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Solute segregation to individual grain boundaries is used by design to produce strong and stable nanocrystalline metallic alloys. Grain-boundary segregation, however, is known to cause adverse embrittlement effects from a strain-localization failure mechanism that imposes significant material limitations for structural applications. Here, using atomistic simulations, it is discovered that heterogeneous Ni segregation in nanocrystalline Ni-mixed Ag alloys dramatically shuts down localized shear bands during plastic deformation, while simultaneously increasing the tensile strength. Nanocrystalline Cu-mixed Ag metals are predicted to exhibit standard homogeneous Cu segregation and a tensile strength that saturates above a solute concentration of 8 at.% due to glass-like shear localization induced by grain boundaries. By contrast, it is found that heterogeneous Ni segregation in nanocrystalline Ag-Ni alloys forms solute-rich clusters along interfaces leading to strain delocalization at high strain and continuous strengthening at high solute concentrations up to 15 at.%. This study reveals the importance of heterogeneous versus homogeneous segregation behaviors on strain localization and points to a fundamentally new strategy to design failure-resistant nanostructured materials through grain boundary segregation engineering.

Metals containing abundant coherent twin boundaries (TBs) are able to sustain substantial plastic deformation without fracture due to shear-induced TB migration and sliding. Retaining ductility in these metals, however, has proven difficult because detwinning rapidly exhausts TB migration mechanisms at large deformation, whereas TB sliding was only evidenced for loading on very specific crystallographic orientations. Here, we reveal the intrinsic shear deformability of twins in nanocrystals using in situ nanomechanical testing and multiscale simulations and report extreme shear deformability through TB sliding up to 364%. Sliding-induced plasticity is manifested for orientations that are generally predicted to favor detwinning and shown to depend critically on geometric inhomogeneities. Normal and shear coupling are further examined to delineate a TB orientation-dependent transition from TB sliding to TB cracking. These dynamic observations reveal unprecedented mechanical properties in nanocrystals, which hold implications for improving metal processing by severe plastic deformation.

Nanoscale materials modified by crystal defects exhibit significantly different behaviours upon chemical reactions such as oxidation, catalysis, lithiation and epitaxial growth. However, unveiling the exact defect-controlled reaction dynamics (e.g. oxidation) at atomic scale remains a challenge for applications. Here, using in situ high-resolution transmission electron microscopy and first-principles calculations, we reveal the dynamics of a general site-selective oxidation behaviour in nanotwinned silver and palladium driven by individual stacking-faults and twin boundaries. The coherent planar defects crossing the surface exhibit the highest oxygen binding energies, leading to preferential nucleation of oxides at these intersections. Planar-fault mediated diffusion of oxygen atoms is shown to catalyse subsequent layer-by-layer inward oxide growth via atomic steps migrating on the oxide-metal interface. These findings provide an atomistic visualization of the complex reaction dynamics controlled by planar defects in metallic nanostructures, which could enable the modification of physiochemical performances in nanomaterials through defect engineering., Crystal defects critically influence surface chemical reactions in nanomaterials, yet the basic mechanisms at play are still elusive. Here, the authors show the atomic-scale dynamics of surface oxidation at coherent planar defects in Ag and Pd, revealing how twins and stacking-faults selectively oxidize metallic nanocrystals.

Nanoscale materials modified by crystal defects exhibit significantly different behaviours upon chemical reactions such as oxidation, catalysis, lithiation and epitaxial growth. However, unveiling the exact defect-controlled reaction dynamics (e.g. oxidation) at atomic scale remains a challenge for applications. Here, using in situ high-resolution transmission electron microscopy and first-principles calculations, we reveal the dynamics of a general site-selective oxidation behaviour in nanotwinned Ag and Pd driven by isolated stacking-faults and twin-boundaries. The coherent planar defects crossing the surface exhibit the highest oxygen binding energies, leading to preferential nucleation of oxides at these intersections. Fast diffusion of oxygen atoms along the planar-fault highways is shown to catalyse subsequent layer-by-layer inward oxide growth via atomic steps migrating on the oxide-metal interface. These findings provide an atomistic visualization of the complex reaction dynamics controlled by coherent planar defects in metallic nanostructures, which could enable the modification of physiochemical performance of nanomaterials through defect engineering.

Woven-fiber ceramic materials have shown remarkable results in the design of insulative lay-up structures for flexible thermal protection systems. A deeper understanding of heat transfer through the different insulation layers is key for predicting the performance of heat shields. In this article, a thermo-mechanical multiscale model is developed to predict the out-of-plane thermal conductivity at the micro- and meso-levels of transversely loaded two-dimensional woven ceramic fabrics. Knudsen effects within the multiscale structure are studied by adjusting gas pressure conditions. Alumina-based Nextel-BF20 and silicon carbide Hi-Nicalon with a 5-harness satin weave pattern are modeled by finite-element analysis. The computational results are validated experimentally by applying the anisotropic transient plane source method. We find that out-of-plane thermal conductivity decreases significantly with gas pressure due to Knudsen effects in the confined air within the fibrous structure. The dependence of thermal conductivity of fabrics on fiber volume fraction is shown to decrease markedly with pressure reduction. The proposed multiscale modeling approach yields a notable accuracy improvement, with respect to simplified series-parallel models, when compared with our experimental measurements. The FEA model is applicable to other fabric materials and loading conditions and presents an opportunity to study how changes at the fiber level affect the overall thermal behavior of woven fabrics.

