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Nanoarchitected materials are considered as a promising research field, deriving distinctive mechanical properties by combining nanomechanical size effects with conventional structural engineering. Despite the successful demonstration of the superiority and feasibility of nanoarchitected materials, scalable and facile fabrication techniques capable of macroscopically producing such materials at a low cost are required to take advantage of the nanoarchitected materials for specific applications. Unlike conventional techniques, proximity-field nanopatterning (PnP) is capable of simultaneously obtaining high spatial resolution and mass producibility in synthesizing such nanoarchitected materials in the form of an inch-scale film. Herein, we focus on the feasibility of using PnP as a scalable fabrication technique for three-dimensional nanostructures and the superiority of the resultant thin-shell oxide nanoarchitected materials for specific applications, such as lightweight structural materials, mechanically robust nanocomposites, and high-performance piezoelectric materials. This review will discuss and summarize the relevant results obtained for nanoarchitected materials synthesized by PnP and provide suggestions for future research directions for scalable manufacturing and application.

The microstructural evolution of Al–Zn–Mg–Cu alloys has been investigated for the homogenization time effect on the texture, grain orientation and dislocation density. The Al–Zn–Mg–Cu alloys were casted and homogenized for 4, 8, 16 and 24 hours. Electron backscatter diffraction (EBSD) analysis was conducted to characterize the microstructural behavior. Micropillars were fabricated using focused ion beam (FIB) milling in grains of specific crystallographic orientations. Coarse precipitations in the grain boundaries are S (Al2CuMg) and T (Al2Mg3Zn3) phases verified by scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) observation. With increasing homogenization time, equiaxed cell sizes increased. The volume fraction of S and T phases decreased with the diffusion of atomic elements into matrix. The Vickers hardness and tensile strength values decreased with homogenization temperature. The micropillar compression analysis was compared to macro tensile test results to understand the size effect and strain burst phenomenon on the mechanical properties of Al–Zn–Mg–Cu alloys.

Nanopillar compression tests were applied on δ-ferrites in austenitic stainless steel welds to measure the thermal ageing activation energy caused by spinodal decomposition. Welds of an austenitic stainless steel were thermally aged at 343, 375, and 400 oC for up to 20,000 h. To avoid the interference of surrounding austenite matrix on embrittlement measurement, austenite matrix was selectively dissolved before the nanopillar fabrication. Using the yield stress changes measured by nanopillar compression tests, the thermal ageing activation energy of δ-ferrite was estimated as 154 KJ/mol, which was compared with literature results and the reasons of discrepancy were discussed.

A parametric study of several parameters relevant to design safety on the spent nuclear fuel (SNF) rod response under a drop accident is presented. In the view of the complexity of interactions between the independent safety-related parameters, a factorial design of experiment is employed as an efficient method to investigate the main effects and the interactions between them. A detailed single full-length fuel rod is used with consideration of post-irradiated fuel conditions under horizontal and vertical free-drops onto an unyielding surface using finite-element analysis. Critical drop heights and critical g-loads that yield the threshold plastic strain in the cladding are numerically estimated to evaluate the fuel rod structural resistance to impact load. The combinatory effects of four uncertain parameters (pellet-cladding interfacial bonding, material properties, spacer grid stiffness, rod internal pressure) and the interactions between them on the fuel rod response are investigated. The principal finding of this research showed that the effects of above-mentioned parameters on the load-carrying capacity of fuel rod are significantly different. This study could help to prioritize the importance of data in managing and studying the structural integrity of the SNF. Keywords: Spent nuclear fuel, Drop impact, Structural response, Design of experiments, Finite-element analysis

Reduced activation ferritic martensitic (RAFM) steel has been regarded as a candidate for blanket material of nuclear fusion plant. Microstructure of RAFM steel, featured by precipitates and hierarchical structure of martensite, contribute to its excellent irradiation resistance. For better understanding of mechanical responses of RAFM steels, microstructure-based simulation has been developed for describing detailed elastic-plastic behavior in grain (or meso-) scale. In this study, a dislocation density based crystal plasticity model including dislocation-precipitate interactions was proposed and implemented in the finite element simulations of meso-scale micro-pillar compression and macro-scale uniaxial tensile tests. For the macroscale simulations, a representative volume element approach with periodic boundary condition was applied for predicting mechanical response of investigated polycrystalline RAFM steel based on measured mesoscale parameters and microstructure. The presented crystal plasticity model could reproduce the mesoscopic local deformation and macroscale flow stress strength in good agreement.

