Symposium Organizers
Ruth Schwaiger, Karlsruhe Institute of Technology
Timothy Rupert, Univ of California-Irvine
Christopher Weinberger, Drexel University
Guang-Ping Zhang, Chinese Academy of Sciences
MB5.1: Deformation Mechanisms
Session Chairs
Ruth Schwaiger
Christopher Weinberger
Monday PM, November 28, 2016
Sheraton, 2nd Floor, Constitution B
9:30 AM - *MB5.1.01
Size Effects of Metals with Real Microstructures
Alfonso Ngan 1 , Rui Gu 2 , Ke Fu Gan 1
1 Department of Mechanical Engineering University of Hong Kong Pokfulam Road Hong Kong, 2 Public Testing and Analysis Center South University of Science and Technology of China Shenzhen China
Show AbstractThe last decade has seen a surge of research efforts into the effects of specimen size on the strength and deformation of single-crystal, monolithic metals in the micron size regime. Notable observations include the power-law dependence of yield strength on size, with a great deal of insights gained into the nucleation of dislocations from a hitherto dislocation-free state, deformation under a continuous dislocation-starved state, stochastic deformation, and so on.
In this paper, the strength and deformation of small metals containing conventional microstructures, such as grain boundaries and second-phase precipitates, are discussed. In these samples, the microstructure imposes an internal length scale that may interplay with the extrinsic length scale due to the specimen size to affect strength and deformation in an intricate manner. For grain boundaries, their presence in a small specimen may significantly affect strength and yet, as a result of the limited specimen size, the effect is far from the Hall-Petch behavior for conventional polycrystalline metals. For precipitates, their interactions with the travelling dislocations in a specimen with confined size may lead to an interesting minimum strength behavior on increasing specimen size.
10:00 AM - MB5.1.02
D2C—Analyzing, Comparing, and Validating Arbitrary Dislocation Microstructure within a Multiscale Data Framework
Stefan Sandfeld 1 , Dominik Steinberger 1 , Nina Gunkelmann 1
1 University of Erlangen-Nuremberg Fuerth Germany
Show AbstractThe mechanical behaviour of metallic materials is governed by properties of the underlying microstructure. Understanding and predicting the structure-property relation is central to experimental and computational materials science. Over the last decades a large number of different simulation methods on various time and length scales has evolved. At the same time, advanced experimental characterization methods with high resolutions are able to reveal a rich variety of details about microstructural features. This offers a number of interesting possibilities, e.g. to use experiments for validating computational results on a microstructural level, or to use microstructure data from a 'lower scale' method (e.g. atomistics) - directly or indirectly - as input or for validation purposes for simulation method on larger scales. Up to date, however, systematic and detailed methodologies for comparing and validating data from different methods are still lagging behind.
In this presentation we introduce our D2C (=discrete to continuous) approach, which can be used as novel 'language' for computationally characterizing dislocation microstructures. This data format can be used to bridge between different methods in a multiscale approach and allows to directly compare dislocation microstructures from, e.g., MD simulations, TEM microscopy or tomography, continuum or DDD simulations. We additionally show how our approach might serve as the foundation for a unified approach towards dislocation data, where one of the strengths of the D2C framework is that ensemble averages of statistically equivalent simulations/experiments can easily be performed. This is ideal for validation and data mining of in particular discrete methods (MD or DDD simulations as well as specialized experiments), whose microstructural data are often not easily accessible.
10:15 AM - MB5.1.03
Influence of Grain Size, Grain Shape and Dislocation Density Distribution on the Viscoplasticity of Thin Nanocrystalline Metallic Films
Hareesh Tummala 1 2 , Guerric Lemoine 1 , Marc Fivel 3 , Laurent Delannay 1 , Thomas Pardoen 1
1 Université Catholique de Louvain Louvain-la-Neuve Belgium, 2 Université Grenoble Alpes Grenoble France, 3 Université Grenoble Alpes/CNRS Grenoble France
Show AbstractA dislocation-based crystal plasticity model has been developed in order to study the mechanical and creep/relaxation behaviour of polycrystalline metallic thin films [1]. The model accounts for the confinement of plasticity due to grain boundaries and for the anisotropy of individual grains, as well as for the significant viscoplastic effects associated to dislocation dominated thermally activated mechanisms. Numerical predictions are assessed based on experimental tensile test followed by relaxation on freestanding Pd films, based on an on-chip test technique [2]. The dislocation-based mechanism assumption captures all the experimental trends, including the stress-strain response, the relaxation behaviour and the dislocation density evolution, confirming the dominance of a dislocation driven deformation mechanism for the Pd films with high defects density. The model has also been used to address some original experimental evidences involving back stresses, Bauschinger effect, backward creep and strain recovery.
The assumptions made in the crystal plasticity model have been challenged based on 3D discrete dislocation dynamics simulations of freestanding multigrain films using a modified version of the code TRIDIS to account for the grain boundaries. The code is coupled to the finite element method [3] in order to account for the image forces from free surfaces. The parameters of the models are identified based on the experimental results obtained on the Pd films. One of the main questions answered by these simulations concern the separation of the different contributions to back stress: grain to grain strength variations as related to size, grain to grain orientation variations and intra-grain back stress. The influences of grain aspect ratio and dislocation density distribution on polycrystalline thin films are also quantified.
References
[1] Lemoine G., Delannay L., Colla M-S., Idrissi H., Schryvers D, Pardoen T., Dislocation and back stress dominated viscoplasticity in freestanding sub-micron Pd films, Acta Materialia 111 (2016) 10-21
[2] M. -S. Colla et al., Dislocation-mediated relaxation in nanograined columnar palladium films revealed by on-chip time-resolved HRTEM testing, Nature. Comm., 6:5922 (2015)
[3] M. C. Fivel and G. R. Canova, Developing rigorous boundary condition to simulations of discrete dislocation dynamics, MSMSE, 7, 753 (1999)
10:30 AM - MB5.1.04
The Thermally Activated Deformation of Microcast Metallic Wires
Suzanne Verheyden 1 , Lea Deillon 1 , Jerome Krebs 1 , Andreas Mortensen 1
1 École Polytechnique Fédérale de Lausanne Lausanne Switzerland
Show AbstractWe explore the thermally activated deformation of aluminium (99.99%/99.999%) and AlMg2% microwires having a diameter down to 10 µm. The wires are prepared using a microcasting process that currently allows for the production of single-crystalline microwires of diameter down to 6 µm, with a smooth surface finish and aspect ratio above 30. Microwires produced in this manner have an initial dislocation density of the order of 1011 m-2, and show extensive plasticity when deformed in tension, their tensile flow curves being characterized by a size and orientation dependent flow stress. The deformation progresses heterogeneously, in the form of large strain bursts; when imposing relaxation tests on Al and Al-Mg microwires it appears that this heterogeneous behavior continues, relaxation consisting of continuous parts superposed with large strain jumps. We present here data collected on both aluminium and Al-Mg alloy samples, characterizing both the continuous and burst-like relaxation mechanisms in these systems.
10:45 AM - MB5.1.05
Investigation of Size Effects in Bi-Crystalline Micro-Pillar—
A Comparison of Length-Scale Strengthening from Grain Boundary and the Free Surface
Hannah Zhang 1 , Xiaodong Hou 1
1 National Physical Laboratory London United Kingdom
Show AbstractInterfaces such as grain boundary play a critical role in determining the mechanical behaviour of metallic materials for advanced engineering. The free surface of material components is also an important factor to influence the plastic deformation especially when the component size is reduced to micro-range. It was demonstrated repeatedly in the literature that the strength increases when the specimen dimension is reduced (e.g. the pillar diameter). Clearly, the free surface of micro-pillars acted as restrictions for dislocation nucleation and dislocation motion when the surface to volume ratio is increased; this is very similar to the common understanding of the grain boundary strengthening mechanism. However, it is yet to be proved if the free surface is indeed providing the same strengthening ability as the grain boundary; furthermore, how these two size effects (pillar size and grain size) are combined.
In this work, micro-pillars containing a large angle grain/twin boundary were fabricated by focused ion beam; pillars were subjected to nano-indentation testing using a sharp indenter and compression using a flat punch indenter. By carefully controlling the pillar size and the interface area, the size effect contribution from free surface and ground boundary were separated and compared so that a solution can be proposed to predict the pillar strength including grain/twin boundaries at small scales.