The mechanisms of strengthening and plasticity in columnar-grained metals with preferentially oriented nano-sized twins have been examined traditionally by considering dislocation processes, but rarely from the perspective of grain boundary (GB) deformation. Here, the effects of GB strain accommodation on plastic deformation in four different columnar-grained nanocrystalline nanotwinned (nt) face-centered-cubic metals (Cu, Ag, Al, and Ni) were studied by large-scale molecular dynamics simulations. It is observed that in tensile deformation parallel to coherent twin boundaries (CTBs), the dislocation mechanisms in each metal are identical and associated with GB emissions of jog and threading dislocations at small and large CTB spacings, respectively. However, CTB strengthening effects are increasingly more pronounced in columnar-grained nt metals as their shear modulus increases, which is rationalized by the dependence of GB stress concentrations on twin size, metal type and strain rate. Also, while flow stresses in nt-Cu, nt-Ag, and nt-Al metals increase linearly with decreasing CTB spacing, a maximum strength limit is reached in nt-Ni below a critical CTB spacing of 6 nm. The strength limit in nt-Ni results from columnar GB cracking induced by prominent GB sliding. For columnar-grained microstructures, GB sliding is equivalent in nt-Ag and nt-Al and slightly lower in nt-Cu but markedly higher in nt-Ni. These findings underscore the importance of new GB deformation mechanisms on plasticity and fracture in columnar-grained nt metals and enrich our understanding of CTB strengthening in fcc metals synthesized in the literature.

The interaction mechanisms between a dislocation and a twin boundary in nanotwinned face-centered cubic metals are well understood in terms of perfect coherent interfaces. Processes involving intrinsic incoherent twin boundary defects, however, remain largely unexplored, despite recent evidence suggesting that imperfect twin boundaries containing short kink-like step defects contribute notably to plastic deformation and twin stability in large nanotwinned grains. Here, molecular dynamics simulation is used to study the underlying interaction of screw dislocations with either 0° or 60° atomic-scale kink twin boundary defects, in order to understand how the presence of imperfect twin boundaries can alter hardening and ductility mechanisms in nanotwinned copper. It is found that kinked twin boundaries are effective in changing the mechanisms from direct dislocation transmission to dislocation absorption when the applied shear strain exceeds 1.06%, with pronounced hardening arising from such transformation. Hardening by dislocation pinning from individual kink steps is also manifest for 60° intersections. On the contrary, twin boundary defects produce no hardening at low applied strains when dislocation absorption at the twin boundary is already prevailing.

The aim of this investigation is to predict the interface separation behavior of silicon carbide (SiC) reinforced magnesium (Mg) matrix composites via multiscale simulations. Interface models for SiC/Mg composites with different interface orientations were first established. The interface crack propagation behaviors and interfacial mechanical properties in the SiC/Mg composites under pure tensile and mixed loadings were then investigated by molecular dynamics simulations. It is found that there are four typical asymmetric crack propagation modes for different SiC/Mg interfaces under pure tension. The interfacial mechanical properties are affected by interfacial bonding characteristics, interfacial orientations and loading modes. A cohesive zone model (CZM) for the SiC/Mg interface was established under mixed loadings and predicted macroscopic mechanical properties of SiC/Mg composites by incorporating the defined CZM in finite element methods are in good agreement with the experimental results. Keywords: Magnesium matrix composites, Molecular dynamics, Interface crack propagation, Cohesive zone model

This study presents the development and validation of a two-scale numerical method aimed at predicting the mechanical behavior and the inter-granular fracture of nanocrystalline (NC) metals under deformation. The material description is based on two constitutive elements, the grains (or bulk crystals) and the grain-boundaries (GBs). Their behaviors are determined atomistically using the quasicontinuum (QC) method by simulating the plastic deformation of [ 1 1 ‾ 0 ] tilt crystalline interfaces undergoing simple shear, tension and nano-indentation. Unlike our previous work (Peron-Luhrs et al., 2013) however, the GB thickness is here calibrated in the model, providing more accurate insight into the GB widths according to the interface misorientation angle. In this contribution, the new two-scale model is also validated against fully-atomistic NC simulation tests for two low-angle and high-angle textures and two grain sizes. A simplified strategy aimed at predicting the mechanical behavior of more general textures without the need to run more QC simulations is also proposed, demonstrating significant reductions in the computational cost compared to full atomistic simulations. Finally, by studying the response of dogbone samples made of NC copper, we show in this paper that such a two-scale model is able to quantitatively capture the differences in mechanical behavior of NC metals as a function of the texture and grain size, as well as to accurately predict the processes of inter-granular fracture for different GB character distributions. This two-scale method is found to be an effective alternative to other atomistic methods for the prediction of plasticity and fracture in NC materials with a substantial number of 2-D grains such as columnar-grained thin films for micro-scale electro-mechanical devices.

