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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Nanocomposite wires composed of a multi-scale Cu matrix embedding Nb nanotubes are in-situ cyclically deformed in tension under synchrotron radiation in order to follow the x-ray peak profiles (position and width) during mechanical testing. The evolution of elastic strains vs. applied stress suggests the presence of phase-specific elastic-plastic regimes in direct relation with size characteristics. The nature of the elastic-plastic transition is uncovered by the ''tangent modulus" analysis and correlated to the microstructure of the Cu channels and the Nb nanotubes. Finally, a new criterion for the determination of the macroyield stress is given as the stress to which the macroscopic work hardening, theta; = dσ/d ε, becomes smaller than one third of the macroscopic elastic modulus [Acta Mat., 57 (2009) 3157-3169]. This criterion appears to be valid to determine the transition from elastic-microplastic to macroplastic regimes in several nanocrystalline materials in contradiction to the traditional 0.2%-strain offset criterion. Finally, different thermal treatments are applied to heavily cold drawn Cu/Nb nanocomposite wires to study their effect on the occurrence/extension of the microplastic regime [Adv. Eng. Mat., in press (2012)].

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Tungsten is the most promising material for use in the plasma facing and structural components in the divertor of a nuclear fusion reactor. This is due to its high melting point, good sputtering rate, low activation and low tritium retention. What is less well characterised is the resistance to radiation damage in these materials, especially at high damage levels. The understanding of how the radiation damage affects the mechanical properties is of utmost importance in the design of future structures. Ion implantation can be used as a good analogue for neutron damage due to the similarities in the damage structures produced. It also has the advantages that significant damage levels can be built up in a few hours, and that the samples are not active after implantation so can be easily handled. In these experiments two types of tungsten-rhenium sheet were used. The first is in the as received condition, which had a highly textured nanostructure, with a grain size of 400nm and a high dislocation density inside the grains. The second was the same material annealed at 1673K for 24hours, which resulted in a grain size of 100µm, with no cold work left inside the grains. Tungsten based sheets have been proposed as structural materials for use in tungsten-tungsten composites for use in the divertor. Tungsten-ion implantation was performed on each of these samples, at 573K, at 5 different doses, ranging from 0.07 displacements per atom (dpa) to 33dpa, with a damage depth of 300nm. The highest doses are significantly higher than in reported neutron irradiations. Nanoindentation, was used to study the change in hardness in the implanted region. As this is shallow the method of Hainsworth and Page was used to de -convolute the effect of the undamaged underlying material for the implanted layer, this shows that the hardness between 100nm and 125nm is unaffected by tip imperfections and by the deformation of the underlying material. The increase in hardness in the annealed material is seen to be dramatic. At 0.07dpa the hardness between 100nm and 125nm increases by 0.8GPa, at 0.4dpa it increases by 0.9GPa which remains constant to 1.2dpa. At 33dpa the hardness significantly increases, which is explained by the formation of rhenium rich clusters, as seen by atom probe tomography. The nano-structured tungsten-rhenium alloy shows a significantly different response. At 0.07 dpa the hardness increase is only 0.25GPa, this climbs steadily until at 33dpa the hardness 0.9GPa, which is similar to the hardness change seen in the annealed material at 0.4dpa. This shows the significant increase in radiation resistance that nano-structured tungsten sheet has over coarse grained tungsten and that it will be advantageous to use such nanostructured sheet in composite structures.

In this study, we systematically investigated the Euler buckling behavior coupled with the electrical contact resistance of a single crystalline gold nanowire using by in-situ nanotesting. The single crystalline FCC metallic defect free nanowires have recently obtained huge attention due to its ultra-strong and ductile behavior, which can be utilized for the possible application of nanoprobing tip with AFM and SPM. To be used in the scanning probe system, it is essential to guarantee electrical conductivity, mechanical robustness, and high flexibility, etc. However, the flexibility and electric conductivity of the Au nanowires has not been systematically studied. Therefore, we quantitatively characterize the flexural behavior of the single crystalline Au nanowires. Furthermore, we evaluated the change of contact resistance of nanowires during its elastic deformation, which is crucial to the SPM application. Our quantitative compressive testing manifested super flexible behavior of single crystalline gold nanowires with various classical Euler buckling modes depending on the boundary conditions. Systematic investigation then revealed the transition of electron transport behavior from Quantum ballistic transport to classical diffusive transport regime while compressing the nanowires, which led to a stable electrical measurement at nanoscale. Buckling has been considered as catastrophic and sudden mechanical failure due to elastic instability which causes mechanical reliability issues. However, we took advantage of the mechanical instability beneficiary to contact probing at nanoscale. Having these features, we successfully demonstrated AFM scanning and electrical probing of nanostructures with ultra-strong and super-flexible single crystalline Au gold nanowires.

Nanoscale multilayer metallic (NMM) composites represent a class of advanced engineering materials whose scientific significance and technological potential as high performance materials has drawn world - wide attention in recent years. NMM composites are shown to exhibit high structural stability, mechanical strength, high ductility,toughness and resistance to fracture and fatigue; these properties suggest that these materials can play a leading role in the future micromechanical devices. However, before those materials are put into service in any significant applications, many important fundamental issues remain to be understood, like the question of the role of the interfacial geometry and chemistry on strengthening of NMM. This issue is addressed in this work by performing molecular dynamics simulations on NMM with interfacial discontinuities (steps) of various sizes and spacings, as well as altered chemistries using various concentrations of hydrogen atoms close to the interface. For that purpose, Cu/Nb bilayer thin films were examined and show to exhibit improvement on their mechanical behavior, compared to similar structures without steps or chemically altered interfaces. Furthermore, an analytical model is developed to explain the strengthening behavior of an NMM with steps on its interface. The theoretical results show a qualitative agreement with the finding of the atomistic simulations.

Metal-matrix nanocomposites are hybrid materials that consist of a combination of a metallic matrix with a rigid ceramic reinforcement, which results in a ductile and high-strength material. Among several metals, aluminum-based nanocomposites are of particular interest for structural applications due to their high strength-to-weight ratio, which makes them good candidates to substitute steel-based alloys in transportation vehicles and aerospace applications. In this context, titanium diboride (TiB₂) nanoparticles (NPs) are the ideal reinforcement material for Al-matrix nanocomposites because they do not react with liquid Al during the casting and they can be spontaneously captured inside the grains during solidification. However, the current strategies to synthesize TiB₂ NPs in the 5-100 nm range are based in non-scalable methods. In this work, we present the sonication-assisted synthesis of Al-matrix nanocomposites reinforced with phase-pure TiB₂ nanoparticles of 10-20 nm in diameter. The method is based in the reduction of potassium fluoroborate and potassium fluorotitanate salts in molten Al at 700 °C combined with high-intensity ultrasonic treatment. This combination allows the in-situ growth of TiB₂ NPs embedded within the Al matrix and results in superior nanoparticle distribution, a critical parameter for the mechanical performance of nanocomposites. The short ultrasound processing treatment (5-30 min) has the dual role of controlling the particle size at the nanoscale as well as dispersing the TiB₂ NPs homogeneously within the Al melt. It is demonstrated that the TiB₂ particle size increases at least in one order of magnitude in absence of sonication and larger aggregates are obtained. After sonication processing, the melt is casted into a permanent mold to obtain the bars for mechanical tests. The resulting TiB₂-reinforced Al-matrix nanocomposites show a significant enhancement in microhardness, tensile strength and yield strength respect to TiB₂-free sonication-processed Al.