11:30 AM - *MB5.1.06
Defect Nucleation, Interaction and Mobility in Au Nanowires
Christian Brandl 1
1 Karlsruhe Institute of Technology Eggenstein-Leopoldshafen Germany
Show AbstractThe strength of micrometer-to-nanometer sized metallic structures is increasing with decreasing size and can approach the theoretical shear strength limit – down to a regime, where the strength is theoretically predicted to be flaw insensitive.
Using molecular dynamics (MD) simulations, we investigate the effect of predefined flaws, i.e. notches, in initially defect-free Au nanowires, which require defect nucleation at the free surfaces to initiate plastic flow. The resulting defect evolution, flow stresses and the strain hardening behavior in the MD simulations are compared to a complementary experimental studies, where the defect-free Au nanowires are structured by He-ion beams with sub-50 nm diameter holes. The microstructure after deformation - as also seen by (high-resolution) transmission electron microscopy - suggests besides the nucleation of leading and trailing partial dislocations the formation of mobile grain boundaries. The aspects of dislocation nucleation, microstructure formation by defect-defect interaction and grain boundary migration are discussed in context of yield stress, strain hardening and the onset of shear fracture.
More generally, the implications of the observed flaw insensitive strength and ductility, which operate at similar stress levels in experiment and MD simulation, are discussed in the framework of thermally-activated defect nucleation, defect mobility and their limitations.
12:00 PM - MB5.1.07
Achieving Ultrahigh Tensile Strength of Metallic Nanowires by Controlling Ni/Ni-Au Multilayer Nanocrystalline Structures
In-Suk Choi 1 , Boo Hyun An 2 , Young Keun Kim 2 , Jae-Pyoung Ahn 1 , Oliver Kraft 3
1 Korea Institute of Science and Technology Seoul Korea (the Republic of), 2 Korea University Seoul Korea (the Democratic People's Republic of), 3 Inst for Applied Materials, Karlsruhe Institute of Technology Karlsruhe Germany
Show AbstractWe report that our micro-alloying-based electrodeposition method creates a strong and stable Ni/Ni-Au multilayer nanocrystalline structure by incorporating Au atoms that makes nickel nanowires (NWs) strongest ever under tensile loads even with diameters exceeding 200 nm. Superior mechanical properties of nanolayered structures have attracted great interest recently. However, previously fabricated multilayer metallic nanostructures have high strength under compressive load, but never reached such high strength under tensile loads. When the layer thickness is reduced to 10 nm, the tensile strength reaches the unprecedentedly high 7.4 GPa, approximately 10 times that of metal NWs with similar diameters and exceeding that of most metal nanostructures previously reported at any scale.
12:15 PM - MB5.1.08
Mechanically Assisted Self-Healing of Ultrathin Gold Nanowires
Yang Lu 1
1 City University of Hong Kong Kowloon Hong Kong
Show AbstractUltrathin gold nanowires have emerged as one of the most promising candidates for nanoelectronics and next-generation interconnect applications. However, due to their exceedingly small sizes (diameter < 10nm) , their structures and morphologies could also be damaged under real service conditions. For example, we found that Rayleigh instability can significantly change their morphologies upon Joule heating, greatly hindering their applications as interconnects. In this talk we show that, upon random mechanical perturbations, pre-damaged, non-uniform ultrathin gold nanowires could quickly recover their original uniform diameters and smooth surfaces, via a unique mechanically assisted self-healing process. By examining the local self-healing process through in situ high-resolution transmission electron microscopy (HRTEM), we concluded that the underlying mechanism was associated with the surface atomic diffusion, which was further evidenced by molecular dynamic (MD) simulations. In addition, mechanical manipulation could provide the necessary driving force, as suggested by the ab initio calculations, to activate more surface adatoms to diffuse and consequently speeds up the self-healing process. This result may provide a facile method to repair ultrathin nanowires directly in functional devices, and quickly restore their uniform structures and morphologies by simple global mechanical perturbations.
12:30 PM - MB5.1.09
Mechanical Properties of Silver Nanowires Under Rate-Dependent and Cyclic Loading Conditions
Horacio Espinosa 1 , Rajaprakash Ramachandramoorthy 1 , Wei Gao 1 , Rodrigo Bernal 1 , Yanming Wang 2 , Wei Cai 2
1 Northwestern University Evanston United States, 2 Stanford University Stanford United States
Show AbstractThin films of conductive electrodes are a vital part of many electronics such as touch screens, flexible antennas and e-readers/papers. Traditionally in such applications Indium Tin Oxide (ITO) has been used as the electrode by the industry. But recently, ITO is getting replaced by metallic nanowires, owing their low cost production and better electrical properties. In all these applications, the silver nanowires will be subject to a variety of loadings, possibly at different strain rates. Thus, understanding the nanowire mechanical properties such as strength and failure mechanisms, especially under rate-dependent and cyclic loading conditions, becomes vital in how these technologies will be optimized for reliability, robustness and failure tolerance. Nonetheless, high strain rate and cyclic behavior of nanomaterials remains largely unexplored.
In this work, we report in situ Scanning Electron Microscope (SEM) strain rate dependent tensile tests on bicrystalline silver nanowires spanning six orders of magnitude from 2e-4/s to 2/s, using microelectromechanical system (MEMS) based devices. The strain rate tensile tests on silver nanowires reported in this work were made possible by the low mass of the MEMS devices and the electronically controlled actuation and load sensing. The experiment revealed observed a remarkable rate dependent brittle-to-ductile transition. We further investigated the atomistic mechanisms using High Resolution Transmission Electron Microscopy (HRTEM) imaging as well as MD simulations and dislocation nucleation theory. HRTEM imaging revealed that the dislocation density and spatial distribution of plastic regions, along the nanowire, increases with increasing strain rate. Plastic deformation mechanisms such as grain boundary migration and dislocation interaction were experimentally observed and studied by MD simulations. Finally, the experimental and MD results were correlated using dislocation nucleation theory.
Using the same MEMS testing platform with closed-loop feedback control, we investigated the cyclic loading of penta-twinned silver nanowires. Load and unload cycles revealed hysteresis with increasing unloading strain, which proved the existence of Bauschinger effect in nanowires. A combination of TEM observations and MD simulations revealed that these processes occur due to the penta-twinned structure and emerge from reversible dislocation activity. While the incipient plastic mechanism through the nucleation of stacking fault decahedrons (SFDs) can be fully reversed, when the tensile stress dropped below a threshold value, plasticity was found to be only partially reversible, as intersecting SFDs led to dislocation reactions and entanglements.
12:45 PM - MB5.1.10
Plasticity in Bent Au Nanowires Studied by Laue Microdiffraction
Thomas Cornelius 1 2 , Zhe Ren 2 , Cedric Leclere 2 , Odile Robach 3 4 , Jean-Sebastien Micha 3 4 , Olivier Ulrich 3 4 , Gunther Richter 5 , Olivier Thomas 2
1 Centre National de la Recherche Scientifique Marseille France, 2 Aix-Marseille Université Marseille France, 3 CEA Grenoble France, 4 European Synchrotron Grenoble France, 5 Max-Planck Institute for Intelligent Systems Stuttgart Germany
Show AbstractIn the recent past, the mechanical properties of low-dimensional materials attracted enormous attention showing increasing yield strengths reaching the ultimate limit of the respective material for defect free nanostructures [1, 2]. Plasticity and the storage of geometrically necessary dislocations (GNDs) may also vary in micro- and nanostructures. While for uniaxial tests dislocations tend to slide through the structures before they can interact with each other [3], bending tests result in the storage of GNDs due to strain gradients present during the deformation [4].
To shed additional light on the plasticity at the nanoscale, ex situ and in situ Laue microdiffraction experiments on <110> Au nanowires (with diameters in the few 100 nm range) bent in three-point configuration have been performed using a newly developed scanning force microscope for in situ nanofocused X-ray diffraction (SFINX) [5, 6]. The orientation (rotation and bending) of the deformed Au nanowires was inferred from Laue microdiffraction patterns recorded along the complete nanostructure. The deformation profile eventually gives access to the activated slip system revealing the bending is accommodated by [01-1](111) dislocations. Activation of secondary slip systems is also observed and may be related to the torsion of the nanowire induced by slight misalignments (~50 nm) of the AFM-tip with respect to the nanowire center. In situ three-point bending tests allow for studying the effect of AFM-tip misalignment on the nucleation of dislocations and give access to the onset of plasticity [7].
This work was funded by the French National Research Agency through project ANR-11-BS10-01401 MecaniX.
