The field of nanostructured materials has seen significant progress in the past decade due largely to exciting advances that span innovative synthesis, atomic-scale characterization, to the strong coupling of theory, experiment, and predictive modeling. These advancements exemplify a major milestone of the field where the atomic and nanoscale insight and control has led to a new era of materials discovery and design benefitting a wide range of advanced applications, from energy to information technology. Building on these key advances is the opportunity to take the nanoscale insights to the mescoscale where the nanoscale unit processes meet the continuum world. The architectures and functionalities emerging from the quantum world of nanoscience hold the promise of impacting the bulk world of macroscopic materials and behavior. This talk will illustrate the recent advances in the mechanical behavior of nanostructured materials, highlighting the innovation pathways of coupled synthesis, characterization, and modeling. It will also discuss the future opportunities of mesoscale science bridging the nanoscale and the continuum.

Cold drawn steel wires can reach an exceptional high tensile strength of 6 GPa (G/13) and they can be plastically deformed at room temperature. Structural parameters and strengthening mechanisms are discussed for structures with a length scale reaching from about 100 to 20 nm and good agreement has been found between experimental and calculated values for the tensile strength which is is about 3.6 GPa at a strain of 3.7. Extrapolation of structural paramters and strengthening mechanisms (boundary and dislocation strengthening and solid solution hardening) to a length scale about 10 nm is analyzed and discussed. This length scale is reached at a strain of about 6, where a caculation of the flow stress is in good agreement with an experimental value of about 6 GPa.

In-situ diffraction experiments performed on nanocrystalline (nc) Ni demonstrate an evolution of intergranular stress with plastic deformation that is markedly different from coarse-grained counterparts. These X-ray diffraction footprints are linked to violent intergranular stress redistributions and special distributions of critical strengths for plastic slip events. This is achieved through close coupling between experiments and quantized crystal plasticity simulations, whereby single intragranular slip events impart large distortions in nc grains, amounting to ~1% jumps in plastic strain. The results suggest that the markedly different mechanical response in nc Ni owes to very inhomogeneous grain-to-grain distributions of critical strengths for slip events and the ability to store large residual strain in deformed samples. The quantized crystal plasticity approach captures diffraction footprint features that cannot be rationalized based on grain boundary sliding alone.

Interface or grain boundary effects in materials have been widely investigated since the discovery of nanocrystalline materials. Another interesting class of materials are nanoglasses or nanostructured amorphous solids. These materials can be produced by synthesis of amorphous nanoparticles in an inert-gas-condensation (IGC) process and subsequent compaction in UHV. The resulting solid material is characterised by large fractions of the material exhibiting a drastically reduced density in the regions between the cores of the former particles. Detailed structural characterization using XRD, TEM and Mössbauer spectroscopy will be reported to clarify the structural details of the novel materials, for the case of FeSc-alloys. The structural modifications give rise to new mechanical, transport and magnetic properties which will be reported and discussed in the presentation.

Research in the past 25 years has revealed a wealth of interesting altered behavior when materials dimensions are reduced to the nanometer regime. For example, studies led by Prof. Julia Weertman pioneered our understanding of the links between microstructure, defects, and resulting mechanical behavior of nanostructured metals, and demonstrated, for example, that porosity can have a profound impact on measured properties. An important outcome of Prof. Weertman&’s work was the insight that careful control of synthesis is a key to revealing and understanding intrinsic size-dependent behavior. This talk will focus on describing the synthesis behavior and structural characteristics of a variety of nanostructured materials, ranging from nanocrystalline metals produced by inert gas condensation to epitaxial oxide thin film nanostructures produced by techniques including sputter deposition and metal-organic chemical vapor deposition. Synchrotron x-ray studies have provided insight into both growth behavior and the relationships between structure and properties. Early x-ray studies probed the relationships between grain boundary structural properties and the physical behavior of nanocrystalline metal or oxide bulk compacts. I will also describe recent studies using in-situ x-ray techniques to probe and control the growth behavior and properties of epitaxial nanostructures, including thin film nanocomposites and nano-layered structures. Opportunities for controlling and enhancing mechanical behavior through careful control of synthesis will be discussed.

Magnetron sputtering has proved to be an effective method for developing highly nanotwinned (nt) metals. These types of materials have shown to posses high strengths while maintaining other attributes, such as ductility and thermal stability. However, due to the interdependence of processing parameters and a strong microstructural dependence on the sputtering conditions, the formation of growth twins during magnetron sputtering is not fully understood. In past studies, it has been shown that the percent of twinned grains can be increased by increasing the deposition rate. However, the deposition rate is a function of processing parameters, such as power, pressure, and target diameter, which may all individually influence the microstructures of sputtered films and hence, twin development. Therefore, in this study, these parameters were systematically changed in order to explore their effects on the growth twin development and overall microstructures within various sputtered FCC metals.

Nanotwinned metals have been shown to have a number of properties that are comparable or improved over those of their nanocrystalline counterparts (e.g., similar high strength, and improved microstructural stability, ductility, conductivity). The present paper examines the effect of shear straining on the internal structure of high purity Cu consisting of parallel columns of highly aligned coherent twin boundaries. A disk, 10 mm in diameter and 150 microns thick, was twisted by high pressure torsion through ½ turn and the results examined in a cross-sectional plane oriented for maximum shear strain. The overall shear strain was 21, but varied greatly throughout the thickness of the sample, falling to less than one in the central 93% of the sample. A previous study showed three regions through the sample thickness: extremely fine grains near the surface, large grains further inward, and finally sheared columns of the remaining twins starting at about 1 micron from the surface (which corresponds to a shear strain of about 1). However recent careful TEM measurements have shown that the “large grains” actually consist of twins and matrix. The twin structure has survived shear strains of several hundred percent. Analysis of apparently detwinned regions near the twin columns and the fine grain region at the surface will be described. The effect of changing the angle of twist, and of cycling the twisting will be shown.

This talk focuses on designing and processing of nanostructured materials of controlled size and uniformity, and defects and interfaces. Controlled size and uniformity is utilized to investigate hardness as a function of grain size, and to show that there is a critical size at which inter or interface deformation starts to play a dominant role. These observations establish unequivocally the phenomenon of inverse Hall-Petch relationship. Process-induced defect content within the grain plays an important role in the onset of inter-grain deformation and resulting grain softening. We show that interfacial energy can be controlled by alloying the boundaries to improve the stability of nanostructured materials, which is critical to practical applications. We also review exciting modifications in mechanical, optical, magnetic and electrical properties of ceramics by embedding metallic nanodots, which incorporate useful properties of metals into ceramics. In-situ deformation studies of these materials shed light on the mechanisms of improvements of fracture toughness and ductility by the incorporation of metallic nanodots. We also show that a combination of nano- and micro-grains plays an important role in designing, processing and obtaining materials with unique and improved properties.

The traditional view of grain boundaries envisions them as mechanically static, immovable structures. Room temperature grain growth in nanocrystalline metals, molecular dynamics simulations, and recently proposed theories of coupled boundary migration all suggest that grain boundaries are not nearly as static as generally assumed. Observations of stress-assisted grain growth will be reviewed and linked to theories of coupled boundary migration. In situ experiments designed to investigate the details associated with stress-coupled grain boundary motion will also be outlined. These in situ observations are being developed to quantify the effect of grain boundary character, morphology, size and connectivity on stress-assisted grain boundary migration. This work was supported by the U.S. Department of Energy under grant number DE-FG02-07ER46437.