Recent studies have shown that nanocrystalline metallic thin films have different material properties compared to their bulk counterparts such as increased in strain rate sensitivity and partial to full recovery of plastic strain over a period of time. In this work, we study these differences in detail by presenting experimental results on free-standing nanocrystalline face-centered cubic (FCC) metal films of sub-micron thickness using Bulge Test setup. Experiments were performed at both room temperature as well as an elevated temperature. At each temperature, both creep test at 400 Mpa and plastic strain recovery were performed at two strain rates of 10^-6 s^-1 and 10^-7 s^-1. Numerical modeling is performed and compared to the experimental observations. The nanocrystalline thin film is modeled as a two dimensional assembly of grains separated by grain boundaries. In addition to deformation mechanisms within the grains, deformation within grain boundaries is accommodated by grain boundary diffusion.

The grain size is a major microstructural parameter influencing mechanical properties of metals and alloys. According to the well known Hall-Petch relationship, one may expect significant improvement in mechanical strength with grain size reduction down to nanometers scale. However, the possibility of strengthening via grain size has some limitations due to the technological difficulties in grain size refinement and the phenomena leading to so called inverse Hall-Petch relationship observed below a certain critical value of the grain size. In this work, we explore the possibility to combine strengthening by grain size refinement with those by second phase particles. To this end, a comprehensive FEM model has been developed and experiments with precipitation strengthening of nanocrystalline aluminium alloys have been carried out. Two different mechanisms of the influence of second phase particles on the flow stress have been considered: (1) blocking of the moving dislocations by the particles distributed in grain interiors and (2) inhibiting of the grain boundary sliding by particles located at the grain boundaries. The obtained results show that second phase particles may efficiently improve the strength of nanometals. However, the overall effect strongly depends on their locations and corresponding strengthening mechanism. The contribution of particles located in grain interiors is only valid under assumption that flow stress is controlled by dislocation sliding. However, their effect is smaller than in microcrystalline counterparts, primarily due to enhanced precipitation at grain boundaries. On the other hand, when plastic flow is controlled by grain boundary sliding, particles at grain boundaries play a significant role. Their contribution to strengthening compensates the loss of strength observed for the inverse Hall-Petch relationship regime. The results obtained in the present study provide an understanding of the combined effects of grain boundary and particles strengthening in nano-crystalline metals. As such, they can be used to optimize heat treatment of nanoalloys.

In last few years a considerable number of experimental and theoretical works have been carried out to investigate nanostructures. The refinement of experimental techniques (such as, scanning tunneling microscopy, mechanically controllable break junction and high resolution transmission electron microscopy (HRTEM)), has lead to the discovery of new and unexpected structures at nanoscale, as for instance, the observation of Ag nanotubes with a square cross-section [1,2]. Among nanostructures, metallic nanowires (NWs) have attracted great interest due to the observation of very interesting physical phenomena as spin filtering, quantized condutance, etc. They can be used as fundamental building blocks in diverse domains, such as nanoelectronics, friction, biological sensors, etc. In spite of many years of investigations, most of the work on NWs has been focused on pure metals, such as gold, silver, copper and platinum. Only recently, the first realization of suspended atomic chains (SACs) from NWs composed of different metals (gold and silver) was achieved [3]. The behavior of these nanoalloys under mechanical stretching exhibited unusual features before the limit of mechanical rupture, such as; change of nominal alloy composition and the self-formation of complex nanostructures involving pentagonal Ag motifs encapsulating linear arrangements of Au atoms [3]. In this work we present experimental and theoretical results for the first experimental realization of SACs composed of Au and Cu atoms. Au-Cu NWs of different compositions were experimentally generated in situ using High Resolution Transmission Electron Microscopy (HRTEM) techniques and theoretically investigated using a methodology based on tigh-binding molecular dynamics simulations using second moment parameters (TB-SMA). We have investigated the process of NWs stretching leading to the SAC formation. Our results show a significant change of mechanical behavior as a function of Cu content and the presence of planar defects at 300 K. We have also observed, for the first time, the phenomenon of NWs self-purification, a gradual evolution of the arrangement of the different atomic elements in their narrowest region, where gold atoms are expelled to the surface or enclosed during elongation. The similarities and differences of SACs formed from Au-Ag and Au-Cu nanoalloys are also addressed. [1] M. J. Lagos, F. Sato, J. Bettini, V. Rodrigues, D. S. Galvatilde;o e D. Ugarte, Nature Nanotechnology v4, 149 (2009). [2] P. A. S. Autreto, M. J. Lagos, F. Sato, J. Bettini, A. R. Rocha, V. Rodrigues, D. Ugarte, and D. S. Galvatilde;o, Phys. Rev. Lett. v106, 065501 (2011). [3] J. Bettini, F. Sato, P. Coura, S. Dantas, D.S. Galvao, D. Ugarte, Nature Nanotechnology, v1, 182 (2006).

The aim of this research is to study, control, and manipulate the properties of a passivating surface oxide (Ga2O3) layer (skin) that forms spontaneously at ambient temperature on a room temperature liquid metal alloy, eutectic gallium indium (EGaIn). This semiconducting skin (or nanofilm) acts like a flexible membrane that provides mechanical stability to the high surface tension, low viscosity liquid metal such that it can be molded into useful shapes (e.g., antennas, interconnects, self assembled monolayer and electrodes). Rupturing this semiconducting nanofilm allows the metal to flow (e.g., into microfluidic channels to define its shape) and provides a method to create conductive structures that change shape, and therefore function, in response to stimuli. We seek to understand these properties to better control the shape of the metal. Rheology and electrochemistry provide fundamental and quantitative information about the behavior of this skin under various mechanical and chemical environments. In a stress controlled rheometer, parallel plates sandwich EGaIn and measure the modulus and yield stress of the skin by oscillatory stress-strain measurements. The mechanical properties of EGaIn on the micro scale can be tuned by modifying the skin. A home built chamber on the rheometer controls the chemical environment surrounding the metal (e.g., water, aqueous solutions, and organic solvents) and allows for the measurement of the modified mechanical properties of the interface of EGaIn in these environments. The results suggest that certain environments (water, acid) weaken the skin and others (polyvinylalcohol) reinforce the skin. To detect the real time formation (oxidation) and rupture of this oxide layer (Ga2O3), we utilize an amperometric technique. We hypothesized that electrochemical measurements could detect the oxidation of freshly exposed metal. We describe a new amperometric technique for measuring the oxidization of metal that takes advantage of the liquid nature of EGaIn. We measure and analyze the amperometric signals arising from this technique. This work is a first step toward ultimately integrating electrochemistry with rheology in a novel manner so as to elucidate the rupture of the oxide skin under controlled mechanical conditions (i.e., in the rheometer) and correlate the resulting changes in mechanical properties in real time.

