundaries in Aluminium.
Marek Muzyk 1 , Krzysztof Kurzydlowski 1
1 Faculty of Materials Science and Engineering, Warsaw University of Technology, Warsaw Poland
Show AbstractAluminium alloys are widely used in industry due to the possibility for precipitations hardening of. Copper and magnesium are the important alloying elements added to the aluminium matrix, which form Al2Cu and Al8Mg5 phase, respectively. These inter-metallic phases have low enthalpies of mixing, which cause make them favourable with regard to solid solutions of Cu and Mg in Al. However, it has been experimentally found that Cu can segregate to grain boundaries in aluminium [1,2]. The resulting depletion of Cu near to the grain boundaries may have important implications to the structure/properties of Al-Cu alloys, in particular with nano-grain size.In this work the segregation of Cu and Mg to the grain boundaries in aluminium was analysed using the Density Functional Theory. The calculations have been carried out to investigate the properties of symmetric twist boundaries in aluminium with and without Cu/Mg atoms. The phenomena of inter-granular segregation of Cu and Mg are discussed and its effect on the stability of precipitates containing these elements.[1] M. de Hass, J.Th.M. De Hosson, Scripta Materialia 44 (2001) 281-286.[2] G. Svenningsen, M.H. Larsen, J.Ch. Walmsley, J.H. Nordlien, K. Nisancioglu, Corrosion Science 48 (2006) 1528-1543.
9:00 PM - P10.32
Strengthening Effect of Twin Boundaries in bcc Crystal Evaluated through a Micro-bending Test.
Yuki Karasawa 1 , Akinobu Shibata 1 , Masato Sone 1
1 , Tokyo institute of technology, Yokohama, Kanagawa Japan
Show AbstractThe introduction of internal boundaries into a material is an important strengthening method. The introduction of nanoscale twins, which divide a grain into nanoscale twin/matrix lamella, is one approach to obtain the high strength without losing the uniform elongation. However, most of the previous studies concerning the strengthening effect of twins have focused on face centered cubic (fcc) crystals, and few studies on body centered cubic (bcc) crystals because of difficulty in obtaining twins in bcc. One way to obtain the nanoscale twinned structure in a bcc material is martensitic transformation. Lenticular martensite, a kind of ferrous α’ martensite (bcc), is composed of three substructures, midrib, twinned and untwinned region. Midrib is composed of nano-scaled transformation twins [1]. Macroscopically, midrib can be regarded as an aggregate of bcc twin boundaries. We conceived a method of micro-bending test using micro-sized specimen where an aggregate of twin boundaries, i.e., midrib, was the only obstacle to dislocations. By using this method, we could evaluate strengthening effect of twin boundaries in a bcc material. Fe-33mass%Ni was used in the present study. By sub-zero cooling to liquid nitrogen temperature, lenticular martensites were partly formed. Orientation of midrib plane of each martensite plate was determined by using electron backscattering diffraction. A micro-sized cantilever specimen (10x10x50 μm) was fabricated from a martensite, where midrib plane was parallel to width direction of test specimen, by using a focused ion beam. The midrib plane was located at about 0.7 μm from specimen surface. Micro-bending test was conducted by using the test machine for micro-sized specimen [2]. The load point was 40 μm from fixed end. The displacement rate was constant, 0.1 μm/s, and bending test was ended at displacement of 20 μm. The result from micro-bending test shows that the introduction of nanoscale transformation twins into a bcc crystal is an effective strengthening method, although more detailed studies are needed.[1] A. Shibata et al., Mater Trans 2008; 49:1242 [2] K. Takashima et al., Mater Trans 2001; 42:68
9:00 PM - P10.33
Mechanical Behavior on Micro-compression Test in Ultra-low Carbon Steel Produced by High Pressure Torsion.
Takashi Nagoshi 1 , Akinobu Shibata 1 , Todaka Yoshikazu 2 , Masato Sone 1
1 , Tokyo Institute of Technology, Yokohama Japan, 2 , Toyohashi University, Toyohashi 331-8580, Aichi, Japan
Show Abstract High-pressure torsion (HPT) is one of severe plastic deformation (SPD) techniques for the fabrication of bulk ultra-fine grained (UFG) materials by inducing large strain. Interesting mechanical properties which can be achieved by HPT processing have been reported recently. However, HPT processed materials have inhomogeneous structure, which is caused by large strain gradient along radial direction. Conventional tensile test with bulk specimen, which contains this inhomogeneity, cannot evaluate mechanical properties specifically. Our mechanical test with micro-sized compression pillar minimizes the influence of strain gradient. In general, strength on micro test is not comparable with which on conventional testing due to sample size effect. But, in our test, specimen size is relatively large (20 μm on a side with square cross-section) [1], and specimen contains several hundreds of grains. On this condition, we are able to obtain the effective mechanical behavior of the bulk material. It means that the micro test is comparable to bulk mechanical test. In this study, we applied the micro compression test with micro-sized pillar to investigating the compressive properties at a particular area with precise value of strain amount. Ultra-low carbon steel (11C, <30Si, <30Mn, <20P, <3S, <2B, 8N, 14O, 300Al, <20Ti, <30Cr, <30Cu, mass ppm) were used in this study. Material was annealed at 1273 K for 6 minutes in pure Ar atmosphere. The HPT process was carried out using two anvils holding disk and torsion strained at a rotation speed of 0.2 rpm under a 5 GPa compressive pressure at room temperature. As a result of HPT process, disks of 10 mm in diameter and 0.6 mm in thickness were produced. Microstructure observation was performed with scanning ion microscopy (SIM). From the region observed by SIM, micro-sized pillar was fabricated by focused ion beam (FIB). Using conventional fabrication method with ion beam from normal to specimen is not feasible in terms of pillar dimension (taper) and damage to fixed end. On the other hand, we propose new fabrication method that using irradiation of Ga ion from side of the specimen allows for the production of pillar with uniform dimensions (non-tapered, non-filleted). The dimension of pillar is 20 μm on a side with square cross-section. Following compression test was carried out using a test machine designed for micro-sized specimens with flat-ended indenter. It is suggested that micro-compression test combined with our fabrication method can be used to clarify the mechanical behavior at a particular area with precise value of strain amount. 1. M.D. Uchic, D.M. Dimiduk, J.N. Florando and W.D. Nix, Science 305 (2004), pp. 986–989
9:00 PM - P10.34
The Effect of Grain Size and Film Thickness on the Thermal Expansion Coefficient of Cu and Ag Thin Films.
Youngman Kim 1 , Seulgi Hwang 1
1 Materials Science and Engineering, Chonnam National University, Gwangju Korea (the Republic of)
Show AbstractThin films have been used in a large variety of technological applications such as solar cells, optical memories, photolithographic masks, protective coatings, and electronic contacts. If thin films experience frequent temperature changes, thermal stresses are generated due to the difference in the coefficient of thermal expansion of the film and substrate. Thermal stresses developed in the thin films are known to be extremely important factor for the reliability and stability of electronic devices. Cu and Ag thin films that are used in this study are common for the applications to electronic devices as metallization of integrated circuit interconnects, which may experience frequent temperature changes.Thermal expansion is more related to the nature of material itself, and in addition the microstuctures, especially when the material is manufactured in the form of thin films, may also play an important role in the thermal expansion.In this study, thermal cycling of Cu and Ag thin films was employed to assess the coefficient of thermal expansion of the films. During thermal cycling, the thermal stresses of Cu and Ag thin films with various microstructures (different grain size and film thickness) were measured using the curvature measurement system. From the slope of stress-temperature curve, the coefficient of thermal expansion of the Cu and Ag thin films were calculated with the knowledge of Young's modulus and Poisson's ratio of the films.The slopes had a tendency to decrease with the increase in the grain size of Cu thin films, thus the coefficient of thermal expansion increased. As the thickness of Cu thin film increased, there was a little effect on the thermal stress, thus the no remarkable change in the coefficient of thermal expansion. Thermal expansion may be absorbed by the grain boundaries, which have a relatively more open structure than the grains. Thus, larger grained materials have fewer grain boundaries than smaller grained materials, and less likelihood of the grain boundaries absorbing the thermal expansion.