Mg-alloys are becoming increasingly researched based on the technological advantages given their low density. However, little has been done to investigate the processing, deformation mechanisms and properties of nanocrystalline hcp Mg-based alloys, unlike their fcc and bcc counterparts. This presentation will overview advanced computationally-aided, processing of nanostructured Mg-alloys via "bottom-up" powder processing and "top-down" severe plastic deformation methods. Thoughts on the deformation mechanisms and preliminary resultant mechanical properties at high and low-strain rates will be given along with the benefits and limitations of each processing approach. The initial results point to unprecedented increases in strength, control of texture and anisotropy, and increased formability at low temperatures. These results forecast promising approaches for the design of nanocrystalline Mg-alloys with superior strength and ductility for advanced structural and defense applications.

We report grain size effect on the densities of edge and mixed dislocations in nanocrystalline Mo prepared by high-pressure torsion: with decreasing grain size the density first increases and then decreases with the highest dislocation density. The trend of grain size effect on the density of edge and mixed dislocations mirrors the reported grain size effect on the strain rate sensitivity of bcc metals, suggesting that the high density of edge and mixed dislocations contribute to the reduction of strain rate sensitivity. We also observed for the first time the ½<111> and <001> pure edge dislocations under high-resolution transmission electron microscopy (HRTEM). Crystallographic analysis and simulations reveal that the best way to study the edge dislocations or the edge components of mixed dislocations is to take HRTEM images along a <110> zone axis. The <001> pure edge dislocations can be easily identified from a <100> zone axis. This work not only sheds lights on the deformation mechanisms of nanocrystalline bcc metals, but also provide a guidance on future HRTEM studies of their dislocation structures.

The mechanical behavior of nanocrystalline materials with grain sizes below 100nm has been extensively investigated. Grain size effects manifest themselves not only in strength but also in a characteristic elasto-plastic transition. Many models exist now to describe the stress-strain curve of nanocrystalline metals and to reproduce eventual deviations from Hall-Petch behavior. Most models describe the deformation as composed of an interplay between dislocation mechanisms and GB-mediated mechanisms, the latter gaining in importance with decreasing grain size. Support for both mechanisms can be found in experiments as well as in simulations. Recent observations of grain coarsening during deformation have sharpened the interest in GB mediated mechanism and especially in stress-driven grain boundary motion. This talk debates the need for having competing scaling laws for dislocation mediated plasticity and GB mediated plasticity in order to be able to reproduce the mechanical behavior of nanocrystalline metals. To do that, results obtained from molecular dynamics computer simulations and in-situ Xray diffraction experiments are discussed and their synergies revealed. Such an approach results in a series of parameters useful for implementation in mesoscopic models.

Al-Sc alloys, strengthened by nanoscale Al₃Sc precipitates (L1₂ structure), exhibit outstanding coarsening and creep resistance up to 300 °C, which can be improved to 400 °C with ternary additions of the neighboring Group 4 elements, Zr or Ti. These elements have a much smaller diffusivity than Sc, resulting in Al₃(Sc_1-x,Zr_x or Ti_x) precipitates that are enveloped in a thin (approximately 1 nm thick) Zr or Ti-rich shell. The slower-diffusing Zr or Ti atoms limit coarsening and, since they substitute for Sc in the precipitates, also reduce the high cost of Sc additions. To increase the coarsening resistance temperature, other alloying additions should be considered. For example, the Group 5 elements (M = V, Nb, Ta) may also be beneficial alloying additions to Al-Sc alloys. They each form an Al₃M trialuminide and also exhibit some solubility in Al₃Sc. Furthermore, the Group 5 elements are anticipated to be much slower diffusers than Zr, providing a higher thermal stability (that is, a smaller precipitate coarsening rate) than Al-Sc-Zr or Al-Sc-Ti alloys. This study investigates, using transmission electron microscopy and 3-D atom-probe tomography, the nanostructures and compositions of the core/shell Al₃(Sc_1-x,M_x) precipitates formed during aging at 200 to 600 °C of quenched supersaturated alloys. The measured compositions, radii, volume fractions, and number densities of the Al₃(Sc_1-x,M_x) core/shell precipitates are used to quantify the strengthening increments from the precipitates at ambient temperature (Orowan strengthening) employing nano-indentation hardness measurements.

Equal channel angular extrusion (ECAE) is one of the methods to obtain smaller grain sizes along with better mechanical properties. During this process, large plastic deformation takes place and the mechanisms responsible for the change in properties, especially the thermodynamics and kinetics are being investigated extensively by many researchers. One indicator, by which information about plastic deformation can be obtained, is the strain rate sensitivity (SRS) of a material that can be found by performing the strain rate jump test in conventional mechanical testing or by nanoindentation testing as discussed in this work. In the last five decades or so, the objective of different works has been to characterize the effects of strain rate in metals. Ultrafine grain (UFG, grain size d>100nm but <1000nm) and nanocrystalline (NC, d<100nm) regimes have been the focus of attention during the past decade where most of the works have attempted differentiating behaviors according to crystal structure, i.e. face centered cubic (FCC), body centered cubic (BCC) and hexagonal close-packed (HCP) metals, as well as the dependence on the method of processing, such as severe plastic deformation (SPD) or powder metallurgy. In the current work, bulk copper subjected to ECAE at different number of passes is probed in order to extract information regarding SRS via nanoindentation and the relation to its internal structure.

Due to the thermodynamics behaviors of the materials which are far from equilibrium state, the nano-scale crystallization is a well-known phenomenon of glassy metallic alloys. While mechanical response of metallic glass has received a significant amount of attention in the theoretical physics and molecular simulation literature over the past decade, significantly less attention has been devoted to the mechanical response of the crystal/amorphous composites metallic alloys. Here, a series of Zr2Cu crystal/amorphous nano-composites with different sizes crystalline grain are established based on the embedded atom method potential. The shear response of these materials was studied by the molecular dynamics simulations and grain-size dependent yield strains, stresses and shear modulus, as well as the shear localization properties are investigated. Also, the shear-strain rate-dependent behaviors are discussed in these studies. The results provide the starting point for constructing the bridge between the amorphous materials and the nano-crystalline materials.

Nano-twinned materials have a great interest in recent years as one of the approach to obtain the incredibly high strength without losing uniform elongation. Many researchers have reported mechanical properties of nano-twinned materials with fcc structure. The deformation behavior of nano-twinned materials other than fcc structure, however, has yet to be studied, due to difficulties in formation of nanoscale twins. One of the ferrous α&’ martensites (bct structure), thin plate martensite, contains nano-spaced transformation twins extended from one martensite/austenite interface to the other. In the present study, we fabricate square shaped pillar from a selected martensite variant and following compression test was conducted to evaluate mechanical properties and clarify deformation mechanisms of the fully nano-twinned materials with bct structure. An Fe-31Ni-10Co-3Ti (mass%) was used in this study. Thin plate martensites were thermally transformed by sub-zero cooling to 4K and rolling at 77K to increase the volume fraction of thin plate martensite. Orientations of austenite and martensite were determined by scanning electron microscopy (SEM) equipped with electron backscattered diffraction pattern detector. A micro-sized square pillar (20 x 20 x 40 mu;m) was fabricated from thin plate martensite by using focused ion beam without any tapering. Channeling contrast image in scanning ion microscope showed that the fabricated pillar was composed of fully nano-twinned structure incorporated with few different martensite variants. In micro-compression test of the fabricated pillar, yield drop from 1200 MPa to 750 MPa was observed which suggested that the catastrophic twin deformation and following deformation occurred inside the deformation twin. Trace analysis conducted using SEM images of each side of the square pillar showed that the deformation twin and transformation twin has (-12-1)[111] and (-211)[111], respectively.

