Symposium MB5 : Size Effects and Small-Scale Mechanical Behavior of Materials

The 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.

The 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.

A 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)

We 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.

Interfaces 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.

The 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.

We 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.

Ultrathin 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.

Thin 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.

In 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

While 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.

The 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 Pd_77.5Cu₆Si_16.5 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.

The 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.

A 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.

Hierarchical 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)

Porous 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.

Nanoporous 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.

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.

Nanoporous 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.

Ceria 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.

Nanostructures 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.

Elemental 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.

The 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.

Demands 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.

Dynamic 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.

Electro-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.

The 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.

For 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 (R_p0.2). 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.

In 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.

Coherent 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.

Under 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).

Recent 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.

We 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.

In 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.

Plastic 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.

In 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.

This 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.

Fatigue crack growth has long been investigated via post mortem analysis, leading to a phenomenological understanding of crack initiation at stress concentrators. However, post-mortem investigations can be very difficult for ultrafine grained materials, such as the Cu thin film in this study, and give little insight as to the dynamic changes in the material under cycling. In situ TEM studies can give a wealth of information, such as grain size, grain orientation, continuous monitoring of crack length/direction/radius, and plasticity present at the crack tip. Here, in situ fatigue is demonstrated using cyclic mechanical loading experiments at frequencies up to several hundred Hz. More than 10⁶ cycles can be reached within one hour. Moreover, the nanometer-scale spatial resolution of the TEM allows the observation of “incipient” crack growth rates of less than 10^-12 m●cycle^-1 very near to the minimum threshold stress intensity factor.

Work performed by K.H., B.L.B., and D.C.B. was fully supported by the Division of Materials Science and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy. Work by W.M.M. was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science under proposal #U2014A0026. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

The addition of nanoparticles into polymeric materials has changed dramatically the properties of the host polymers, promising a novel class of composite materials with different properties and added functionalities. We report on the thermo-mechanical properties and morphology of polyacrylic-nanoclays (Bentonite) composites prepared in-situ via emulsion polymerization, using a batch mode. The latex emulsion thus obtained was stable for at least six months. Moreover, this process produced un-controlled molecular weight in the final latex and high formation of agglomerates. The synthesized emulsion hybrids were characterized by Fourier transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC), contact angle, uniaxial tension, nano-indentation and scanning electron microscopy. Films drawn from the latex exhibited excellent optical transparency, suggesting good dispersion of the nanoclay, and confirmed by scanning electron microscopy (SEM). There was an increase in glass transition temperature, T_g, suggesting a modification of molecular dynamics and decrease in hydrophobic behavior, as probed by water contact angle, was also promoted. Moreover, the Young’s modulus and hardness of the nanostructured latex films increased up to 12 and 10 times with only 3 wt% nanoclay, respectively, therefore denoting a reinforcing effect of the nanoparticles onto polymeric matrix.

The present study is focused to investigate influence of short fibers such as Alumina Microfibers (AMFs), Silk Microfibers (SMFs) and Ceria Nanofibers (CNFs) as reinforcements in Bis-GMA/TEGDMA resin towards development of composite dental filler. Morphologies of AMFs, SMFs, CNFs and their representative fracture surfaces of the reinforced dental resins/composites were examined by SEM. X-ray Diffraction Analysis was done to analyse the phase of the fibers used in this study and degree-of-conversion of the fiber incorporated base resin was studied by FTIR. Viscosity study of fiber resin mixture, depth of cure and mass change behaviour of the fibers resin composites in artificial saliva were done to analyse the flow ability and physical properties of the fiber resin composites. Mechanical properties of the composites were tested by a universal testing machine. This study demonstrated that incorporation of 10% AMFs, 5% SMFs, and 3.33% CNFs individually in Bis-GMA/TEGDMA dental resin resulted in similar degree of conversion compared to the control. Also the fiber reinforced composites (10% AMFs, 5% SMFs, and 3.33% CNFs) demonstrated significant improvement in mechanical properties compared to Bis-GMA/TEGDMA resin (Control). However, depth of cure was significantly reduced due to incorporation of fibers in the resin. The reinforcement effect of AMFs, SMFs in dental resin was superior due to their uniform distribution and good interfacial bonding between fibers and resin matrix. In case of CNFs, rapid increase in viscosity during mixing of fibers with resin and inhomogeneous mixing were the major problem encountered during formulation, which was mainly associated with high surface to volume ration of the nanofibers. The resultant composite containing CNFs had less improvement in mechanical properties which may be due to less fiber content, formation of agglomerates and improper distribution of the fibers in the composite which subsequently resulted in reduction of adhesive strength.

In today’s world where cementitious materials make up the majority of US infrastructure including roads and bridges, it is necessary to better understand the nano-scale dynamics that lead to bulk failures and billions of dollars in repair costs. In this study, we take a bottoms-up approach to investigating these nano-scale dynamics and potential mitigation strategies. Here, we synthesize cement-based nanoparticles (CNPs) of controlled size, structure, and composition. Using nanoindentation, we determine the strength and ductility of individual CNPs. After CNP assembly, hydration (bonding), mechanical stressing, and subsequent characterization, we correlate mesoscale mechanical properties and failure mechanisms to individual CNP qualities, providing a database of knowledge for the future design of superior engineered materials.

The exceptional mechanical, electronic, and thermal properties of graphene render it a highly promising filler for polymer-matrix nanocomposites. We report results of molecular-dynamics simulation studies on the mechanical behavior of polymer (high-density polyethylene) nanocomposites reinforced by graphene and fullerenes with the aim of elucidating the underlying mechanisms that govern the mechanical response of these composite materials. Using a united-atom-based model of the glassy polymer matrix, we show systematic trends in the enhancement of the mechanical stiffness of the polymer composite as a function of filler concentration, filler size, and matrix–filler interfacial interaction strength that is governed by dispersive forces. For comparison of different filler reinforcement effects, we also present systematic mechanical behavior studies of fullerene–polymer composites and show that, for fullerene fillers, the response is only weakly dependent on filler size with relatively high loadings required for a considerable improvement in the composite’s stiffness. In contrast, graphene-polymer nanocomposites show an appreciable improvement in the stiffness reinforcement with increasing filler size. We explain the elastic response of the graphene-reinforced polymer-matrix composites through detailed atomic-scale characterization of the nanocomposites in conjunction with the predictions of an analytical continuum-mechanics model for understanding the filler-size dependent response of the polymer nanocomposites. We also derive size dependence scaling relations of the composite modulus in polymer-graphene nanocomposites at given filler concentration. Moreover, we report results of our continuum mechanical analysis for polymer-matrix reinforcement on the basis of shear-lag modeling and show a parametric dependence of the axial and interfacial shear stress at the graphene-polymer interface on the graphene flake size. Based on our atomistic simulations and this shear-lag analysis, we identify the range of filler length scale over which classical shear-lag theory becomes most effective for describing the polymer-matrix reinforcement.