9:00 PM - P10.35
First-principle Constitutive Equation for Nonlinear Elasticity.
Shigenobu Ogata 1 , Hajime Kimizuka 1
1 Department of Mechanical Science and Bioengineering, Osaka University, Osaka Japan
Show AbstractRecent progress of nanoscale materials processing, testing and observation techniques enables to discover anomalous mechanical behaviors of materials having nanoscale characteristic length. The nanoscale materials and specimens in many cases exhibit extraordinary high strengths which are comparable to the ideal strength. Thus, since we expect large elastic deformation more than 10% strain before plastic deformation starts, nonlinear stress-strain relation should be explicitly considered for describing whole elastic behavior quantitatively. Although there exist many studies for understanding plastic deformation behavior, no theories and methods can be found for describing the nonlinear elastic behavior. We here propose a recipe to construct the nonlinear constitutive equation of any crystals using results of crystal deformation tests based on first principles density functional theory calculations and demonstrate it for diamond and aluminum crystals.
9:00 PM - P10.36
Effects of Controllable Directed Self-assembly on the Contact Mechanics of Gold Nanoparticle Monolayers.
John Kelley 1 , Michael Jespersen 1 , Steven Patton 1 , Richard Vaia 1 , Andrey Voevodin 1
1 Nanostructured & Biological Materials, WPAFB, Dayton, Ohio, United States
Show AbstractWhile there have been several studies to date on the mechanical properties of nanoparticle crystals, aggregated nanoparticles, and single particles, the contact mechanics of nanoparticle films formed by directed self-assembly are not fully understood. A quantitative understanding of contact mechanics for rationally designed nanoparticle thin films is required in order to design and optimize functional surfaces for use in a variety of applications, including nanoscale sensors, surface coatings, and nanostructured devices. As one example, microelectromechanical systems (MEMS) have a wide range of uses in the aerospace, defense, automotive, and wireless communications industries, but widespread commercialization of next generation MEMS devices has been hindered by reliability problems. This is primarily due to incorporation of extremely smooth surfaces that increase the strong adhesion forces between contact surfaces. Consequently, wider application of MEMS technology requires development of surface modification technologies to mitigate these effects. In previous studies, bio-derived noble metal nanoparticles (NPs) and nanoparticle liquids (NPLs) employed as surface lubricants significantly improved junction durability, likely as a consequence of the nanoscale surface roughness introduced by the adsorbed nanoparticles. In order to more closely examine the impact of controllable surface roughness on contact surface mechanics, gold nanoparticles (AuNP) were immobilized onto gold and silicon dioxide surfaces by a simple directed self-assembly process employing bifunctional linker chemistry. Nanoindentation was utilized to ascertain mechanical properties (e.g., hardness, elastic modulus, etc.) of the Au NP films as a function of NP areal density, particle radius, and surface linker chemistry. Finally, MEMS switch simulations were performed to determine the effect of the immobilized AuNPs on cycle life for contact junctions. The results of these studies, implications for next generation MEMS technology, and other functional opportunities for deliberately designed nanoscale contact surfaces will be discussed.
9:00 PM - P10.37
Homogeneous and Highly Ordered Lamellar-Structured Silica Thin Films and Their Physical Properties.
Jeong-Gyu Park 1 , Ki-Rim Lee 1 , Young-Seon Ko 1 , Yong-Tae Kim 1 , Young-Uk Kwon 1
1 Chemistry, Sungkyunkwan University, Suwon Korea (the Republic of)
Show AbstractIn our previous study1, various mesostructured silica thin films were synthesized in optimized conditions with reproducibility. In this study, we focused on one of them, which has the unusual lamellar structure, and performed a comprehensive investigation on the structural and physical properties. The silica thin films were synthesized by spin-coating of precursor solutions of TEOS and F-127 as the silica source and the structure directing agent, respectively, followed aging and calcination. The X-ray diffraction (XRD), grazing induced small angle x-ray scattering (GISAXS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM) data all indicated that these films have homogeneous and ordered lamellar structures with alternating high silica density layer and low silica density layer with the repeating distance of 6-9nm. Mesostructured silica thin films with the lamellar structure have been reported by other groups but the structure collapsed after calcination. On the other hand, the lamellar structure of our films was maintained even after high temperature treatment of up to 800 degrees celsius. Nanoindentation measurement data of these films showed 2.0±0.3 GPa for the hardness and 30±2.0 GPa for the Young’s modulus, which are unusually high for pure silica films. Typical sol-gel silica films have 0.2-0.3 GPa of hardness and 5-10 GPa of Young’s modulus. The thicknesses of the high density and low density layers could be controlled by adjusting composition of the coating solution. In this presentation, the formation mechanism and the details of the characterization data of our lamellar structured silica thin films will be reported.* Reference1. U. H. Lee et al. J. Mater. Chem., 2008, 18, 1881-1888
9:00 PM - P10.38
Deformation Behavior of Au Single-crystalline Nanowires: A Molecular Dynamics.
Na-Young Park 1 2 , Ho-Seok Nam 1 , Pil-Ryung Cha 1 , Seung-Cheol Lee 2
1 School of Advanced Materials Engineering, Kookmin University, Seoul Korea (the Republic of), 2 Computational Science Center, Korea Institute of Science and Technology (KIST), Seoul Korea (the Republic of)
Show AbstractWe performed molecular dynamics simulations to study the deformation behavior of Au nanowires subjected to tensile loading. Au nanowires have FCC structures with a <110>-crystallographic orientation along tensile loading direction and with four {111} lateral surfaces, and the ratio of length to width is nine. In order to investigate the effect of nanowire dimension and different empirical potentials on the deformation behavior, nanowires with the widths ranging from 4nm to 20nm were considered and three different empirical embedded atom method potentials were considered. The yield stress was observed to increase with decreasing nanowire dimension and the change of deformation mechanism was observed between 4nm and 10nm widths. During tensile loading, nanowires with the width larger than 4nm showed the full re-orientation from <110>-oriented wire to <100>-oriented one by the propagation of twin boundaries while nanowire with 4nm width broke down in <100>-oriented region during the migration of twin boundaries. The deformation behavior of nanowires also shows strong dependence on the empirical potentials, which will be also presented in this study.
9:00 PM - P10.39
WITHDRAWN 12/21/10 Structural and Mechanical Properties of Ti66-xNb13Cu8Ni6.8Al6.2Bx (x=0.5 and 1.0 at.%) Alloy.