Ni-Mn-Ga based ferromagnetic shape memory alloys have attracted tremendous interest since their discovery in the mid 1990s. Yielding theoretical strains of up to 10% in particular the 14M modulated martensite phase offers new prospects as functional material in on-chip actuator and sensor applications. Yet, miniaturization as single crystalline thin films remains a greatly unresolved challenge. With the aim of getting insights into the physics of twinning, we investigate the nanometer-resolved mechanical properties of single-crystalline 14M modulated martensitic Ni-Mn-Ga films, which are fabricated by sputter deposition on magnesium oxide substrates at elevated temperatures - in relation to their microstructure. Located beyond the resolution limit of nanoindentation, contact resonance atomic force microscopy (CR-AFM) is employed for nanomechanical characterization [1]. Comparing experimental indentation moduli obtained with CR-AFM with our theoretical predictions based on density functional theory (DFT) indicates a central role of pseudoplasticity and intermartensitic phase transitions for mechanical response. Spatially highly resolved mechanical measurements allow for quantification of mechanical properties around twin boundaries, while imaging of mechanical contrast identifies them as mechanical heterogeneities of low indentation moduli on the surface. As the CR-AFM technique constitutes a rather new approach, it is also briefly reviewed, while advantages, drawbacks and possible imaging artifacts are carefully addressed. [1] A.M. Jakob, M. Müller, B. Rauschenbach and S.G. Mayr, New Joural of Physics, 14, 033029 (2012)

The strengths of micro- and nano- scale metallic structures exhibit significant size dependencies. This phenomenon has attracted a great deal of attention among the materials science and mechanics communities and various experimental techniques have been utilized to address this issue. The micro-compression, micro-bulge and micro-tensile tests are commonly used techniques. Interestingly, the authors have observed considerable scattering (a relatively wide range of strengths corresponds to a single size) in the experimental data reported from micro-compression tests. This scattering is commonly attributed to the geometrical factors of the specimen (whether tapered or not, different cross sections, etc.) and the base support. In this work, our multiscale dislocation dynamics analyses of plastic deformation in small volume single crystal Cu show strong dependence of the yield stress not only on size and geometry but also on the statistical nature of the initial microstructure. Both the numerical and experimental data show significant scattering and the two sets of data are in good agreement. Furthermore, a statistical analysis that the percent uncertainty decreases with the increasing size for all the dislocations morphologies considered. Specifically, the percent uncertainties of the micro-size specimen that have smaller dislocation densities tend to drop faster with increasing specimen size. Both the dislocation dynamic simulations and the experimental data show that the percent uncertainty decreases as a power law with increase of the size of the micro-specimen.

Nanoscale metallic multilayer thin films are often characterized by techniques that provide average rather than underlying constituent properties. Yet, a better understanding of the constituents—particularly in confined geometries—can foster intelligent multilayer design. This work examines the biaxial stress-strain response of Cu and Ni layers embedded within Cu/Ni multilayers, over a range of varying bilayer thickness and volume fraction. This range includes coherent and semi-coherent interfaces. After fabrication by magnetron sputtering on Si substrates, the film-substrate system is heated from room temperature to 325°C and back to room temperature. In plane x-ray diffraction scans are taken at 50°C intervals during the process, revealing the elastic (lattice) strain at different temperatures. Coherent samples follow the expected thermo-elastic response during the thermal cycle. However, semi-coherent samples depart from this response, indicating the development of open loop (remnant) plasticity during a thermal cycle. This open loop strain is sometimes produced by forward and reverse plastic flow during the heating vs. cooling stages, and it occurs without a change in the sign of layer stress. Such unusual Bauchinger behavior is attributed to the forward and backward motion of threading dislocations, which can serve to store and remove dislocation content. The resulting stress vs. plastic strain response for each layer type suggests that such motion is not fully reversible, with pinning strengths that may depend on the coherent vs. semi-coherent nature of the interface.

Laser Induced Periodic Structures (LIPS), a corrugation of the material surface after laser irradiation, typically form in materials only after multiple laser shots. We hypothesize that for laser intensities above the material&’s damage threshold, the crater edges from the first shot diffract light from subsequent laser shots and form LIPS. To test our model, Au nanostructures were deposited onto Si substrates and irradiated by single ultrafast laser shots. LIPS formed on both the Au and Si surfaces; suggesting the presence of any edge allows for Fresnel diffraction of light parallel to that edge during a single laser shot. This diffracted light interferes with the incoming laser beam, creating a diffraction pattern in the material surface and forming LIPS after the deposition of energy. The intensity of diffracted light was found to be dependent on the orientation of the edges with the laser polarization. A model invoking Fresnel diffraction from the nanostructure edges will be presented and applied to various nanostructure geometries.

The combination of amorphous nano-layers with crystalline nano-layers can archive ultra-high strength in conjunction with high elongation-to-failure, when the constituent layers approach a critical minimum thickness. The plastic deformation, moreover, is confined by the crystalline-amorphous interface, which additionally has to maintain deformation compatibility to mediate homogeneous plastic flow. Contrary to crystalline-crystalline interfaces, the amorphous structure is characterized by a lack of crystalline long-range order. Using atomistic simulation methods, the compensation mechanism of this reduced long-range order is studied at the interface for the Cu (FCC) / CuxZr(1-x) (amorphous) system. The interface structure in terms of preexisting line defects is relate to the shear deformation of the interface, which mediated by the motion of interface dislocations. The interface structure are discussed in context of 1) the weak interfaces for dislocation transmission through the interface region to form a shear band in the amorphous phase, and 2) the implications on the experimentally observed co-deformability and associated layer size effect.

Efficient global optimization algorithms are of general interest and have applications in many fields. The Basin Hopping (BH) method is one of the widely used methods to predict the lowest energy structure of atomic clusters. BH is based on the Monte Carlo algorithm with an additional minimization step before the application of the Metropolis criterion, effectively transforming the energy landscape into a collection of interpenetrating staircases. We introduce a new variant of the BH algorithm suitable for systems and configurations typically used to study deformation mechanisms. The applicability of the method on a large number of atoms relies on the division of the system into smaller regions during the application of the acceptance rule. Additionally, the acceptance criterion used in the algorithm has to be adapted when working with configurations that exhibit internal stresses, through the evaluation of enthalpy. The results of the Confined Basin Hopping method will be discussed for cases such as the diffusion of vacancies around dislocations and the migration of grain boundaries. The benefits of the new method will be compared with standard molecular dynamics simulations.