[1] B. Wu et al., Nature Materials 4 (2005) 525
[2] G. Richter et al., Nano Lett. 9 (2009) 3048
[3] S.H. Oh, M. Legros, D. Kiener, G. Dehm, Nature Materials 8 (2009) 95
[4] B. Roos, B. Kapelle, G. Richter, C.A. Volkert, Appl. Phys. Lett. 105 (2014) 201908.
[5] Z. Ren, F. Mastropietro, S. Langlais, A. Davydok, M.-I. Richard, O. Thomas, M. Dupraz, M. Verdier, G. Beutier, P. Boesecke, T.W. Cornelius, J. Synchrotron Radiat. 21 (2014) 1128
[6] C. Leclere, T.W. Cornelius, Z. Ren, A. Davydok, J.-S. Micha, O. Robach, G. Richter, L. Belliard, O. Thomas, J. Appl. Cryst. 48 (2015) 291
[7] Z. Ren, PhD thesis, Aix-Marseille Université, France (2015
MB5.2: Deformation of Non-Crystalline and Porous Materials
Session Chairs
Megan Cordill
Stefan Sandfeld
Monday PM, November 28, 2016
Sheraton, 2nd Floor, Constitution B
2:30 PM - *MB5.2.01
Relevant Length Scales, Size Effect, and Nanomechanics of Metallic Glasses
Mo Li 1 2 , Yun Luo 1
1 Georgia Institute of Technology Atlanta United States, 2 State Key Laboratory of New and Advanced Metallic Materials University of Science and Technology Beijing Beijing China
Show AbstractWhile bigger is perceived as better in many fields, smaller sized materials and devices have become the favorite in materials science and engineering. The goal here is also on leveraging the small size to gain bigger and better performance. In crystalline materials, shrinking the size would lead to strength increase, sometimes orders of magnitude higher. The primary reason is the dislocations or the dislocation related activities in small crystalline samples. What happens if we have a material that does not even have dislocation? One example is metallic glass. In this case, one faces a series of questions: how does the decrease in size affect the materials’ strength and ductility? Would one still see the trend of “the smaller, the stronger”? Or are there fundamental principles that operate and govern the mechanics of small sized materials in particular?
In this talk, I will go over several arguments to show that in metallic glass, where the extended structural defects such as dislocations and grain boundaries are absent, the strength is related to the compatibility of the characteristic length scales between the sample size and some intrinsic material process during deformation. But what are these relevant characteristic length scales? Are we able to identify them? And how are they related to the mechanical properties of metallic glasses at small scale? I will show that there are several characteristic length scales and try to identify which is most relevant.
3:00 PM - MB5.2.02
Size-Effect in Pd
77.5Cu
6Si
16.5 Metallic Glass Micro-Wires—More Scattered Strength with Decreasing Size
Guannan Yang 1 2 3 , Zhun Li 1 4 , Fengmei Guo 1 , Yang Shao 1 2 , Kefu Yao 1 2
1 School of Material Science and Engineering Tsinghua University Beijing China, 2 Key Laboratory for Advanced Materials Processing Technology, Ministry of Education Beijing China, 3 City University of Hong Kong Hong Kong Hong Kong, 4 China Iron and Steel Research Institute Group Beijing China
Show AbstractThe properties of metallic glasses at down to micro and nano-scale have attracted long-term attentions as an important and interest scientific topic. The mechanical behaviors of polycrystalline alloys are found to be strongly size-dependent, due to the decrease of grain boundary and dislocations at small scale. The metallic glasses, however, have no characteristic microstructures beyond atomic scale. It is therefore concerned that if a similar size effect can still exist in metallic glasses.
Previous studies have discovered lower modulus, higher plasticity, and a trend of homogeneous flow instead of localized shear band deformation in metallic glasses at nano and submicron scales. Both trends of increasing and decreasing strength at smaller scale have been observed in different alloy systems. However, the effectiveness of these results was still under debate, as the sample structure might have changed during fabrication. Such as by focused ion beam machining, nanoimprinting and superplastic deformation. To further study the size effect in metallic glasses with new fabrication methods and in other scales is still an important task.
In this study, we fabricated a series Pd77.5Cu6Si16.5 metallic glass microwires by melt-spinning method. Through such a method, the properties of small-scaled metallic glass in original rapid-cooled glassy structure could be measured. The as-prepared microwires with diameters ranging from 15 to 114 μm were then tested by uniaxial tension. Statistical result showed that in the measured diameter range, smaller samples would show more scattered strength. This trend was very different from the size effect observed in previous metallic glasses at smaller scales, or in polycrystalline alloys. Such phenomena possibly result from the more scattered flaw density distribution at smaller scale, and the higher flaw sensitivity under tension condition. The experiment also showed that at smaller scale, the alloy could possibly reach higher strength and lower elastic modulus. For example, the microwire with a diameter of 15 μm reached a strength of 2136 MPa, which was ~60% larger than that of bulk samples, and an elastic modulus of 68.9 GPa, which was ~30% lower than that of bulk samples.
These results provide new experimental evidences for the size effect in metallic glasses at micro-scale, which might help to understand the substantial deformation mechanism of these materials.
3:15 PM - MB5.2.03
Anisotropic Mechanical Response and Failure of Spider Silk Reveals Its Hierarchical Structure
Qijue Wang 1 , Hannes Schniepp 1
1 College of William and Mary Williamsburg United States
Show AbstractThe origin of spider silk’s outstanding mechanical properties, combining high strength and high extensibility, has been intensively studied and discussed for several decades. Various experimental and theoretical efforts have been made to establish structure–property relationships of silk. Although the protein sequence and macroscopic morphology of the silk fiber are known, there is currently no consensus model regarding structural organization of the protein for length scales in between. Protein micelles have been favored by some, whereas a nanofibrillar organization of the protein has been suggested by others.
For a better understanding of the silk structure we study the silk of recluse (Loxosceles) spider. In contrast to most other silks, its morphology is not cylindrical, but ribbon-like, with a thickness of less than 50 nm and a width of 6–8 μm. Being only a few protein layers thin, this unique structure is much simpler, and thus ideal to learn more about the molecular makeup of silk.
Like many silk fibers, the Loxosceles ribbons exhibit a nanofibrillar structure at their surface. Bringing these silk ribbons to failure in different ways and observing the structure of the rupturing locations via atomic force microscopy (AFM), we found that Loxosceles ribbon silk is entirely composed of 20-nm thin nanofibrils. On one hand, Loxosceles silk shares very similar mechanical properties with some of the best-performing spider silks. On the other hand, our experiments fully revealed the hierarchy of its structure: proteins organized into nanofibrils that are further organized into ribbons. We also tested the anisotropic mechanical properties of the hierarchically organized ribbons by force spectroscopy and compared the results to models and finite element simulations. We were able to assess both the mechanical properties of individual nanofibrils, as well as the binding force between the fibrils. On this basis we aim to develop a fully comprehensive understanding of the mechanical properties of spider silk.
3:30 PM - *MB5.2.04
Commonalities in the Signatures of Plasticity in Disordered Solids
Daniel Strickland 2 , Daniel Gianola 1 , Robert Ivancic 3 , Andrea J. Liu 3
2 Department of Materials Science and Engineering University of Pennsylvania Philadelphia United States, 1 Materials Department University of California, Santa Barbara Santa Barbara United States, 3 Physics Department University of Pennsylvania Philadelphia United States
Show AbstractA microscopic understanding of plasticity and failure is considered one of the grand challenges of condensed matter physics and materials science, and progress for disordered solids lags far behind their crystalline (ordered) counterparts. Elucidating the structural origins of the onset of plastic deformation in disordered solids would have far-reaching implications, from mitigating catastrophic failure in materials such as metallic glass, amorphous carbon, functional nanoparticle films and concrete, to predicting earthquakes and avalanches. Recent progress has surmounted a major stumbling block for disordered solids, namely, the inability to identify flow defects from the microscopic structure comparable to dislocations in crystals. In particular, machine learning methods have been developed to identify the “softness” of each particle, where softness is the linear combination of quantities characterizing the structure of the neighborhood of the given particle that correlates most strongly with particle rearrangements.
We show that for disordered solids there is a remarkable universality in microscopic structure and dynamics, as characterized respectively by spatial correlations in softness (the size of flow defects), and by spatial correlations in the non-affine displacement of a particle (the size of particle rearrangements). At the same time, we demonstrate universality in a macroscopic property, namely the value of strain at the onset of macroscopic failure. We demonstrate the universality of these features for disordered solids with constituent particles spanning seven orders of magnitude in particle size (from atomic to granular scales), and thirteen orders of magnitude in elastic modulus, with vast differences in bonding character (from metallic to covalent to van der Waals), and various loading states (from indentation to tension to shear). These remarkable commonalities, which transcend details of constituent size and bonding, do not exist for crystals, suggesting that the disorder itself is responsible for programming both the microscopic structure and dynamics of defects and the resistance to plastic flow. These results point to the possibility of a unifying framework and vast simplification of our understanding of plasticity and failure in disordered solids, which paradoxically may not be possible for crystals.