Dislocation–twin interactions critically control the plastic deformation and ultrahigh strength of nanotwinned metals. Here, we report a strong twin-thickness dependence of dislocation–twin interaction mechanisms from the tensile deformation of face-centered cubic metallic nanocrystals by in situ nanomechanical testing. Direct observations at atomic scale reveal that the predominant dislocation–twin interaction abruptly changes from dislocation transmission on the {111} slip planes to the unusual (100) slip plane of the twin, when the twin thickness is smaller than 4 layers. Using atomistic simulations, we find that the energy barrier for {100} slip transmission mechanism gradually decreases, with decreasing twin thickness, below the energy level required for normal (111) slip transmission, which remains identical for all twin sizes. Our in situ observations and simulations provide atomistic insights into a fundamentally new mechanism of plasticity in nanotwinned metals, down to the lowest twin size limit.

Molecular dynamics simulations are used to show a new type of pseudo-elasticity and shape memory effects in small-scale nanotwinned metals. Nanotwinned Au thin-films and nanowires are found to achieve full recovery of up to 20% tensile and −6.25% compressive strains upon reverse loading when the twin boundaries make a special angle of 70.53°from the principal axis. This phenomenon results in superelastic recoverable strains up to 5 times larger than the useful range of deformation that can be induced in some advanced bulk shape-memory-alloys and small-scale ceramics, with a tensile strength above 1 GPa. The pseudo-elastic behavior stems from a unique interplay between deformation twinning and slip in grains composed of non-{1 1 1} free surfaces and discontinued twin boundary migration in those exposing only {1 1 1} free surfaces. This finding could open up new opportunities for small-scale nanotwinned metals as advanced materials for vibration damping and mechanical energy storage applications.

Nanotwins require little energy to form in metals, but their impact on strength and ductility is dramatic. New mechanisms of strengthening, strain hardening, ductility, and strainrate sensitivity have been observed in nanowires, films, and bulk materials containing nanoscale twins as the twin-boundary spacing decreases. These mechanisms can act in concert to produce interface-dominated nanomaterials with extreme tensile strength and plastic deformation without breaking. This article reviews recent theoretical and experimental understanding of the physical mechanisms of plasticity in nanotwin-strengthened metals, with a particular focus on the fundamental roles of coherent, incoherent, and defective twin boundaries in plastic deformation of bulk and small-scale cubic systems, and discusses new experimental methods for controlling these deformation mechanisms in nanotwinned metals and alloys.

Strengthening of metals through nanoscale grain boundaries and coherent twin boundaries is manifested by a maximum strength—a phenomenon known as Hall–Petch breakdown. Different softening mechanisms are considered to occur for nanocrystalline and nanotwinned materials. Here, we report nanocrystalline-nanotwinned Ag materials that exhibit two strength transitions dissimilar from the above mechanisms. Atomistic simulations show three distinct strength regions as twin spacing decreases, delineated by positive Hall–Petch strengthening to grain-boundary-dictated (near-zero Hall–Petch slope) mechanisms and to softening (negative Hall–Petch slope) induced by twin-boundary defects. An ideal maximum strength is reached for a range of twin spacings below 7 nm. We synthesized nanocrystalline-nanotwinned Ag with hardness 3.05 GPa—42% higher than the current record, by segregating trace concentrations of Cu impurity (

Flexible thermal protection materials made from two-dimensional woven ceramics fibers are of significant interest for hypersonic inflatable aerodynamic decelerators being developed by NASA for future missions on Mars and other planets. A key component of the thermal shield is a heat-resistant outer ceramic fabric that must withstand harsh aero-thermal atmospheric entry conditions. However, a predictive understanding of heat conduction processes in complex woven-fiber ceramic materials under deformation is currently lacking. This article presents a combined experimental and computational study of thermal conductivity in 5-harness-satin woven Nextel 440 fibers, using the hot-disk transient plane source method and computational thermo-mechanical modeling by finite-element analysis. The objective is to quantify and understand the effect of compressive strain on anisotropic heat conduction in flexible two-dimensional ceramic materials. We find, both experimentally and theoretically, that thermal conductivity of woven fabrics rises in both in-plane and out-of-plane directions, as the transverse load increases. Air gap conduction and fiber-to-fiber contacts are shown to play a major role in this behavior. Our finite-element simulations suggest that the thermal conductivity anisotropy is strong because heat transfer of air confined between fibers is reduced compared to that of free air. The proposed modeling methodology accurately captures the experimental heat conduction results and should be applicable to more complex loading conditions and different woven fabric materials, relevant to extreme high temperature environments.

This paper focuses on the molecular dynamics (MD) simulation of the tensile response of Ni-coated CNT-reinforced magnesium matrix composites (Ni-CNT/Mg) subjected to uniaxial tension at different temperatures and strain rates. The results show that Ni-CNTs can improve the mechanical properties of the composites effectively. The maximum stress of Ni-(6,6)CNT/Mg is, respectively, increased by 25.66 and 11.13%, while the elastic modulus is increased by 23.69 and 14.43% compared with those of the single-crystal Mg and uncoated (6,6)CNT/Mg at 300 K and 1 × 109 s−1. In addition, the calculated elastic modulus of the Ni-(6,6)CNT/Mg composite is consistent with the prediction based on the rule-of-mixture. The Ni-CNT/Mg composites still have better mechanical properties at 500 K but exhibit a significant temperature softening effect in the temperature range 100–500 K and a strong positive strain-rate sensitivity at a strain rate greater than 1 × 109 s−1. The various failure modes of the composite at the nano-scale are mainly determined by the combined effects of the different factors such as the atomic disordering, the void and dislocation nucleation, structural phase transformations in Mg nanocrystals near the interface, and the subsequent fracture of the Ni-coated CNT close to the voids.