Uwe Siegel 1 2 , Arne Helth 2 , Uta Kuehn 2 , Jens Freundenberger 2 , Alexander Kauffmann 2 , Tom Marr 2 , Juliane Scharnweber 3 , Juergen Eckert 1 2 , Werner Skrotzki 3 , Ludwig Schultz 2
1 Institute of Materials Science, Technical University, Dresden Germany, 2 Institue for Complex Materials, Leibniz Institute for Solid State and Materials Research , Dresden Germany, 3 Institue for Structural Physics, Technical University, Dresden Germany
Show AbstractWe report on the phase formation and mechanical properties of a Ti66-xNb13Cu8Ni6.8Al6.2Bx alloy upon copper mould casting. The microstructure consists of a dendritic TiNbAl beta titanium phase and an interdendritic TiCuNi phase. Minor boron additions of 0.5 and 1.0 at.% lead to a decrease of the secondary dendrite arm spacing of the beta titanium phase. Therefore, yield strength and hardness increases with boron content. The maximum strength in compression reaches up to 2500 MPa coupled with a plastic strain of more than 25 %. These mechanical properties are improved compared to the well known Ti90Al6V4 wt.% alloy which was prepared under the same conditions.
9:00 PM - P10.40
Microstructure Evolution of Amorphous Hollow Al2O3 Micro/Nanoparticles by Thermal Annealing.
Zijie Yan 1 , Douglas Chrisey 1
1 Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States
Show AbstractAlumina (Al2O3) is an important ceramic material. γ-Al2O3, in particular, has important industrial applications as catalysts or catalyst carriers, adsorbents and additives in nanofluids to improve the thermal conductivity [1-3]. All of these applications would benefit from particles with high surface-to-volume ratio, thus, hollow γ-Al2O3 particles are attractive due to their specific structure [1]. Pulsed laser ablation could produce Al2O3 micro/nanoparticles from bulk Al, for example, laser ablation of an Al target in an oxygen environment has shown the ability to fabricate γ-Al2O3 particles, but they are solid [2]. Quite recently, we developed a novel method to directly produce hollow Al2O3 micro/nanoparticles by using excimer laser ablation of bulk Al in water or an aqueous solution [4]. The Al2O3 particles have an amorphous structure due to the quenching effect in liquid environment. The amorphous structure provides the basis to generate crystalline structures, such as γ-Al2O3 and α-Al2O3 by thermal annealing. Herein, we report the microstructure evolution of the amorphous hollow Al2O3 micro/nanoparticles by annealing at different temperatures. The hollow structure is maintained but the microstructure evolves upon the temperature rise. The mechanical properties of the crystalline hollow Al2O3 particles are studied by nanoindentation tests using a Hysitron TI900 Triboindenter. In this presentation, we will describe the laser fabrication of hollow γ-Al2O3 nanoparticles and describe the mechanism for their formation and phase transition.References:[1]Kou, H. M.; Wang, J.; Pan, Y. B.; Guo, J. K. J. Am. Ceram. Soc. 2005, 88, 1615-1618.[2]Pan, C. N.; Chen, S. Y.; Shen, P. Y. J. Phys. Chem. B 2006, 110, 24340-24345.[3]Li, Y. J.; Zhou, J. E.; Tung, S.; Schneider, E.; Xi, S. Q. Powder Technol. 2009, 196, 89-101.[4]Yan, Z.J.; Bao, R.Q.; Huang, Y.; Chrisey, D. B. J. Phys. Chem. C, in press (DOI: 10.1021/jp104884x).
9:00 PM - P10.41
Infrared Spectroscopic Investigation of Water Confined in Mesoporous Silica.
Yoshie Aoki 1 , Junko Hieda 1 , Osamu Takai 1 , Nagahiro Saito 2
1 Materials Science and Engineering, Nagoya University, Nagoya Japan, 2 EcoTopia Science Research Institute, Nagoya University, Nagoya Japan
Show AbstractMesoporous silica is defined as a material with size-regulated mesopores (2-50 nm). It has highly-ordered arrangement of pores. Mesoporous materials are remarkableness as a novel field of molecule reactions. On the other hand, some researchers reported that the internal water trapped by mesopores behaved extraordinarily, compared with bulk water. For examples, the water showed the lower melting point and the increasing effect of non-freezing water. These phenomena proceed from the increasing ratio of interactions to water molecules from pore surface. These extraordinary behaviors of water result in fluctuation in reaction field states, and several problems for its industrial application. In this research, we aimed to reveal water behaviors in mesoporous silica by infrared spectroscopy. The spectroscopy can provide us the state separation of the localized water with significant sensitivity. Mesoporous silica was synthesized from several organic templates by conventional thermal calcination. Namely, organic template from organic-inorganic material was thermally removed and left in the pores surrounded by silica matrix. The pore size was regulated with the variation of reaction temperatures. The samples were fabricated with different pore sizes and/or with different chemical surface states led from different calcination times because the behaviors of water inside pores were affected by them. The synthesized mesoporous silica was characterized by XRD, TEM and N2 adsorption-desorption isotherm curve. The internal water of mesoporous silica placed in a chamber was measured by IR spectroscopy while the partial pressure of water vapor was controlled. The chamber was vacuumed up to ca. 1 Pa with a rotary pump. Given the partial pressures was adjusted by the introduction of water vapor into the chamber. The stretching mode of OH bonds in water was acquired by IR spectroscopy. The shape of OH bands was clearly varied with the increase of partial pressure. This variation indicates that it depends on the pore volume and/or the chemical surface state, for an example, the presence of silanol group. Moreover the breadth of band indicates the presence of both types of non-freezing and freezing water. The both state was separated and indentified by Gaussian deconvolution.
9:00 PM - P10.42
Experimental and Numerical Investigation of Plastic Strain Recovery in Thin Film Nanocrystalline Metals.
Nastaran Ghazi 1 , Christian Niordson 2 , Jeffery Kysar 1
1 Department of Mechanical Engineering, Columbia University, New York, New York, United States, 2 Department of Mechanical Engineering, Technical University of Denmark, Kgs. Lyngby Denmark
Show AbstractNanocrystalline thin film metals have significantly different behavior than their coarse-grained counterparts. For instance, plastic strain in microcrystalline metals is typically considered to be irreversible. However, recent experimental studies have shown that plastic strain can be recoverable in nanocrystalline thin films over a period of time. The underlying physical mechanisms of this behavior are not well understood. In this talk we discuss experiments on free-standing thin copper films with average grain size of 40 nm that are deformed using a standard bulge test technique. The specimens are loaded at strain rates that range from 10-8 s-1 to 10-5 s-1. After unloading the specimen to atmospheric pressure the film adopts a wrinkled configuration as a consequence of the induced plastic strain. As the plastic strain is recovered, the wrinkle amplitude in the film slowly decreases to zero at which time the film is again flat signifying that the plastic strain has been fully recovered. The length of the wrinkled film is measured periodically using a laser scanning confocal microscope in order to characterize the rate of the plastic strain recovery. To investigate the plastic strain recovery mechanism computationally, we model grain boundary sliding and grain boundary diffusion in a two-dimensional assembly of nanocrystalline metal with a user subroutine in ABAQUS. The constitutive equations for grain boundary diffusion are defined based on diffusion driven by chemical potential.
9:00 PM - P10.43
Microscopic Phase Field Study of Hydrogen Transport by Dislocation in BCC Iron.