Ti/Ta-reduced activation ferritic/martensitic (RAFM) steel used in this study composes one class of lath martensite that has a hierarchically-arranged crystallographic structure. Due to the multi-scale complexity in the microstructure, mechanical characterization at the appropriate level is required to understand the role of each hierarchical feature. In this study, uniaxial micro-pillar compression tests combined with rigorous crystal orientation analysis were performed to figure out the specific influences of the lath and sub-block boundaries on the plasticity in the lath martensite-structured Ti/Ta-RAFM steel. By conducting a postmortem slip trace analysis and comparing the critical resolved shear stress of each activated slip system, we confirmed that there is no strengthening effect from either the lath or sub-block boundaries.

Traditionally, the manipulation of contact mechanisms has been adopted as the primary strategy to tailor the friction properties of surfaces. On the contrary, the detaching process involving the local deformation and failure at the interface has been considered relatively less important. Here, we present a new approach toward the friction control of amorphous carbon through the plasticity and resultant transition of deformation mode on nanopatterned surfaces. Depending on the topography of the nanopatterns, the mechanical responses of the surfaces alter from elastic fracture to plastic flow, through which the friction coefficient changes by a factor of 5 without manipulation of the intrinsic structure of the material.

Highly controlled biomineralization of calcium carbonate is via non-classical mesocrystallization of amorphous precursors. In the present study, a simple in vitro assay was developed to mimic the biological process, which involved stabilized amorphous calcium carbonate and a single crystal substrate of calcite. The microcoating layer formed on the calcite substrate displayed mesocrystalline characteristics, and the layers near the substrate were strongly influenced by the epitaxy to the substrate. This behavior was preserved even when the morphology of the coating layer was modified with poly(acrylic acid), a model anionic macromolecule. Interestingly, the extent of the epitaxy increased substantially with poly(ethylene imine), which barely affected the crystal morphology. The in vitro assay in the present study will be useful in the investigations of the biomineralization and bioinspired crystallization of calcium carbonate in general.

In the hierarchically-arranged crystallographic structure of reduced-activation ferritic/martensitic (RAFM) steel, the smallest but the most abundant microstructural unit, i.e., the lath, plays an important role in dislocation plasticity. Due to the multi-scale complexity in the lath-martensitic microstructure, miniaturized mechanical characterization at very fine scale is required to understand the deformation mechanism associated with laths. In this study, uniaxial micro-compression tests combined with rigorous crystallographic analysis were performed to figure out the plastic deformation mechanism of lath boundary sliding in RAFM steels. These experimental results were further interpreted via molecular dynamics simulations to discover the underlying dislocation mechanisms. We found that the amount of lath boundary sliding is controlled by the crystallographic orientation of the lath boundary plane, the direction of Burgers vectors of the interfacial dislocations, and the magnitude of resolved shear stress on the lath boundary plane. Also, the effect of normal stress on the lath boundary plane was investigated.

Any transition toward an era of flexible electronics will have to overcome the mechanical limitations of materials. Specifically, the attainment of both strength and flexibility, which are generally mutually exclusive, is required including glass-like wear resistance, plastic-like compliance, and a high level of strain. Here, we fabricate a urethane-methacrylate-siloxane hybrid (UMSH) material. It is found that UMSH, with molecule-level hybridization of urethane linkage and methacrylate-siloxane conetworks, demonstrates ceramic-like high strength (574 MPa), yet polymer-like low modulus (8.42 GPa), and even high strain (6.3%) at fracture with excellent optical transparency. This combination of high strength, flexibility, and optical transparency indicates that this is a suitable material for glass substitution and can be used as a transparent flexible cover window for foldable display.