GENERAL PRINCIPLES AND THEORETICAL CONSIDERATIONS Transverse Thermoelectric Effects and Their Application H. J. Goldsmid Thermoelectric Induction in Power Generation: Prospects and Proposals L. I. Anatychuk Thermoelectric Devices as Heat Engines: Alternative Thermodynamic Cycles L. E. Bell Functionally Graded Thermoelectric Generator and Cooler Elements E. Muller, K. Zabrocki, C. Goupil, G. Jeffrey Snyder, and W. Seifert Thermodynamics and Phase Transformations in Thermoelectric Materials: Applications to the Development of New Materials Jean-Claude Tedenac First Principles Calculations of Electron Transport Properties in Disordered Thermoelectrics Janusz Tobola and Laurent Chaput New Thermoelectric Materials with Precisely Determined Electronic Structure and Phonon Dispersion Tsunehiro Takeuchi 8 Entropy Flow in Interactive Semiconductor/Metal Nanoensembles Dieter M. Gruen Ab Initio-Based Band Engineering and Rational Design of Thermoelectric Materials Jiong Yang, Xun Shi, Wenqing Zhang, Lidong Chen, and Jihui Yang Band Structure Guidelines for Higher Figure-of-Merit: Analytic Band Generation and Energy Filtering Espen Flage-Larsen and Ole Martin Lovvik Introduction to Modeling Thermoelectric Transport at High Temperatures Andrew F. May and G. Jeffrey Snyder The Effect of Resonant Energy Levels on the Thermoelectric Power and Thermoelectric Power Factor Joseph P. Heremans Graphene-Like Exfoliated Quasi-2D Thermoelectric Crystals Alexander A. Balandin 14 The Bottom-Up Approach to Bulk Thermoelectric Materials with Nanoscale Domains Anuja Datta, Adrian Popescu, Lilia Woods, and George S. Nolas Surface and Interface Effects on Thermoelectric Behavior in Crystalline Nanowires Frederic Sansoz MATERIALS PREPARATION AND MEASUREMENT High-Performance Nanostructured Thermoelectric Materials Prepared by Melt Spinning and Spark Plasma Sintering Xinfeng Tang, Wenjie Xie, Han Li, Baoli Du, Qingjie Zhang, Terry M. Tritt, and Ctirad Uher Fabrication Routes for Nanostructured TE Material Architectures Muhammet S. Toprak, Shanghua Li, and Mamoun Muhammed Preparation and Thermoelectric Properties of Iron Disilicide Yukihiro Isoda and Haruhiko Udono The Deposition of Bi2Te3 and Sb2Te3 Thermoelectric Thin Films by Thermal Coevaporation and Applications in Energy Harvesting L. M. Goncalves Thermoelectric Materials, Measurements, and Opportunities for Energy Harvesting Patrick J. Taylor Thermal and Thermoelectric Characterization of Individual Nanostructures and Thin Films Li Shi Microchips and Methods for the Characterization of Thermoelectric Transport Properties of Nanostructures Friedemann VOlklein, Daniel Huzel, Heiko Reith, and Matthias Schmitt Neutron Scattering Investigations of Thermoelectric Materials Mogens Christensen

High-temperature aero-thermal heating in a 30 kW inductively coupled plasma torch was used to replicate the effects of harsh oxidizing environments during hypersonic atmospheric entry on fracture behavior and microstructure of two-dimensional woven SiC fibers. Hi-Nicalon SiC woven cloths were exposed to surface temperatures over 1400°C with different high-enthalpy dissociated oxygen and nitrogen plasma flows, and were subsequently deformed in pure tension at room temperature. Changes in fiber microstructure and surface chemistry after thermal exposure were examined by scanning electron microscopy. Pure nitrogen plasmas resulted in a 50% decrease of strength in woven SiC fibers with minimal effects on the fiber structure, except for highly localized surface pitting caused by partial decomposition of silicon oxycarbonitride phase at high temperature. In contrast, exposure to dissociated oxygen and air plasmas led to severe strength reduction and embrittlement over significantly short time scales, corresponding to degradation rates up to 200 times higher than those reported with static heating at equivalent temperatures. The origin of accelerated embrittlement at microscopic scale was found related to complex gas-surface interactions and high-temperature oxidizing processes involving the formation of SiO2 bubbles and microcracks on the surface. These findings are important for the development of outer fabric materials for new flexible thermal protection systems in space applications.