Hideki Mori 1 , Yuita Takenaka 1 , Hajime Kimizuka 1 , Shigenobu Ogata 1
1 Mechanical Science and Bioengineering, Osaka University, Osaka Japan
Show AbstractFor understanding the hydrogen diffusion behavior and transport mechanism in BCC iron, it is important to evaluate the cooperative interactions among combinations of hydrogen atoms, hydrogen-trapping defects (dislocation, grain boundary, surface etc.) and stresses during the deformation process. We investigate dislocation dynamics coupled with hydrogen diffusion behavior in BCC iron using microscopic phase-field (MPF) modeling, which is combination of analytical elastic term and atomic term. Using microelasticity theory, we can evaluate the long-range elastic interaction for stress field induced by dislocations and inhomogeneous hydrogen concentration distribution. On the other hand, using density-functional-theory (DFT) calculations, we can evaluate the short-range atomic interaction between hydrogen atoms and lattice misfit of BCC iron for dislocations. It is clearly observed that the hydrogen is significantly localized and concentrated around dislocation cores, so that the remarkable difference exists in hydrogen concentration between in the bulk region and in the vicinity of dislocation cores, ranging from several weight ppm to several thousands weight ppm. Therefore, dislocations have a potential as hydrogen transporter.From our DFT results, the misfit energy of iron is remarkably lowered by the hydrogen impurity at high concentration of hydrogen. This fact brings the result that the distribution of hydrogen concentration affects the dislocation configurations mutually, and the width of dislocation core becomes broader due to the trapped hydrogen.
9:00 PM - P10.44
Plasticity Controlled Nanoscale Wear in SiC.
Maneesh Mishra 1 , Izabela Szlufarska 1 2
1 Materials Science Program, University of Wisconsin Madison, Madison, Wisconsin, United States, 2 Materials Science and Engineering, University of Wisconsin Madison, Madison, Wisconsin, United States
Show AbstractWear resistance of ceramics can be improved by suppressing fracture, which can be accomplished either by decreasing the grain size (increasing fracture toughness) or by reducing the size of the deformation zone (increasing ductility). Surprisingly, we found that simultaneous decrease of the grain size and the deformation zone leads to reduction in wear resistance of silicon carbide. We performed large scale molecular dynamics simulations of nanoscale wear in single crystal and nc-SiC with 5nm grain diameter. Wear in single crystal is controlled by dislocation nucleation and glide on the closely packed (111) planes. Dislocations that reach the free surface form steps leading to formation of a pile up. Plowing of the pile up requires dislocation plasticity and therefore in this regime single crystal SiC has a relatively high scratch resistance. Wear in nc-SiC shows a rich response with grain boundary sliding as the primary deformation mechanism. Grain boundary sliding is accommodated by heterogeneous nucleation of partial dislocations, formation of voids at the triple junctions, and grain pull-out. We estimate the stresses required for heterogeneous nucleation of partial dislocations at triple junctions and shear strength of grain boundaries. Pile up in nc-SiC consists of grains that were pulled out during deformation. Plowing of this pile-up is accommodated by grain boundary sliding and consequently scratch hardness of nc-SiC is lower than that of single crystal SiC. Our results demonstrate that machining of nc ceramics can be performed with nanometer-sized tools because in this regime brittle ceramics are pliable.
9:00 PM - P10.45
Grain Size Refinement in Polycrystalline Yttrium Oxide (Y2O3) via Reversible Phase Transformation Transformation Mechanism.
Jafar Al-Sharab 1 , Bernard Kear 1 , Stuart Deutsch 1 , Oleg Voronov 1 , Stephen Tse 1
1 Materials Science & Engineering, Rutgers University, Piscataway, New Jersey, United States
Show AbstractYttrium oxide is a key component in current optical systems, especially for mid-IR (2-8µm) applications. To increase mechanical strength while maintaining high optical transmittance, polycrystalline Y2O3 with grain size <200 nm is of interest. However, current processing requires a high sintering temperature (>1500°C), which results in grain size >2000 nm. In this research, we describe a reversible phase transformation process where a fully dense polycrystalline Y2O3 is converted directly into the nanocrystalline state. The process involves a forward transformation from cubic-to-monoclinic under a high pressure (8GPa) and a backward transformation from monoclinic-to-cubic under a lower pressure (1GPa). An example is given of a reduction in grain size of cubic-Y2O3 from 300 µm to 0.1 µm in a single pressure-induced reversible phase transformation at 1000°C. All processed samples were subjected to atomic scale characterization in order to correlate structure with optical and mechanical properties. Moreover, internal stresses and lattice defects, such as dislocations, were analyzed by high resolution TEM. Further work is underway to establish the optimal processing parameters to achieve the finest possible nano-grain size in the fully transformed material.
9:00 PM - P10.47
Formation of the First Stable Frank-read-type Source in Pristine Gold.
Zhangjie Wang 1 , Zhiwei Shan 1 , Ju Li 1 2 , Evan Ma 1 3 , Jun Sun 1
1 Center for Advancing Materials Performance from the Nanoscale (CAMP-Nano) & Hysitron Applied Research Center in China, State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an, Shaanxi, China, 2 Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States, 3 Department of Materials Science and Engineering, The Johns Hopkins University, Baltimore, Maryland, United States
Show AbstractGold is one of the softest and most ductile metals. However, the mechanical behavior of gold changes dramatically when the deformed volume is at the submicron scale, often showing ultrahigh yield strength, discrete plastic flow, and reduced mechanical shapeability. Better understanding of the discrete plastic behavior is important not only for gaining fundamental knowledge of small-scale plasticity but also for the micro processing of materials at the submicron scale. In order to uncover the underlying physical mechanism, computer simulations have been carried out and several analytical models have been developed. However, due to the lack of direct experimental evidence, all the proposed mechanisms are at best reasonable speculations. By using a quantitative in situ TEM deformation technique, we compressed initially pristine gold particles fabricated through high-temperature (1150°C) de-wetting of gold thin films on top of SiO2/Si substrate. At the early deformation stage (Stage I), the particles display large strain bursts upon very high stress (over 1 GPa). Conceivably, large amount dislocations (N≈100) should be nucleated during the burst process to account for the sample geometry change. However, very surprisingly, few storage dislocations can be found inside the particle following the large bursts (post-mortem observation). Nevertheless, once the contact radius increase to a critical value (~250 nm, stage II)), the deformation comes into continuous and stable stage under relative lower stress. At the same time, high density tangled dislocations are observed inside the particle (post-mortem observation). Inspired by the above observations, we propose that the formation of the first stable Frank-Read-type dislocation source (characterized by internal dislocation multiplication under low stress) is the pivotal point to distinguish the two deformation stages. In stage I, the plastic deformation will be controlled by surface and/or interface dislocation nucleation. Due to the small crystal volume and given loading geometry, the newly generated dislocations will move out of the crystal before they can tangle with each other. Correspondingly, the mechanical behavior of the materials at this deformation stage is characterized by size-independent ultrahigh strength and uncontrollable geometry change. In stage II, due to the improved temporal and spatial correlation, the nucleated dislocations are able to tangle with each other to form Frank-Read-type dislocations sources. As a consequence, the programmed plastic flow can be carried out through internal dislocation multiplication and propagation under a size-dependent stress level. The present work suggests how to introduce stable Frank-Read-type sources will be a key issue for improving the shapeability of small-scale materials.