Polymer nanocomposites with inclusion of ceramic nanofillers have relatively high yield strength, elastic moduli, and toughness that therefore are widely used as functional coating and films for optoelectronic applications. Although the mechanical properties are enhanced with increasing the fraction of nanofiller inclusion, there generally is an upper limit on the amount of nanofiller inclusion because the aggregation of the fillers in the polymer matrix, which typically occurs, degrades the mechanical and/or optical performances above 5 vol % of inclusions. Here, we demonstrate an unconventional polymer nanocomposite composed of a uniformly distributed three-dimensional (3D) continuous ceramic nanofillers, which allows for extremely high loading (∼19 vol %) in the polymer matrix without any concern of aggregation and loss in transparency. The fabrication strategy involves conformal deposition of Al2O3 nanolayer with a precise control in thickness that ranges from 12 to 84 nm on a 3D nanostructured porous...

HT9, a ferritic/martensitic steel, is a candidate structural material for next-generation advanced reactors. Its microstructure is a typical tempered martensite showing a hierarchical lath-block-and-packet structure. We investigate the specimen size effect and strengthening contribution of various microstructural boundaries manifested in the compression tests of micropillars with diameters ranging from 0.5 to 17 μm. It is observed that micropillars with diameters larger than 3 μm show uniform deformation and plastic flow curves comparable to the bulk flow curve. Localized deformation by a few pronounced slip bands occurs in micropillars with diameters smaller than 1 μm, and the yield strength is reduced. Careful examination of the sizes of the microstructural features and cross-sections of the micropillars shows that the block boundaries are the most effective strengthening boundaries in tempered martensitic microstructure. The bulk mechanical properties of HT9 can be evaluated from a micropillar with diameter as low as 3 μm.

This paper proposes a systematic risk evaluation framework for one of the most significant impact events on an interim dry storage facility, an aircraft crash, by using a probabilistic approach. A realistic case study that includes a specific cask model and selected impact conditions is performed to demonstrate the practical applicability of the proposed framework. An event tree analysis of an occurred aircraft crash that defines a set of impact conditions and storage cask response is constructed. The Monte-Carlo simulation is employed for the probabilistic approach in consideration of sources of uncertainty associated with the impact loads onto the internal storage casks. The parameters for representing uncertainties that are managed probabilistically include the aircraft impact velocity, the compressive strength of the reinforced concrete wall, the missile shape factor, and the facility wall thickness. Failure probabilities of the impacted wall and a single storage cask under direct mechanical impact load caused by the aircraft crash are estimated. A finite element analysis is applied to simulate the postulated direct engine impact load onto the cask body, and a source term analysis for associated releases of radioactive materials as well as an off-site consequence analysis are performed. Finally, conditional risk contribution calculations are represented by an event tree model. Case study results indicate that no severe risk is presented, as the radiological consequences do not exceed regulatory exposure limits to the public. This risk model can be used with any other representative detailed parameters and reference design concepts for other comparable direct or indirect impact conditions onto the cask body, which may provide an efficient way to investigate storage facility capacity to withstand an aircraft crash and thereby protect public health.

Most monolithic brittle materials are vulnerable to the failure by cracks because of a lack of intrinsic toughening mechanisms, such as the plasticity in the vicinity of the crack front. As a result, most of the efforts to mitigate the sudden failure of brittle ceramics have been focused on developing the extrinsic toughening mechanisms that hinder crack propagation behind the tip, such as the fiber bridging. In this work, we experimentally demonstrate that the intrinsic toughening arises even in the brittle monolithic ceramic material such as diamond-like carbon (DLC) when its external dimension reduces down to sub-micron scales. This unique phenomenon owes its origin to the decrease of the crack driving force in the small samples, which in turn enables them to bear high enough stresses to activate the local atomic plasticity. Through nanomechanical tensile and bending experiments, electron energy loss spectroscopy analysis, and finite element method for stress distribution calculation, we confirmed that the local atomic plasticity associated with sp3 to sp2 rehybridization is responsible for the intrinsic toughening., 2 tables, 7 figures in main, 5 figures in supplementary information

The structural integrity of spent fuel assembly is a key element for flexible and economic management of spent nuclear fuel (SNF) considering handling and transportation operations in most of the management scenarios. In order to better understand the SNF rod behavior under drop accident condition and to study the effects of various uncertain parameters on the fuel rod structural integrity, a detailed finite element (FE) model for pressurized water reactor (PWR) fuel rod is being developed using ABAQUS. This paper briefly describes the FE model and discusses the influence of the parameters relevant to design safety with concern ofpostirradiated fuel conditions such as degradation of cladding mechanical properties, pellet-tocladding mechanical interaction (PCMI), spacer grids (SGs) spring stiffness degradation, rod internal pressure (RIP). Through this study, we can initially prioritize the importance of data in managing and studying the structural integrity of the SNF in the next assessment stage. This work is part of a bigger research on the structural integrity of SNF in cask-loaded condition, whereby it might help in proposing guidelines for safe handling of SNF.