Pushing the limits of elastic deformation in nanowires subjected to stress is important for the design and performance of nanoscale devices from elastic strain engineering. Particularly, introducing nanoscale twins has proved effective in rising the tensile strength of metals. However, attaining ideal elastic strains in nanotwinned materials remains challenging, because nonuniform twin sizes locally affect the yielding behavior. Here, using in situ high-resolution transmission electron microscopy tensile testing of nanotwinned [111]-oriented gold nanowires, we report direct lattice-strain measurements that demonstrate a strong Hall-Petch type relationship in the elastic strain limit up to 5.3%, or near the ideal theoretical limit, as the twin size is decreased below 3 nm. It is found that the largest twin in nanowires with irregular twin sizes controls the slip nucleation and yielding processes in pure tension, which is in agreement with earlier atomistic simulations. Continuous hardening behavior without loss of strength or softening is observed in nanotwinned single-crystalline gold nanowires, which differs from the behaviors of bulk nanocrystalline and nanotwinned-nanocrystalline metals. These findings are of practical value for the use of nanotwinned metallic and semiconductor nanowires in strain-engineered functional microdevices.

Molecular dynamics simulations are used to understand the role of surface C–C bonds formed by heat treatment on plastic deformation and fracture mechanisms in cone-stacked carbon nanofibers. The simulations predict that the surface bond density linearly relates to the heat treatment temperature. As the surface bond density increases, it is found that tensile strength and ductile fracture energy in carbon nanofibers rise dramatically by more than 65% and 622%, respectively; hence unveiling a regime of superplastic deformation that is unmatched by standard carbon nanotubes and graphitic nanofibers. We demonstrate that both strengthening and superplasticity effects are enabled by the occurrence of surface bond-induced fiber splaying processes that can effectively resist interlayer sliding and crack propagation during deformation. The findings of this computational study have important implications for designing flaw-tolerant nanocomposite systems.

Small-scale mechanics of solute atom segregation and incipient plasticity in nanotwinned Ag containing trace concentrations of Cu were studied by using large-scale hybrid Monte Carlo and molecular-dynamic simulations. It is found that solute Cu atoms are segregated concurrently to grain boundaries and intrinsic twin-boundary kink-step defects during thermal annealing. Low Cu dopant contents below 1 at. % are predicted to substantially increase twin stability in nanotwinned Ag, accompanied with a pronounced rise in yield strength at 300 K. Incipient plasticity is associated with kink-step migration, grain-boundary sliding, and dislocation nucleation from grain boundaries and twin-boundary defects, which are affected by doping. Cu-dependent yield strengthening in doped nanotwinned Ag is shown to correlate with the critical stress required to initiate crystal slip emitted from grain boundaries and twin-boundary defects. These findings provide fundamental insight into the roles of twin-boundary imperfections on plastic yielding, and offer clues to further extend the extraordinary stability and strength of nanotwinned metals by microalloying.

We investigated the effect of geometrically constrained stress–strain conditions on the formation of nanotwins in α-brass phase reinforced Ni59Zr20Ti16Si2Sn3 metallic glass (MG) matrix deformed under macroscopic uniaxial compression. The specific geometrically constrained conditions in the samples lead to a deviation from a simple uniaxial state to a multi-axial stress state, for which nanocrystallization in the MG matrix together with nanoscale twinning of the brass reinforcement is observed in localized regions during plastic flow. The nanocrystals in the MG matrix and the appearance of the twinned structure in the reinforcements indicate that the strain energy is highly confined and the local stress reaches a very high level upon yielding. Both the effective distribution of reinforcements on the strain enhancement of composite and the effects of the complicated stress states on the development of nanotwins in the second-phase brass particles are discussed.

The crucial role that slip events emitted from free surfaces play in the overall plasticity and strength of low-dimensional crystals such as metallic nanowires (NWs) is well documented; however, the influences of stacking fault energy (SFE) and sample diameter on these local deformation processes are not clearly established. Experimental characterization by nanomechanical bending or tensile testing of NWs, in particular, may not be applicable to NWs made of different metals or exhibiting non-uniform dimensions. In this study, atomic force microscopy nanoindentation is used to probe the local plastic behavior and hardness properties of electrodeposited bimetallic Ni–Au NWs ranging from 60 to 358 nm in diameter and fixed on functionalized-glass substrates. Hardness measurements in individual NW segments are found to be larger in Ni than in Au owing to the difference in SFE and shear modulus between these two metals. However, the characteristic length scale associated with indentation size effects is shown to be material independent and directly linked to the NW diameter. Atomistic study of deformation mechanisms in single-crystalline NWs by molecular dynamics simulations further confirms that the interaction mechanisms between newly emitted dislocations and free surfaces are fundamentally different between Ni NWs and Au NWs during nanoindentation. By decoupling the intrinsic diameter dependence from indentation size effects in the hardness of bimetallic Ni–Au NWs, we find a marked reduction in size effects with a power-law scaling exponent of n = 0.18 during the incipient yielding of pristine NWs, in contrast to n = 0.8 in plastically pre-strained NWs.