Additive manufacturing (AM) is transforming the way we design and manufacture components.

Whereas macroscale AM of metallic materials is an established industrial process by now, present techniques are not suited for micro- and submicrometer scale fabrication. Hence, several different metal AM techniques enabling feature sizes well below 10 micrometer have been proposed and are presently evaluated.^[1]

The chemical composition and the microstructures of the deposits synthesized by the single techniques vary widely. As a result, the material properties of the fabricated structures are a pronounced function of the utilized AM method. Because the material properties are a major characteristic of each technique, besides resolution and geometric capabilities, the study of the resulting properties is important. While most research is focused on the electrical characterization of deposited structures, mechanical properties are often ignored, although they will be of importance for possible MEMS and sensor applications.

We presently study mechanical properties of structures fabricated by different micro metal AM techniques. We here present first results of nanoindentation and microcompression tests of deposits synthetized by electrohydrodynamic printing (EHD printing)^[2] as well as electrochemical FluidFM.^[3] These two techniques produce two distinct microstructures: a nanoparticle agglomerate in the case of EHD-printing and a dense, metallic microstructure in the case of FluidFM.

We show that the mechanical properties alter drastically. Additionally, we demonstrate the effect of annealing for the case of EHD-printing. Finally, we compare our results to available literature values for alternative metal AM techniques and present an outlook for our study of other metal AM techniques.

References

[1] A. Reiser et al., Adv. Mater, accepted.
[2] J. Schneider et al., Adv. Funct. Mater., 26, 833 (2015)
[3] L. Hirt et al., Adv. Mater., 28, 2311 (2016)

Intrinsically conducting (or p-conjugated) polymers and graphene related materials are attractive for manufacturing hybrid nanocomposite materials and devices owing to their multifunction at the constituents’ interface i.e. graphene/ conducting polymer. To utilize these materials, understanding of and relating their microscopic internal structure, surface morphology, and physical properties become indispensable. In this work, we report on the electrochemically synthesized conducting polymers with electrochemically processed graphene nanosheets forming hybrid films and the enhancement of mechanical properties of polymers arising due to synergistic effects promoted by nanostructured morphology and vicinal polymeric chain ordering induced by embedded or impregnated graphene nanosheets. We investigated surface topography, chain ordering, residual stress distribution, force curves and force volume imaging using micro-Raman spectroscopy, Raman mapping, atomic force microscopy and force spectroscopy gaining insights into structural and interfacial bonding, conjugation length distribution, and intermolecular forces. Traditional force curves measure the force felt by the tip as it approaches and retracts from a point on the sample surface (i.e. tip-sample adhesion force), whereas force volume is an array of force curves over an extended sample area. Moreover, detailed structural studies are able to demonstrate that the bonding configurations and structural conformations intertwined with crystallinity and surface chemistry in both the polymers and graphene derivatives nanosheets have a strong effect on nanoscale intermolecular forces and surface elasticity maps. Meanwhile the spring constant (k) estimated from the force curves using elastic contact model was synchronously enhanced for hybrid composites with interspersed graphene sheets on polymer chains forming a kind of ordered stacked structure. Alternatively, the mechanical properties of polymers are improved following in the order: PAni/ErGO > PPy/ErGO > PAni/GO > PPy/GO > PAni > PPy. The electrostatic force between the tip and residual charged polymers surface, are also discussed in terms of multilayer model consisting of three layers analogous to electrostatic double-layer. Furthermore, the information in the force volume measurement was decoupled from topographic data offering new insights into the materials’ surface and mechanical properties of hybrid nanocomposites. These measurements are complemented with electron microscopy and X-ray diffraction revealing their surface morphology, intrinsic structure and crystallinity having far reaching implications and impacts on alternative renewable electrochemical energy, photovoltaic and aerospace device applications. [1] Gupta eta. al. J. Composites Part B 92, 175 (2016).

Polymeric thin films have been received much attention as new materials for high-techs like semiconductor and flexible displays. Though the optical and electrical properties of polymeric thin films have been studied intensively, the mechanical properties of polymeric thin films have not. Inter alia, the adhesion strength of polymeric thin film as its mechanical properties has been reported in only few studies.
The adhesion strength is regarded as the major property in determining the reliability of the product in a thin film system. However, most of the conventional methods based on the interfacial peeling and fracture method have limitations. This study is for evaluating the quantitative adhesion using nano indentation as a non-destructive test method with polymeric thin film on the metallic substrate. Since its procedure is relatively simple and performed locally small part, indentation has its own merits.
Indentation test at the thin film/substrate combined system shows a load-displacement curve that including the effects of the interface like adhesion strength with its hardness and plastic zone size. The plastic zone is derived using the Expanding Cavity Model and interfacial constraint effect. The assumption that the plastic volume of the relatively soft materials is constrained by the hard materials is needed. In this case, polymeric thin film would be constrained as the relatively softer materials than substrate. Next, the interface parameter needs to be defined. The interface parameter means the ration of constrained plastic zone volume to original plastic zone assumed that there is no adhesion strength.
The hardness of adhesion strength would be obtained at the relative indentation depth 0.4 for composite system and 0.1 for the thin film. Finally, because of the variety of the adhesion strength with the experimental conditions, the work of adhesion is normalized to the final adhesion evaluation method.
With this study, the expectation that the adhesion strength of deposited thin film test using nano indentation would be made.

We investigated mechanical failure mechanism of highly transparent and flexible Ag-grid-graphene films by using electrohydrodynamic (EHD) jet printing process on PET substrate. To realize high-quality transparent conductive films, the invisible Ag grid with a line width of 7-8 μm was directly printed on graphene electrode by EHD jet-printing. The optimized Ag-grid on graphene films with Ag-grid pitch of 400 um showed a sheet resistance of 21 Ohm/square, a transmittance of 84.18 % at visible range (400 nm~800 nm). We examined the mechanical properties of Ag-grid on graphene films using various bending test mode such as the outer/inner bending test, twisting test, and rolling test to investigate mechanical failure mechanism of Ag-grid-graphene hybrid films. We observed that the resistance of Ag grid-graphene hybrid films abruptly increased at outer and inner bending radius of 3 and 2 mm respectively. Based on field emission scanning electron microscope examination results obtained from Ag grid-graphene hybrid film after outer/inner bending test, we suggested a possible mechanism to explain mechanical failure of Ag grid-graphene hybrid films. These properties expect that the Ag-grid on Graphene film is promising transparent flexible interconnect for using in electronic devices.