An austenitic stainless steel containing high density of nanosized NbC precipitates was subjected to micropillar compression tests. Based on the negligible size effect of micropillars with 1–5 μm diameter, the strengthening effect of NbC precipitates was measured. Further compression tests with various terminal strain were performed to understand the dynamic evolution of NbC precipitates during compressive deformation. The dissolution of nanosized NbC precipitates was observed even in the elastically deformed micropillar, caused by the interaction between the pre-existing dislocations and NbC precipitates. In the plastically deformed micropillar, the re-precipitation of NbC precipitates along the slip band was observed, while the partially dissolved NbC precipitates were observed in the region away from the slip band. In addition, the measured strengthening effect of NbC precipitates was compared with existing prediction models, but large discrepancy was observed between them. Finally, with modification of an existing hardening model by incorporating relaxation at the interface and consideration of dissolution of NbC precipitates before plastic deformation, the measured and predicted strengthening effects became comparable.

An austenitic stainless steel weld was thermally aged at 400 °C for 20,000 h, and subsequently proton irradiated up to 10 displacement per atom (dpa) at 360 °C. After thermal ageing, only spinodal decomposition was observed without G-phase precipitates in δ-ferrite. However, ~ 1023 m−3 of fine (~ 3 nm in diameter) G-phase precipitates were observed after proton irradiation. Also, wavelength of spinodal decomposition was increased with proton irradiation. The additional hardening in δ-ferrite after proton irradiation was attributed to enhanced spinodal decomposition and G-phase precipitation. After irradiation, the contribution to addition hardening was greater for G-phase than enhanced spinodal decomposition.

Quantitative characterization of mechanical properties of many nano- or micro-scaled materials is of crucial importance for many technological applications. Among various experimental methods, the in-situ nanomechanical tensile testing is of particular significance due to its capability of direct observation of mechanical behavior. Especially, the tensile loading mode enabling the precise displacement control is desirable for brittle nanomaterials that often suffer from the catastrophic failure by mechanical instability. In this study, we developed the MEMS-based nanomechanical tensile device optimized for stable displacement control even in the presence of the rapid reduction of sample stiffness. The MEMS device is designed to stabilize the motion of the actuating rod in the testing equipment via the high stiffness of springs and the large mass of the movable part. To verify the performance of the device, we conducted tensile testing on the pre-notched metallic glass samples and successfully observed fast load drops during crack propagation.

By employing atomistic simulation, we investigated the deformation mechanism of the Zr-ZrH2 bi-crystal pillar containing coherent the Zr-ZrH2 interface under uniaxial compressive loading. The plastic deformation of the bi-crystalline sample first commences in the Zr part with tension twinning, followed by dislocation nucleation on the basal planes of the twinned area. Then plasticity transfers to ZrH2 with the assistance of lateral dislocation pile-up at the Zr side of the coherent Zr-ZrH2 interface, which promotes dislocation nucleation on {001} slip system in ZrH2 side. The proposed deformation mechanism sheds light on the deformation behavior of inter-granular and trans-granular hydrides in the Zr matrix.

Density-strength tradeoff appears to be an inherent limitation for most materials and therefore design of cell topology that mitigates strength decrease with density reduction has been a long-lasting engineering pursue for porous materials. Continuum-mechanics-based analyses of mechanical responses of conventional porous materials with bending-dominated structures often give the density-strength scaling law following the power-law relationship with an exponent of 1.5 or higher, which consequentially determines the upper bound of the specific strength for a material to reach. In this work, a new design criterion capable of significantly abating strength degradation in lightweight materials is presented, by successfully combining the size-induced strengthening effect in nanomaterials with the architectural design of cellular porous materials. Hollow-tube-based 3D ceramic nanoarchitectures satisfying such criterion are fabricated in large area using proximity field nano-patterning and atomic layer deposition. Experimental data from micropillar compression confirm that the strengths of these nanoarchitectural materials scale with relative densities with a power-law exponent of 0.93, a hardly observable value in conventional bending-dominated porous materials. This discovery of a new density-strength scaling law in nanoarchitectured materials will contribute to creating new lightweight structural materials attaining unprecedented specific strengths overcoming the conventional limit.