4:30 PM - MB5.2.05
Analysis of the Size-Dependent Storage Modulus in Bio-Inspired Micro-to-Nano Porous Polymeric Foam
Julia Syurik 1 , Ruth Schwaiger 2 , Prerna Sudera 3 , Stephan Weyand 2 , Siegbert Johnsen 4 , Gabriele Wiegand 4 , Hendrik Hoelscher 1
1 Institute for Microstructure Technology Karlsruhe Institute of Technology Karlsruhe Germany, 2 Institute for Applied Materials Karlsruhe Institute of Technology Karlsruhe Germany, 3 Institute of Nanotechnology Karlsruhe Institute of Technology Karlsruhe Germany, 4 Institute of Catalysis Research and Technology Karlsruhe Institute of Technology Karlsruhe Germany
Show AbstractHierarchical porous structures found in nature play an important role in functional adaptation serving mainly for passive mechanical functions. A very illustrative example is the pomelo peel consisting of a hierarchical porous structure. Such fruits can survive a fall from a 15-meter tree without a sign of damage demonstrating very effective energy dissipation achieved through an optimized pore structure and the related variation in mechanical properties [1].
Inspired by this and other examples we produced a foam from poly(methyl methacrylate) (PMMA) with a gradual change of pore size by saturation in supercritical carbon dioxide. By controlling the fabrication parameters we created a gradual change from microcellular to nanocellular within one sample. The nanocellular areas of the foam feature very homogeneous pores and during the transition to the microcellular areas the pore fraction decreases by a factor of 2 (from 35% to 17%). We investigated the size-dependent viscoelastic properties of the sample by flat punch indentation along the pore size gradient. The storage modulus gradually decreases by 15% over the transition from micro- to nano- sized pores which corresponds to a smooth decrease of the thickness of the polymeric walls. Being rather a material property, the loss factor does not change with the pore size. Interestingly, the storage modulus increases with increasing pore fraction. This observation appears counterintuitive at first sight but can be explained by an increase in the cell wall thickness. The presented foaming process is applicable to any thermoplast being an easy way to tune elasticity of a broad range of polymers for various applications.
[1] Thielen et al. Bioinspir. Biomim. 8, 025001 (2013)
4:45 PM - MB5.2.06
Investigation of Time-Dependent Deformation of Porous Silicon Through Nanoindentation
Tyler Vanover 1 , T. John Balk 1
1 University of Kentucky Lexington United States
Show AbstractPorous silicon (π-Si) is an intriguing material that has been the subject of intense study in fields involving mechanical behavior, catalysis, sensing, MEMS and drug delivery, due to its large surface-area-to-volume-ratio. It is well known that size effects owing to the bi-continuous network of pores and ligaments – typically smaller than 100 nm – have interesting effects on the mechanical behavior of the porous structure. It is, however, yet to be fully understood in terms of deformation mechanics. Here, π-Si films have been fabricated through a novel dealloying technique with a nominal thickness of 1 µm and probed with a Berkovich indenter to investigate the fundamental nature of ligament size constraints and deformation mechanics. Comparisons in the phase angle shift, elastic modulus and hardness of fully dealloyed films, as well as dealloyed films that have subsequently been annealed in vacuum, will be presented. Surprisingly, all films appear to exhibit time dependence and also recover fully upon removal of the load. Here, the authors attempt to understand the deformation mechanics in terms of the crystal structure of the films, the reported relative density and ligament size.
5:00 PM - MB5.2.07
Microstructural Characterization and Crystal Orientation Independence on the Mechanical Properties of Nanoporous Gold
Xiaotao Liu 1 , Nicolas Briot 1 , T. John Balk 1
1 Chemical and Materials Engineering University of Kentucky Lexington United States
Show AbstractNanoporous gold (np-Au) has attracted considerable attention owing to a high surface-to-volume ratio, leading to a wide range of potential applications in fields such as catalysis, sensing, MEMS, etc. Bulk, polycrystalline, millimeter-size np-Au samples were created using a two-step dealloying process combining free and electrochemical dealloying of a gold-silver precursor alloy. Before dealloying, the samples were polished to provide a flat surface for nanoindentation tests to determine mechanical properties. Near complete removal of the sacrificial element, silver, was verified by energy dispersive x-ray spectroscopy in the scanning electron microscope (SEM).
The influence of sample preparation (polishing and annealing) and dealloying conditions on the resulting np structure (ligament shape and size) was studied by imaging the np-Au ligaments in SEM. An evolution of the average ligament size was observed, in cross-section, from the surface toward the center of the samples. These results will be discussed in an effort to optimize sample preparation techniques, necessary for the nanoindentation tests.
In addition, electron back-scattered diffraction in the SEM was used to determine the orientation of neighboring grains before and after dealloying. We hypothesized that the mechanical properties of np-Au would be independent of the original crystal orientation of the precursor alloy before dealloying. The mechanical properties (Young’s modulus and hardness) of several grains with different crystal orientations were measured by nanoindentation before and after dealloying. The results of these investigations will be discussed in the context of ligament structure, orientation and size effects.
5:15 PM - MB5.2.08
Mechanical Properties and Failure Mechanisms of Nanoporous Gold Studied through In Situ Tensile Testing
Katherine Frei 1 , Josh Stuckner 1 , Mitsu Murayama 1 , Bill Reynolds 1 , Sean Corcoran 1
1 Material Science Virginia Tech Blacksburg United States
Show Abstract
Nanoporous structured metals is an exciting topic that has been highly researched due to its potential in applications including sensing, catalysts, gas storage, and heat exchangers, made possible by its high surface area to volume ratio and high porosity. However, this material, especially nanoporous gold, generally shows a brittle behavior despite it consisting of a normally ductile constituent element, limiting these many commercial applications. This contrasting structure – mechanical property relationship appears to be significant when the ligament size reaches less than 15 nm. There have been multiple simulated studies on the tensile mechanical properties and the fracture mode of this material, but limited physical tensile testing research exists due to technical difficulty of conducting such experiments with small fragile samples. We examine the tensile mechanical properties of nanoporous gold with ligament sizes ranging from 10 to 15 nm using in situ tensile testing under an environmental scanning electron microscope (ESEM). A specially designed tensile stage and sample holders are used to deform the sample inside the ESEM, allowing us to observing both the macro and microscopic structure changes in both 2D and 3D. Our experimental results will advance our understandings of how the ligament size and its structure (both internal and surface) influence the mechanical properties of nanoporous gold, and they also serve as a statistically relevant multi scale input parameters to increase the accuracy of future simulated studies that will take this material a step towards commercial use by providing a thorough understanding of its size-dependent mechanical limitations.
5:30 PM - MB5.2.09
Indentation Size Effect Dependent on Ligament Size in Nanoporous Gold
Young-Cheon Kim 1 , Seung-min Ahn 1 , Ju-Young Kim 1
1 Ulsan National Institute of Science and Technology Ulsan Korea (the Republic of)
Show AbstractNanoporous gold (np-Au) is a kind of metallic foam that has nanoscale ligaments and pores. Based on its high specific surface area and chemical stability, it has been widely applied as a functional material in sensors, actuators, catalysis, and the like. According to previous research, in contrast to solid gold, np-Au shows catastrophic brittle fracture after elastic deformation rather than plastic behavior. Assessing the mechanical characteristics of np-Au is critical in estimating its structural reliability. In this study, we looked for the origin of the indentation size effect (ISE) in nanoporous gold and its dependence on ligament size in two different fracture modes: collapse and shearing beneath an indenter. We derived a theoretical ISE model as an inverse function of indentation depth. To verify the model, uniaxial compression and shear tests were performed on four np-Au samples of different ligament sizes. The model appropriately estimated a decrease in indentation hardness of np-Au with indentation depth. We found ligament-size-dependent ISE in np-Au by normalizing hardness and indentation depth, which can be explained by different size effects in compressive and shear deformation of np-Au, and by dislocation movement in ligaments junctions of each sample, resulting in strain-hardening by dislocation pileup.