Problem-solving strategies of naturally growing composites such as nacre give us a fantastic vision to design and fabricate tough, stiff while strong composites. Bone and nacre are prime examples of natural ceramic-based composites with high strength and toughness. Previous studies on mechanical performance of these structural materials show that their outstanding properties are direct results of the nano-scale features and the optimized arrangement of the elements. Moreover, to provide the outstanding mechanical functions, nature has evolved complex and effective functionally graded interfaces. Particularly in nacre, organic-inorganic interface in which the proteins behave stiffer and stronger in proximity of calcium carbonate minerals provide an impressive role in structural integrity and mechanical deformation of the natural composite. However, further research on the toughening mechanisms and the role of the interface properties as a guide on design and synthesize new materials is essential. In this study, a micromechanical analysis of the mechanical response of “Brick-Mortar” and “Brick-Bridge-Mortar” composites is presented considering interface properties. The well-known shear-lag theory was employed on a simplified two-dimensional unit-cell of the multilayered composite. The closed-form solutions for the displacements in the elastic components as a function of constituent properties can be used to calculate the effective mechanical properties of composite such as elastic modulus, strength and work-to-failure. The results solve the important mysteries about nacre and emphasize on the role of organic-inorganic interface properties. The effect of mineral bridges is also studied. Our results show that the properties of proteins in mineral bridges proximity are also significant especially in increasing the elastic modulus of the structural composite. Detailed relationships are presented to identify future directions for advanced material design and development. More studies need to be performed on the effect of imperfect interface in manmade materials and on the strategies to enhance the interface properties.

Thin polymer films are widely used in diverse fields from microelectronics to medical devices. While it is widely accepted that properties of thin polymer films such as their glass transition temperature or elastic modulus deviate dramatically from their bulk values, there is not a consensus regarding the nature of this size effect. For example, the modulus of nanoscale polymer films has been reported in different studies to be larger and smaller than bulk values depending on the method used. Specifically, nanoindentation commonly measures an increased stiffness of materials such as poly(methyl methacrylate) (PMMA), while buckling experiments suggest a softening of thin films. It is known that the presence of a stiff supporting substrate artificially raises the apparent modulus of thin films, but no systematic way exists for separating this substrate effect and the true material properties of the system of interest.

Here, we present a combined computational and experimental method for measuring the true modulus of nanoindented thin polymer films. In particular, through extensive finite element simulations, we have determined a correction to the Hertzian contact model that depends upon a dimensionless film thickness and the Poisson’s ratio of the material of interest. Using this correction, we can in principle extract the modulus of soft films on hard substrates. To experimentally test this approach, we deposited and experimentally interrogated films of three model polymers with thicknesses ranging from 20 nm to 300 nm. When our atomic force microscope-based nanoindenting experiments were interpreted using this computationally-derived correction factor, we indeed found the modulus of sub-100 nm PMMA films to be smaller than that of bulk PMMA. Given the agreement between this method and the results of buckling experiments, we propose that this approach can serve as a general method for unambiguously determining the modulus of thin polymer films.

Detailed experimental characterization of the interactions between microscale full-field deformations and the corresponding microstructural characteristics in polycrystalline materials is critical in understanding the heterogeneous deformation mechanisms active in these materials at the grain and sub-grain level, and their impact on macroscopic mechanical properties. In this study, the microscale deformation behavior of a polycrystalline WE43 magnesium alloy was characterized using an experimental approach combining in-situ mechanical testing, electron backscattered diffraction (EBSD), and scanning electron microscopy combined with distortion-corrected digital image correlation (SEM-DIC). The evolution of full-field surface deformations was continuously tracked during in-situ mechanical loading over areas covering hundreds of grains, and the displacement and strain fields were obtained by SEM-DIC. Highly localized deformation phenomena including slip and twinning were identified in the high-resolution strain maps, and subsequently correlated with the active deformation modes through quantitative analysis of the displacement and strain fields. Statistically significant information was obtained for analyzing the distribution, evolution, and interaction of the deformation modes, was well as their interaction with the underlying microstructure during loading. The deformation fields and the activity of individual deformation modes were applied for tuning of a crystal plasticity finite element model developed in an integrated computational materials engineering (ICME) framework.

In this work, the cure kinetics of the two-component epoxy-based polymer DGEBA/DETA is investigated using concurrent Raman and Brillouin light scattering. Raman scattering allows one to monitor the in-situ reaction and quantitatively assess the degree of cure, while the Brillouin scattering measurements yield the elastic properties of the material, which provides a measure of the network connectivity. We show that the adiabatic modulus of the polymer evolves non-uniquely as a function of cure degree, depending on the cure temperature and the molar ratio of the epoxy system.

Accordingly, there are two mechanisms that contribute to the increase in the elastic modulus of the material during curing. First, there is the formation of covalent bonds in the network during the curing process. Second, once the bonds are formed, the structure undergoes structural relaxation toward an optimally packed configuration of the network, which enhances non-bonded interactions. Both contributions are apparent in the adiabatic modulus derived from Brillouin scattering, as it reflects the elastic response of the polymer network in thermodynamic equilibrium. Here we investigate to what extent the non-bonded interaction contribution to structural rigidity in cross-linked polymers is reversible, and to what extent it corresponds to the difference between adiabatic and isothermal moduli obtained from static tensile, i.e., the so-called relaxational modulus. To this end we subject the epoxy to various degrees of strain using a miniature tensile tester mounted in the beam path of the light scattering setup, and simultaneously measure the adiabatic and isothermal elastic moduli as a function of the applied strain and deformation rate.

Joining of nanostructured materials by conventional processes, such as welding, destroys the functionality of the material by disrupting its microstructure within the heat-affected zone. We present an approach to joining of nanolayered Cu/Nb composites without compromising the integrity of the nanolayered architecture by using a microstructure-preserving lap joint. The two parent materials are interconnected by a nanolayered composite section of the same total thickness and same nanolayered architecture as the parent material. The gap between the sections of parent material is kept constant at 20 um while the overlap of ends of the lap joint varies in length. Cu/Nb multi-layered nanostructure is synthesized by DC magnetron sputtering on Si substrate at room temperature. This study presents a structural characterization of the sputter-deposited Cu/Nb nanocomposite lap joint using Scanning Electron Microscopy and nano-indentation, exploring potential variation of structural properties between parent material, overlap area, and gap infill region. Our work advances future applications of nanocomposite materials by paving the way toward practical, microstructure-preserving methods of joining them.