Experimental determination of characteristic length scale of dislocation plasticity Joshua D Gale and Ajit Achuthan*, Department of Mechanical & Aeronautical Eng. Department Clarkson University Recrystallization of the grain structure of metals into nano-sized grains by using mechanical means, has received wide attention in the last two decades. It is well known that materials with a fine-grain crystal structure have favorable properties compared to the same materials with course-grained crystal structure. Surface Mechanical Attrition Treatment (SMAT), a technique developed in the early part of this decade, has been successfully used to recrystallize the surface grains of metals into nanocrystals of the order of 10 to 100 nanometers from their original grain sizes in the order to 10 to 30 microns. Resulting enhancement in surface properties has quite interesting applications, varying from materials with improved fatigue resistance to medical devices. In this study, our focus is to experimentally determine the characteristic length scale associated with dislocation plasticity using SMAT copper samples. The load displacement behavior under nano-indentation loading is obtained on SMAT samples of different grain sizes. By correlating the characteristic features of load displacement behavior with the grain sizes the characteristic length scale of dislocation plasticity is determined. A crystal plasticity based model has been developed to verify the experimental observations.

There are inherent challenges in the high-resolution microstructural analysis of nanocrystalline materials. Transmission electron microscopy can provide information about the grain size and morphology, but overlapping of grains in the thin foil prevents direct examination of the boundaries between the grains. Atom probe tomography can overcome many of these issues, providing 3D information about the distribution of atoms within the structure, but the data requires careful interpretation to extract accurate and useful information. Importantly, it is possible to obtain detailed information about the segregation of solute atoms to the grain boundaries, phase boundaries and triple junctions in nanocrystalline alloys. This information is of particular interest for nanocrystalline samples due to the high number of grain boundaries. Even small amounts of segregation to boundaries are likely to influence the concentration of the grains themselves, meaning that the alloying elements may behave differently to how they would in the corresponding larger-grain-size alloy, information that should be used to inform alloy design. However, grain boundaries captured in atom probe data can have complex shapes that are curved at the scale of the dataset, making analysis of the composition via simple line profiles inaccurate. The interfacial excess can also vary considerably over the boundaries. Different approaches for the accurate determination of average interfacial excess and excess mapping from boundaries and triple junctions and will be presented and compared. In addition, new methods for extracting crystallographic information from atom probe data now make possible the measurement or the orientation of individual grains in atom probe data, allowing measurements of the ‘nano-texture&’ of the samples via 3D orientation mapping. It is even possible to compare the disorientation of grain boundaries to the extent of segregation observed. We will present the application these new analysis methods to the study of various nanocrystalline materials, including low-solute aluminium thin films and a super duplex stainless steel prepared by high pressure torsion.

Dealloying is a simple process used to fabricate various nanoporous metals which have numerous potential applications as functional materials. Among all the nanoporous metals, nanoporous gold (np-Au) has the most potential applications including sensors, actuators, super-capacitors, catalytic substrates, enhanced Raman scattering, and anode substrate for Li-ion battery. The dealloying process for most nanoporous metals involves use of acid to selectively dissolve the less noble element from a binary alloy, ideally a solid solution, under free corrosion or applied voltage. For np-Au, one of the simplest and most used fabrication method is by free-corrosion dealloying of Ag-Au alloys in concentrated nitric acid, removing the silver atoms while gold atoms self-rearranged into the porous structure, with typical pore size of 5-20 nm. Here, we systematically investigate the dealloying rate of Ag-xAu alloy for a range of alloy composition (x= 20-40 at.%) and nitric acid concentration (- 7.3-14.9 M) using in situ transmission x-ray microscopy with synchrotron source. With a customized acid feeding sample environment cell, high-resolution in situ x-ray projections and ex situ tomographic reconstructions allow imaging of the dealloying front propagation during dealloying. The dealloying front velocity is constant with time, and depends exponentially on the alloy Ag/Au atomic ratio and the acid molar concentration. The leanest alloy, Ag-20% Au, shows a large macroscopic shrinkage in sample diameter (~ 38%) after dealloying, which leads to crack nucleation and growth observed by in situ high resolution x-ray imaging in real time during dealloying. Finite element modeling is used to estimate dealloying-induced stresses and strains, and shed light on the cracks created by the diameter shrinkage.

Nanostructured materials often exhibit nanoscale anisotropy and/or heterogeneity, which are expected to influence strength, ductility, damage tolerance, fracture toughness, and fatigue resistance. Hence advanced nanomechanical characterization, which enables both electron microscopy (such as SEM and TEM) and/or atomic force microscopy (AFM) imaging of sample&’s micro/nanostructure evolution as well as precise measurement of the mechanical properties at nanoscale, has recently emerged as a powerful tool to interrogate nanostructured metallic materials and biological materials, providing guidelines to design and optimize material micro/nanostructures for improved properties and functions. For example, nanotwinned copper (NT Cu) exhibits strong plasticity anisotropy and nonuniform plastic deformation within each grain; our recent results further suggested that NT Cu with increased twin density has enhanced damage tolerance, improved fracture toughness and increased fatigue life, while their underlying mechanisms were studied by visualization of structural evolution in response to monotonic and cyclic loading. The size dependent trend of these nanoscale characteristics were also quantified. As an interesting comparison, we have also investigated the nanoscale heterogeneity in the hierarchical structure of bone, and indentified the heterogeneity-promoted energy dissipation and related characteristic length scale. Thus the holistic study of nanostructured material systems at multi-scale could provide a potential path for the next-generation advanced materials processing and design.

Cross-sectional transmission electron microscopy (TEM) provides essential atomic scale imaging and probing for unmatched characterization of nanoscale structures, interfaces, alloy phases, subsurface damage, and more. The most common way to prepare site-specific TEM cross sections is via focused-ion-beam (FIB) lift outs. FIB lift outs of nanoscale features can be extremely challenging, because it is typically difficult to properly position the FIB slice with a specific feature or row of features, particularly if the < 100 nm features cannot be resolved in the FIB/SEM. Here, this problem is addressed by making ~3 µm x 12 µm grid patterns of features (nanoindentations). The grid pattern is rotated on the surface, while keeping the desired crystallographic orientation normal to the FIB slice. By rotating the array at the optimal angle, which is a function of feature spacing, the FIB slice is guaranteed to contain the center of at least one of the features in the array. Furthermore, adjacent features in the TEM specimen contain snapshots of different sections of similar features. These sections are combined using imaging software to create 3-D tomographic images. This method for FIB lift outs can be extended for TEM analyses of any nanoscale feature that can be oriented in an array pattern. In this study, the above method for high-throughput FIB lift outs and 3-D tomography is applied to elucidate the deformation mechanisms of InAs thin films (< 200 nm) on GaAs. In nanolaminate composites, the strength can be several times greater than that of the bulk constituents, due primarily to two mechanisms: (1) increased stresses necessary for nucleation and migration of dislocation cores with decreasing layer thickness, and (2) for incoherent nanolaminates, by interfacial strain and blocking of mobile dislocations by the interface between adjacent layers. These two mechanisms are coupled in incoherent nanolaminates. This research seeks to decouple the strengthening mechanisms attributed to layer thickness from those attributed to an incoherent interface, by studying the effect of a single interface on the deformation mechanisms for a single layer of InAs on GaAs, instead of multiple layers. The InAs/GaAs system is a zincblende semiconductor system with a lattice mismatch of ~7%, and has applications in optoelectronics such as lasers and infrared sensors. The deformation mechanisms of InAs and GaAs have been studied in depth, but not those of layered InAs/GaAs. Herein, nanoindentation is used to create repeatable indents ranging from depths of 10-100 nm on InAs (100) films of varying thickness on GaAs (100). The dislocation networks beneath the nanoindentations are analyzed using cross-sectional TEM, and the influence of an incoherent interface on the impinging dislocation network is directly observed. TEM images of adjacent nanoindentations are then used to create 3-D reconstructions of a representative nanoindentation plastic zone from similar indents.