Room-temperature uniaxial compressions of 900-nm-diameter aluminum bi-crystals, each containing a high-angle grain boundary with a plane normal inclined at 24° to the loading direction, revealed frictional sliding along the boundary plane to be the dominant deformation mechanism. The top crystallite sheared off as a single unit in the course of compression instead of crystallographic slip and extensive dislocation activity, as would be expected. Compressive stress strain data of deforming nano bicrystals was continuous, in contrast to single crystalline nano structures that show a stochastic stress strain signature, and displayed a peak in stress at the elastic limit of ∼176 MPa followed by gradual softening and a plateau centered around ∼125 MPa. An energetics-based physical model, which may explain observed room-temperature grain boundary sliding, in presented, and observations are discussed within the framework of crystalline nano-plasticity and defect microstructure evolution.

Metallic glasses typically exhibit high strength and a high elastic limit but suffer from poor fatigue resistance. This work demonstrates that 1.6 micron diameter Zr-based metallic glass samples subjected to compressive fatigue cycling did not fail after 40 × 10^6 cycles. The fatigue endurance limit was shown to increase to more than 110% of bulk yield strength under compression–compression and up to 90% under bending; those of the same material with macroscopic dimensions are typically at 50% of bulk yield strength.

Creating new materials with novel properties through structural modification is the Holy Grail of materials science. The range of targetable structures for amplification of mechanical properties in metallic glasses would include types of atomic short range orders at the smallest scale through compositions or morphologies of phases in composites. Even though the usefulness of the latter approach has been successfully demonstrated in the past decades, the feasibility of the former has been incompletely proved with only marginal property improvements reported within experimentally-accessible atomic-level structural changes. Here, we report the significant enhancement of deformability in Zr-based monolithic metallic glass only through the atomic disordering by proton irradiation without altering any other structural traits. Metallic glass nanopillars that originally failed catastrophically without any notable plasticity become capable of attaining more than 30% uniaxial plastic strain accommodated by homogeneous deformation when irradiated to ~1 displacement per atom (DPA). We discuss the atomistic origin of this improved plasticity in terms of density and spatial distributions of icosahedral short range order influenced by irradiation.

Useful properties of structural materials generally depend on their bulk microstructure. For centuries, improvements in structural materials relied heavily on processing, which in turn determined the resulting microstructure and properties. Materials sciences are entering an era in which specific properties of a material are obtained not only from its processing but also by controlling of the architecture of its constituents, often with sub-micron dimensions. To utilize this newly achievable nanoscale engineering precision in structural applications, it is imperative to quantify the deformation processes at each relevant scale, with special attention focusing on the importance of internal and external heterogeneities, for example grain boundaries, bi-material interfaces, phase boundaries, etc., on mechanical loading. It has been shown for single crystals that yield (and fracture) strengths increase with power-law dependence on sample size reduction when the micron scale is reached, and therefore, can no longer be inferred from bulk response or from the literature. Although these studies provide a powerful foundation for fundamental deformation processes operating at small scales, they are far from representing real materials used in structural applications, whose microstructure is often complex, containing boundaries and interfaces. Both homogeneous (i.e. grain and twin boundaries) and heterogeneous (i.e. phase and precipitate–matrix boundaries) interfaces in size-limited features are crucial aspects of the structural reliability of most modern materials. They are also of particular importance to damage initiation. This article provides a comprehensive overview of the state-of-the-art experimental and computational methods used to investigate mechanical behavior and microstructural evolution in small-scale metallic systems, deformation of which depends on intricate interactions of defects with internal interfaces and with free surfaces. Attention is focused on the effects of multiple grain boundaries spanning the sample volume (nanocrystalline and polycrystalline metals). This overview sheds light on the relative importance of intrinsic versus extrinsic length scale limitations on deformation mechanisms in nanostructured metals, which has significant implications for the development of new materials with tunable mechanical properties.