The use of thin metal structures to add strength to metallic systems due to dislocation interactions with interfaces has been well documented in laminate blanket bi-layer composites such as Cu-Nb or Cu-Ni thin films. Adding a third layer, such as Cu-Ni-Nb, alters the landscape for dislocation motion and results in added strain hardening which is not possible in bimetallic systems. This current study extends the impact of interfaces into two different systems of reduced dimension: one of adding a nanolaminate on a nanoporous gold film, and one where the defects of nanoscale clusters of Cr are incorporated within the softer FCC layers. A combination of microtensile testing, bulge testing, and nanoindentation are used to demonstrate hardening effects as a function of layer thickness, and the strength enhancement beyond what would be expected using a classical Hall-Petch model is described in terms of the confined layer slip model. The ability to strain harden in the tri-layer structures is well beyond that of the bi-layer films both with and without additional impurities that increase strength (but not necessarily strain hardening coefficients). In the nanoporous foams the strengthening of adding Ni to Au has been simulated using molecular dynamics and compared directly to the nanoindentation experiments. The hardness of the coated nanoporous Au was five times of that of the pure nanoporous Au. Simulations of the Au-Ni ligaments suggest that the strengthening mechanisms of the are comparable to those for fcc nanocomposites. Deformation in the composite foam is dominated by twinning and partial dislocation activity, while in the monolithic material dislocation motion is dominant. Finally, the addition of harder precipitates of Cr in the FCC layers, adding an additional obstacle to dislocation motion, is shown both through simulation and experiment to add significant strength to these new structures.

The fabrication of materials comprised of nanoparticles gives the opportunity to achieve materials which possess peculiar optical, magnetic, electronic, catalytic and mechanical properties that lead to promising applications. The knowledge of the mechanical properties of nanocomposite materials, i.e. elastic modulus and hardness, is necessary to understand their stability, reliability and machinability. As to the difficulties of measuring the mechanical properties of thin films of nanoparticles without the interference of the substrate only little is known about mechanical behavior.[1] Films composed of nanoparticles crosslinked by organic molecules have been studied intensively in the recent past: Charge transport properties of thin and freestanding films show a dependency on the crosslinking molecule used. The chemical structure and the length of the crosslinker as well as the chemical environment have an influence on the conductance. Therefore, these films can be used as chemical sensors by taking advantage of the altered resistance in response to vapors.[2] Gold nanoparticles of differing sizes have been reacted with organic dithiols of different lengths and kinds as linking molecules in our research project. For the formation of thin films layer-by-layer spin-coating as a new efficient method has been used.[3] Instrumented Indentation Testing has been chosen to perform the mechanical testing of these ensembles of nanoparticles. A Micro Materials NanoTest was used to measure the nanomechanical properties of broad regions of nanoparticles. Films of varying thicknesses have been characterized as well as films crosslinked by different dithiols. With this setup we could determine the change of the hardness and the elasticity according to the variation of the linker. Small angle x-ray scattering was used to investigate the structural properties of these films. The length and the chemical structure of the linkers do have a direct influence on the spacing of the gold nanoparticles inside the films. Under the premise of a constant film density shorter linkers result in more closely packed film and higher hardness of the film, which leads to a more stable system. [1] H. Hu, L. Onyebueke and A. Abatan; J. Minerals & Materials Char. & Eng., 9, 4. (2010), pp. 275-319 [2] Y. Joseph, A. Peicacute;, X. Chen, J. Michl, T. Vossmeyer and A. Yasuda; J. Phys. Chem. C, 111, (2007), pp. 12855-12859 [3] H. Schlicke, J.H. Schroeder, M. Trebbin, A. Petrov, M. Ijeh, H. Weller and T. Vossmeyer; Nanotechnology, 22, (2011), 305303

Cu/Nb metallic multilayers with nominal layer thickness (h) of 5 nm (150 bilayers), 15 nm (50 bilayers) and 30 nm (25 bilayers) were manufactured by PVD. The evolution of the hardness and elastic modulus of the samples with temperature was determined by means of nanoindentation in the range 300 K to 673K in flowing Ar atmosphere. Nanoindentation tests were carried out using fast loading cycles in order to minimize thermal drift effects on the elastic modulus determined from the unloading stiffness. Loading rates of 6 mN/s were used with a hold period of 0.5 s at a maximum load of 12 mN and unloading times of 0.5 s. At least 10 indents were performed at each temperature and the samples were kept at the test temperature for at least 3 hours. After cooling down, nanoindentations were carried out at room temperature on each sample to measure the post-annealing hardness and modulus. The ambient temperature results showed the expected behavior. The multilayer elastic modulus was in the range 120-125 GPa and independent of the layer thickness. The hardness increased as the layer thickness decreased from 5.2 GPa (h = 30 nm) to 6.1 GPa (h = 5 nm). Both hardness and modulus decreased with temperature but the reduction in the former was more significant, particularly at 473 K and above. While the layer thickness did not influence of the reduction of the elastic modulus with temperature, the drop in hardness with temperature was maximum for the thinnest multilayers. The effect of temperature on the mechanical properties was discussed to the light of the deformation mechanisms induced by the layer thickness, the thermal residual stresses and the oxidation.

Multilayered materials are gaining increasing attention and interest because they can exhibit better properties than their individual components, especially at the nanoscale. In this study, we have used magnetron sputtering to deposit magnesium and titanium layers alternately onto a single crystal silicon substrate with equal individual layer thickness to form the multilayered Mg-Ti films. The individual layer thickness was varied from 2.5 nm to 200 nm in order to investigate its effect on the mechanical behaviors of the films. We have examined the structure of the deposited multilayer films using cross-sectional transmission electron microscopy (TEM). We have measured the nano-mechanical behaviors of the films with an instrumented nanoindenter, including the elastic modulus, the nano-hardness and strain rate sensitivity of the films. Our results show that the hardness of the deposited films increases with decreased individual layer thickness, following the Hall-Petch law very well, when the individual layer thickness is greater than 10nm. However, the Hall-Petch slope appears be lowered when the thickness is further reduced below a threshold thickness value of about 5 nm. The maximum hardness value was achieved at the smallest individual layer thickness, i.e., ~2.5 nm. The inverse Hall-Petch relationship which was usually observed in other nano-multilayered thin films has not been observed in these Mg-Ti multilayered films. We have discussed the experimental results based on our TEM observations.

It is well known that the sintering temperature of metallic nanoparticles is much lower than the melting temperature of corresponding bulk materials because of their high surface energy. Recently, there is an increasing interest in applying low-temperature sintering of metallic nanoparticles for bonding of semiconductor chips and inkjet printing of fine wiring on plastic films in the research field of electronics packaging technology. The many studies about developing Ag nano particles for these applications are already reported because of the highest electronic and thermal conductivity among metals. Ag nanoparticles have an advantage that small particles can be synthesized easily due to their stability in air, but Ag is high cost and low resistible against ion migration caused by the presence of moisture. Therefore, development of Cu nanoparticles is strongly expected because the electronic and thermal conductivity is almost the same between Cu and Ag even though Cu is much cheaper than Ag. However, a desirable synthesis method of Cu nanoparticles for these applications is difficult to be achieved because Cu nanoparticles are very sensitive to oxidation and easily aggregated as a particle size decreases. Many researchers synthesize Cu nanoparticles using polymers, such as PVP and Gelatin in order to prevent them from oxidizing and aggregating. However polymers cannot be completely decomposed after thermal process especially in inert or reductive atmosphere. Remained polymers inhibit sintering of particles so that properties of sintered structure would be deteriorated by microvoids. On the other hand, capping layers by surfactants can be decomposed more easily than polymers because they have lower molecular mass generally. We have developed the one-pot synthesis method to produce Cu nanoparticles capped by fatty acids and amines which are ones of typical surfactants. Our synthesis method features high yield rate for short time. The mean size of synthesized Cu nanoparticles could be controlled from 100 to 10nm as the alkyl carbon number increased from 10 to 22. The main component was Cu, but they exhibited slight oxidation when the particle size was smaller than 20nm. We confirmed the thermal decomposition peaks of synthesized Cu nanoparticles were less than 300 degrees C even under an inert atmosphere in all cases. This decomposition peak was lower than those of polymers. The sintering and bonding properties by synthesized Cu nanoparticles will be presented in this study.