5:45 PM - MB5.2.10
Shape Memory Zirconia Foams through Ice Templating
Xueying Zhao 1 , Alan Lai 1 , Christopher Schuh 1
1 Materials Science and Engineering Massachusetts Institute of Technology Cambridge United States
Show AbstractCeria doped zirconia has been shown to exhibit enhanced shape memory properties at small scales in micro-pillar form, and open-cell foams could potentially make use of this effect across larger scales, with each foam strut functioning like a shape memory micro-pillar. This work presents the fabrication of zirconia foams via ice templating and seeks to understand the relationship between processing conditions, microstructure and shape memory properties. Directional freezing is used to synthesize zirconia-based foams with pores and struts on the order of microns and relative densities in the range 0.09-0.44. The foams are subjected to thermal cycling and x-ray diffraction analysis to evaluate the martensitic transformation that underlies shape memory properties. The prospects for multi-cycle shape memory and superelasticity in such foams are evaluated and directions for improving cyclic transformation properties are discussed.
Symposium Organizers
Ruth Schwaiger, Karlsruhe Institute of Technology
Timothy Rupert, Univ of California-Irvine
Christopher Weinberger, Drexel University
Guang-Ping Zhang, Chinese Academy of Sciences
MB5.3: Size Effects I
Session Chairs
Christian Brandl
Timothy Rupert
Tuesday AM, November 29, 2016
Sheraton, 2nd Floor, Constitution B
9:30 AM - *MB5.3.01
Size and Environmental Effects on Small-Scale Mechanical
Behavior of Materials
Xiaodong Li 1
1 University of Virginia Charlottesville United States
Show AbstractNanostructures have a high surface to volume ratio. The surface plays critical roles in the unique properties and functionalites of nanostructures. The surface atomistic lattices, stress and strain fields have rarely been experimentally unveiled. The surface atomistic strain fields were mapped by coupled lattice imaging and digital imaging correlation techniques. The surface strain fields were used to explain the unusual mechanical properties and superior sensing performance of nanostructures. On the other hand, nanostructured devices are often constructed with building blocks of dissimilar materials. The devices experience thermal and mechanical deformations in packaging and operation. It is of critical importance to obtain full-field deformation data of these heterogeneous materials in the controlled thermal and mechanical conditions. Digital image correlation based thermal and mechanical strain mapping techniques were used to probe nano/atomic scale thermal and mechanical deformations of nanostructures/devices. These techniques provide new guidelines for the design, packaging and reliability control of nanodevices. The newly discovered internal electron tunneling enabled electromechanical coupling in a single nanowire provides position and force sensing at the pico-nanometer and pico-Newton levels.
10:00 AM - MB5.3.02
Plasticity and Size Effects
in Germanium and Silicon
Ming Chen 1 , Ralph Spolenak 1 , Jeff Wheeler 1
1 Department of Materials ETH Zurich Zurich Switzerland
Show AbstractElemental semiconductor materials with a diamond-cubic structure, e.g. Ge and Si, are widely used in the microelectromechanical systems (MEMS) and functional semiconductor components. These materials are brittle at ambient temperature and pressure, while ductility is observed by miniaturizing sample to micron or sub-micron scales in order to prevent the onset of cracking and to allow for plastic deformation. In the present study, micro-compression of FIB-machined micropillars is conducted to obtain a thorough understanding of the micro-mechanical properties of Ge and Si in their brittle regimes. Recent advances in nano-mechanical testing systems enable the measurements of the temperature- and time-dependent deformation parameters of these materials over wide temperature range.
The brittle-to-ductile transition in Ge and Si is investigated as a function of temperature and sample size to study the transition of deformation mechanisms, i.e. partial to perfect dislocation motion on the glide set in the elevated temperature range. This transition is quantitatively analyzed using strain rate jump tests on micropillars with different orientations to correlate crystal orientation with the activation volume for plastic deformation. Deformed regions in micropillars are extracted by preparing TEM lamella, and subsequently characterized using TEM to track dislocations and microtwins. An unambiguous interpretation of dislocation processes in the diamond-cubic structure shared by Ge and Si will be presented.
10:15 AM - MB5.3.03
Cold Nanoindentation of Crystalline Ge
Jodie Bradby 1 , Larissa Hutson 1 , Brett Johnson 2 , Kiran Mangalampalli 1 , Tuan Tran 1 , Lachlan Smillie 1 , Jim Williams 1
1 Australian National University Canberra Australia, 2 Department of Physics University of Melbourne Carlton Australia
Show AbstractThe end phases formed via pressure-induced transformations of Ge possess technically-interesting properties. Nanoindentation is a convenient tool for inducing high-localized pressures to enable phase transformation. However, previous studies have showed that nanoindentation of crystalline Ge results in other deformation mechanisms such as twinning or generation of crystalline defects. The indentation-induced phase transformation of Ge is only observed intermittently and only using certain loading conditions such as a very sharp tip or very fast loading.
In this work we use a low-temperature indentation stage together with in-situ electrical measurements to probe the deformation behavior of Ge at low temperatures. Standard loading conditions were selected which resulted in deformation only via the generation of crystalline defects at room temperature. However, at temperatures below 0°C, the deformation pathway was instead dominated by pressure-induced phase transformations. Raman microspectroscopy of the residual impressions shows evidence of the formation of r8-Ge and amorphous Ge after indentation at low-temperatures. This was supported by transmission electron microscopy studies that identified regions of hexagonal-diamond Ge. This phase is known to form from the r8 phase when annealed at room temperature for several hours. These results show nanoindentation at temperatures below 0°C can reliably promote phase transformations in Ge.
10:30 AM - MB5.3.04
Investigation of Size Effects in Ion-Irradiated 800H Steel at High Temperatures Utilizing Small-Scale Mechanical Testing
Anya Prasitthipayong 1 , Scott Tumey 2 , Shraddha Vachhani 3 , Andrew Minor 1 4 , Peter Hosemann 5
1 Materials Science and Engineering University of California, Berkeley Berkeley United States, 2 Center of Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory Livermore United States, 3 Hysitron, Inc. Minneapolis United States, 4 National Center for Electron Microscopy, The Molecular Foundry, Lawrence Berkeley National Laboratory Berkeley United States, 5 Nuclear Engineering University of California, Berkeley Berkeley United States
Show AbstractDemands to improve the reliability of structural components used in nuclear applications necessitate the search for novel structural materials that will be able to tolerate the extreme environments in reactors that lead to undesirable changes in microstructure and mechanical performance. Due to the non-radioactive nature and the relatively short irradiation exposure time, ion irradiation is often used as a surrogate for neutron irradiation. However, the low penetration depth of ions restricts the amount of irradiated materials available to study, necessitating the development of small-scale mechanical testing techniques. Nanoindentation and in-situ microcompression have served as powerful small-scale testing tools for evaluating the mechanical properties of ion-irradiated alloys. However, size effects remain a major obstacle to obtaining meaningful mechanical properties from small irradiated volumes, especially regarding any temperature effects.
Austenitic Fe-Ni-Cr Alloy 800H is one of the potential candidates for structural materials in light water reactors and is thus selected for study. 800H has been irradiated with 70 MeV Fe9+ at 4500C to the total dose of 20.68 dpa. The damage layer of approximately 6 μm into the irradiated surface predicted by SRIM calculations is confirmed by cross-section nanoindentation performed orthogonal to the irradiated surface. Utilizing nanoindentation and microcompression, we aim to investigate the influence of temperature on size effects and understand the fundamentals leading to indentation size effect and sample size effect, respectively.
The effect of sample size on the measured mechanical properties as a function of temperature is an outstanding question. For instance, size effects are expected to be less significant at high temperatures due to the larger plastic zone size. Conversely, owing to more contribution of the source length, sample size effects are expected to be more pronounced at high temperatures. The contrasting influences of temperature on size effects in nanoindentation and microcompression imply that testing methods determine how significant size effect is at high temperatures. Therefore, our study will directly compare these effects using nanoindentation and microcompression at high temperatures, combined with in-situ observations of the deformation mechanisms.
10:45 AM - MB5.3.05
Does Dynamic Nanoindentation Testing Influence the Acquired Mechanical Properties
Alexander Leitner 1 , Megan Cordill 2 , Daniel Kiener 1 , Verena Maier-Kiener 3
1 Department Materials Physics Montan University Leoben Leoben Austria, 2 Erich-Schmid Institute of Materials Physics Austrian Academy of Sciences Leoben Austria, 3 Department of Physical Metallurgy and Materials Testing Montanuniversity Leoben Leoben Austria
Show AbstractDynamic indentation techniques, particularly the continuous measurement of stiffness (CSM) employing a superimposed oscillating force signal, came to the center of attention in the indentation community within the past two decades. The main benefit is that within a single indentation test, it is possible to obtain mechanical properties continuously over the entire penetration depth. For elastic isotropic materials this enables to calculate the actual area in contact with the tip, thus allowing to perform advanced drift corrections or tip shape corrections.