Indium columnar structures with diameters near 1 μm were deformed by uniaxial compression at strain rates of approximately 0.01 and 0.001 s −1 . Defect density evolution in the nanopillars was evaluated by applying synchrotron Laue X-ray microdiffraction (μSLXRD) on the same specimens before and after deformation. Results of the μSLXRD measurements indicate that the dislocation density increases as a result of mechanical deformation and is a strong function of strain rate. These results suggest that the rate of defect generation during the compression tests exceeds the rate of defect annihilation, implying that plasticity in these indium nanostructures commences via dislocation multiplication rather than nucleation processes. This is in contrast with the behaviors of other materials at the nanoscale, such as, gold, tin, molybdenum, and bismuth. A hypothesis based on the dislocation mean-free-path prior to the multiplication process is proposed to explain this variance.

Uniaxial tension and compression experiments on [0 0 1] and [0 1 1] oriented molybdenum nano-pillars exhibit tension–compression asymmetry, a difference in attained stresses in compression vs. tension, which is found to depend on crystallographic orientation and sample size. We find that (1) flow stresses become higher at smaller diameters in both orientations and both loading directions, (2) compressive flow stresses are higher than tensile ones in [0 0 1] orientation, and visa versa in [0 1 1] orientation, and (3) this tension–compression asymmetry is in itself size dependent. We attribute these phenomena to the dependence of twinning vs. antitwinning deformation on loading direction, to the non-planarity of screw dislocation cores in Mo crystals, and to the possibly lesser role of screw dislocations in governing nano-scale plasticity compared with bulk Mo.

Homogeneous plasticity in metallic glasses is generally only observed at high temperatures or in very small structures (less than ≈100 nm), so their applications for structural performance have been very limited. Here, nanolaminates with alternating layers of Cu_(50)Zr_(50) metallic glass and nanocrystalline Cu are synthesized and it is found that samples with an optimal composition of 112-nm-thick metallic-glass layers and 16-nm-thick Cu layers demonstrate a maximum strength of 2.513 GPa, a value 33% greater than that predicted by the rule-of-mixtures and 25% better than that of pure Cu_(50)Zr_(50) metallic glass. Furthermore, ≈4% strain at fracture is achieved, suppressing the instantaneous catastrophic failure often associated with metallic glasses. It is postulated that this favorable combination of high strength and deformability is caused by the size-dependent deformation-mode transition in metallic glasses, from highly localized plasticity, leading to immediate failure in larger samples to homogeneous extension in the smaller ones.

We report results of uniaxial compression tests on Zr_(35)Ti_(30)Co_(6)Be_(29) metallic glass nano-pillars with diameters ranging from ~1.6 μm to ~100 nm. The tested pillars have nearly vertical sidewalls, with the tapering angle lower than ~1° (diameter >200 nm) or ~2° (diameter ~100 nm), and with a flat pillar top to minimize the artifacts due to imperfect geometry. We report that highly-localized-to-homogeneous deformation mode change occurs at 100 nm diameter, without any change in the yield strength. We also find that yield strength depends on size only down to 800 nm, below which it remains at its maximum value of 2.6 GPa. Quantitative Weibull analysis suggests that the increase in strength cannot be solely attributed to the lower probability of having weak flaws in small samples – most likely there is an additional influence of the sample size on the plastic deformation mechanism.

Nanocrystalline metals generally exhibit high strengths and good fatigue resistance Their strengthening scales with the inverse of grain size through square root dependence down to grain sizes of ~20 nm, representing the well-known Hall-Petch relation Here we show that in surface-dominated structures with sub-micron dimensions, i e nanopillars, 60 nm grained Ni-W alloys exhibit lower tensile strengths with decreasing pillar diameter, form shear bands and undergo mechanical twinning Moreover, there appears to be a transition in the deformation mechanism from dislocation-driven deformation in pillars with diameters larger than 100 nm to grain-boundary mediated deformation in pillars of 100 nm and below, including grain rotation and grain-boundary migration, processes previously observed only in grain sizes below 20 nm in materials of the same composition We postulate that the presence of free surfaces activates these grain-boundary mediated deformation processes at much larger grain sizes than observed before and results in lower attained strengths.