Ordered arrays of particles interacting via Hertzian contacts are attracting increased attention due to a range of unique linear and nonlinear acoustic phenomena observed in these systems, which are referred to as granular crystals. While most work on granular crystals has been done with macroscopic particles, a new direction has been opened by recent acoustic experiments on monolayers of micron-sized particles. Vibrational dynamics of these microgranular assemblies differ significantly from those of their macroscopic counterparts, primarily due to the critical role played by adhesion forces on the microscale. Microgranular monolayers studied in early experiments possessed local hexagonal packing but lacked long range order on the scale of the measurement. In the present work we study acoustic dynamics of a fully ordered, “single crystal” hexagonal monolayer of polystyrene microspheres adhered to a glass substrate coated with a thin aluminum layer. A laser-induced transient grating technique is employed to generate and detect acoustic modes of the structure across the entire Brillouin zone (BZ) of the granular crystal. Measurements of the acoustic dispersion of the system reveal the presence of three distinct families of acoustic modes: low-frequency contact-based collective modes, high-frequency vibrations originating from spheroidal vibrational modes of individual microspheres, and Rayleigh surface waves in the substrate. Zone-folding of the Rayleigh wave dispersion at the BZ boundary indicates a perfect long-range order of the granular crystal. We identify three low-frequency contact-based modes predicted in a recent theoretical study and investigate their dispersion across the BZ. We observe a number of interesting phenomena in the behavior of the spheroidal modes: the interaction with the Rayleigh wave resulting in an avoided crossing, mode-splitting due to the sphere-substrate interaction and dispersion due to the sphere-sphere interaction. These observations are compared with the theory we develop within the small perturbation approach.

The flexural properties of reduced graphene oxide (rGO)-cellulose nanocrystals (CNC) composite nanomembranes have been characterized by using both AFM and bulging test. We found that the flexural properties of rGO-CNC nanomembranes depend on rGO weight contents (wt%). The improved mechanical properties of the nanomembrane highlight strong bonding between the rigid nanorod components and the flexible rGO sheets. The viscoelastic flexural behavior of rGO-CNC nanocomposites was investigated by applying different ramp rates, and it was found that the relaxation time gradually decreased with an increase in the ramp rate. It proved deformational-rate dependent interfacial bonding strength existing between rGO sheets and CNC. The bulging test was also performed to assess the flexural properties of rGO-CNC nanocomposites under conditions of uniform pressure. The maximum elastic bending modulus of the rGO-CNC nanocomposite, 141 GPa, was obtained by the AFM bending test, and it corresponded with the elastic tensile modulus measured from the bulging test. It indicates both methodologies have high credibility to characterize the flexural rigidity of ultrathin nanomembranes. We demonstrated that the freely suspended ultrathin rGO-CNC nanomembranes are highly flexible as well as highly resilient in comparison with other engineering materials. The findings of this research provide guidance for designs and applications for rGO-CNC based multi-functional and flexible devices.

A large number of materials which are being investigated for future use in energy and transport applications are hard and brittle at room temperature. Their plastic deformation is often poorly understood in the low temperature regime due to the low fraqcture toughness impeding conventional mechanical testing. However, it is this understanding which is soften necessary to assess the effects of alloying or processing strategies.
Here, examples will be shown of how to use nanomechanical testing to study plasticity mechanisms in this kind of materials and extend our knowledge of plasticity in more complex crystals. Microcompression allows the plastic deformation of the most brittle materials due to an effect of size favouring plasticity at small scales. This allows individual slip systems to be activated and quantiatively investigated, which is not possible in indentation. These experiments are complemented by nanoindentation at variable rates and at up to 1000 °C and the resulting deformation analysed by EBSD, TEM and AFM. Examples of materials to be discussed are Mg₁₇Al₁₂, for which a change in deformation mechanism is discussed in the light of reinforcement of Mg alloys, MoSi₂, in which the effect of alloying on the activation of additional slip systems has been studied and an MCrAlY superalloy coating which is tested for the first time in terms of hardness and creep by nanoindentation at up to 1000 °C.

The crystalline and amorphous phases are the two principal states of silicon, and the transformation between them has attracted great attention. One route to realize the crystalline-to-amorphous transition (CAT) in Si is mechanical loading. This has relevance in practical applications; for example, an amorphous layer always forms after mechanical polishing of single crystalline Si (c-Si) wafers, causing damage that degrades the performance of integrated circuits and electronic devices. The CAT has also been recognized to play a role in the incipient plasticity of bulk c-Si at room temperature. Stress-induced amorphization in Si has in fact been widely observed under various mechanical loading conditions, such as indentation, ball milling, scratching and bending. Despite of numerous efforts for decades, the physical mechanism of mechanical stress-induced CAT in Si is still under considerable debate owing to the absence of direct experimental observation and quantitative analysis. Here, we have devised a novel and effective c-Si (crystalline silicon)/a-Si (amorphous silicon) core/shell sample configuration, which enabled large plastic deformation and in situ monitoring of the CAT process inside a transmission electron microscope (TEM). The malleable a-Si shell helped to inhibit brittle fracture occurring in the submicron-sized sample, and provided the confinement to significantly raise the stress level and extend the plastic flow in the c-Si core. Our in situ TEM uniaxial compression experiment unambiguously demonstrated that diamond cubic Si transforms into a-Si through slip-mediated generation and storage of stacking faults, without involving any intermediate crystalline phases. By employing density functional theory (DFT) simulations, we find that energetically unfavorable single-layer stacking faults create very strong antibonding interactions, which trigger the subsequent structural rearrangements. Our findings thus resolve the interrelationship between plastic deformation and amorphization in silicon. Our experimental protocol also opens up the possibility for in situ investigation of plastic deformation and deformation-induced amorphization in other brittle solids.

Magnesium (Mg) and its alloys have been garnering significant interest as structural materials for many technologically relevant applications due to the their lightweight, high strength, and superior damping capacity. However, the use of Mg alloys as a structural material has been limited to date due to their poor ductility and formability. The poor ductility of Mg originates from the hexagonal closed packed (HCP) lattice structure, which exhibits low crystal symmetry. As a result, Mg does not have sufficient active slip systems at room temperature, which dictates that deformation twinning plays an important role in accommodating plastic deformation. Thus, quantifying the competitive nature between slip and twinning is necessary to understand the deformation behavior of Mg and subsequently improve its properties through alloying. Here, we present a comprehensive experimental study to understand crystal size and temperature effects on the transformation in deformation modes in twin oriented Mg single crystals. Single crystal micropillars with size ranging from 2 μm to 23 μm were fabricated using FIB milling, then tested by in-situ SEM nanoindentation. First, we report crystal size effect on the transformation of deformation mechanisms at room temperature. The experiments reveal two regimes of size effects: (1) single twin propagation, where a typical “smaller the stronger” behavior was dominant in pillars ≤ 18 μm in diameter, and (2) twin-twin interaction, which results in anomalous strain hardening in pillars > 18 μm. Molecular dynamics simulations further indicate a transition from twinning to dislocation mediated plasticity for crystal sizes below a few hundred nanometers. In the second set of experiments, we report temperature effect (room temperature, 100^oC, and 150^oC) on the deformation mechanism of micropillars with fixed crystal size (6 μm in diameter). Unusually high flow stress was observed at 100^oC while twinning still governs plasticity. Surprisingly, at 150^oC, we observed strain softening due to dislocation mediated plasticity. Transmission electron microscopy (TEM) imaging was utilized to understand the mechanical response and the governing deformation mechanism operating at different temperatures.