9:00 PM - P10.48
In-situ Observation of Buckling and Rippling in Multi-wall Boron Nitride Nanotubes.
Hessam Ghassemi 1 , Chee Hui Lee 1 , Yoke Khin Yap 1 , Reza Shahbazian Yassar 1
1 , Michigan Technological University, Houghon, Michigan, United States
Show AbstractMulti-wall boron nitride nanotubes (BNNTs) upon severe mechanical deformation undergo a different morphological pattern in comparison with carbon nanotubes. Our in-situ transmission electron microscopy observations reveal that bent BNNTs form reversible rippling that reduces the inner diameter of nanotubes. The rippling wavelength depends on outer diameter and wall thickness of individual nanotubes. Presence of Van der Waals interaction between the nanotube walls prevents inner diameter reduction to less than 0.34nm. As opposed to carbon nanotubes, BNNTs do not form a series of wavy buckles next to each other, but exhibit fully reversible deformation.
9:00 PM - P10.49
Structure and Microhardness of Nanocrystalline Composite Alloys Based on Al and Ti.
Nina Noskova 1 , Ravil Churbaev 1 , Lev Korshunov 1 , Yuri Filippov 1
1 , Institute of Metal Physics, Yekaterinburg Russian Federation
Show AbstractA nanotechnology has been developed for making of layered nanocrystalline composite alloys Al-Si, Al(1Hf+0.2Nb+0.2Sn)-Si, Al(0.5Ce+0.5Re+0.1Zr)-Si (wt. %) and Ti-Si, with the starting materials being nanocrystalline foils of aluminum, aluminum alloys or titanium (nanograins 50 nm in size on the average) and a nanoscale silicon powder (particles 30 nm in size on the average) [1]. Experimental data on the structure and microhardness of layered nanocrystalline composite alloys based on aluminum and titanium have been obtained. The analysis of the nanoscale silicon powder for impurity elements revealed the presence of the following metals (wt. %): 1*0.1 Mg, 1*0.01 Cu, 1*0.01 Fe, 1*0.003 Bi, 1*0.001 Mn, 1*0.0001 Ag, < 0.001 Al, < 0.001 Ca. The transmission electron microscopic examination revealed sparse nanotubes of pure silicon (3 nm in diameter and up to 20 nm long) in the nanoscale silicon powder. The basis metal was pure submicrocrystalline aluminum, nanocrystalline Al-1%Hf-0.2%Nb-0.2%Sn alloy, nanocrystalline Al-0.5%Ce-0.5%Re-0.12%Zr alloy (wt. %) or pure nanocrystalline titanium. Complex methods – a method for making of the nanocrystalline basis metal and a method for intensive plastic deformation by high-pressure shear (making of the compact) – proved to be useful for production of layered nanocrystalline composites. The basis metal of the composite was thin foils (0.3 mm thick) interleaved with thin layers of a nanoscale Si powder (in the ratio 96:4 wt. %), which were subject to intensive deformation by high-pressure shear (P = 5 GPa, 2 full turns of the Bridgman anvils). An electron microscopic examination of the structure of the two-layered Al-Si composites and Al-Si alloys and the analysis of the electron energy spectra, which were measured during scanning from the cross section of the Ti-Si composite, demonstrated that a nanostructured composite state was realized in all the cases (the Si concentration changed from 0.05 to 2.5 wt. % in going from the surface to the middle of the cross section). Structural lamellas, whose boundaries accumulated silicon nanoparticles, appeared in the middle of the cross section. The microhardness of the two-layered Al-Si, Al(Hf,Nb,Sn)-Si and Al(Ce,Re,Zr)-Si composites increased 3 to 5 times as compared to the microhardness of nanocrystalline aluminum and the alloys. The microhardness of the two-layered nanocrystalline Ti-Si composite increased 6 times. The increase in the number of the layers (up to 30) in the composites had little effect on the microhardness value. The effect of a composite coating on the functional properties of commercial Al-Sn-Pb alloys, which are used in friction assemblies, has been studied. The result is positive.
9:00 PM - P10.5
Nanomechanics, Bending Stability, and Failure of Layered Silicates as Function of CEC and Stress.
Yao-Tsung Fu 1 , Gregory Zartman 1 , Hua Liu 1 , Ras Pandey 2 , Lawrence Drummy 3 , Hendrik Heinz 1
1 Department of Polymer Engineering, University of Akron, Akron, Ohio, United States, 2 Department of Physics, University of Southern Mississippi, Hattiesburg, Mississippi, United States, 3 Nanostructured and Biological Materials Branch, Air Force Research Laboratory, Wright-Patterson AFB, Ohio, United States
Show AbstractNanomechanical properties of layered silicates such as mica, montmorillonite, and pyrophyllite have remained uncertain across multiples in spite of extensive use in polymer nanocomposites. We employed multiscale simulation techniques including DFT and molecular dynamics on the basis of classical force fields to characterize elastic moduli of single lamellas of clay minerals, and compared the results to available experimental data for macroscopic samples. We find an in-plane modulus of 160 GPa independent of cation exchange capacity (CEC). Perpendicular moduli are strongly stress dependent in the range 5 to 60 GPa and in the upper range for high CEC. Parallel to the layers, shear flow was possible beyond shear stresses of 0.2 to 1 GPa and appears to be a major failure mechanism of layered silicates under increased stress. Under extreme stresses (>10 GPa), an increase in perpendicular modulus close to in-plane values occurs and cation intrusion into the clay layers is possible. The nanoscale properties seen in the simulation are consistent with macroscale properties and largely determined by lamellar anisotropy and by the cation density of the minerals. We also analyze the bending stability of single clay mineral layers on the basis of molecular simulation in comparison to an analysis of radii of curvature of clay layers embedded in various polymer matrices (silk and epoxy) using an extensive library of TEM images. The frequency of low radii of curvature is expectedly much lower than that of high radii of curvature (>>20 nm) both in experiment and in simulation. However, molecular dynamics simulation suggests that slow bending of single clay layers to low radii of curvature (<10 nm) is possible without failure under a comparatively high energy penalty.Zartman et al. J. Phys. Chem. C. 2010, 114, 1763.
9:00 PM - P10.50
Multiscale Atomistic/Coarse-grained Simulation of Fast Crack Propagation.