However, in-depth knowledge about the influence of this oscillating sinusoidal force signal is still lacking, and some reports in literature indicate an influence on material properties. Therefore, this study will contrast dynamic with static nanoindentation tests, where no superimposed force is applied. In order to consider the effect of lattice type and microstructure, various body-centered cubic (bcc) and face-centered cubic (fcc) metals with single crystalline and ultra-fine grained microstructures have been examined. Based on this we show that the impact of CSM is insignificant at common indentation depths of several 10 nm. However, we find that another important experimental parameter is often neglected. Static indentation tests are commonly not performed with a constant indentation strain-rate, in particular the hold segment before unloading results in a drop of the strain-rate dependent on the examined material. This will lead to distinct errors in obtained mechanical properties for materials with a high strain-rate sensitivity, which could by mistake be ascribed to the use of CSM.
11:30 AM - *MB5.3.06
Size Effects on the Electro-Mechanical Behavior of Flexible Film Systems
Megan Cordill 1
1 Erich Schmid Institute Leoben Austria
Show AbstractElectro-mechanical properties of metal thin films on polymer substrates are important to understand in order to design reliable flexible electronic devices. Ductile films and lines are an integral part of flexible electronics because they allow current flow between semiconducting islands and other operating features. When ductile films on polymer substrates are strained in tension the substrate can suppress the catastrophic failure that allows for their use in flexible electronics and sensors. However, the charge carrying ductile films must be of an optimum thickness and microstructure for suppression of cracking to occur. In order to improve mechanical and electrical properties of these complex material systems, more work at characterizing the processing-structure-property relationships should be performed. Studies of strained metal films on polymer substrates tend to emphasize only the electrical properties and thickness effects more than the role of film microstructure or deformation behavior. The microstructure of the film not only determines the mechanical behavior but also influences the electrical behavior and could be optimized if studied in connection with the mechanical behavior. To address both the electro-mechanical and deformation behavior of metal films supported by polymer substrates, in-situ 4 point probe resistance measurements were performed with in-situ atomic force microscopy imaging and X-ray diffraction during straining. The combination of electrical measurements, surface imaging, and mechanical strain measurements allow for a complete picture of electro-mechanical behavior needed for the improvement and future success of flexible electronic devices. These combined in-situ techniques will be discussed as well as results on the role of thickness, residual stress and microstructure of Au, Cu, Ag, and Mo films on polyimide substrates. From the investigation, the ideal deposition techniques and microstructure which produces electro-mechanical film properties of high fracture and delamination stresses will be determined for improved device reliability.
12:00 PM - MB5.3.07
Electro-Mechanical Behavior of Single and Multilayer Metal Thin Films on Polymer Substrates
Tanja Jorg 1 , Megan Cordill 2 1 , Robert Franz 1 , Christoph Kirchlechner 3 1 , Joerg Winkler 4 , Christian Mitterer 1
1 Montanuniversität Leoben Leoben Austria, 2 Austrian Academy of Sciences Leoben Austria, 3 Max-Planck-Institut für Eisenforschung GmbH Düsseldorf Germany, 4 Plansee SE Reutte Austria
Show AbstractThe in-situ characterization of the deformation behavior of thin films and multilayers is of great technological interest for many applications. A prominent example is the field of flexible electronics, where thin metal layers are used as electrodes and are stacked together in order to fulfil the required functionality. Those stacked electrodes usually consist of ductile metals with a low electrical resistivity like Cu, Al, Au or Ag and are combined with brittle metals like Mo, Ti, Ta or Cr, which are used as diffusion barriers or adhesion promotion layers. The aim of this work is to study the electro-mechanical performance of ductile/brittle metal film combinations with different layer architectures. Three metals (Cu, Ag and Mo) and a series of multilayers consisting of bilayers and trilayers with alternating ductile and brittle layers were deposited onto 50 µm thick polyimide substrates (UBE Upilex®-S) by magnetron sputtering. In-situ synchrotron X-ray diffraction was employed to determine the film stress and fracture strength of the individual metal layers during uniaxial tensile straining, while simultaneously measuring the change in electrical resistance for the whole multilayer system. The in-situ experiments show that the film stress in the individual layers decreases rapidly at around 1% strain, as cracks initiate and reaches a plateau at the crack saturation regime. Crack initiation usually corresponds to a rapid increase in the film resistance. However, the multilayer systems were able to withstand considerably higher tensile strains than their individual layers and remained electrically conductive up to 3-5% strain. The results indicate that the layer architecture rather than the choice of metals governs the deformation behavior of multilayer systems.
12:15 PM - MB5.3.08
A Comparison of Reflection Anisotropy Spectroscopy and Synchrotron X-Ray Diffraction for Yield Point Determination of Thin Metallic Films on Compliant Substrates
Andreas Wyss 1 , Nilesha Mishra 2 , Alla Sologubenko 1 , Patric Gruber 2 , Ralph Spolenak 1
1 Department of Materials ETH Zurich Zurich Switzerland, 2 Institute for Applied Materials Karlsruhe Institute of Technology Karlsruhe Germany
Show AbstractFor optimal electric performance, a flexible electronic device has to sustain large mechanical stresses without losing its structural integrity. The thin film geometry exerts hard constraints or even excludes a number of conventional techniques for mechanical testing. For the determination of yield strength, synchrotron x-ray diffraction (sXRD) is commonly applied. However, it is restricted by beam time availability and scattering volume. Therefore new labscale characterization techniques are desirable.
In this study we present a rather new technique (Reflection Anisotropy Spectroscopy (RAS)) for mechanical characterization of metallic thin films on polyimide substrates and compare it with sXRD. Both techniques are known to be sensitive to changes in the strain state of the specimen, its phase and microstructural configurations. Since the mechanical behavior of thin metallic films is known to be thickness dependent specimens of thicknesses ranging from 50 to 500 nm were investigated.
In an earlier work Wyss et al. showed that the evolution of the RA-signal at a specific energy is proportional to the elastic strain in the material. However a full correlation and yield point determination has not been demonstrated yet.
In classical stress-strain curves, obtained for example by sXRD, the yield point is usually determined in the loading regieme by shifting the linear slope (in the elastic regieme) by 0.2% and determining the intersection with the stress strain curve (Rp0.2). To do so one has to shift the startpoint according to the residual stresses measured for example with sin2(ψ) which yields a lot of new problems
In this work, unloading curves were used for yield point determination since they are independent of the initial residual stresses and errors caused by the mounting procedure of the specimen.
Our results show that RAS, compared to sXRD, is a suitable and reliable method that allows dynamic monitoring of thin film strain states during deformation and can be used for yield point determination of thin metallic films. Additionally data acquisition and therefore strain resolution with RAS is faster (1 sec per datapoint at a single energy) with still exhibiting an improved signal to noise ratio. This is due to the fact that the penetration depth of white light in metallic films is in the order of ten nanometers which is well below the thickness of the thinnest film.
12:30 PM - MB5.3.09
In Situ X-ray Diffraction Study of Strain Path Change Effects in Al-5wt% Mg using a Miniaturized Multiaxial Deformation Machine
Karl Sofinowski 1 2 , Maxime Dupraz 1 , Steven Van Petegem 1 , Helena Van Swygenhoven-Moens 1 2
1 Paul Scherrer Institute Villigen PSI Switzerland, 2 Ecole Polytechnique Fédérale de Lausanne Lausanne Switzerland
Show AbstractIn situ diffraction studies yield invaluable information on microstructure evolution, texture development, and residual stress development during multiaxial plastic deformation of metals. [1] However, few machines exist that can perform such experiments on ultra-fine grain and nanocrystalline metals. Here, a novel miniaturized multiaxial deformation machine is presented for measuring strain path changes in situ with x-ray diffraction. The machine is designed to measure cruciform samples at various load ratios, the first of its kind to be implemented in a synchrotron light source. The cruciforms are prepared using picosecond pulsed laser ablation to mill the gauge section to 50 μm. By using an ultra-short pulsed laser and fluence just above the ablation threshold, a thinned test section can be milled without significant damage of the sample. [2]
The results of in situ x-ray diffraction experiments on ultra-fine grain Al-5wt% Mg (AlMg5) are presented. Three load path changes are examined—0 degrees, 90 degrees, and 61 degrees—as well as uniaxial dogbone samples for comparison. For all three load path changes, the characteristic strain bursts of the Portevin Le Chatelier (PLC) Effect are observed during both the first and second loading. The diffraction pattern evolution shows a distinct response to the PLC effect, which is presented and discussed. Additionally, the final microstructure of the deformed samples are compared to the initial microstructure using transmission electron microscopy (TEM), and the differences are explained with respect to the lattice strain evolution measured during the test.