Crystalline, super-elastic materials typically exhibit large recoverable strains due to the ability of the material to undergo a reversible crystallographic phase transition between martensite and austenite phases. Applicable to various alloys, ceramics and intermetallic compounds, this reversible transition serves as a general mechanism for superelasticity. In our recent work, a new superelasticity mechanism has been observed in a novel ternary intermetallic compound, CaFe₂As₂. CaFe₂As₂, a ThCr₂Si₂-type intermetallic compound, exhibits a reversible phase transition between either a tetragonal (above 170 K) or orthorhombic (below 170K) phase to a collapsed tetragonal phase. Additionally, a cryogenic linear shape memory effect near 0 K was observed. From this work, we presented that the collapsed tetragonal phase transition may also serve as a general mechanism, which accounts for superelasticity and shape memory effects for various ThCr₂Si₂ structured intermetallic compounds.
In this work, two very different ThCr₂Si₂ structured intermetallic compounds, LaRu₂P₂ and CaFe₂As₂, were investigated and a unified mechanism accounting for superelasticity and shape memory effect in this class of materials is presented. Single crystals of LaRu₂P₂ and CaFe₂As₂ were solution-grown in Sn flux. Micropillars were fabricated using focused-ion beam machining. Continuous contact stiffness measurement of micropillars compressed using an in-situ nanomechanical device, as well as in-situ X-ray diffraction studies on bulk crystals, produces clear evidence of a reversible phase transition at room temperature during uniaxial and hydrostatic compression, respectively. Density functional theory calculations demonstrate that the atomic radius of the Th-type element and the electrostatic attraction between layers of the Si-type element determines the capability and availability of superelasticity and the cryogenic linear shape memory effect. Our results offer the possibility of developing cryogenic linear actuation technologies which boasts high precision and ultrahigh values of actuation power per volume for deep space exploration. Additionally, results presented suggest a mechanistic path to design of a new class of shape memory materials.

Crystals of the ThCr2Si2-type structures have been investigated for the superconductivity. Recently nano-indentation experiments have shown that small-scale crystals of CaFe₂As₂ can exhibit superelastic behavior with recoverable strains of over 10%. In this talk, we demonstrate that this behavior can be attributed to the phase transition inherent to these materials using density functional theory as in conjunction with analytical models. The models are then able to demonstrate that the super-elastic behavior depends on the type of loading and that is confined to uniaxial loading. The behavior of CaFe₂As₂ is then compared to LaRu₂P₂ which shows differences in its superelastic response, which are compared to recent experimental results.

This presentation will review the recent progress in the understanding of plastic deformation in ultra-fine scale metal-based composites. Examples will be presented from a variety of material systems: laser-processed Al-Al₂Cu lamellar eutectic, vapor deposited Cu-TiN and Al-TiN thin films, and Cu-Nb multilayers rolled to large plastic strains. The common aspects in interface-dominated mechanical behavior in ultra-fine scale metallic composites such as unusually high flow strengths, high strain hardening rates and plastic co-deformability will be elucidated through in situ TEM straining experiments and analyzed using atomistic modeling, dislocation theory and crystal plasticity. The strain hardening behavior of confined systems will be interpreted using a three-dimensional crystal elastic–plastic model that describes plastic deformation based on the evolution of dislocation density in the constituent phases. The conditions that lead to morphological and crystallographic stability of interphase boundaries in certain composite systems at extremes of mechanical straining will be highlighted.

Al/SiC metal-ceramic multilayers were manufactured by magnetron sputtering. Different nanolaminates were manufactured with the same nominal values for the Al and SiC layer thickness (in nm): 10/10, 25/25, 50/50 and 100/100 and cylindrical micropillars of 2 µm in diameter and 4 µm in height were milled with a focused ion beam (FIB). Nanoindentation and micropillar compression tests were carried in the Al/SiC multilayers in the direction perpendicular to the laminate at 25C and 100C. In addition, the deformation mechanisms in the 100/100 nanolaminate were ascertained by means of in situ micropillar compression tests in the transmission electron microscope. It was found that deformation was controlled by the plastic deformation of the Al layers that took place by the nucleation of the dislocations at the metal-ceramic interface. The dislocations were absorbed in the opposite interface and the Al plastic flow was constrained by the stiff SiC layer.
The hardness of the multilayers at ambient and elevated temperature was fairly independent of the layer thickness, while the strain hardening and yield strength of the Al layers increased significantly as the layer thickness decreased at both temperatures. Numerical simulations of the hardness and micropillar compression tests were in agreement with the experimental results and showed that hardness was independent of the layer thickness because the Al deformation was fully constrained regardless of the layer thickness. However, the constrain imposed by the ceramic layers during micropillar compression was much higher in the case of the thin nanolaminates (10/10 and 25/25), leading to a very large strain hardening which was not found in the thicker nanolaminates. Under constrained deformation, the mechanical response of the nanolaminate was weakly dependent on the Al yield strength and the influence of the temperature on the mechanical properties was limited.

In recent years two-phase nanolayered composites with individual layer thicknesses varying from 200-300nm down to 1-2 nm have been the subject of intensive study because of their unusual physical, chemical and mechanical properties. For example, with decreasing layer thicknesses (down to nanometer length scales) the mechanical response of these nanocomposites becomes increasingly interface dominated, and they exhibit ultrahigh strengths approaching the theoretical limit for ideal crystals. Moreover if the constituent phases present large differences in strength, elastic modulus and ductility, these multilayers give rise to new possibilities for the deformation mechanisms and properties of the composite as a whole. In this work we explore the possibility of synthesizing multilayered composites where one constituent phase has a low ductility, with a final goal of enhancing both the strength and ductility of the system.

Using physical vapor deposition (PVD) techniques we synthesized a hexagonal close-packed (HCP) – body-centered cubic (BCC) Mg-Nb system (where twinning in Mg leads to its lack of ductility), over a range of layer thicknesses ranging from 5 nm to 200 nm. Testing of such miniaturized poses significant challenges. We utilize a combination of nanoindentation, in-situ SEM compression testing of micro-pillars, and in-situ SEM fracture toughness testing of 3 point bend micro-beams containing these multilayered nano-composites to evaluate their deformation mechanisms. Micropillar testing for three different orientations, with the interfaces oriented normal, parallel and oblique (45^o) to the compression axis, enable us to explore the anisotropy in the mechanical response of the multilayer system, while the fracture toughness of the specimens are measured using the notched 3-point bend tests. These results are compared for varying layer thicknesses as well as under varying ambient temperatures.