Monolithic high-Z nanoporous materials with ultralow-densities (less than about 10% of the full density) are attractive for a number of energy-related applications. However, their synthesis remains a challenge. Here, we use carbon-nanotube-based aerogels as scaffolds for the synthesis of novel metal/carbon composites with densities of 100 mg/cc and below. Our main focus is on the uniformity and mechanical properties of resultant nanofoams and on the effect of the metallic component on gelation chemistry in the limit of high metal loadings. This work was performed under the auspices of the U.S. DOE by LLNL under Contract DE-AC52-07NA27344.

Surfaces play important roles in deformation of crystals at the nanometer scale. Plasticity of nano-crystals is dominated by dislocation nucleation from surfaces [Weinberger and Cai, J. Mater. Chem. 22, 3277 (2012)]. Atomistic simulations have shown that surface stress can even trigger the phase transformation and plastic deformation in metal nanowires when the diameter was reduced to below 5 nm [Diao et al., Phys. Rev. B 70, 075413 (2004)]. Here we present an experimental observation of surface induced plastic deformation in gold nano-crystals, more specifically, in nano-ligaments (~ 10 nm in diameter) of a millimeter-sized nanoporous gold prepared by dealloying. We found that nanoporous gold can undergo spontaneous contraction at room temperature and in the absence of external stresses, in an electrochemical environment. More interestingly, we found that the shrinkage is very sensitive to the applied electrochemical potential. And a correlation between the surface diffusivity and shrinkage was observed: sample volume contraction is larger and faster while the surface diffusion (coarsening) is faster. Shrinkage can be completely stopped by suppressing the surface diffusion, by covering the gold surface with oxygen or by addition of a small amount of Pt (in nano-ligaments). Since the surface diffusion itself could not lead to the contraction of nano-ligaments and nanoporous gold, the results presented here imply that both the surface diffusivity and the surface dislocation nucleation are governed by the same surface structures or properties, as will be discussed in the talk. This observation may also have some relation to a recent report that the strength and ductility of nanoporous gold can be tuned by applying an electrochemical potential [Jin and Weissmuller, Science 332, 1179 (2011)].

When materials are deformed plastically via dislocations, a general finding is that samples with smaller dimensions exhibit higher strengths but with very limited amount of plasticity in tension. Here we report that one-dimensional coherent nanostructures with tilted internal twins exhibit anisotropic size-effect: their strengths show no apparent change if only their thicknesses reduce, but become stronger as the sample sizes are reduced proportionally. Large-scale molecular dynamics simulations show that such NWs deform primarily through twin migration mediated by partial dislocations in one active slip system, and a large amount of plasticity could be achieved in such nanowires via twin migration. The unique structure shown here is suitable to explore strengthening mechanisms in metals when plasticity is controlled by a single dislocation slip system. This study also suggests a novel approach to modulate strength and ductility in one-dimensional coherent nanostructures.

Ultra-fine-grained (UFG) metals offer significant advantages in strength and ductility over metals with conventional grain sizes [1, 2]. The typical method for producing UFG metals is by severe plastic deformation (SPD) using either accumulative roll bonding (ARB) or equal channel angular pressing (ECAP) techniques. The ECAP technique was pioneered by Segal in the early 1980s, developed by Valiev and coworkers in the 1990s, and is now the most prominent SPD technique. Indentation testing has expanded similarly in the last few decades. Since the development of nanoindentation systems and the Oliver and Pharr analysis in the early 1990s, nanoindentation has become the premier method for nanomechanical testing. Recent developments have now expanded instrumented indentation&’s capabilities into reliable assessment of strain rate- [3, 4] and temperature-dependent [5, 6] behavior. The hardness and strain rate sensitivity of ECAP UFG Al was examined as a function of temperature using in situ strain-rate-jump indentation in the SEM at temperatures up to 250°C. Rapid stabilization of the water-cooled indentation rig at elevated temperatures allowed testing of the UFG Al before significant grain growth could occur. A transition from plastic to superplastic deformation processes was observed in situ at lower temperatures than for coarse grained material. This was observed to be linked to the activation of grain boundary and lattice diffusion. Strain rate jump tests were effectively demonstrated to measure strain rate sensitivity as a function of temperature below the onset of superplasticity. References [1] R.Z. Valiev, I.V. Alexandrov, Y.T. Zhu, T.C. Lowe, Journal of Materials Research, 17 (2002) 5-8. [2] H.W. Höppel, J. May, M. Göken, Advanced Engineering Materials, 6 (2004) 781-784. [3] J. Alkorta, J.M. Martínez-Esnaola, J. Gil Sevillano, Acta Materialia, 56 (2008) 884-893. [4] V. Maier, K. Durst, J. Mueller, B. Backes, H.W. Höppel, M. Göken, Journal of Materials Research, 26 (2011) 1421-1430. [5] S. Korte, R.J. Stearn, J.M. Wheeler, W.J. Clegg, Journal of Materials Research, 27 (2011) 167-176. [6] J.M. Wheeler, P. Brodard, J. Michler, Philos. Mag., (2012) in press.

We propose a new mechanism to reduce the hysteresis and widen the temperature range of superelasticity of shape memory alloys (SMAs) through altering the transformation pathway and domain structure via impurity doping. The basic idea is to use local lattice distortion created by point defects to prevent long-range ordered polytwin domain structures and "freeze" the ferroelastic system into a microstructural state of randomly distributed nanodomains of individual variants. By assuming that anti-site defects alter the thermodynamic stability of austenite and create local lattice distortions, we show by computer simulations using the phase field method that such a transformation pathway is possible and the unique microstructural state generated does have a nearly zero hysteresis, superelastic behavior over a wide temperature range, and possible Invar effect. We then confirm experimentally that extra impurity doping of a conventional SMA does convert the normal long-range ordered poly-twin domain structure into a nanodomain structure of individual variants, change the large hysteresis into a slim one and widen significantly the temperature range of superelasticity. Our predictions are not limited to just point defects. Any extended defects of sufficient quantity could serve the same purpose.

A unique type of interfaces, twin interface, is identified in a variety of sputter-deposited fcc metals, such 330 stainless steel, Cu and Ag films. Both coherent {111} and incoherent {112} twin boundaries are observed in nanotwinned metal films. Average twin spacing, on the order of ~ 10 nm, can be achieved by varying deposition conditions. Twin interfaces are effective barriers to the transmission of dislocations, and thus lead to high strength in monolithic metals. Meanwhile twin interfaces are effective sources and sinks for dislocations during deformation. Twin interface may also offer a solution to the challenge of achieving high strength and high electrical conductivity and enhanced thermal stability as demonstrated in epitaxial nanotwinned films. In situ nanoindentation studies reveal that certain twin interfaces are mobile during deformation due to rapid movement of Shockley partials, and consequently detwinning occurs.