This research is performed within the ERC Advanced Grant MULTIAX (339245).
[1] S. Van Petegem, J. Wagner, T. Panzner, M. V. Upadhyay, T. Trang, and H. Van Swygenhoven. Acta Materialia 105 (February 15, 2016): 404–16.
[2] A. Guitton, A. Irastorza-Landa, R. Broennimann, D. Grolimund, S. Van Petegem, and H. Van Swygenhoven. Materials Letters 160 (December 1, 2015): 589–91.
12:45 PM - MB5.3.10
Detection of the onset of Plasticity in Micro-Crystals—In Situ Deformation of InSb Micro-Pillars under Synchrotron Coherent X-Ray Nanobeam
Ludovic Thilly 1 , Vincent Jacques 2 , Dina Carbone 3 , Rudy Ghisleni 4 , Christoph Kirchlechner 5
1 University of Poitiers-Pprime Institute Futuroscope France, 2 Laboratory of Solid State Physics Orsay France, 3 European Synchrotron Radiation Facility Grenoble France, 4 Swiss Federal Laboratories for Materials Science and Technology Thun Switzerland, 5 Max-Planck-Institut Düsseldorf Germany
Show AbstractCoherent x-ray micro-diffraction was used to detect and count phase defects (stacking faults, SFs, left in the crystal after the glide of partial dislocations) preliminarily introduced by deformation of InSb single-crystalline micro-pillars. Diffraction patterns were recorded by scanning the coherent nanobeam along the pillars axis: peak splitting is observed in the diffraction pattern associated to the top region, in agreement with the presence of a few SFs located in the upper part of the deformed pillars. Simulations of coherent diffraction patterns were also performed considering SFs randomly distributed in the illuminated volume: they show that not only the number of defects but also the size of the defected volume influences the maximum intensity of the pattern, allowing for a precise counting of defects [Physical Review Letters, 111 (2013), 065503].
Recently, diffraction measurements were performed in-situ, during compression, to detect the first lattice defects, i.e. the first events of the plastic deformation appearing in InSb micro-pillars.
MB5.4/MB6.5: Joint Session: In Situ TEM
Session Chairs
In-Suk Choi
Sandra Korte-Kerzel
Tuesday PM, November 29, 2016
Sheraton, 2nd Floor, Constitution B
2:30 PM - *MB5.4.01/MB6.5.01
Diagnose and Heal Defected Submicron-Sized Al Single Crystal through Low Amplitude Cyclic Loading
Zhiwei Shan 1 , Zhangjie Wang 1
1 Xi'an Jiaotong University Xi'an China
Show AbstractUnder stress amplitude that is lower than nominal yield stress, the loading and unloading curves of materials with and without internal mobile defects will overlap with each other and form a loop, respectively given the instrument used having enough high resolution. The area enclosed by the loop can directly reflect the defects state of the tested materials. In this work, we demonstrate that the loading and unloading curves can be used to diagnose the state of the defects inside the tested samples. In addition, we demonstrate a mechanical healing phenomenon, i.e., when submicron-sized single crystal aluminum samples are subjected to low amplitude cyclic straining, the density of those pre-existing dislocations can be dramatically reduced with virtually no change of the overall sample geometry. This is at odds with traditional wisdom that when a metal is subjected to cyclic loading, defects are prone to accumulate progressively, leading to crack initiation and even failure. In situ transmission electron microscope (TEM) tensile tests reveal that dislocation lines behave very different in response to the external applied stress. In addition, samples experienced various degree of mechanical healing exhibit different mechanical behaviors in strain-to-failure test. Our findings are expected to find applications in submicron sized devices, as their property and performance can now be optimized by mechanically tuning the defect density in a controllable manner (Wang ZJ et al, PNAS, 2015).
3:00 PM - *MB5.4.02/MB6.5.02
Nanoscale Strain Mapping of Individual Defects during In Situ Deformation
Andrew Minor 1 2 , Thomas Pekin 1 2 , Colin Ophus 2 , Christoph Gammer 2 3 , Jim Ciston 2
1 University of California, Berkeley Berkeley United States, 2 National Center for Electron Microscopy, Molecular Foundry, Lawrence Berkeley National Laboratory Berkeley United States, 3 Erich Schmid Institute of Materials Science Leoben Austria
Show AbstractRecent advances in local strain mapping using nanobeam electron diffraction (NBED) has demonstrated the ability to observe single defects and the strain fields around them at a resolution of single nanometers. In addition to measuring the strength of small-volumes, measuring the evolution of strain during plastic deformation is of great importance for correlating the defect structure with material properties. By observing dislocations, their strain fields, their movement under stress, as well as their interactions with each other and precipitates, we aim to provide insight into fundamental mechanisms of deformation in metals. This work will highlight our latest results from in situ strain mapping in an Al-Mg alloy and stainless steel using both contact loading methods (such as in situ nanopillar compression and nanoindentation) as well as non-contact methods such as tensile straining. Our method of local strain mapping consists of recording large multidimensional data sets of nanodiffraction patterns during the test. The resulting dataset contains diffraction data for every point of the STEM image, from which strain maps can be extrapolated on a scale not previously possible during in situ deformation.
3:30 PM - MB5.4.03/MB6.5.03
In Situ TEM Observation of the Onset of Plastic Deformation by Prismatic Dislocation Loop Emission
Subin Lee 2 1 , Aviral Vaid 3 , Erik Bitzek 3 , Sang Ho Oh 2
2 Depatrment of Energy Science Sungkyunkwan University Suwon Korea (the Republic of), 1 Pohang University of Science and Technology Pohang Korea (the Republic of), 3 University of Erlangen-Nuremberg Erlangen Germany
Show AbstractWe present direct observations on the incipient plasticity of dislocation-free single crystal Au [110] nanowires by in situ transmission electron microscopy nanomechanical testing. The diameter of the tested nanowires was in the range of ~80 nm-300 nm and the length-to-diameter ratio was larger than 5. The top end of the Au nanowires is bound by two inclined {111} faces in a wedge shape, on the other hand the side faces consist of four large {111} and two small {100} planes, resulting in a truncated rhombic cross-section. In our deformation setup where the wedge-shaped growth end of nanowire was compressed with a flat diamond punch, the strain becomes localized to the region under the contact. Under such a strong strain gradient condition, the initial compressive deformation began with the successive emission of prismatic dislocation loops from the top corner. Direct observations revealed that prismatic dislocation loops are formed by cross-slip mechanism; a single dislocation loop wrapping itself around a glide prism by multiple cross slip and finally reacting with itself. Not only closed prismatic loops, but also helical prismatic loops or half prismatic loops were generated when the both ends of glide loops missed each other at the final step of cross slip. The diameter of the loops was around 20 nm depending on contact area, and the Burgers vector was determined to be a/2[-1-10], which generates the vertical downward displacement of the inner area encompassed by the prismatic loops, so that it can be regarded as geometrically necessary dislocations. Detailed atomic-scale nucleation mechanism was studied by complementary MD simulations, showing that formation of stair-rod dislocations prevents further expansion of glide loop and thus promots cross slip. Right after the nucleation, these prismatic loops glided immediately down to reach a certain position where it remained stationary until newly generated loops force to glide downward in jerky manner. Once a certain number of loops were punched out (usually less than ten), they are coaxially aligned along the growth direction of the nanowire with preserving an equilibrium spacing between loops, which is determined by the stress fields of loops. Assuming the equi-sized loops, the lattice friction stress can be estimated from the inter-loop spacing, which is ~0.3 MPa. Since the nucleated loops continuously glided downward without direct interaction with other dislocations, it seems that prismatic dislocations, when confined in a small dislocation-free volume, do not necessarily attribute to strain hardening unlike geometrically necessary dislocations formed in bulk. Instead, these prismatic loops, approaching free surfaces in close proximity and under influence of surface image force, suddenly escaped through free surface with leaving slip steps, indicating that they assist surface nucleation and multiplication of ordinary dislocations.