In submicron or nc metals or alloys the volume fraction of the grain boundary regions become substantial, therefore, the grain-boundary (GB) mediated processes play a more decisive role than in coarse grain materials. Recent studies on the microstructure of nc metals and alloys under various deformation conditions show that grain growth occurs in these types of materials when severely deformed. The simultaneous appearance of dislocations and GB processes needs special care in explaining the grain-growth behavior of nc materials during deformation. In our three earlier consecutive papers, it was shown that in a nanocrystalline Ni-18wt.%Fe alloy cold rolled either at room temperature (RT) or at liquid nitrogen temperature (LNT) the grain size increases, whereas the dislocation density and the twin boundary frequency decreases. Though all the results and discussions in these studies were sound and conclusive, the rolling conditions were rather simple and ad-hoc not taking into account the columnar grain shape or the initial texture of the starting material. In the as-deposited state, the material has a well defined <100> fiber texture normal to the plane of the electro-deposited sheet, where the long axes of the columnar grains are also along the same <100> directions. Rolling deformation can be carried out in many different directions relative to the directions of both, the texture and the long-axes of the columnar grains. In the present work we select two very different specimen orientations for determining the relevant changes of the sub-structure. In one specimen orientation, the normal-direction (ND) of rolling is identical to the <100> fiber texture direction. In the other specimen orientation, the ND direction is perpendicular to the <100> fiber direction. Our this systematic investigation shows that strong deformation reduces the average dislocation density and causes detwinning, especially if twins exist initially, however, the grain-growth depends strongly on the relative orientations between (i) the texture direction, (ii) the long-axes directions of the columnar grains and (iii) the ND of rolling. High energy X-rays are used, which can penetrate relatively thick specimens. The relatively small diffraction angles and the full Debye-Scherrer rings in the detector enable to evaluate the diffraction patterns corresponding to two perpendicular diffraction vector directions at the same time. The third direction of the diffraction vector is accessed by changing the specimen orientation in the diffractometer. This synchrotron technique is used to obtain size parameter values in three dimensions for bulk materials, without bothering the surface-microstructure relaxation from TEM thin foils.

Nanowires have great potential for applications in sensors, actuators and nano electromechanical systems (NEMS)[1-3]. In order to integrate nanowires into such functional devices it is essential to fully understand nanowire mechanics, electrical characteristics, and how electrical transport evolves with an applied stress- the electromechanical response. There is a wide understanding of how to characterise nanowire mechanical and electrical properties [4-5], however measuring the electromechanical response has been hampered by the lack of reliable tools and systems. We have developed a reliable scanning probe measurement system that allows for dynamic characterisation of both mechanical and electrical properties. Utilising dual lateral atomic force microscopy and four probe electrical measurement, we have shown how resistance and resistivity evolve under an applied strain for penta-twinned silver nanowires (Ag NW&’s). A single suspended Ag NW is laterally loaded through the elastic and plastic regime in a three point bending geometry whilst obtaining the force-displacement curve, and 4-probe IV characteristics from a constant source voltage. We observed relative resistance increases of 7 % from 0 % - 100 % strain and resistivity increases up to 29 %. Resistance and resistivity changes may be attributed to increased surface scattering due to tensile elongation, although the exact mechanism is unclear at this time. Tensile elongation reduces the nanowires cross sectional area in accordance with Poisson&’s ratio. This lateral dimensional decrease perpendicular to the applied load means the effective nanowire diameter evolves as the nanowire bends, hence increasing surface scattering. This effect allows us to probe resistance changes above and below the electron mean free path. (References: 1. Xia, Y., et al., One-Dimensional Nanostructures: Synthesis, Characterization, and Applications. Advanced Materials, 2003. 15(5): p. 353-389. 2. Lieber, C.M., Nanoscale science and technology: Building a big future from small things. MRS Bull., 2003. 28(7): p. 486-491. 3. Cui, Y., et al., Nanowire Nanosensors for Highly Sensitive and Selective Detection of Biological and Chemical Species. Science, 2001. 293(5533): p. 1289-1292. 4. Bin Wu, A.H., John J. Boland, John E. Sader, XiaoMing Sun, YaDong Li, Microstructure-Hardened Silver nanowires. Nanoletters, 2006. 6(3): p. 468-472. 5. Garnett, E.C., et al., Self-limited plasmonic welding of silver nanowire junctions. Nat Mater, 2012. 11(3): p. 241-249.)

In situ tensile tests of Cu nanocrystals in a high-resolution TEM have revealed an effect of sample/crystal dimensions on plasticity mechanisms. When the single crystal size was reduced to <~150 nm, the normal full dislocation slip was taken over by partial dislocation mediated processes (PDMPs). For the first time, we demonstrate this “full-to-PDMP” transition in a quantitative manner by assessing the relative contributions to plastic strain from PDMPs and full dislocations (with the former contributing ~75% to the overall strain at crystal sizes <100 nm). The ability to quantify is made possible by the double-tilt capability of our in situ TEM tensile holder, which allowed the rotation of the crystal to proper zone axis to clearly identify the planar defects using direct atomic-scale imaging as well as diffraction patterns. This represents yet another “size effect”, beyond other reported influence of sample/crystal dimensions, such as the Hall-Petch type (or inverse H-P) relationship, the “smaller is stronger” trend, and the evolution towards dislocation starvation or source truncation. These findings also lend credence to the similar switch from full dislocations to PDMPs previously postulated for grain size reduction into UFG and NC regimes in polycrystalline metals.

Ni-Fe multilayered samples with grain size modulation periodicity of 0.05-1 µm were synthesized by pulsed electrodeposition technique. The unique microstructure of these samples consists of alternate layers of nano and ultrafine grained layers with the thickness ratio of 1:1. This provides a bimodal grain size distribution with coherent interfaces between the nano and ultrafine grained layers. Tensile test results revealed that decreasing the layer thickness results in an increase in yield strength. Furthermore, microstructure analysis with transmission electron microscopy was used to investigate the high elongation to failure in some samples. The microstructure and mechanical properties at different layer thicknesses are reported and size dependent strengthening mechanisms including Hall-Petch relation are discussed accordingly.