Atomistic simulations using the quasicontinuum method are used to study the role of vacancy defects and angstrom-scale voids on the mechanical behavior of five tilt bicrystals containing grain boundaries (GBs) that have been predicted to exhibit characteristic deformation processes of nanocrystalline and nanotwinned metals: GB-mediated dislocation emission, interface sliding and shear-coupled GB migration. We demonstrate that such nanoscale defects have a profound impact on interfacial shear strength and underlying deformation mechanisms in copper GBs due to void-induced local stresses. In asymmetric high- and low-angle GBs, we find that voids become preferential sites for dislocation nucleation when the void size exceeds 4 A. In symmetric Σ 9 ( 221 ) GBs prone to sliding, voids are shown to shield the local shear stress, which considerably reduces the extent of atom shuffling at the interface. In symmetric Σ 5 ( 210 ) and Σ 27 ( 115 ) GBs, we find that the effect of voids on shear-coupled GB migration depends on the GB tilt direction considered, as well as on the size and number of voids. Remarkably, large voids can completely abate the GB migration process in Σ 27 ( 115 ) GBs. For all GB types, the interfacial shear strength is shown to decrease linearly as the volume fraction of voids at the interface increases; however, this study also suggests that this decrease is much more pronounced in GBs deforming by sliding than by dislocation nucleation or migration, owing to larger void-induced stresses.

Classical molecular dynamics simulations are used to study the effects of cone angle on mechanical properties and failure mechanisms in thermally-treated cup-stacked CNFs. We find a 22-fold reduction in elastic modulus and 4-fold decrease in tensile strength of cup-stacked CNFs with a wide range of cone angles between 19.2 and 180. Our results show significant elastic stiffening for intermediate angles between 38.9 and 112.9, as well as a minimum in tensile strength at a critical cone angle, due to the competition between weak van-der-Waals forces between layers and strong strengthening mechanisms from surface bonds introduced during thermal treatment. Different failure modes in CNFs subjected to tensile deformation are also predicted as a function of cone angle. This study constitutes an important step toward understanding the origin of strength dispersions observed experimentally in CNFs, and suggests that the design of high-strength CNFs can be optimized structurally by appropriately tuning the cone angle.

Coherent twin boundaries (CTBs) are widely described, both theoretically and experimentally, as perfect interfaces that play a significant role in a variety of materials. Although the ability of CTBs in strengthening, maintaining the ductility and minimizing the electron scattering is well documented, most of our understanding of the origin of these properties relies on perfect-interface assumptions. Here we report experiments and simulations demonstrating that as-grown CTBs in nanotwinned copper are inherently defective with kink-like steps and curvature, and that these imperfections consist of incoherent segments and partial dislocations. We further show that these defects play a crucial role in the deformation mechanisms and mechanical behaviour of nanotwinned copper. Our findings offer a view of the structure of CTBs that is largely different from that in the literature, and underscore the significance of imperfections in nanotwin-strengthened materials.

Bonding mechanisms and rates between the active edges of a cone-stacked CNF are examined by molecular dynamics simulations at temperatures up to 2273 K. Thermally treated nanofibers subjected to tensile deformation show a substantial increase in the elastic strain limit, albeit no change in elastic modulus, due to the resistance of surface bonds to crack propagation. Two bonding mechanisms; i.e., the formation of energetically stable loops from single dangling atoms and the folding of zigzag and armchair graphene bilayer edges, are shown to display predominant, yet distinct kinetics. This study reveals a critical transition temperature at 1000 K beyond which bilayer edge folding dominates over the formation of single atom loops in strengthening the surface of CNFs. This study also underscores the critical roles played by surface bond types, numbers, and distributions on the large failure strength dispersion observed experimentally in CNFs.

Polycrystalline materials, with nanosized grains (< 100 nm), exhibit superior strength exceeding those of their coarse-grained counterparts. With such small grains, the deformation mechanisms taking place at grain boundaries (GBs) become dominant compared to the intragranular crystal plasticity. Recent studies have revealed that the deformation mechanisms are influenced by the GB network. For instance, a high yield stress in nanostructured metals can be obtained by choosing the relevant grain boundary character distribution (GBCD). In this paper we present an original numerical multiscale approach to predict the mechanical behavior of nanostructured metals according to their GBCD composed of either high angle (HA) GBs (HAB) or low angle (LA) GBs (LAB). Molecular simulations using the quasicontinuum method (QC) are performed to obtain the mechanical response at the nanoscale of GB undergoing simple shear (GB sliding behavior) and tensile loads (GB opening behavior). To simulate the grain behavior, a mechanical model of dislocation motions through a forest dislocation is calibrated using a nanoindentation simulation performed with QC. These QC results are then used in a finite element code (direct numerical simulation-DNS) as a GB constitutive model and as a grain constitutive model. This two-scale framework does not suffer from length scale limitations conventionally encountered when considering the two scales separately. © 2013 Elsevier Ltd. All rights reserved.

Nanoscale metallic crystals have been shown to follow a ‘smaller is stronger’ trend. However, they usually suffer from low ductility due to premature plastic instability by source-limited crystal slip. Here, by performing in situ atomic-scale transmission electron microscopy, we report unusual room-temperature super-elongation without softening in face-centred-cubic silver nanocrystals, where crystal slip serves as a stimulus to surface diffusional creep. This interplay mechanism is shown experimentally and theoretically to govern the plastic deformation of nanocrystals over a material-dependent sample diameter range between the lower and upper limits for nanocrystal stability by surface diffusional creep and dislocation plasticity, respectively, which extends far beyond the maximum size for pure diffusion-mediated deformation (for example, Coble-type creep). This work provides insight into the atomic-scale coupled diffusive–displacive deformation mechanisms, maximizing ductility and strength simultaneously in nanoscale materials. In situ atomic-scale imaging of deformation in silver nanocrystals reveals that it is possible to achieve deformability and high strength, attributed to a coupling mechanism between crystal slip and surface diffusional creep.