Additionally our work shows that at low enough layer thicknesses the crystal structure of Mg can be transformed and stabilized from simple hexagonal (hexagonal close packed hcp) to body center cubic (bcc) at ambient pressures through interface strains,. We show that when introduced into a nanocomposite bcc Mg is far more ductile, 50% stronger, and retains its strength after extended exposure to 200 C, which is 0.5 times its homologous temperature. These findings reveal an alternative solution to obtaining lightweight metals critical needed for future energy efficiency and fuel savings.

Two-phase nanolaminate thin film composites have demonstrated an unusually broad number of desirable properties under extreme environments, such as high strength, high strain to failure, thermal stability, and resistance to light-ion radiation. The microstructures that arise from different synthesis routes such as Physical Vapor Deposition or Severe Plastic Deformation can vary widely in terms of layer morphology, local structrure, texture, and resulting mechanical behavior. Recently we have shown that bi-phase HCP/BCC nanolaminates with individual layer thicknesses approaching 10 nm can be made via severe plastic deformation (SPD) in bulk sizes suitable for structural applications. Mechanical testing of these HCP/BCC nanolaminates shows exceptionally high strength and characterization via a suite of techniques including neutron diffraction, EBSD, and TEM indicates that the crystals are highly oriented. While the cause of these unusual properties can easily be associated with a high density of bimetal interfaces, how the interfaces physically control microstructural evolution and macroscopic properties remains an area of intense research. This presentation highlights our modeling and experimental efforts to understand and link the evolution of the nanostructure, the interface properties, and preferred texture during the SPD process.

Metallic nanowires show potential for use in a range of applications, including but not limited to sensing, actuators and NEMs¹ (Nano-Electromechanical systems) devices. However, before nanowires can be incorporated into functional devices, it is important to fully understand their mechanical properties. There is a pre-existing stress associated with wires deposited on an Si/SiO₂ surface in the form of tension or compression². It is essential to establish whether this pre-existing stress is inherent in the wire or if it is a result of deposition onto a substrate. This must be understood prior to the incorporation of wires into functional devices as the stress can have a profound effect on the measured stiffness.

This project focuses on the use of a three point AFM bending experiment previously developed in our lab³ whereby the Young’s Modulus of individual nanowires was extracted. Nanowires are suspended over pre-defined trenches etched in SiO₂. This allows lateral manipulation of the wire perpendicular to its long axis without friction effects between the wire and the substrate. Through careful tip calibration⁴ the lateral deflection on the photodiode can be converted to produce a Force-Displacement curve. This Force-Displacement curve is then fitted to the Sader model⁵ for a clamped-clamped beam. The effect of the pre-existing stress on the fit to the Sader model is examined along with an investigation into the origin of the stress. One certain way to remove the stress from the wires is to create a cantilever beam out of the wire by focused ion beam (FIB) cutting. Through lateral AFM manipulation of the now singly clamped wires the force response can be fitted to Euler beam theory to extract the correct Young’s Modulus values. However further investigation into the basis of this pre-existing stress is required before these wires can be incorporated into devices such as NEMs or interconnects.

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Metallic microwires with a controlled diameter in the range from 7 to 100 µm are produced by a scaled-down investment casting process. Molds are first produced by the slow crystallization of high-purity salt around polymer or carbon preforms that are subsequently pyrolized. The liquid metal is then infiltrated into the salt cavities under a pressure of 1.5 MPa and directionally solidified. After dissolution of the salt, thin single crystalline wires are obtained, with a low dislocation density and a high aspect ratio (≈50), making them particularly suitable for tensile testing. Plastic deformation of these microscale wires is characterized by jerky flow and the emergence of slip steps at the free surface, the number of activated slip systems depending on the initial crystallographic orientation of the wire with respect to the load axis. We will present here results for different compositions, namely pure Al, pure Mg, and Al-Mg alloys, and focus on how plastic deformation is affected by the crystal structure and the presence of solute atoms or second phases. Characterization of the deformation behavior involves statistical analysis of the strain burst distributions and post-mortem SEM observation of the wires.

Deformation twinning mediated pseudo-elasticity with recovery tensile strain exceeding 40% has been previously reported in <110> oriented rhombic FCC metal nanowires with {111} surface facets based on molecular dynamics simulations. The {111} surface facets play critical roles during the deformation process, therefore the same mechanism does not operate to enable pseudo-elasticity in <110> oriented cylindrical nanowires made of the same materials. In this work, we report two forms of pseudo-elasticity in cylindrical FCC metal nanowires based on atomistic simulations, which are also based on deformation twinning but in nanowires with dramatically different microstructures. The first type of microstructure to enable pseudo-elasticity is cylindrical nanowire with tilted coherent twin boundaries, while the second type of nanowires are single crystalline but oriented at a direction deviated from <110>. Since one dimensional nanostructures manufactured from FIB milling or templated assisted electrodeposition are mainly cylindrical, the findings from this research may open door for new applications of cylindrical metallic nanostructures.

Molecular dynamics simulations are performed to investigate the deformation behavior of an ultralight architecture consisting of hollow Ni nanotubes arranged as a 3-D kagomé structure. As a precursor, we have also investigated mechanical response of a single hollow Ni nanotube under homogeneous and flat-punch compression. We observe that 1/6(112) Shockley partial dislocations and twin formation at 3.5% compression collapses the nanotube. Hollow nanotube architecture can withstand much larger compression than a single Ni hollow nanotube. In the case of hollow nanotube architecture under flat punch compression, most of the deformation is observed near the node of the kagomé structure. The deformation is caused by 1/6(112) Shockley partial and 1/2(110) dislocations. No twin formation is observed in the strained kagomé structure. At 12.5% compression, we observe plastic buckling of the kagomé structure.

To date, most studies on nanotwinned materials have focused on single element systems which have limited engineering applications. Introducing alloyed systems allows for a wide range of new materials thus expanding the synthesis space for nanostructure materials. In this work, the mechanical properties of fully nanotwinned Cu-Al and Cu-Ni alloys are evaluated by tension testing utilizing digital image correlation. By individually varying twin thickness, grain size, and composition, the isolated effect of each of these parameters on both strength and ductility will be presented

Thin metal films may both contain extremely high densities and support extremely high stresses, which may lead to microstructural instabilities. Structural transformations may occur at room or elevated temperatures and may include grain growth, orientation changes, densification, and morphology changes, which, in turn, lead to changes in properties such as strength, stiffness, resistivity and reflectivity. We have developed a method for inducing microstructural transformations by inducing high concentrations of twin boundaries in a series of thin FCC metal films. Such nanotwinned structures are of interest because of the potential to create materials with high strength and high ductility. However, these defects represent additional stored energy in the film. Our current experiments suggest that these structures will not be stable against deformation, time and temperature. We discuss the magnitudes of the available driving forces as well as strategies for stabilizing these defective structures.