Crystal plasticity finite element (CPFE) simulations were performed on the representative volume elements (RVE) modeling body centered cubic (bcc) single, bi- and tri-crystals. The RVE model was designed to include a void inside a grain, at a grain boundary and at a triple junction. The effect of single crystal orientation on the flow strength and growth rate of the void was discussed under prescribed boundary conditions for constant stress triaxialities. CPFE analyses could explain the effect of inter-grain orientations on the anisotropic growth of the void located at the grain boundaries. The results showed that the rate of void growth had preferred orientation in a single crystal, but the rate could be significantly different when other orientations of neighboring crystals were considered.

We provide an overview of our work where carbon-based nanostructures have been applied to two-dimensional (2D) planar and three-dimensional (3D) vertically-oriented nano-electro-mechanical (NEM) switches. In the first configuration, laterally oriented single-walled nanotubes (SWNTs) synthesized using thermal chemical vapor deposition (CVD) were implemented for forming bridge-type 2D NEMS switches, where switching voltages were on the order of a few volts. In the second configuration, vertically oriented carbon nanofibers (CNFs) synthesized using plasma-enhanced (PE) CVD have been explored for their potential application in 3D NEMS. We have performed nanomechanical measurements on such vertically oriented tubes using nanoindentation to determine the mechanical robustness of the CNFs. Electrostatic switching was demonstrated in the CNFs synthesized on refractory metallic nitride substrates, where a nanoprobe was used as the actuating electrode inside a scanning-electron-microscope. The switching voltages were determined to be in the tens of volts range and van der Waals interactions at these length scales appeared significant, suggesting such structures are promising for nonvolatile memory applications. A finite element model was also developed to determine a theoretical pull-in voltage which was compared to experimental results.

In situ nanomechanical tests are carried out to investigate the tensile and compressive behavior of -oriented body-centered cubic (bcc) metals W, Mo, Ta and Nb with nanometer dimensions. We find that the strength of these metals exhibits strong size dependence. The compressive size effect in Nb, as evaluated by the log–log slope of strength vs. nanopillar diameter, is −0.93, a factor of 2.1 greater than that for the other three metals W, Mo and Ta (−0.44). In tension, however, Ta and Nb show higher size effect slopes (−0.80 and −0.77) as compared with W and Mo (−0.58 and −0.43). We also report that while the yield strength of these metals is a strong function of size, the strain-hardening behavior does not present any size-dependent trends. We further discuss the effects of strain-rate on deformation behavior and provide transmission electron microscopy analysis of microstructural evolution in the same Mo nanopillar before and after compression.

Julia R. Greer received her S.B. in Chemical Engineering from the Massachusetts Institute of Technology (1997) and a Ph.D. in Materials Science from Stanford University, where she worked on the nanoscale plasticity of gold with W. D. Nix (2005). She also worked at Intel Corporation in Mask Operations (2000-03) and was a post-doctoral fellow at the Palo Alto Research Center (2005-07), where she worked on organic flexible electronics with R. A. Street. Greer is a recipient of TR-35, Technology Review's Top Young Innovator award (2008), a NSF CAREER Award (2007), a Gold Materials Research Society Graduate Student Award (2004), and an American Association of University Women Fellowship (2003). Julia joined Caltech's Materials Science department in 2007 where she is developing innovative experimental techniques to assess mechanical properties of nanometer-sized materials. One such approach involves the fabrication of nanopillars with different initial microstructures and diameters between 25 nm and 1 µm by using focused ion beam and electron-beam lithography microfabrication. The mechanical response of these pillars is subsequently measured in a custom-built in situ mechanical deformation instrument, SEMentor, comprising a scanning electron microscope and a nanoindenter.

Size-dependent deformation behavior of niobium single-crystalline samples 200–900 nm in diameter is investigated by in situ compression and tension testing. Stress–strain curves in compression show discrete plastic flow associated with dislocation escapes at the surface. In tension, plastic flow is more continuous with pronounced strain hardening, and fracture occurs at the low true strain of ~3%. Transmission electron microscopy analysis of the same 100 nm nanopillar before and after deformation provides insights into dislocation activity during compression deformation.