As the characteristic dimension in metallic thin films and micro-pillars decreases from 10 micron down to approximately 100 nm plasticity increasing flow stress. This behavior has been attributed to dislocation motion/multiplication mechanisms, primarily through the so-called “source shut-down” mechanism. In contrast, face-centered-cubic (FCC) metal nanowires (NWs) with diameters that are below 100 nm, are dislocation free prior to deformation resulting in plastic deformation mechanisms that are controlled by dislocation nucleation. Although there have been many molecular dynamics simulations on the plastic deformation of metal nanowires, most of the predictions have not been tested experimentally due to the difficulty of conducting in-situ transmission electron microscopy (TEM) tensile measurements which can directly identify plastic deformation mechanisms. The nucleation-controlled nature of plastic deformation of single crystal metallic NWs typically leads to high strength accompanied by limited strain hardening and ductility. Here we present the results of a combined study of in-situ TEM tensile testing and molecular dynamics (MD) simulations conducted on penta-twinned Ag NWs. We demonstrate that the coherent internal twin boundaries present in the NWs lead to unique size-dependent strain hardening that achieves both high strength and ductility. We find that thin Ag NWs deform via the surface nucleation of stacking fault decahedrons (SFDs) in plastic zones distributed along the NW. The internal twin boundaries which run along the axis of the penta-twinned NWs act as barriers for dislocation propagation leading to the formation of SFD chains that locally harden the NW and promote subsequent nucleation of SFD at other locations. In contrast, thick NWs exhibit lower flow stress accompanied by a reduction in the number of distributed plastic zones due to the onset of necking, which can be understood by the formation of more irregular and complex dislocation structures observed in MD simulations.

Using Al-TiN as a model system, we explore the mechanisms that enable maximum strength in the composite as well as co-deformability of the ceramic and metal nanolayers. Compression tests revealed that at extremely small layer thicknesses (< 5 nm), the nano-scale multilayers exhibit unusually high flow strengths (~ 4.5 GPa maximum) significantly higher than the hardness divided by a factor of 3, high compressive deformability (5-7% plastic strain) and high strain hardening rates in the Al layers, which are on the order of 16-35 GPa (~E/4 - E/2). Indentation fracture experiments are used to study crack nucleation and propagation in the multilayers. Density functional theory is used to compute the interface shear properties, and dislocation theory models are developed for the work hardening and maximum flow strength of the composite. Nanoscale metal-ceramic multilayers allow the design of nano-composite materials with appreciable deformability and both flow strength and work hardening rates approaching the theoretical limits. This research is sponsored by DOE, Office of Science, Office of Basic Energy Sciences.

The high strength of nanocrystalline alloys suggests that these materials are promising for wear-sensitive applications, where engineering components must be able to survive the repetitive application of high contact stresses. However, nanocrystalline metals also demonstrate novel grain boundary-dominated deformation mechanisms and a tendency for mechanically-driven structural evolution. This talk addresses the tribological response of nanocrystalline Ni-W alloys across a range of grain sizes spanning the Hall-Petch breakdown, with a focus on understanding how the extreme conditions produced during wear can lead to dynamic nanostructures and properties. Multiple wear testing methodologies including sliding and abrasive wear are studied in electrodeposited coatings with grain sizes ranging from 3 to 100 nm, to understand the influence of initial microstructure, mechanical properties, and testing conditions on wear resistance. The experiments reveal evidence of grain growth and grain boundary relaxation during repetitive loading of the finest nanocrystalline specimens, which improves the wear properties of the alloys. These results are analyzed in the context of other reports of mechanically-driven structural change in nanocrystalline metals, to provide a roadmap for predicting such evolution and understanding its effect on subsequent mechanical behavior.

Existing metallic muscles made of nanoporous metals with high surface-area-to-volume ratios can exert work due to changes in their interface electronic charge densities. However, they suffer from serious drawbacks such as the usage of aqueous electrolyte that is needed to inject electronic charge at the nanoporous metal/electrolyte interface. An aqueous electrolyte prohibits metallic muscles to operate in dry environments and hampers a high actuation rate due to the low ionic conductivity of the electrolytes. In addition, redox reactions involved in actuation severely coarsen the ligaments of nanoporous metals, leading to a substantial loss in performance of the actuator. Here an innovative electrolyte-free approach to put metallic muscles to work via a metal/organic semiconductor interface is presented. We exploit the concept of a space-charge built up at that interface to design a single-component electrolyte-free nanoporous metal/polymer bulk heterojunction actuator featuring strain rates that are three orders of magnitude higher than that of three-component nanoporous metal/electrolyte hybrid actuators.

Deformation of nanostructured materials has been extensively studied in recent years through both experimental investigations and computer simulations, and quite a few new understandings are established regarding the deformation mechanism. However, as a group of emerging materials, the fatigue deformation of nanostructured materials has remained a poorly-explored territory. Recently, we have conducted series of studies on the fatigue deformation of a nanocrystalline NiFe alloy. We use advanced characterization techniques, such as electron transmission microscopy and in-situ high-energy X-ray diffraction, to study the fatigue crack propagation behavior. It is our observation that significant grain growth is induced by the local plastic deformation along the crack path. The in-situ diffraction technique further demonstrates a unique deformation feature near the fatigue crack tip that can be correlated to the local stress state. Acknowledgements: SC, SYL, LL, and PKL very much appreciate the financial support of the US National Science Foundation, under DMR-0231320, CMMI-0900271, CMMI-1100080, and DMR-0909037, with Drs. C. Huber, C. V. Cooper, D. Finotello, A. Ardell, and E. Taleff as contract monitors.

Nanocrystalline metals are commonly typified as having high strength and poor ductility. Tensile experiments to characterize the mechanical behavior of electrodeposited nanocrystalline Ni-Fe foils have been performed and at first glance Ni-Fe follows this trend. Peak strength exceeds 1.5 GPa and elongation to failure is less than ten percent. However, after detailed examination of the fracture surface, it is noted that the material exhibits considerable reduction in cross sectional area. This observation suggests that nanocrystalline metals can accommodate substantial amounts of plasticity contrary to the initial perception. The tensile response of nanocrystalline Ni-Fe and how the traditional metrics for ductility may lead to assessments of limited plasticity will be discussed.

Nanocomposite 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 elastic-plastic regimes in direct relation with size characteristics. The nature of the elastic-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, theta; = dσ/d ε, becomes smaller than one third of the macroscopic elastic modulus [Acta Mat., 57 (2009) 3157-3169]. This criterion appears to be valid to determine the transition from elastic-microplastic to macroplastic regimes in several nanocrystalline materials in contradiction to the traditional 0.2%-strain offset criterion. 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 [Adv. Eng. Mat., in press (2012)].

The primary objective of this work was to determine the influence of 1 to 4 at% Zr additions on the thermal stability of mechanically alloyed nanocrystalline Fe-Cr alloys containing 10 at% Cr. Grain sizes based on XRD, along with microhardness changes, are reported for isochronal annealing treatments up to 1000 °C. Microstructure investigations were done using optical microscopy, channeling contrast FIB imaging, and TEM. Grain size stabilization in the nanoscale range was maintained up to 900 °C by adding 2 at% Zr. Kinetic pinning by nanoscale intermetallic particles was identified as one source of high temperature grain size stabilization. Intermetallic particles also contribute to strengthening in addition to the Hall-Petch effect. The analysis of microhardness, XRD data, and measured values from the TEM image for Fe-10 at% Cr with 2 at% Zr suggested that both thermodynamic and kinetic mechanisms would contribute to grain size stabilization. Nanoscale grain size stabilization of the Fe-Cr alloys is being extended to the Fe-Cr-Ni alloys in the current research.