Nanostructured polycrystals in the form of films can be produced by first crystallizing the “grains” as ligand-capped nanocrystals, depositing them by self-assembly, and then removing the ligands by plasma processing. This approach bypasses the probabilistic nature of bulk nucleation, and can therefore can form polycrystals with uniform, nanoscale grain sizes
We describe the evolution of the structure, composition, and mechanical properties of these materials during the removal of the ligands. We elucidate how the organic binder is fully removed, and how crack-formation due to volume loss can be completely prevented by controlling the arrangement of the particles in the assembly. We then characterize the mechanical properties of the assemblies during ligand removal and during sintering.
Characterization of the mechanical properties through substrate-corrected nanoindentation and the subsequent analysis through the mechanical model of Kendall et al. indicates that the arrays behave as granular matter before processing as well as after the complete removal of the ligands, albeit with radically different moduli and hardnesses (modulus, E: 2.2 ± 0.3 vs 44.5 ± 2.6 GPa; hardness, H: 0.1 ± 0.02 vs 2.2 ± 0.1 GPa). This intermediate state displays non-granular behavior with ~4 times the strength of a granular system of same porosity.
During the ligand removal by plasmas we show how the mechanical properties go from granular (before plasma) to non-granular (during plasma), and back to granular (upon complete ligand removal). The grain boundary composition can be controlled by the ligand chosen for the synthesis of the particles and has a tremendous influence on the thermal processing of the resulting materials and its mechanical properties.
References:
Nanocrystals as Precursors for Flexible Functional Films
L. Cademartiri*, G. von Freymann, A. C. Arsenault, J. Bertolotti, D. S. Wiersma, V. Kitaev, G. A. Ozin*
Small 2005, 1 (12), 1184-1187
Building Materials from Colloidal Nanocrystal Arrays: Preventing Crack Formation During Ligand Removal by Controlling Structure and Solvation
S. Shaw, B. Yuan, X. Tian, K. J. Miller, B. M. Cote, J. L. Colaux, A. Migliori, M. G. Panthani, L. Cademartiri*
Advanced Materials, accepted
Evolution of the Structure, Composition, and Mechanical Properties of Colloidal Nanocrystal Films Upon Removal of Ligands by O₂ Plasma (communication)
Santosh Shaw, Julien L. Colaux, Jennifer L. Hay, Frank C. Peiris, Ludovico Cademartiri*
Advanced Materials, revision submitted

Significant attention has been given to size-dependent plasticity in metals with sub-micrometer grain sizes but other deformation mechanisms, including time-dependent stress relaxation mechanisms, are also active in these materials. The ability to control viscoplastic and viscoelastic deformation in metallic films may prevent failures in micro-electro-mechanical systems (MEMS) devices so there is technological value as well as scientific value in improved understanding. If the active mechanisms are related to the presence of grain boundaries and nearby free surfaces then a strong dependence of relaxation rate on grain size and film thickness might be expected. Conversely, if the primary mechanisms occur within the lattice then other length scales may be more important. In this work we will explore the mechanisms that dominate the low-strain (0.1%), viscoelastic portion of stress relaxation once any initial viscoplasticity has been depleted. Deformation dependence on a wide range of grain sizes (10s up to 1000 nanometers) of sputtered gold films tested via the bulge method will be presented. Compositional and thermal effects will be included in the discussion of the size-dependence of relaxing thin films.

Axial compression was conducted on micro-pillars, in which polycrystalline Cu thin films were sandwiched between CrN and Si. Extensive plastic flow of Cu was achieved, when the Cu films are inclined either at 90° or 45° with respect to the pillar axis. The texture of Cu films was altered by changing the template on which film growth occurred. The Cu microstructure was further altered by post-deposition annealing. The flow stress shows little dependence on the film texture in the as-deposited state. However, annealing influences the flow stress of confined Cu films significantly. Experimentally measured flow stresses are compared to output of a simple strain gradient plasticity model. Implications of present results on plasticity modeling are discussed.

References:
3. Y. Mu, X. Zhang, J.W. Hutchinson, W.J. Meng, “Dependence of confined plastic flow of polycrystalline Cu thin films on microstructure”, MRS Comm., accepted (2016).
2. Y. Mu, J.W. Hutchinson, W.J. Meng, “Micro-pillar measurements of plasticity in confined Cu thin films”, Extreme Mechanics Letters 1, 62-69 (2014).
1. Y. Mu, K. Chen, W.J. Meng, “Thickness dependence of flow stress of Cu thin films in confined shear plastic flow”, MRS Comm. 4, 129-133 (2014).

The mechanical properties and deformation behavior of materials at the nanoscale have been intensively studied in the past decade to clarify the effects of nanosize and nanostructure on mechanical behavior as well as to verify the structural stability and operation reliability of nanoscale materials. However, most of the studies were focused on examining the mechanical behavior of nanoscale metallic materials. Whether the deformation mechanism of nanoscale ionic materials is different from that of bulks remains unclear. Therefore in this study, in-situ compressions of single-crystalline, face-centered cubic ionic MgO nanopillars along a [111] direction were conducted in a transmission electron microscope, and the activities and slip system of dislocations were examined. Conventionally for bulk ionic materials, dislocation activities on the charge-balanced {110}〈110〉 slip system are recognized, whereas activities on the non-charge-balanced {111}〈112〉 system are prohibited. However in this study, for the ionic nanopillars under compressions along the [111] direction, the traditional {110}〈110〉 slip system with a resolved shear stress of zero was replaced by the unusual {111}〈112〉 system for partial dislocations and the {100}〈110〉 for complete dislocations, yielding a certain extent of plastic deformation. The activation of dislocation glides on the unusual {111}〈112〉 system of ionic materials at the nanoscale is possibly attributed to a nanosize effect on the change of lattice friction and the transition of slip system, equivalent to a high pressure effect on the critical shear stress of different slip systems in MgO single crystals as suggested by modeling.