Atomisitc simulations and continuum dislocation models have advanced the understanding of plastic deformation in crystalline materials. However, these two methods have limitations that prevent the advancement of predictive models. Atomisitc simulations are restricted to small length scales and large strain rates unattainable with experiments. While dislocation dynamics simulations depend on heuristic representations of key dislocation mechanisms such as nucleation, reactions and core energy. In this talk we present an adaptive multiscale method that enhances the phase field dislocation dynamics method with atomistic simulations. The energy and forces from atomistic simulations will be used by the phase field model. This approach replaces the complex, yet approximate, interaction laws in dislocation dynamics for defect reactions with actual atomistic simulations around defects, while long range interactions and dislocation glide are solved with the phase field model. We show 3D simulations of dislocation evolution that exhibit the advantage of the proposed approach over traditional atomistic and continuum dislocations methods.

Nano-grained (50 nm) Mg composites were fabricated by a simple method of pressureless melt infiltration of Mg and Mg alloys into the porous preforms of Ti₂AlC and Cr₂AlC, the latter two are members of MAX phase family. The MAX phases are machinable layered ternary carbides and nitrides that are relatively light and stiff. When a commercially available Mg alloy, AZ61, is used to infiltrate the Ti₂AlC preform, ultimate tensile stresses of asymp; 800 MPa are achieved. In addition, the composites can damp about 25% of mechanical energy at high stresses. The Mg nanograins are extraordinary thermally stable; annealing at 90% of melting point of Mg did not lead to grain growth. Since the fabrication method is quite simple and scalable, it is possible to make Mg nanocomposite parts as large as parts made today by powder metallurgy easily and economically.

One-dimensional structures have the prospect to change the physical properties of materials used in contemporary devices. Physical properties change with dimension and size enabling a tailoring of performance in nanometer sized devices. Ceramics, semiconductor and carbon materials are easily synthesized as one-dimensional structures with typical diameters of several nanometers and length-diameter ratios of 1000:1. Only the metals as one of the oldest are difficult to fabricate in similar geometries. Recently we developed a process to grow perfect defect and flaw free nanowires with diameters of several ten nanometers, attached on substrates based on a process published in 1574. An initiator mediated filamentary crystal growth process based on the physical vapour deposition technique was developed. Typical diameters of the whiskers are 100 nm and lengths of up to 200 µm are observed, giving aspect ratios of up to 2000:1. Microstructure characterization of the nanowires was carried out by electron microscopy, revealing a perfect, flaw and defect free bulk and surface crystal structure. No dislocations, stacking faults, or grain boundaries were detected. The growth direction is generally along the <110> crystallographic direction of the face centered cubic lattice. Low indexed crystallographic planes, the {111} and {100}, form the nanowire surface. The overall geometry is dominated by the minimization in terms of surface energy and resembles the Wulff shape. It was not possible to detect impurities from the growth process on the surface or in the bulk of the nanowires. In situ bending experiments utilizing a micromanipulator inside a SEM allows mechanical probing of small volumes. This reduces the stochastic effect of slight deviations, especially on the surface facets, on the measured mechanical properties. Nanowhiskers, which are attached on a standard TEM Cu grid, were deflected in a cyclic manner with increasing load. After each deflection the load was reduced to check for plastic deformation. From the last bending experiment with elastic deformation the bending stress was calculated as a lower limit for the yield strength from the curvature of the deformed nanowhisker. The resulting bending stress reached values between 4 to 6 GPa. Almost no dependence on the nanowhisker diameter is observed. Knowing the crystallographic orientation of the whisker axis, a dislocation model is proposed for the deformation mechanism.

We report on the mechanical behaviour of nanostructured W/Cu (3 nm/1 nm grain sizes) thin films deposited on Kapton® under controlled biaxial loadings thanks to a biaxial testing device developed on DiffAbs beamline at SOLEIL synchrotron (Saint-Aubin, France) [1]. In situ tensile tests were carried out combining 2D synchrotron x-ray diffraction (XRD) and digital-image correlation (DIC) techniques. First, the elastic behaviour of the composite metallic film - polymeric substrate was investigated under equi-biaxial and non-equi-biaxial loading conditions. The results show that the strain measurements (in the crystalline film by XRD and the substrate by DIC) match to within 10-4. This result demonstrates the full transmission of strains in the elastic domain (Domain I) through the film-substrate interface and thus a good adhesion of the thin film to the substrate although no adhesion layer was used. Higher strains response was investigated under equi-biaxial tensile tests. The elastic limit of the nanostructured W/Cu thin films was determined at the bifurcation point between strains obtained by XRD and DIC at 0.50 %. Two regimes were observed : after bifurcation the film applied strain still increases linearly up to an applied load of ~100 N (i.e. a corresponding strain of 0.67%, Domain II). Domain III starts when the elastic strain saturates. Deformation mechanisms such as strain localisation and film fragmentation are proposed. [1] G. Geandier, D. Thiaudière, R. N. Randriamazaoro, R. Chiron, S. Djaziri, B. Lamongie, Y. Diot, E. Le Bourhis, P.O. Renault, P. Goudeau, A. Bouaffad, O. Castelnau, D. Faurie, F. Hild, Rev. Sci. Instrum. 81 (2010) 103903.

We report deformation twinning in Cu within accumulative roll-bonded (ARB) Cu-Nb nanolamellar composites and shocked physical vapor deposition (PVD) Cu/Nb multilayers. Twins appear connected to the Nb{112}//Cu{112} interface with the Kurdjumov-Sachs orientation relationship, which we show to be ordered and faceted. The interface adopts a new faceted structure after twinning. Our analysis suggests that deformation twinning involves facet dissociation and slip-transfer from the Nb layer to the Cu layer due to a geometrically favorable slip transmission pathway. This research is supported by US DOE, Office of Science, Office of Basic Energy Sciences.

Nanocrystalline metallic films have outstanding yield strength but their long-term inelastic behavior is significantly affected by sensitivity to loading rate and temperature. Grain refinement in nanocrystalline metals leads to increased influence of grain boundary driven creep strain and various dislocation based processes. In this work the authors studied the mechanical behavior of annealed nanocrystalline Au films of 1-2 mu;m thickness and average grain size of 64 nm over a wide range of strain rates from 10^-5 to 10 /s at four different temperatures between room temperature and 110 °C. The elastic modulus of the Au films was not affected by temperature or strain rate in the range investigated. On the contrary, the yield strength was highly sensitive to both temperature and strain rate: at room temperature it increased by ~100% within the range of applied strain rates, while it decreased by as much as 50% in the given temperature range for most strain rates. The trends in ductility and activation volume pointed to two distinct regimes of plastic deformation namely creep driven and dislocation mediated plasticity with the transition occurring at a higher strain rate for increasing temperature. The activation volume for creep-influenced deformation increased monotonically from 6.4 b^3 to 29.5 b^3 between 298 K and 383 K, signifying grain boundary (GB) diffusion processes and dislocation mediated creep, respectively. Dislocation climb, as an accommodation mechanism for GB sliding, provided an explanation for the increased activation volume at higher temperatures. The activation volumes calculated at high strain rates decreased from 19.7 b^3 to 11.4 b^3 between 298 K and 383 K. A model for thermally activated dislocation depinning was applied to explain the abnormal trend in activation volume, resulting in activation energy of 1.2 eV which is in agreement with existing experimental data. The strong indication from monotonic tensile tests for creep based deformation was validated by uniaxial creep experiments on Au films in the same temperature range. The athermal component of dislocation driven deformation at higher strain rates was also quantified via calculations based on data from strain rate jump experiments at different temperatures.