The recently introduced shear-compression-specimen geometry has been miniaturized and applied to small-sized nanocrystalline samples, here Pd-Au alloys prepared by inert gas condensation. Varying strain rate, temperature, and pressure enables to deduce thermal acitvation parameters as well as the strain rate dependent onset of micro- and macroyielding. From TEM and in-situ diffraction and deformation, we are able to identify and assign relevant deformation mechanisms to the distinct regimes of stress strain curves of nanocrystalline metals. We discuss the obtained results in comparison with the deformation behavior of conventional fcc metals and metallic glasses.

High-pressure torsion (HPT) straining is one of the shape-invariant severe plastic deformation methods and is capabile of extensively straining brittle materials. The present research focuses on the formation of nanostructures by HPT-straining in L1₂ ordered Ni₃Al and its impact on mechanical properties. Discs (10 mm diameter) of polycrystalline Ni₃Al were subjected to HPT-straining under a compressive stress of 5 GPa at room temperature. Microstructural characterization was done using X-ray diffraction (Cu-Kα), optical microscopy, scanning electron microscopy and transmission electron microscopy. Tensile samples having a gauge section of 4 mm in length and 1 mm in width were cut from the discs. Tensile tests were done on a screw-driven tensile machine with a strain rate of about 10^-3 s^-1 at room temperature. Microstructures after HPT-straining was found to be very heterogeneous. Transmission electron microscope (TEM) observations revealed that HPT-strained samples were composed of disordered, equiaxed grains of about 50 nm and plate-like subgrains, still retaining L1₂ order, of several 100 nm in width and several micrometers in length. The plate-like subgrains were separated by thin (~20 nm) twin plates on {111} planes. Conventional and high-resolution TEM observations revealed that these twins were disordered. Increasing the number of turns (N) in HPT led to an increase in the amount of the equiaxed nanograins at the expense of the plate-like subgrain. Formation of such heterogeneous nano-structures had a significant impact on the room temperature mechanical properties. A sample before HPT exhibited no plasticity due to intergranular fracture. After HPT-straining of N = 1, the sample exhibited pronounced work-hardening; ultimate tensile strength over 2 GPa with a total elongation of 5 % were attained. Apparently the heterogeneous nanostructure is responsible for the dramatic improvement of the mechanical properties.

Numerous studies have been devoted to characterizing spherical nanoparticles (nanospheres) because of their emerging applications, e.g., in nanocomposite and multifunctional coatings. Currently, the only way of characterizing the mechanical properties of individual nanoparticle is nanoparticle indentation, for which, however, there is no quantitative data analysis methodology available in the literature. In this study, a first-ever closed-form analytical model, near perfectly matching finite element analysis, is provided based on large-deformation contact mechanics to determine the modulus, hardness, yield strength, and fracture properties. In the model, two contacts in series are considered: (1) the indenter/nanosphere contact, and (2) the nanosphere/substrate contact. The methodology is used to analyze the experimental results of gold nanosphere nanoindentation. We believe that this first-ever methodology can not only be used to characterize the mechanical properties of individual nanoparticles but also impact the understanding of size dependence of mechanical properties at small scales.

Low-carbon, lath martensite is an important microstructural component in many advanced high strength sheet steels; due to the hierarchical microstructure composed of grains, packets, blocks and laths as well as a high dislocation density, it&’s mechanical response is complex but has a direct bearing on the properties and formability of the sheet product. In this study, the deformation behavior of martensite was evaluated in a fully martensitic and two dual-phase (DP) sheet steels consisting of ferrite and martensite phases using micropillar compression and nanoindentation methods. Pillar diameters in the fully martensitic steel ranged from 5 mu;m to ~300-400 nm. In DP steels however, the prior austenite grain size limits the martensite pillar size (maximum 2 mu;m). Typical lath size of martensite (in DP steels) is in the range of 50-150 nm and is therefore less than the pillar diameter in consideration whereas block and sub-block sizes are usually within the range of the pillar dimensions, and packets can be in the 1-20 micron range. As a consequence, a range of response may be expected; for example, some pillars exhibit no work-hardening whereas others do; whereas micropillars enable a comparison of martensite response in a family of dual phase steels, nanoindentation measurement shows a distribution in hardness, that precludes meaningful comparisons and interpretation. Results from these studies will be presented and implications will be discussed.

Instrumented indentation technique was applied to investigate an indentation-induced plasticity initiation behavior associated with a variety of defects such as grain boundary and in-solution atoms in metallic materials. The plasticity initiation behavior is detected in a pop-in phenomenon on a loading curve of indentation technique. The critical load Pc of the pop-in phenomenon is lower in the case of probing on the grain boundary than that in a grain interior, indicating that the grain boundary acts as an effective dislocation source with a lower critical shear stress for plasticity initiation. In-solution elements such as carbon and silicon in Fe alloys have a significant effect for pushing up Pc values. A presumable mechanism of the effect of the elements is discussed based on a dislocation source activation model. Maximum shear stress tau;max underneath the indenter is calculated from Pc through Hertz contact theory, and shear modulus G is given by conversion from Young&’s modulus measured in an unloading curve, then we found the tau;max is directly proportional to the G with a coefficient close to 1/2π that is consistent with a classic model of an ideal strength on a slip plane. TEM in-situ deformation technique was applied to Fe alloys for understanding the relationship between an individual dislocation motion and mechanical response.

Multilayered metals exhibit increasing strength with decreasing layer thickness, because layer interfaces inhibit dislocation motion. However, mechanical differences can be subtle when comparing, say, a multilayer comprising 5nm layers with one comprising 10nm layers. In this work, we apply a novel characterization technique to determine the hardness and elastic modulus of (111) and (100) Cu/Ni multilayers having individual layer thicknesses of 1nm, 5nm, 10nm, 50nm, and 100nm. This new characterization technique is an advanced form of instrumented indentation that allows testing at a rate of 1 indentation per second. On each sample, we perform 1600 indentations at 1600 different sites. We report the hardness and elastic modulus for each sample. Also, for each pair-wise comparison of samples, we report the number of indentations required to discern a significant difference in hardness at 95% confidence.

In this work we address size dependence of plasticity in multiasperity metal contacts. Asperities are first flattened and subsequently sheared by a rigid platen. Plastic deformation is studied within the framework of discrete dislocation plasticity with plastic deformation arising from the collective motion of discrete dislocations and results are compared with crystal plasticity simulations. Besides the expected size dependence of individual micron-scale asperities, an additional contribution to the mean contact pressure is found to stem from the interaction between neighboring contacts. The computed local pressure distribution for closely spaced contacts is significantly higher than what is predicted by classical plasticity. During shearing, attention focuses on the competition between plastic deformation, dominant in micron scale contacts, and de-adhesion, which is controlling nanoscale contacts.