Surface faceting on sidewalls is ubiquitously observed during crystal growth of semiconductor nanowires. However, predicting the thermal transport characteristics of faceted nanowires relevant to thermoelectric applications remains challenging. Here, direct molecular dynamics simulations show that thermal conductivity is considerably reduced in crystalline111Si nanowires with periodic sawtooth faceting compared to nanowires of same size with smooth sidewalls. It is discovered that surface phonon scattering is particularly high with {100} facets, but less pronounced with {113} facets and remarkably low with {111} facets, which suggests a new means to optimize phonon dynamics for nanoscale thermoelectric devices. This anomaly is reconciled by showing that the contribution of each facet to surface phonons is due to diffuse scattering rather than to backward scattering. It is further shown that this property is not changed by addition of an amorphous shell to the crystalline core, similar to the structure of experimental nanowires.

The size dependence of strength observed in submicrometer face-centered-cubic (fcc) metallic crystals under uniform deformation depends on the interaction of pre-existing dislocations with surfaces. To date, however, the dislocation processes controlling flow stress scaling in fcc crystals less than 100 nm in size have remained an open question due to limited knowledge on microstructural evolution during deformation in such small volumes. Here, molecular dynamics computer simulations employing a technique of high-temperature annealing and quenching on porous crystals were used to generate complex dislocation microstructures in sub-75 nm Cu pillars with high initial dislocation densities of 10 16 m � 2 , which made it possible to quantitatively examine their evolution during compression as a function of pillar diameter. These simulations reveal a transition from a state of dislocation exhaustion, where mobile dislocations are lost at the free surface and the dislocation density steadily decreases, to a regime of intermittent plastic flow between elastic loading and sourcelimited activation inside the pillars. It is shown that plastic flow stresses predicted during dislocation exhaustion regime exhibit little to no size dependence, while pronounced size effects are found during source-limited activation. Remarkably, the relationship between flow stress predicted at 5% strain and diameter is found to follow closely the power-law dependence reported in past experiments with larger Cu crystals and smaller densities. A deformation mechanism map, expressed in terms of diameter, is developed and used to elucidate the origin of size-dependent plasticity in nanoscale fcc crystals.

We examine the processes of spherical indentation and tension in Ni nanowires and thin films containing random distributions of nanoscale grains by molecular dynamics simulations. It is shown that the resistance to nanoindentation of nanocrystalline Ni nanowires with diameters of 12 and 30 nm tends not to depend on the wire diameter and free surfaces, contrary to nanoindentation in single-crystalline nanowires. Accommodation of plastic deformation by grain boundary sliding suggests a mitigation strategy for sample boundary effects in nanoscale plasticity.

The effects of twin size and sample diameter on yield stress and surface dislocation emission in twinned metal nanowires deformed uniformly were studied using classical dislocation theory and the concept of image force from twin boundaries. This theoretical study is shown to quantitatively capture the linear increase in yield stress as twin size decreases in periodically twinned Au nanowires predicted by atomistic simulations. The implication of this model as a yield criterion for realistic metal nanostructures with nanoscale growth twins is discussed.

The role played by nanoscale twins is becoming increasingly important in order to understand plasticity in nanowires synthesized from metals. In this paper, molecular dynamics simulations were performed to investigate the synergistic effects of stacking fault energy and twin boundary on the plasticity of a periodically twinned face-centered cubic (fcc) metal nanowire subjected to tensile deformation. Circular nanowires containing parallel (1 1 1) coherent twin boundaries (CTBs) with constant twin boundary spacing were simulated in Au, Ag, Al, Cu, Pb and Ni using different embedded-atom-method interatomic potentials. The simulations revealed a fundamental transition of plasticity in twinned metal nanowires from sharp yield and strain-softening to significant strain-hardening as the stacking fault energy of the metal decreases. This effect is shown to result from the relative change, as a function of the unstable stacking fault energy, between the stress required to nucleate new dislocations from the free surface and that to overcome the resistance of CTBs to the glide of partial dislocations. The relevance of our predictions to realistic nanowires in terms of microstructure, geometry and accuracy in predicting the generalized planar and stacking fault energy curves is also addressed. Our findings show clear evidence that the plastic flow of twinned nanowires under tension differs markedly between fcc metals, which may reconcile some conflicting observations made in the past.