Ryo Kobayashi 1 2 , Takahide Nakamura 1 2 , Shuji Ogata 1 2
1 Department of Scientific and Engineering Simulation, Nagoya Institute of Technology, Nagoya Japan, 2 Japan Science and Technology Agency, CREST, Saitama Japan
Show AbstractBrittle materials which include cracks or flaws fail under loading much smaller than the ideal strength because of the stress concentration around the crack-tip. Since cracks grow very fast with successive breakage of atomic bonds at the crack-tip, real-time observation is difficult. Thus in this field the atomistic simulation is very useful tool. On the other hand the stress field around the crack-tip extends remarkably wide, so the continuum approaches are often used instead of the atomistic ones compromising discreteness and anharmonicity. Recently a lot of concurrent hybrid atomistic and continuum approaches, which adopt the atomistic method for the region near the crack-tip where discreteness and anharmonicity are significant and the continuum method for the surrounding region, have been suggested. However applications of most of these approaches are limited to the static problems, because short wave-length wave generated at the crack-tip reflects at the interface of atomistic and continuum regions. Some hybrid approaches such as the bridging scale method (BSM) and bridging domain method (BDM) can successfully reduce the reflection. But since the reduction of short wave-length wave causes reduction of temperature in the atomistic region, these approaches can be applicable to simulation of zero temperature.We have developed novel hybrid approach that can be applicable to dynamic and finite-temperature simulation of the fast crack propagation. We adopt the coarse-grained particle (CGP) method for the surrounding region of the atomistic region instead of the continuum method. Since the stiffness in the CGP method is obtained systematically based on the statistical mechanical average, elastic properties and dispersion relation of the CGP system agree well with those of the atomistic system. Thus the CGP method is suitable to be coupled with the atomistic method. Futhermore we have suggested a new coupling method, which extra atoms/particles are laid beyond the interface of two regions and these atoms/particles work as glue to couple two regions and as heat bath to control temperature. Present coupling method provides smallest reflection of short wave-length wave compared to the exsiting methods and computational very efficient. We have applied the hybrid atomistic-CGP method for the simulation of fast crack propagations in brittle materials. In the case of the fast crack propagation, a crack-tip position moves fast. And in order to reduce the degrees of freedom of the total system, the atomistic region should be successively changed following the moving crack-tip. Thus we have developed adaptive selection and change algorithm of the atomistic region around the crack-tip. The hybrid atomistic-CGP method with present coupling method and adaptive selection algorithm provides dynamic and finite-temperature simulation of fast crack propagation with accuracy of the atomistic approach and efficiency of the continuum approach.
9:00 PM - P10.51
Multiscale Modeling of Misfit Dislocation Networks at Semi-Coherent Interfaces.
Aurelien Vattre 1 , Michael Demkovicz 1
1 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractWe present a multiscale modeling based on the Frank-Bilby equation (FBE), dislocation dynamics (DD) and molecular dynamics (MD) to model misfit dislocation networks at semicoherent heterophase interfaces. In this context, analytical solutions to the FBE are used to initialize the network structure. The DD model, based on the elasticity treatment of dislocations, with self-stress effects included, is used to calculate the energy and predict the structure of relaxed dislocation networks. From the insights gained, we propose a criterion for predicting lowest energy fcc/bcc interfaces for orientation relationships ranging form Nishiyama-Wasserman to Kurdjumov-Sachs. Misfit dislocation network structures predicted by DD are compared to MD results for selected model interfaces. Extensions of this multiscale method to model interface-point defect interactions will be discussed. This material is based upon work supported as part of the Center for Materials at Irradiation and Mechanical Extremes, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number 2008LANL1026.
9:00 PM - P10.52
Yttria-stabilized Zirconia (YSZ) Thin-film Modified Surfaces for Enhanced Fracture Behavior.
Ryan Chan 1 2 , B. Stoner 1 , R. Scattergood 2 , R. Smith 3 , J. Thompson 3 , J. Piascik 1
1 Center for Materials and Electronic Technologies, RTI International, Research Triangle Park, North Carolina, United States, 2 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States, 3 Department of Prosthodontics, NOVA Southeastern University, Ft Lauderdale, Florida, United States
Show AbstractThis study aims to evaluate the use of yttria-stabilized zirconia (YSZ) thin film coatings as a viable approach for improving the fracture behavior of biomaterial ceramics. Recent investigations have reported that sputtering of thin-films can yield an improvement in flexural strength of approximately 10-32%. This strengthening effect was attributed to toughening mechanisms; such as, blunting of crack tips, crack-bridging of surface defects, stress relaxation, and/or other energy-dissipating mechanisms. Utilizing substrate bias during rf magnetron sputtering, our group has shown the ability to deposit YSZ thin films consisting of an interrupted columnar microstructure, characterized by the formation of randomly distributed lateral defects. It is hypothesized that such a microstructure would show a further increase in strength through the absorption of energy by way of a crack deflection mechanism. YSZ films (3µm) were deposited using rf magnetron sputtering onto 3-point bend specimens, (2mm x 2mm x 15mm) cut from porcelain blocks. Films were characterized using scanning electron microscopy (SEM), optical microscopy, x-ray diffraction (XRD), and wafer bow measurements (for film stress analysis). 3-point bend specimens were tested in an electromechanical testing system in DI water at 37°C to simulate in-vivo conditions. Mechanical data displayed an increase in flexural strength of up to 34% over uncoated specimens. Fractography of all failure surfaces was then conducted to characterize fracture origin and estimate apparent fracture toughness values. These data coupled with a simple model describing lateral defect distribution will be presented to further describe the strengthening of thin film modified bioceramics. This work is supported through NIH-NIDCR grant number NIH – 2RO1DE013511-10.
9:00 PM - P10.53
Grain Size Dependency of Void Formation in Nanocrystalline Cu at Room Temperature.
Junya Inoue 1 , Shoichi Nambu 1 , Toshihiko Koseki 1
1 Department of Materials Engineering, The University of Tokyo, Tokyo Japan
Show AbstractVoid and twin formations in nanocrystalline Cu during equibiaxial relaxation experiment were quantitatively studied to clarify the effect of grain size distribution on the deformation behavior in nanocrystalline Cu at room temperature. Cu films with various thicknesses were deposited on Si/SiO2/TaN substrates by magnetron sputtering deposition method. On the top and bottom surfaces of the Cu layer, Ta layers were deposited to suppress the surface diffusion of Cu during the relaxation test. The Cu alloy films were annealed after the deposition to achieve complete grain growth to the size equivalent to the film thickness at various elevated temperatures in a vacuum of 1.0x10-5Pa, then subsequently quenched to room temperature in order to induce a uniform equibiaxial thermal strain in the Cu film. Isothermal microstructural evolution as well as void formation was studied after the equibiaxial relaxation experiment.While no apparent initial voids were found in the as-deposited films, voids were found at the grain boundary triple points in the films subjected to the equbiaxial relaxation experiment. A clear grain-size distribution dependency of the number of void and the area fraction of void was demonstrated. No severe deformation twinning could be found in the films with a thickness less than 100nm even though the applied stress level was almost equivalent to the strength predicted from the Hall-Petch type relationship derived from the conventional twin growth mechanisms in the regime of grain size > 1μm. A moderate grain size dependency of the deformation twining could be observed for the films with average grain size above 100nm. The competition between the intergranular and intragranular accommodation processes will be further discussed.
9:00 PM - P10.54
Quantum Simulations of Cement.
Engin Durgun 1 , Jeffrey Grossman 1
1 Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States
Show AbstractCement is the essential component for making concrete by hydration and widely used as a construction material. Cement is the cause of ~10% of global CO2 emissions, and yet, while it is one of the most common materials in use, we have remarkably little understanding of its microscopic properties. Toward this end, we use atomic scale, quantum mechanical simulations to examine the electronic properties and structure of cement crystals and to understand the surface stability (reactivity) of various cement clinker phases. Using these results, our aim is to clarify the mechanisms of cement dissolution, which is the initial stage of hydration and also one of the key processes that leads to the need for high temperature (and therefore energy) manufacturing. As a first step we modeled the crystal structure of two major clinker phases, alite (Ca3SiO5) and belite (Ca2SiO4). The results that we obtained for these bulk phases are in very good agreement with experimental X-ray diffraction data. Next, we cleaved the clinker crystal in the simulation along different symmetry directions in order to obtain a prediction of the most stable surfaces. Dissolution occurs at the surface so accurate determination of the surface pattern is crucial. Using the computed surface energies, we can predict the full structure of the clinker nanocluster. This allows us to examine the interaction of water molecules with different nanocluster phases, in order to shed light on the dissolution mechanism and use this new understanding to predict possible novel routes for modifying and controlling the dissolution reactions.