We present results from molecular dynamics simulations of nano-crystalline Tantalum thin films that illuminate the variety of atomic-scale mechanisms of incipient plasticity. These thin films geometries are loaded in uniaxial tension at various rates, and display phenomena including emission of perfect 1/2<111>-type dislocations, formation of twin boundaries, and rotation and migration of grain boundaries. Features of these defect mechanisms are characterized using various metrics and tools including common neighbor analysis, slip vector, the dislocation extraction algorithm (DXA), and continuum stress and deformation fields evaluated using atom-to-continuum (AtC) expressions. Detailed analysis of nano-scale deformation using these tools enhances our understanding of deformation mechanisms in Tantalum, and provides information vital towards the construction of higher length scale deformation models. Sandia is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed-Martin Corporation, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Thermodynamic stabilization is achieved when segregation of solute atoms to grain boundaries produces a metastable equilibrium state with respect to grain growth. This has been proposed as a mechanism to stabilize a nanocrystalline grain size at high temperatures. Alternate stabilization mechanisms are based on retarding grain boundary mobility by grain boundary pinning (Zener pinning, etc.). The work presented here was motivated by the need to develop a predictive model for thermodynamic stabilization, applicable to strongly segregating solutes, that uses available input data. This will serve as a benchmark for selecting solutes and assessing the possible contribution of thermodynamic stabilization in the development of high-temperature nanocrystalline alloys. Following a regular solution model recently developed by Trelewicz and Schuh [1], the grain boundary region is distinguished from the grain interior region by a transitional interface. The elastic part of the enthalpy due to the solute atomic size misfit is not taken into account in [1] and we include this using the Wynblatt and Ku approximation [2] for linear superposition of the elastic and chemical contributions to the mixing enthalpy. It is important to recognize that the elastic term always contributes to segregation whereas the chemical term may contribute or detract. The total Gibbs free energy of mixing is minimized with respect to simultaneous variations in the grain-boundary content and the solute concentrations within the grain boundary and grain interior regions using the Lagrange multiplier method. The model predictions are presented as a parametric study of the key input variables. Applications to the temperature dependence of the grain size and solute segregation will be given for selected systems where experimental results suggest that thermodynamic stabilization can contribute. 1. J. R. Trelewicz and C. A. Schuh, Phys. Rev. B 79, 094112 (2009) 2. P. Wynnblatt and D. Chatain, Met. and Matl. Trans. A, vol. 37A, 2595 (2006)

The mechanical properties of materials are dominated by their micro/nano-structure, including grain boundaries, and precipitates. Therefore, to develop materials with superior mechanical properties, it is important to examine the mechanical properties and deformation behavior of each microstructural constituent. However, the measurement of mechanical properties and observation of the deformation behavior of each micro/nano-structural constituent are difficult because the size of these constituents is on the order of less than microns. We have developed a testing machine that enables the measurement of mechanical properties and in-situ observation of deformation behavior of microsized materials. This testing machine is equipped with a white-light interferometer that can measure with a resolution of 0.1 nm the surface profile of a microsized specimen during its deformation. We have measured the mechanical properties and observed the deformation behavior of microsized specimens prepared from microstructural constituents of multiphase alloys, including steels, non-ferrous alloys, and intermetallics. We present our micromechanical testing machine, along with some examples of the mechanical characterization and observation of the deformation behavior of microsized specimens prepared from microstructures of several materials.

The mechanical and electronic properties of materials have been significantly advanced by reducing the characteristic sample size or grain size to nanoscale regime (typically <100 nm). However, the enhanced creep of nanostructured materials limits their utilities at mediate and high temperature of service. Recently, the mechanical strength and ductility of copper have been greatly optimized by inserting layered growth nanotwins inside of grains[1, 2]. Two competing deformation modes governed by twin-boundary spacing (TBS) are discovered to be responsible for the strengthening and softening of such heterogenous structured metals[3]. Here we use atomistic modeling to highlight this strategy as an effective pathway to slow down the creep deformation of bulk nanostructured metals (BNM). The steady-state creep rate is found to be a function of TBS at constant uniaxial stress. The deformation mechanism transits from inclined dislocation nucleation to parallel dislocation nucleation with decreasing TBS, which leads to a slowest creep rate at a critical TBS of a given grain size. We understand the creep mechanisms by varying the applied stress, temperature, grain size, and TBS. The derived activation parameters, e.g., activation volume, activation energy, and power-law stress exponent provide basic understanding on the deformation of nanotwinned BNM. We hope such a strategy will shed light on more advanced technics to broaden the application of BNM. [1] L. Lu, Y.F. Shen, X.H. Chen, L.H. Qian, K. Lu, Science 304 (2004) 422. [2] L. Lu, X. Chen, X. Huang, K. Lu, Science 323 (2009) 607. [3] X.Y. Li, Y.J. Wei, L. Lu, K. Lu, H.J. Gao, Nature 464 (2010) 877.

In coarse-grained polycrystalline metals, plastic deformation occurs via glide of lattice dislocations and their interaction with grain boundaries (GBs). In order to ensure the continuity in strain at GBs, grains exercise a mutual constraint on one another during deformation. This mutual constraining interaction among grains requires multiple slip to operate, having a large influence on the mechanical properties. Many experiments have shown that flow stresses for thin films of coarse-grained polycrystalline metals with grain size d, decrease with decreasing film thickness (t) when the number of grains contained along the thickness direction becomes smaller than a critical value (t/d*) [1]. This “thinner is weaker” trend indicates that the range of the mutual constraining interaction is finite. The critical value t/d* has been reported to increase with decreasing grain size for pure copper thin films [1]. Since the grain size in these experiments was large enough, the dominant mechanism of the plastic deformation is considered to be dislocation slip motion. Then, a question arises as to how the mechanical properties change depending on the specimen size when the grain size is so small that the deformation mechanism based on dislocation slip is no longer operative. In the present study, we investigated compression deformation behavior in electrodeposited nanocrystalline copper of three different average grain sizes ranging from 34 to 360 nm. We examined not only the grain-size dependence but also the specimen-size dependence of the deformation behavior using micrometer-size pillars to determine the critical value t/d* in nanocrystalline metals. Nanocrystalline copper films were prepared by pulsed electrodeposition. Square column specimens 1minus;20 mu;m on a side with an aspect ratio of 1:3 were machined by the focused ion beam method. Uniaxial compression tests were conducted with a flat punch indenter tip in a micro compression tester. For nanocrystalline copper with d = 360 and 100 nm, the yield stress decreases with the decrease in the micropillar size with the critical values of specimenminus;grain size ratio of 35 and 85, respectively. On the other hand, the yield stress for d = 34 nm is independent of specimen size. TEM observation of the deformation microstructure indicated that the dominant plastic deformation mechanism shifts from the dislocation slip to the grain boundary diffusional creep (Coble creep) with decreasing grain size from 100 to 34 nm. Plotting the critical value of specimenminus;grain size ratio at which the yield stress decreases as a function of grain size reveals a power law scaling for nanocrystalline and coarse grained copper. The concept of mutual interaction among grains established in coarse grained metals is applicable to nanocrystalline metals as far as the dominant deformation mechanism is the dislocation slip. [1] S. Miyazaki, K. Shibata and H. Fujita. Acta Metall., 27 (1979) p.855.

The stable mechanical property of metal thin film under repeating deformation is a key issue for reliable metal electrode. Mechanical degradation with crack evolution causes the increase of electrical resistance and electrical degradation is critical for reliability of flexible devices. For applying of metal electrode on flexible electronics, the precise understanding of mechanical behavior of metal electrode during repeating stress and the design of stable electrode without significant increase of electrical resistance are important. In this study, the fatigue stability of metal thin film on flexible substrate was investigated under repeating tensile and compressive bending strain. Various over-layers were applied to suppress the crack evolution for stability of metal electrode. The 1 um thick Cu film was deposited on 125 um thick polyimide by thermal evaporation at high vacuum condition. The metal film on flexible substrate was cut by 4 mm width and 60 mm length and the ends of the specimens were fixed with metal grips for in-situ electrical measurement. While the upper side was fixed, the lower side repeated sliding motion which induced the damage zone. Cyclic tensile and compressive bending was performed 500,000 cycles in 1.1 % strain with 10 mm sliding distance at 5 Hz frequency. Fatigue failure was defined by in-situ monitoring of the electrical resistance and observation of microstructure with respect to crack initiation and propagation. To study the effect of overlayer on the fatigue behavior, various materials with different mechanical properties (Al, Ti, graphene, etc.) has been deposited. As for Al, a 10 nm film is deposited on Cu film. Cracks were nucleated at the early deformation cycles and propagated with repeating strain. The fatigue damages such as intrusion and extrusion were formed by dislocation motion and the density of cracks increased with the increasing of cycles. These cracks affected on the increase of the electrical resistance of metal thin film. Initially, the electrical resistance change was very small but after approximately 1,000 cycles, the resistance increased abruptly. As the number of cracks was increased, the resistance of metal electrode was more increased over 200 % of initial resistance. However for Al over-layered thin film, the resistance little increased and only 18 % of resistance was increased for the compressive strain mode after 500,000 cycles of deformation. The crack or extrusion was little observed at the surface of Al over-layered thin film after 500,000 cycles and this shows the surface modification using thin Al layer improved the fatigue resistance by preventing damage evolution at the surface of Cu film. Because the crack nucleation is closely related to surface, thin Al over-layer could suppress the crack evolution of the surface of Cu thin film. In addition the effects of Ti and graphene over-layer will be discussed.