The ideal strength of crystalline solids refers to the stress at elastic instability of a hypothetical defect-free crystal with infinite dimensions subjected to an increasing load. Experimentally observed metallic wires of a few tens of nanometers in diameter usually yield far before the ideal strength, because different types of surface or structural defects, such as surface inhomogeneities or grain boundaries, act to decrease the stress required for dislocation nucleation and irreversible deformation. In this study, however, we report on atomistic simulations of near-ideal strength in pure Au nanowires with complex faceted structures related to realistic nanowires. The microstructure dependence of tensile strength in face-centered cubic Au nanowires with either cylindrical or faceted surface morphologies was studied by classical molecular dynamics simulations. We demonstrate that maximum strength and steep size effects from the twin boundary spacing are best achieved in zigzag Au nanowires made of a parallel arrangement of coherent twin boundaries along the axis, and {111} surface facets. Surface faceting in Au NWs gives rise to a novel yielding mechanism associated with the nucleation and propagation of full dislocations along {001}110 slip systems, instead of the common {111}112 partial slip observed in face-centered cubic metals. Furthermore, a shift from surface dislocation nucleation to homogeneous dislocation nucleation arises as the twin boundary spacing is decreased below a critical limit in faceted nanowires. It is thus discovered that special defects can be utilized to approach the ideal strength of gold in nanowires by microstructural design.

Molecular dynamics simulations were performed to gain fundamental insight into crystal plasticity, and its size effects in nanowires deformed by spherical indentation. This work focused on -oriented single-crystal, defect-free Ni nanowires of cylindrical shape with diameters of 12 and 30 nm. The indentation of thin films was also comparatively studied to characterize the influence of free surfaces in the emission and absorption of lattice dislocations in single-crystal Ni. All of the simulations were conducted at 300 K by using a virtual spherical indenter of 18 nm in diameter with a displacement rate of 1 m·s−1. No significant effect of sample size was observed on the elastic response and mean contact pressure at yield point in both thin films and nanowires. In the plastic regime, a constant hardness of 21 GPa was found in thin films for penetration depths larger than 0.8 nm, irrespective of variations in film thickness. The major finding of this work is that the hardness of the nanowires decreases as the sample diameter decreases, causing important softening effects in the smaller nanowire during indentation. The interactions of prismatic loops and dislocations, which are emitted beneath the contact tip, with free boundaries are shown to be the main factor for the size dependence of hardness in single-crystal Ni nanowires during indentation.

Atomistic simulations using the quasicontinuum method are performed to examine the mechanical behavior and underlying mechanisms of surface plasticity in nanocrystalline aluminum with a grain diameter of 7 nm deformed under wedge-like cylindrical contact. Two embedded-atom method potentials for Al, which mostly differ in their prediction of the generalized stacking and planar fault energies, and grain boundary (GB) energies, are used and characterized. The simulations are conducted on a randomly oriented microstructure with 〈1 1 0〉-tilt GBs. The contact pressure–displacement curves are found to display significant flow serration. We show that this effect is associated with highly localized shear deformation resulting from one of three possible mechanisms: (1) the emission of partial dislocations and twins emanating from the contact interface and GBs, along with their propagation and intersection through intragranular slip, (2) GB sliding and grain rotation and (3) stress-driven GB migration coupled to shear deformation. Marked differences in mechanical behavior are observed, however, as a function of the interatomic potential. We find that the propensity to localize the plastic deformation at GBs via interface sliding and coupled GB migration is greater in the Al material presenting the lowest predicted stacking fault energy and GB energy. This finding is qualitatively interpreted on the basis of impurity effects on plastic flow and GB-mediated deformation processes in Al.

Twin boundaries exist in bulk metals, and they are even more common in metallic nanomaterials. Molecular simulation has made it possible to achieve a predictive understanding of the atomic mechanisms leading to the enhanced properties of nano-twinned metals. Taking nanowires as prototypes, this paper presents an atomistic view of twin structure and its important role in synthesis and in mechanical deformation.

Nanocrystalline (50-μm-thick) Ni films with controlled surface morphology at the nanoscale were synthesized by direct-current electrodeposition of Ni on an Si substrate under different electrochemical conditions. A relationship between spatial roughness scaling and mean grain size in electrodeposited Ni was established using X-ray diffraction and atomic force microscopy. Fractal analysis showed a transition from self-affine to ultrasmooth surfaces. A non-destructive method is demonstrated to estimate the grain size distribution of ultrasmooth nanocrystalline Ni surfaces by atomic force microscopy with high-resolution probes.

The nanoindentation of semicrystalline polyamide 6 (PA6) specimens was evaluated by atomic force microscopy with an extremely sharp diamond tip whose apex was nominally less than 10 nm in radius. Nanoindentation was performed under dry and wet conditions on PA6 samples injection-molded at different melt temperatures. The bulk Young’s modulus of dry PA6 was found to be in good agreement with the literature, thereby confirming the experimental approach developed in this study. A major finding is the observation of sudden increases in force during the loading portion of the experimental load–displacement curves, which occurred regardless of water absorption. We also found some irregularity in the pile-up morphology around the indents, linked to the heterogeneous nature of the deformation in PA6. Finite element analysis was used to elucidate the phenomenon of force discontinuity by independently considering the variations in elasticity, yield stress and friction coefficient of the crystalline lamellar aggregates in PA6. It is shown that force discontinuities in nanoindented PA6 result from local differences in yield phenomena at the lamellar level. The present investigation sheds some light on the importance of mechanical heterogeneity emerging from contact interactions between an extremely sharp tip and the nanoscale morphology of semicrystalline polymers.