Electrical contacts require low contact resistance for efficient passage of signals or power without increasing friction. This is achieved by applying a thin layer of hard Au on contact terminals; however, these films tend to soften over time due to stress-induced grain growth during frictional contact. Alloy additions of Ni, Co or Ag increase wear resistance of hard Au but tend to segregate to the top during long-term aging. Currently hard Au coatings are electroplated, producing pore-ridden films that can have a wide range of mechanical, electrical and frictional properties, making them unreliable for use in critical devices. Vapor deposited Au-ZnO nanocomposites show promise for a novel hard Au coating with a pore-free and stable structure under frictional loading, without hindering electrical pathways. Nanoindentation of as deposited Au-ZnO films reveal an increase in the hardness of the films with increasing concentrations ZnO nanoparticles, from 1.67± 0.10 GPa for as deposited nanocrystalline Au to 2.71±0.14 GPa for Au-2.0%ZnO nanocomposite. Mechanical and electrical testing of wear tested Au-ZnO films demonstrate stability during frictional contact and thus the potential for these films to replace current hard Au layers used on contact terminals. This work was supported by Sandia National Laboratories, a Lockheed Martin Company for the USDOE NNSA under Contract DE-AC04 94AL85000.

Electrodeposition plays a key role in current industry by providing metallic coatings and nanoscale materials for electronic, optical, magnetic, wear, corrosion, and even aesthetic applications. Current manufacturing processes rely heavily on chemical additives, whose effects are poorly understood, in order to optimize the mechanical behavior and morphology of the materials. A novel alternative approach using under potential deposited (UPD) monolayers of a mediating metal to produce high quality (large grained and single crystal) metallic materials with low intrinsic stresses without the use of additives will be presented. Specific attention is given to the deposition of Ag and Cu using Pb as the mediator metal. These materials display up to 85% reductions in intrinsic stress and improved crystal quality compared to samples synthesized in the conventional manner. Applications of this approach for thin film and nanostructured materials processing will be discussed. The authors gratefully acknowledge materials support for this work from Dr. Gregory Fritz, IBM Watson Research Center.

A simple thermodynamic model is commonly used to predict the texture of metal thin films based on minimization of interface and strain energy. For FCC films, this model predicts 111 texture in thin films and a sharp texture transition to 100 in thick films. It does not allow for stable mixed texture. However, our experiments show that mixed textures of 111 and 100 grains are stable for a wide range of film thicknesses in silver thin films. Silver films were evaporated in thicknesses between 500 and 2000 nm with and without titanium adhesion layer on silicon substrates. In-situ synchrotron experiments at the Cornell High Energy Synchrotron Source allowed us to monitor both texture transformation and the development of stress within each texture component using a modified sin2psi; method. EBSD was used to analyze grain size and texture boundary spacing. These results provide a better understanding of the stable mixed texture and texture transformation in silver thin films than the simple thermodynamic model.

Tantalum thin films can be deposited in a metastable β-phase and transformed to the stable α-phase. The microstructure of α-Ta films formed by phase transformation shows large grains with continuous orientation gradients and a discontinuous boundary structure. The development of the microstructure during the β to α phase transformation is not well understood. However, previous work has shown that the phase transformed microstructure of α-Ta film depends on deposition parameters such as oxygen content and film thickness. In this work, we have analyzed EBSD micrographs of the film microstructure at different transformation stages and deposition conditions using a model in which the local curvature tensor for the lattice is analyzed to determine possible dislocation structures. We will show how the orientation distribution function changes during transformation and how it depends on deposition parameters. These results provide a better understanding of the phase transformed α-Ta microstructure which is essential for designing α-tantalum thin film components for microelectronic devices with new tailored properties.

Functional properties of metallic nanoparticles are strongly influenced by their size, structure and morphology and our ability to control these structural aspects is crucial for the optimization of the catalytic properties of these particles. Metallic nanoparticles can be synthesized into a wide-variety of structures. A common feature in these structures is the presence of pentagonal nanoparticles with twin boundaries and re-entrant surface modifications. The stability of small penta-twinned nanoparticles (~5-10 nm) has been attributed to the decrease in the total surface energy that arises due to surface energy anisotropy, however, the presence of these motifs at much larger sizes (~50-100 nm) remains unexplained. The contributions of the elastic strain energy are non-negligible at larger sizes and the effect of stress relaxation due to re-entrant surface formation is poorly understood. In this talk, we will discuss the thermodynamic stability analysis of faceted pentagonal nanoparticles, where the strain energy of various shapes is evaluated using finite element (FE) analysis. The FE calculations reveal non-trivial dependence of stress-strain distributions that are essential in understanding the stress relaxation mechanisms in these particles. Not only is the contribution of elastic strain energy necessary to develop reliable kinetic growth models for nanoparticles, the distribution of strain energy also plays a crucial role in understanding the heterogeneity of structure in multicomponent nanoparticles with components of varying stiffness.

Both superelasticity and shape memory properties of shape memory alloys result from the diffusionless and displacive phase transformations between austenite and martensite. During the transformations, elastic strain energy accumulates in the sample and affects subsequent transformation processes. Surface relief of some accumulated strain energy plays an important role in confined volumes such as shape memory pillars, small wires, and thin films. In this talk, we will discuss our recent progress in developing a mesoscale model for simulating the martensitic transformation processes in small-scale shape memory alloys. We calculate the internal stress state after each local transformation event in a series, and use that to determine the subsequent transformation events in the series. Simulation results from both load controlled and strain rate controlled boundary conditions will be presented, and compared to our prior experimental results from thermomechanical tests of shape memory microwires.

The ability to control the growth of metals at nano scale has enabled many interesting observations and applications. Surface enhanced Raman spectroscopy (SERS) incorporating metal nanoparticles have attracted considerable attention due to its great potential for many technological applications. We have recently demonstrated an easy and low cost technique to fabricate high surface area and porous SERS substrates. Nano-fibrous polymer membrane was first produced using the inexpensive electrospinning method. The electrospun nanofibers were subsequently coated with the silver nanoparticles at room temperature and ambient conditions. The deposition of silver was achieved by dipping the nanofibers in an appropriate mixture of silver nitrate, ammonium hydroxide and sodium hydroxide. Dextrose and disodium salt of ethylenediamine tetra acetic acid was used as a reducing agent. This process facilitates controlled deposition of silver nano particles on the surface of nano fibers. This technique would offer an easy means to make SERS templates for various applications and be amenable for bulk production at low cost. The experimental details on the fabrication and characterization of the SERS platforms will be presented. # AK, PD and MN have equal contributions.