Carbon nanotubes (CNTs) possess low density, excellent flexibility, and high tensile strength. In an effort to take advantage of their attractive mechanical properties, CNTs have been used as a reinforcing material for metal or ceramics composites. By mixing CNTs with matrices (metal or ceramics species), we can prepare strong CNT/metal or CNT/ceramics composites. However, the mechanical properties of these composites have not been significantly improved. Do matrices affect the intrinsic mechanical properties of CNTs? Do matrices bind the carbon atoms of CNTs? We need to figure out the influence of the metal or ceramics on the mechanical properties of CNTs. In this paper, we report the influence of surface-supported materials, metal or ceramics nanoparticles, on the tensile strength of an individual multi-walled CNT (MWCNT).
We synthesized vertical aligned MWCNTs by the chemical vapor deposition method. Individual MWCNTs were picked up by pushing up half of a copper grid for transmission electron microscope (TEM) observation to the MWCNT forest, and the TEM grids were clamped on a glass slide. Following this, platinum/palladium (Pt/Pd) or aluminum nanoparticles were spattered to the TEM grids by a magnetron sputtering apparatus. The aluminum nanoparticles were oxidized in an electric furnace at 50 °C for 6 h to form aluminum oxide (Al₂O₃) nanoparticles. We performed tensile tests of individual MWCNTs without and with the supported Pt/Pd or Al₂O₃. The tensile tests were carried out with a nano-manipulator inside a scanning electron microscope.
The average tensile strengths of the individual MWCNTs without and with Pt/Pd were 3.92±1.57 and 3.00±1.26 GPa, respectively. Statistically, these strengths were not significantly different. On the other hand, the average tensile strengths of the individual MWCNTs without and with Al₂O₃ were 1.52±0.73 and 2.06±0.78 GPa, respectively, indicating these strengths were equivalent statistically as the case of the Pt/Pd. Thus, the fracturing loads of the MWCNTs did not change regardless of the presence or absence of the surface-supported materials in this study. We consider that there is no interaction between the electron orbitals of the surface-supported materials (Pt/Pd or Al₂O₃) and the π electron orbital of the MWCNT. Our findings suggest that the matrix, Pt/Pd or Al₂O₃, in the CNT composites does not affect the tensile strength of individual MWCNTs.

Axial buckling of nanostructures on a compliant substrate as a result of mechanical straining has become prevalent due to their promising applications in flexible electronics, sensors and energy harvesting devices. Literature shows that the mechanical buckling of nanostructures may varies between in-plane and out-of-plane mode, depending on the stiffness and amount of pre-strain of the compliant substrate, the geometry of the nanostructure and interfacial adhesion between the two components. In our experiments, 15 µm-long Si nanoribbons with different widths (i.e. 60, 80, 100 and 200 nm) but constant thickness of 30 nm were deposited on a 100 % pre-strained PDMS substrate treated with ultra-violet/ozone (UVO) surface treatment at a duration (t) of 0, 90 and 150 s. UVO treatment increases the surface modulus and hydrophilicity of the PDMS substrate, thus enhancing the chemical bonding between the Si nanoribbons and the PDMS substrate. Without UVO treatment, 60 and 80 nm-wide Si nanoribbons buckled in-plane but no buckling was observed for those wider ones. Buckling did not occur on 100 and 200 nm-wide Si nanoribbons because the applied pre-strain was lower than its critical buckling strain to initiate buckling. When t = 90 s, all nanoribbons exhibited in-plane buckling mode except the 200 nm-wide Si nanoribbons. As the t increased further, those three groups of Si nanoribbons showed possibility of both in-plane and out-of-plane buckling modes. On the other hand, regardless of the UVO treatment duration, out-of-place buckling was favorable in the 200 nm-wide Si nanoribbons. In-plane buckling is a result of sliding between the Si nanoribbon and the PDMS substrate while out-of-plane buckling indicates stronger adhesion between the two components, considering the effect of contact area.

It has been reported that elastic modulus of polymer nanofibers noticeably increases with decreasing diameter of nanofibers due to rigid conformation of polymer chains. In this study, size effect on the crystal structure, thermal behavior and molecular mobility of Nylon 6 nanofibers (NFs) were studied. We found that NFs with smaller diameter tend to form α–phase crystal. The T_m for α–phase crystal increases with decreasing NFs diameter. The higher T_m indicates that NFs with smaller diameter possess more rigid crystal structure. On the contrary, T_m for β–phase crystal decreases with decreasing NFs diameter. The decreased T_m of β–phase which is located on the surface of NFs suggests that the Nylon 6 crystals on the free surface of NFs has lower thermal stability. The molecular mobility of NFs was observed by analyzing spin-lattice relaxation time (T₁). In the low-temperature region (< T_min), correlation time increases with decreasing diameters of NFs. The increase of correlation time is attributed to the rigid structure of polymer chains. In the high-temperature region (> T_min), activation energy of NFs rapidly increases with decreasing temperature because the relatively larger molecular motion at high temperature could be hardly occurred in a rigid chain structure.

Ultrathin gold nanowires, made by bottom-up chemical synthesis, have recently emerged as ideal candidates for future nanoelectronics applications at sub-10nm level. One of the paramount criteria to evaluate the performance of ultrathin gold nanowire-based devices is their stability under real service conditions, such as upon cyclic Joule heating as nanoscale interconnects. In this study, Rayleigh instability, a phenomenon discovered by Plateau and theoretically studied by Rayleigh in 1870s, of ultrathin gold nanowires has been systematically studied: When sizes down to nanoscale, as their surface-to-volume ratio increases considerably, metallic wires are believed to be more sensitive to Rayleigh instability; while in this work, by using in situ transmission electronic microscopy (TEM), we demonstrated that Rayleigh instability is much more prominent for ultrathin gold nanowires upon Joule heating, even the e-beam heating will tremendously alter the surface geometry of ultrathin gold nanowires. We further studied the energy and size dependency of Rayleigh instability in ultrathin gold nanowires, and reveal that the underlying mechanism of this sub-10nm Rayleigh instability should be based on surface atom diffusion.

Dislocation nucleation from interfacial defects dominate plastic deformation of materials in a confined volume, which may have a limited number of plastic deformation carriers within the confined volume. Because dislocation nucleation is a thermally activated rate-controlling process at finite temperature and is usually rate-sensitive, the temperature and strain-rate sensitivities should be carefully studied at realistic timescales. Thus, in this study we extend the timescale of molecular dynamics (MD) simulations using adaptive-boost MD (ABMD) and study the dislocation nucleation with atomic-level resolution at finite temperature and an experimentally reasonable timescale, which offers opportunities for more comprehensive investigation of these processes arising from interfacial defects. The ABMD reveals a mechanism transition and strong temperature dependence of dislocation nucleation from grain boundaries (GBs) in Cu. At stress levels up to ~90% of the ideal dislocation nucleation stress, atomic shuffling at the E structural unit in a GB acts as a precursor to dislocation nucleation, and eventually a single dislocation is nucleated. At very high stress levels near the ideal dislocation nucleation stress, a multiple dislocation is collectively nucleated. The mechanism transition can be found under not only uniaxial but also complex loading conditions. The activation free energy and activation volume depend strongly on temperature. We also calculated the activation parameters of the second nucleated dislocation from intrinsic stacking fault facets on the GB where an initial dislocation has been emitted. The result shows that the dislocation nucleation from the imperfect GB is much easier than that from perfect GB and nucleation process is also sensitive to temperature. The findings of the mechanism transition and strong temperature dependence of GB dislocation nucleation have been missed in the previous conventional molecular dynamics studies.