9:00 PM - P10.56
Characterization of the Interface Properties as a Function of Dislocation/Disconnection Content in fcc/bcc Multilayers.
Niaz Abdolrahim 1 , Ioannis Mastorakos 1 , Hussein Zbib 1 , David Bahr 1
1 Mechanical and Materials Engineering, Washington State University, Pullman, Washington, United States
Show AbstractNanoscale metallic multilayers (NMM) exhibit very high strength compared to bulk materials. For layer thickness less than 50 nm, empirical laws such as the Hall-Petch relation cannot explain the observed high strength in NMM. In these materials the role of the interface in defining the whole behavior of the structure becomes very significant. The interface properties evolve continuously during deformation because of dislocations passing through it or being absorbed by it, causing morphological changes and increasing the density of interfacial dislocations. Generally, when a dislocation passes through the interface it may leave a disconnection at the interfaces which basically can be defined as the composition of a dislocation and a step. The disconnections can act as extra barriers for slip transmission and lead to shear band formation. The effect of such disconnections, with different heights, on the strengthening mechanisms of nanolayers is explored in this research using MD simulations. The interfacial strain energy and shear stresses are computed for a various dislocation configurations. Particularly, we develop energy maps for sheared interface as a function of dislocation content and disconnection density. Such measurements can be used in the development of a hardening law for NMM with incoherent interfaces.
9:00 PM - P10.6
Surface Modification Effect in Polycrystalline Y2O3 Subjected to High Pressure Processing
Stuart Deutsch 1 , Jafar Al-Sharab 1 , Bernard Kear 1 , Oleg Voronov 1 , Stephen Tse 1
1 Materials Science & Engineering, Rutgers University, Piscataway, New Jersey, United States
Show AbstractIn a recent paper, we described a reversible phase transformation process to convert coarse-grained polycrystalline cubic-Y2O3 directly into the nanocrystalline state. The process involves a forward transformation from cubic-to-monoclinic under a high pressure and a backward transformation from monoclinic-to-cubic under a lower pressure. An example was given of a reduction in grain size of cubic-Y2O3 from 300 µm to 0.1 µm via a pressure-induced reversible phase transformation at 1000°C. In the same processed material, we also observed a striking surface modification effect. Specifically, a surface-localized columnar grained structure was observed, which covered the entire sample surface. This paper will describe its structure, defect content, and properties. Preliminary work indicates that the surface layer is composed of an yttrium oxy-carbonate phase, formed by interaction between the graphite heater and sample. A report in the literature indicates that this phase decomposes at ~550°C. Hence, its formation at 1000°C may be attributed to increased stability under high pressure. On-going research should resolve this issue.
9:00 PM - P10.60
Characterizing the Role of Deformation during Electrochemical Etching of Metallic Films.
Anil Kumar 1 , Keng Hsu 1 , Kyle Jacobs 1 , Placid Ferreira 1 , Nicholas Fang 1
1 Center for Nanoscale Chemical-Electrical-Mechanical Manufacturing Systems, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States
Show AbstractElectrochemical dissolution and precipitation of ionic species into a compound is an area of great interest. In Li-based batteries, the charging-discharging process governs the battery life and its capability to store energy. Similarly, solid-state electrochemical etching processes are governed by the ability of etchant material to incorporate metal ions. The incorporation of ions is governed by several factors including work function difference, dislocation density, and grain size and number of grain boundaries. Careful separation of various parameters is required to understand the role of individual parameters.One approach to understand the behavior of metal dissolution into a solid-state ionic conductor is to conduct systematic deformation of the ionic crystal and monitor the resulting change in its ionic properties. Because deformation results into increased dislocation density, this approach allows quantitative understanding of the nature of ionic dissolution into the conductor.Here we report our recent investigations on Ag-Ag2S system to understand the role of mechanical deformation on electrochemical properties of a solid-state ionic conductor. Ag2S is a mixed ionic conductor with yield strength of about 80 MPa. For this work, first, polycrystalline Ag2S was grown by heating an Ag pallet in sulfur atmosphere above 400 °C for 10 hours. Nano-indentation on flat mesa of such a crystal showed a modulus of 30 GPa and hardness of 0.6 GPa indicating good deformation characteristics. The electrochemical nature of this system for direct metal patterning has earlier been explored.To understand the role of deformation, we systematically deformed a cylindrical Ag2S crystal in a closed die format. After each deformation step, a conical tip with given mesa size was machined and electrochemical etching of a thick amorphous Ag film was carried out. Corresponding etch depth was measured using atomic force microscopy to calculate the etch rate. We observe that deformation of crystal to one-third its original size resulted into a three-fold increment in etch rate. This enhanced etch rate is attributed to increased dislocation density and strongly highlights the importance of deformation in solid-state dissolution of metals.By looking at the temporal profile of etching current, important observation related to the kinetic processes can be made. Since highest deformation occurs at the interface of the mesa, highest etching should be observed at the start of etching process and is confirmed through the enhanced initial current densities. Since Ag2S is a mixed ionic conductor, increased dislocation density increases electron scattering and offer higher electronic resistance, which is confirmed by dramatic reduction in the electronic current density.Our findings highlight the important role of deformation in solid-state ionic dissolution and have potential applications in improved nanomanufacturing, faster charging of batteries and high-speed actuation.
9:00 PM - P10.61
Investigation of the Interaction Between an Edge Dislocation and Voids in bcc Iron Thanks to a Multiscale Approach.
Sylvain Queyreau 1 , Brian Wirth 1 , Jaime Marian 2 , Athanasios Arsenlis 2
1 Nuclear Engineering, UC Berkeley, Berkeley, California, United States, 2 , Lawrence Livermore National Laboratory, Rolla, Missouri, United States
Show AbstractThis work focuses on the strengthening associated to nanometer-sized vacancy clusters, or voids, which are formed in bcc Iron under irradiation. Previous Molecular Dynamics (MD) simulations have shown that voids offer strong resistance to dislocation motion and that the interaction is rather complex and involves different atomistic mechanisms. Here, we describe results of a mesoscale model of dislocation – void interactions using dislocation dynamics (DD). The following elementary mechanisms are assumed to control the void strengthening and are explicitly accounted for in our model:- The shearing of the void by the dislocation generates a step of one Burgers vector magnitude, which increases the total energy of the system. The surface energy associated to this step is calculated by MD calculations in a comprehensive manner.- As voids are by definition empty, the core and elastic energy associated to the “dislocation segments” inside the void is recovered. This contribution is easily calculated from elastic theory and atomistic results. - To satisfy the traction-free condition at the void surface, image forces are introduced. A coupling with Boundary Element Method is performed to solve rigorously the stress field in presence of voids. Using the non-singular expressions as in [1], we developed a new analytical formulation for the stress generated by dislocations on a surface element of rectangular shape. The interaction between a ½ <111>{110} edge dislocation and a periodic row of voids is chosen as a benchmark simulation. The temperature dependence of the edge mobility is determined from MD simulations using the recent Mendelev et. al. potential [2]. The edge velocity remains a linear function of the applied stress, even at low temperature. The damping coefficient increases when decreasing the temperature, and no threshold stress is required for the dislocation motion.The DD model results are able to reproduce the effect of void size on obstacle strength obtained from MD simulations at 0 K, and also agree well with the temperature dependence of obstacle strength. For the range of void sizes considered, the strengthening contribution related to the line energy of the removed segment is twice larger than the surface energy contribution. However, the temperature dependence of the strengthening appears to be controlled by the decrease in the (110) surface energy. [1] W. Cai, A. Arsenlis, C. R. Weinberger,V. V. Bulatov, J. Mech. Phys. Sol. 54, 561 (2006).[2] M. I. Mendelev, S.Han, D. J. Srolovitz, G. J. Ackland, D.Y. Sun, M. Asta, Phil. Mag. 83, 3977 (2003).