3:45 PM - MB5.4.04/MB6.5.04
Grain Rotations in Ultrafine-Grained Aluminum Films—Insights from In Situ TEM Deformation with Automated Crystal Orientation Mapping
Ehsan Izadi 1 , Amith Darbal 2 , Rohit Sarkar 3 , Jagannathan Rajagopalan 4
1 SEMTE Arizona State University Tempe United States, 2 AppFive, LLC. Tempe United States, 3 SEMTE Arizona State University Tempe United States, 4 SEMTE Arizona State University Tempe United States
Show AbstractIn situ TEM straining is a widely used technique to investigate the deformation mechanisms of ultrafine-grained (UFG) and nanocrystalline (NC) metals. But obtaining statistically meaningful information to evaluate microstructural changes in these materials using traditional TEM bright-field/dark-field imaging or diffraction techniques is challenging and tedious.
Automated crystal orientation mapping in TEM (ACOM-TEM), in contrast, is highly suitable to perform crystallographic analyses on UFG/NC metals. In this technique, a precessing nanoprobe electron beam is scanned over the specimen to collect spot diffraction patterns. The orientation maps of the sample are extracted after indexing the diffraction patterns using a template matching process. ACOM-TEM enables direct acquisition of orientation/phase map over micron-sized areas while enhancing the ability to identify grains, microtexture and twin boundaries. This technique is particularly useful to monitor the microstructural evolution of UFG/NC metals during deformation.
Here, we use ACOM-TEM in combination with quantitative in situ TEM straining using a custom MEMS device to track orientation changes in hundreds of grains in a freestanding non-textured, UFG aluminum film (thickness 200 nm, mean grain size 180 nm) and correlate those changes with the macroscopic stress-strain response of the film. Our results show extensive grain orientation changes during loading, with both the fraction of grains that undergo rotations and their magnitude increasing with strain. The rotations are reversible in a significant fraction of the grains during unloading, leading to notable inelastic strain recovery. More surprisingly, a small fraction of grains rotate in the same direction during both loading and unloading, even though the applied stress is substantially different. The ACOM-TEM measurements also provide evidence for reversible as well as irreversible grain/twin boundary migration in the film. These microstructural observations point to a highly inhomogeneous and constantly evolving stress distribution in the film during both loading and unloading.
4:00 PM - MB5.4/MB6.5
BREAK
4:30 PM - *MB5.4.05/MB6.5.05
Grain Boundary Processes Involved in Nanocrystals Deformation and Failure
Marc Legros 1 , Frederic Mompiou 1 , Nicolas Combe 1 , Ehsan Hosseinian 2 , Olivier Pierron 2
1 Centre d’Élaboration de Matériaux et d’Etudes Structurales Centre National de la Recherche Scientifique Toulouse France, 2 Georgia Tech Atlanta United States
Show AbstractPlastic deformation of crystals is carried out by the motion of dislocations in most metals and alloys over a wide range of experimental conditions (stress, temperature, …). When this motion is hindered, either due to intrinsic atomic bonding that limits the dislocation mobility (ceramics and semiconductors below a certain temperature), or because of a large number of obstacles (point defects, other dislocations, precipitates…), plasticity is limited and the material may become brittle.
In nanocrystalline metals (d≤100 nm) the large proportion of grain boundaries (GBs) both limits the mean free path of dislocations and diminishes their availability. Two alternative mechanisms have been recently revealed using in situ TEM on nanocrystalline (nc) Al and Au. In nc-Al shear-migration coupling is able to accommodate large strains between two grains, but seems heavily dependent on diffusion when several grains need to be deformed together. The shear-migration coupling mechanism occurs through the motion of step-dislocations, confined to GB and also called disconnections. These disconnections are also observed during the controlled deformation of nc Au free standing thin films. Using a MEMS-based deformation platform, repeated tractions led to the propagation of cracks across the films. In the case of thicker films, some intragranular dislocation activity was evidenced, but the main mode of deformation and crack propagation remained grain boundary sliding. In this mechanism, GB dislocations are also heavily involved. Incompatibilities at triple junctions and shear perpendicular to the film led to crack propagation and failure.
Similarities can be drawn between disconnections propagating along GBs and dislocations shearing a crystal, but the former is far less understood than the later. The reason why shear-migration coupling is favoured in one case and sliding in the other is for example unclear, even if the deformation carrier (disconnection) seems the same. What we have shown is that considering perfect GBs to assess the properties of sliding or coupling may not be relevant as GBs in nanocrystals contain a sufficient amount of disconnections. How these defects are activated, that is how one "GB glide system" is selected over another seems the key question to understand plastic deformation in nanocrystals.
5:00 PM - *MB5.4.06/MB6.5.06
Investigation of Small-Scale Plasticity/Fatigue Mechanisms and Size Effects Using Advanced Transmission Electron Microscopy
Hosni Idrissi 2 1 , Vahid Samaeeaghmiyoni 2 , Jonas Groten 3 , Ruth Schwaiger 3 , Caroline Bollinger 4 , Francesca Boioli 5 , Thomas Pardoen 1 , Patrick Cordier 5 , Dominique Schryvers 2
2 Electron Microscopy for Materials Science University of Antwerp Antwerp Belgium, 1 Institute of Mechanics, Materials and Civil Engineering Université Catholique de Louvain Louvain-la-Neuve Belgium, 3 Institute for Applied Materials Karlsruhe Institute of Technology Eggenstein-Leopoldshafen Germany, 4 Bayerisches GeoInstitut, University of Bayreuth Bayreuth Germany, 5 Unité Matériaux et Transformations, UMR 8207 CNRS/Université Lille1 Lille France
Show AbstractIn the present work, the fundamental plasticity/fatigue mechanisms operating at interfaces in micro/nano-scale Ni samples have been investigated. in-situ SEM fatigue tests have been performed on FIB prepared single and bi-crystal micropillars with well-known orientations as revealed by electron backscatter diffraction (EBSD). Careful characterizations of the nature and the distribution of deformation dislocations, the character and the local structure of the interface as well as the mechanisms controlling the interaction between these defects under cyclic loads were performed using ex-situ TEM techniques including diffraction contrast imaging, automated crystallographic orientation and nanostrain mapping in TEM (ACOM-TEM) as well as electron diffraction tomography. Furthermore, quantified in-situ TEM nanotensile tests were performed on both single and bi-crystal samples in order to directly observe the plasticity mechanisms.
Recently, an original method combining the measurement of dislocation mobility using commercial in-situ TEM nanomechanical testing and dislocation dynamic (DD) simulations has been used to revisit the plasticity of olivine single crystals at low temperature [1]. Cyclic deformation was applied in the load control mode. Load was increased to a given value, which is maintained constant for several minutes before unloading. During the plateau, dislocation motion is observed and characterized (hence, under a known and constant applied stress). Using this method, we found that the intrinsic ductility of olivine at low temperature is significantly lower than previously reported values which were obtained under strain-hardened laboratory conditions. More generally, we demonstrated the possibility of characterizing the mechanical properties of specimens which could be available in the form of sub-millimetre sized particles only.
References
[1] H. Idrissi, C. Bollinger, F. Boioli, D. Schryvers, P. Cordier, Low-temperature plasticity of olivine revisited with in situ TEM nanomechanical testing. Science Advances. 2 (2016) e1501671.
5:30 PM - MB5.4.07/MB6.5.07
Cyclic Pseudo-Elastic Twinning in Small-Scaled BCC Tungsten
Scott Mao 1
1 Department of Mechanical Engineering and Materials Science University of Pittsburgh Pittsburgh United States
Show AbstractThis talk will be based on recent publication on In Situ Atomic-Scale Observation of Twinning Dominated Deformation in Nanoscale Body-Centred Cubic Tungsten, Nature Material (March 2015) by Jiangwei Wang, Zhi Zeng, Christopher R. Weinberger, Ze Zhang, Ting Zhu and Scott X. Mao. Twinning is a fundamental deformation mode that competes against dislocation slip in crystalline solids. Deformation twinning has been well documented in FCC nanoscale crystals. Here, by using in situ high-resolution transmission electron microscopy, we report that twinning is the dominant deformation mechanism in nanoscale bi-crystals of BCC tungsten. Such deformation twinning is found to be pseudoelastic, manifested through reversible detwinning during unloading. We find that the competition between twinning and dislocation slip can be mediated by loading orientation, which is attributed to the competing nucleation mechanism of defects in nanoscale BCC bi-crystals. Our work provides direct observations of deformation twinning under cyclic loading as well as new insights into the deformation mechanism in BCC nanostructures.
5:45 PM - MB5.4.08/MB6.5.08
In Situ TEM Dynamic Testing for Investigation of High-Cycle Fati