As materials and structures are engineered to smaller and smaller dimensions, observations of size-dependent deformation are also becoming more frequent. One of the key issues in materials engineering is the characterization, control and optimization of mechanical properties, which requires a detailed understanding of the deformation processes. Plastic deformation in the nanocrystalline grain size regime is thought to change from bulk to interface mechanisms. Mechanisms that have been suggested include the nucleation and motion of partial dislocations, grain boundary sliding or grain rotation and grain boundary motion. Experimentally, this is reflected by increased strain rate sensitivity at low temperatures and small activation volumes compared to coarse-grained materials. Introducing miscible solutes offers an additional degree of freedom and can be expected to lead to modified mechanical properties compared to the pure metals. Alloying of nanocrystalline metals may stabilize the microstructure if the solute segregates to the boundary but also affect the stacking-fault energies. In this study, nanocrystalline Pd und PdAu thin films were investigated using strain rate sensitive instrumented indentation at different temperatures ranging from 10°C to 90°C. In this temperature range, no microstructural changes were observed. In general, the hardness was observed to increase with increasing alloying content for all temperatures. The temperature had little effect on the strain rate sensitivity for the alloying contents up to 30%.

The brittle nature of metallic glasses limits their applications. The current study applies the idea to place a thin layer of {111} textured face centered cubic Cu nanocrystalline metal film beneath a brittle binary Cu50Zr50 thin film metallic glass layer via a direct co-sputtering synthesis method. In order to better elucidate the co-deformation mechanism of amorphous/crystalline nano-laminates, we synthesized well-defined and controllable amorphous/nanocrystalline Cu50Zr50/Cu multilayers with a range of different layer thicknesses (100 nm/100 nm, 100 nm/50 nm, and 100 nm/10 nm). Displacement-controlled indentation tests reveal a size effect of the deformation behavior of these multilayered films. Shear bands initiated in the Cu50Zr50 thin film amorphous layer can be absorbed and accommodated by increasing the thickness of the nanocrystalline Cu layer without delamination. We also use the nanocompression methodology (sub-micropillar tests) in order to study systematically the effects of the pillar diameter (300-1000 nm) on the co-deformation behavior. The amorphous/crystalline interface may exhibit good strain compatibility after appreciable plastic deformation.

Nanotwinned (nt) and nanocrystalline materials exhibit very high flow strengths compared to their coarse-grained counterparts, but unlike nanocrystalline metals, nanotwinned metals can also exhibit large tensile ductility. We have synthesized free-standing nt-Ag film using magnetron sputtering over a wide-range of deposition rates. As expected, the twin boundary spacing is dependent on the deposition rate, but more importantly the rate strongly influences the formation of a metastable phase in the as-deposited films. To examine the influence of the structure on the mechanical behavior of the nt-Ag films we have utilized real-time synchrotron X-ray scattering coupled with bulk mechanical testing as well as nano-indentation. From these experiments, we find that the strength and ductility are strongly dependent on the fraction of the metastable phase that forms during deposition. For example, films with larger volume fractions of metastable phase have tensile stresses approaching 500 MPa (quasi-static uniaxial tension) with limited ductility, while films that contain minimal metastable phase have lower flow stresses but considerably larger ductility. Furthermore, we have found that both the temperature- and strain-rate-sensitivity of the films is strongly dependent on the metastable phase fraction.

Nanoindentation is being increasingly used to assess the indentation creep response of materials, particularly of thin films and small scale structures. However, creep stress exponents extracted from nanoindentation measurements have proven to be unreliable in terms of interpreting rate controlling deformation mechanisms. No consensus exists on this issue in the nanomechanics community primarily due to the fact that majority of nanoindentation creep tests are confined to room temperature. The dominant deformation mechanism in creep depends primarily on stress, temperature and grain size. Therefore, to gain further insights into creep measurements at small scales, we performed nanoindentation and micro-pillar compressive creep testing on electrodeposited Ni having varying grain sizes (nanocrystalline to ultrafine grained, from 20nm to 500nm) over the temperature range 25-200 degrees C. Elevated temperature nanoindentation creep tests were performed using a novel system that eliminates compliance and minimizes thermal drift. The stress state was varied by utilizing indenter tips of different geometries like Berkovich, spherical and flat punch. Uniaxial compressive creep testing of micro-pillars was performed in-situ in SEM to directly observe grain boundary sliding and grain growth during creep measurements. The creep stress exponents and activation parameters extracted from both types of compressive testing provide interesting insights into the operative deformation mechanisms as a function of stress, temperature and grain size. Additionally, rate equations, pioneered by Ashby for analysis of creep data, were used to model the contribution of various deformation mechanisms towards the overall strain rate and to determine the dominant mechanism in each case. As baseline studies, nanoindentation and micro-pillar compression tests were also performed on bulk Ni samples so as to study the deformation behavior without the influence of grain boundaries. In light of these results, testing, analysis and interpretation of experimental data obtained from nanoindentation for creep characterization will be discussed.

Thermal instability due to grain growth via the motion of grain boundaries is a long-standing issue in metallic nanocrystalline materials. In this study, a method based on molecular dynamics has been adapted to simulate grain boundary motion in the experimental range at temperatures (T) as low as ~0.2 Tm (Tm is the melting point). By using the adapted simulation method, grain boundary motion at velocities (V) and driving forces (P) across more than five orders of magnitude has been studied. Analysis of the V-P relation at different T reveals that grain boundary migration can be dominated by two modes, depending upon how fast the grain boundary moves: a thermally activated diffusional process at low V (e.g., at low T and P) vs. a “ballistic” process at high V (e.g., at high T and P). A rather broad transition range in both P and T lies between the two regimes. These findings suggest that some highly-driven atomistic simulations on grain boundary motion could probe a different kinetic regime (ballistic) as compared to that revealed in most experimental studies (diffusional).

An Al 5083 alloy with a bimodal grain size has been previously synthesized using a low-temperature milling process and consolidation via cold isostatic pressing (CIP). This material has been shown to exhibit greatly improved strength when compared to conventional aluminum alloys. Additionally, this material has shown sensitivity to test conditions. This work studies the effects of temperature on the mechanical properties of this material by examining its elastic and plastic properties through uniaxial tension tests conducted under a variety of conditions at temperatures up to 473 K. Microscopic analysis was used to study the material&’s deformation and failure mechanisms. Dynamic recovery was observed, but analysis of the microstructure failed to show any thermally activated grain growth. Non-monotonic dependence of strain rate sensitivity on temperature was noted. While the material was found to exhibit significant improvements in strength at room temperature, i