Bulk metallic glasses (BMGs) represent a new class of metallic alloys with a wide range of potential applications. However, a major disadvantage of BMGs is the potential for rapid strain localization leading to catastrophic failure, particularly under tensile loading conditions. Among the investigated solutions to this problem is the introduction of crystalline phases to control the propagation of shear bands in the glassy matrix. In this work, we have studied the mechanical behavior of a model metallic glass/crystalline metal composite system using Molecular Dynamics with Embedded Atom Method potentials, with a focus on c-Cu / a-(Cu-Zr-Ti) multilayers. Deformation of the composite system was studied under different applied stress states, crystal orientations with respect to the interface, and crystalline volume fractions. Several systems with sizes ranging from 500,000 atoms to 1 million atoms were studied to understand the effect of system size on the deformation behavior. The systems were annealed at 800K and the deformation simulations were performed using displacement control at 300K. The shear strain and hydrostatic strain were calculated for each atom, and variations in these values were compared with the local atomic coordination at each time step. The modeled crystalline phase was initially defect free, resulting in a yield strength greater than that of the glass. Deformation, therefore, first initiates in the glass, where shear strain localization was observed. High strain regions in the glass correlated with the absence of icosahedral short range order. In contrast, regions with networks of the efficiently packed icosahedra are associated with low strains. As the applied strain increases, the fraction of atoms in an icosahedral environment in the glass decreases. The deformation moves into the crystalline phase, where stacking faults bounded by partial dislocations form at the interface and propagate into the crystal. At even higher applied strains, twin formation is observed in the crystal. Efforts to determine how deformation in the glassy phase precipitates defect formation in the crystalline phase are ongoing.

Metal thin films on elastomeric substrates deposited by e-beam evaporation, ink-jet printing, laser direct write (LDW), sputtering, or surface modifications have been recently used for flexible electronics applications. The cracks in metal thin films on elastomeric substrates can be easily initiated by fabrication process and propagated by stretching, twisting, and bending of substrates. Crack formation in metal thin films on elastomeric substrates has been a significant issue to be avoided since it causes a loss of conductivity and a device failure during the operation. Here, we report the mechanical/electrical behavior of metal nanoparticle thin film on PDMS elastomer from the viewpoint of cracking phenomena and a highly sensitive flexible pressure sensor application using these properties. The metal nanoparticle thin film patterns on PDMS substrate was formed by using a simple single-step transfer patterning of ionic metal nanoparticle precursor solution followed by thermal annealing process. This method is easy, low-temperature and low-cost process by using the three-dimensionally microstructured PDMS membrane as both stamp and target substrate. Uniform and clear microscale metal nano-ink patterns were achieved on the PDMS membrane with metallic properties (sheet resistance=51.6ohm/sq and resistivity=7.7x10^-6 ohm.m). We measured the changes of the resistance of metal nanoparticle thin film on PDMS by applied tensile strain. Because many cracks are originally created during the fabrication process and easily grown by tensile strain, the film exhibit very high strain sensitivity ((ΔR/R)/ε=106.45). Also, they can be easily closed when the strain is relaxed, resulting in full recovery of electrical resistance after unloading. Further, the resistance maintained to be constant after 10,000 cycles of elongation (ε=10%) and relaxation. We also studied the crack growth and closure behavior by in-situ SEM and AFM imaging of metal nanoparticle thin film surface under various elongations up to ε=10%. Using these excellent piezoresistive properties of metal nanoparticle thin films, we fabricated flexible pressure sensors based on patterned metal nanoparticle thin films on thin PDMS membranes (thickness=100, 225, 500um). They showed good detection performances to the deformation of the PDMS membrane by pressure change with short response/recovery times (< 3s). They showed high sensitivity in the low-pressure regime (<10kPa, ΔR/R= 0.35 for P=1kPa) and the sensitivity can be easily controlled by altering the thickness of PDMS membrane. In summary, we transferred metal nanoparticle thin film patterns on 3D micro-structured PDMS membrane by single-step transfer patterning method and studied the mechanical/electrical properties under reversible cracking phenomena. Also, we fabricated a flexible pressure sensor with high sensitivity to low pressure and fast response/recovery time using crack propagation/combination of metal nanoparticle thin film.

Thin metal nanocrystalline films deposited on polymer substrates are often used as conductors and interconnects in flexible electronics. Unlike conventional electronic devices, flexible devices are often subject to large deformation (stretches, bending and twists). The mechanical failure of the polymer-supported thin metal conductors under large deformation poses significant challenge to the functional reliability of flexible electronics. Existing theoretical studies often assume plane strain condition of the deformation of these polymer-supported thin metal films. In reality, however, flexible devices are often subject to large and complicated deformation. For example, the electronic sensitive skins covering the elbow of a robot experience large bi-axial stretches. To decipher the failure mechanisms of polymer-supported thin metal films under arbitrary in-plane loading conditions (i.e., different ratios of tensile strains in two in-plane directions), we perform failure mechanism analysis to determine the critical tensile strain above which necking sets in the thin metal films. We consider two representative material combinations, namely, thin metal films on stiff plastic substrates, and thin metal films on compliant elastomer substrates. Also emerging from the analysis is the orientation of the necking respect to the tensile loading directions. The results quantitatively correlate the critical necking limit strain as well as the necking orientation with the mechanical properties and the thickness of the metal film and the polymer/elastomer substrate. These results offer understandings on the deformability of polymer-supported thin metal nanocrystalline films in flexible electronics; therefore shed light on optimizing the material selection and structural design of deformable metal conductors to achieve better mechanical reliability of flexible electronics.

Unlike polymer composites, metal composites require high temperatures and/or pressures for consolidation and processing. Earlier studies synthesized metal composites using powder metallurgy methods or by means of melting and solidification techniques. In both cases, the high processing temperatures and pressures were found to damage the carbon nanostructures or lead to chemical reactions between the nanostructures and the metal matrix, thus lowering the performance of the composite. Further challenges arise from the formation of nanostructure agglomerates, which lead to localized concentration of reinforcement and anisotropic behavior that ultimately degrade the overall mechanical properties of the composite. Here we report on the cold-spray synthesis of novel CNT/Al and ND/Al metal composites that, unlike other nanocomposites, can offer both high hardness/strength and improved fracture toughness. In order to avoid damage of the nanoparticles during processing and to minimize chemical reactions between the metal matrix and nanostructures, synthesis temperatures must be sufficiently low. During cold spray, a low-temperature powder processing technology, metal and nonmetal particles are consolidated to form a coating or freestanding structure by means of ballistic impingement upon a substrate. Upon impact, the solid particles deform and create a bond with the substrate. Prior to cold spray, carbon nanostructures and aluminum powders were mixed using high-energy ball milling and cryogenic milling techniques in order to control powder particle size and shape, reduce the Al grain size, and maximize the dispersion of the nanostructures in the metal matrix, all of which are crucial parameters that determine the final performance of the composite. To allow for a direct comparison between cold spray and conventional powder metallurgical methods, samples of the milled powders were pressed, sintered, and hot rolled. Structure, composition, and properties of both the cold sprayed and the hot-rolled composites were studied using a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy and nanoindentation. Finally, we conducted shock response and projectile impact studies on the fabricated composites to demonstrate the feasibility of a lightweight, metal matrix composite-based armor material.