9:00 PM - P10.62
Laser Sintering of Nanocomposites Fe-Ni.
Evgeny Kharanzhevskiy 1 , Sergey Reshetnikov 1
1 , Udmurt State University, Izhevsk Russian Federation
Show AbstractSelective laser sintering is one of the new rapid prototyping, tooling and manufacturing techniques. This paper presents result of studying the structure layers, synthesized by the method of laser sintering ultrafine powder, consisting of nanocomposites Fe-Ni using the methods transmission and scanning electron microscopy, X-ray analysis and XPS spectroscopy.Powder materials contains nanocomposite particles of Fe-Ni consist from carbonyl iron and carbon Nickel (NiCO 3). Materials subjected to high-energy milling in planetary ball mill and subsequent annealing. X-ray study, electron microscopic image of the structure and Powder electron diffraction show that the prepared powder is a nanocomposite Fe-Ni.To implement the method of laser sintering using radiation Yb-fiber laser operating in pulsed mode generation of radiation. Treatment was carried out in a chamber with forevacuum evacuation and subsequent flushing with argon grade reagent.Analysis of images of sintered layers obtained in the reflected electrons on Auger electron spectrometer shows that the structure of the sample surface contains an extensive system of interconnected pores of different scales. On the surface there are sintered together units of different structural level, the minimum of which amounts to 100 nm.Structural studies of layers of Fe-Ni exhibit two phases: α-Fe and solid solution of nickel in the γ-Fe, while the number of γ-phase is less than expected on the basis of qualitative analysis equilibrium diagrams of binary alloy Fe-Ni. The explanation of the low content of γ-phase in the sintered samples obtained by method of mathematical modeling of processes occurring during laser sintering of ultrafine powders. As a result of high heating rates and cooling the powder layer (up to 107 K/s) and high value temperature gradient (up to 5 107 K/m), the conditions under which Nickel does not have time to significant shifts in volume of the melt.This work was supported by the Russian Foundation Basic Research, project No 09-02-12110 and the Federal Program "Research and scientific-pedagogical personnel of innovation Russia", No 2009-1.5-507-007-002.
9:00 PM - P10.63
Nanoindentation Induced Deformation Near Grain Boundaries of Corrosion Resistant Nickel Alloys.
William Herbert 1 , Krystyn Van Vliet 1 , Bilge Yildiz 2
1 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractStress-corrosion cracking (SCC) is a complex degradation mechanism afflicting metallic materials in a wide range of industries. Detailed understanding of the chemo-mechanical coupling associated with the incipient stages of SCC is still lacking, and is required for the development of new microstructures that can withstand increasingly extreme environments of corrosive media and applied stress. The goal of this study is to provide a unique insight into the atomic-scale synergy between accumulated strain and electronic work function at the grain boundaries of corrosion resistant alloys. We compared the damage accumulation behaviour of different grain boundary structures in Inconel 690 (Ni-29%Cr-9%Fe) in the presence of large, localised plastic strains induced by nanoindentation. We compared “random” high-angle and twin boundaries with sub-micron spatial resolution from the grain boundary, and quantified local hardness as a measure of damage accumulation. Relative hardening as a function of indentation-boundary distance was not distinguishable at room temperature, and did not differ significantly between the random and twin boundaries. Further, the critical “pop-in” loads indicating the onset of incipient plasticity decreased with decreasing distance from the grain boundary, but did not differ significantly between random and twin boundaries. These results suggest a comparable extent of dislocation mobility and absorption at the Inconel grain boundaries for different grain boundary types, at least under ambient conditions. We discuss the underlying reasons for this suppression of grain boundary-dependent deformation in such experiments, and highlight ongoing work on higher-resolution spectroscopic and electronic property mapping of grain boundary deformation via scanning tunnelling microscopy.
9:00 PM - P10.7
Electronic Transport Properties of Graphene with Extreme Mechanical Deformation.
Haiyuan Gao 1 , Yang Xu 1 , Bin Yu 2 , Zhonghe Jin 1
1 , Institute of Microelectronics and Optoelectronics, Zhejiang University, Hangzhou China, 2 , College of Nanoscale Science and Engineering, State University of New York, Albany, New York, United States
Show AbstractWith atomically-thin thickness and unique mechanical/electronic properties, graphene has a spectrum of potential applications such as transparent conductor and flexible electronics. While most of the reported research to-date focusing on the 2-D (or planar) behavior of graphene, a systematic study of key electronic properties of graphene with various bent deformation is still very much lacking. In this work we explore the local energy band structure modification and ballistic transport in bent graphene using tight-binding model and Non-Equilibrium Green's Function (NEGF) approach. The energy band diagram and gate-controlled transport in mono-/bi-layer graphene with varying bending angle and curvature radius are investigated and compared with that in planar case. Key simulation parameters are chosen from experiments with appropriate linear downscaling. The simulation result reveals that bent graphene with extremely small bending curvature radius exhibit quite different electron & hole conductivities and their dependence on gate biasing (as compared with a planar graphene), which may be caused by misalignment of adjacent pi-bonds at the deformed regions. Our approach suggests that the bending curvatures play an important role on the critical electronic properties of graphene on corrugated surfaces.
9:00 PM - P10.9
Grain Boundary Related Indentation Strain Bursts in Mo and Ta Metals.
Girija Marathe 1 , Rainer Hebert 1
1 Department of Chemical,Materials and Biomolecular Engineering, University of Connecticut, Storrs, Connecticut, United States
Show AbstractNanoindentation measurements have been conducted on annealed polycrystalline molybdenum as well as tantalum metals to examine the indentation strain bursts close to grain boundaries. Strain bursts appear as discontinuities in load-displacement curves and their occurrence is generally attributed to the generation and propagation of dislocations or to the fracture of an oxide layer on surface. Grain boundary related displacement bursts occur at a higher load as well as greater depths than the initial pop-in events which are caused by initial yield-in phenomena, depicting a transition from elastic to plastic deformation. Prior literature results on bcc metals such as Nb or Fe-Si alloy have linked the occurrence of grain boundary related pop-in events to the misorientation across the grain boundary as well as to the back stress at the pile up head of dislocations near the grain boundary leading to an active slip transmission.Nanoindentation measurements were performed on annealed Mo as well as Ta metals at indentation strain rates varying between 0.005 s-1and 0.5 s-1. The analysis of the load-displacement data for annealed Mo showed a strong strain rate dependence of the strain bursts. The occurrence of grain boundary strain bursts in certain Mo grains reflects a link between active slip transmission and the misorientation across grain boundary. The nanoindentation results near grain-boundaries are discussed in the context of slip transmission across grain-boundaries.