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This study presents an irradiation-dependent internal state variable (ISV) elastoviscoplasticity-damage constitutive model that accounts for nuclear irradiation hardening and embrittlement of the irradiated polycrystalline materials. The irradiation effects were added to the coupled plasticity-damage kinetics with consideration of the structure–property relationships. The present irradiation-dependent elastoviscoplasticity-damage model was compared with the lab deformation experimental data of irradiated oxygen-free high conductivity (OFHC) copper, modified 9Cr-1Mo steel, and Ti-5Al-2.5Sn. The results show excellent agreement over the entire stress–strain curves at various irradiation doses. Because the ISV model, before the irradiation plasticity-damage addition, had been used on over 80 different metal alloys, it is anticipated that this nuclear irradiation supplement will also allow for application to many more irradiated metal alloys.

This paper studies the effects of high-pressure torsion (HPT) at ambient temperature on microstructural evolution and mechanical properties enhancement in pure copper. The aim is to introduce gradient microstructure, with various statistical distributions of grain size and grain orientations to examine their effect on strength and ductility. To this end, extruded cylindrical pure copper subjected to HPT for 1, 2, and 3-turns resulted in grain refinement down to the grain size of 500 nm. Combination of microhardness test and EBSD scans through the radial direction confirm the creation of a heterogeneous structure through the thickness and radial directions. The results demonstrate that increasing the shear strain leads to (1) ultra-fine grain (UFG) generation at deformed coarse-grain boundaries, (2) an increase in the fraction of recrystallized grains and high angle grain boundaries, and (3) a homogenous structure in the last step. A unique mixture has been obtained due to the particular shape of the anvils. The mixture included a chain of UFGs and coarse grains contain dislocations and subgrains. The highest level of gradient structure through the thickness was observed after 1-turn, which leads to the best combination of strength and ductility.

Clay minerals are platelike particles that play a critical role in problems involving swelling, deformation, and failure. Fundamental understanding of these phenomena and the parameters tha...

Molecular dynamics simulations were performed to identify the underlying deformation mechanisms controlling the plastic behavior of nanoindented nanoscale multilayered Ti/TiN. MD simulations were conducted on pure Ti and pure TiN as well as on four different layer-thickness ratios of Ti/TiN multilayers, Ti:TiN = 1, 2.5, 4, and 7.5. The Ti layer thickness varied from 2 nm to 15 nm while the TiN layer thickness is kept constant of 2 nm. The plastic deformation of nanoindented pure Ti was dominated by the formation of dislocation loops resulting from basal partial dislocations, while very few perfect dislocations that tie dislocation loops together were observed. The plastic deformation of nanoindented pure TiN was controlled by the activation of perfect dislocation propagation along the ( 1 1 1 ) plane that dissociates into two partials. Depending on the thickness ratio, either dislocation pile-up or single dislocation crossing through the interface was the controlling plastic deformation mechanism of nanoindented Ti/TiN multilayers. For metal layer thicknesses above 5 nm, significant dislocation pile-ups were observed at the interface of the multi-layered samples. The Ti/TiN multilayer with a thickness ratio of 1:1 with individual layer thickness of 2 nm exhibited the highest strain-hardening rate. At this length scale, the activation of dislocation sources requires very high stresses, and the single dislocation crossing process is the most dominant deformation mechanism. The initiation of plasticity in the TiN layer occurs at a high level of stress since there is no dislocation pile-up at the interface.

This paper develops a continuum model of deformation of X70 steel coated with Cu/Nb–nanolaminate metallic systems (NMSs). The developed model is based on an elastic/viscoplastic framework that includes the effects of dislocation density, size effect, and strain hardening on the plastic deformation. The X70 steel is modeled according to a modified Taylor equation that considers the effects of statistically stored and geometrically necessary stored dislocation densities. NMSs obey two different mechanisms, the confined layer slip (CLS) and dislocation nucleation (Nu), during plastic deformation. NMSs are modeled according to the CLS mechanism when the individual lamina thickness is between 100 nm and 5 nm and the Nu mechanism when the individual lamina thickness is 5 nm or below. The model is implemented in a subroutine of a commercial finite element analysis code. Simulating a tensile test is the benchmark to generate flow curves of the model that agree with experimental data found in the literature.

Although metallic materials with gradient microstructure exhibit notable performance in harsh environmental conditions, they can also exhibit unusual mechanical behaviour. This is attributed to bot...

We present an entirely new method of bottoms-up fabrication of polycrystalline micropillars using direct printing and sintering of nanoparticles in 3D and study their behavior under compression for different microstructures. The pillars showed brittle behavior with higher effective modulus for small grain sizes with high porosity, while highly ductile behavior with a lower effective modulus and larger grain sizes but low porosity. These unusual trends are explained by a porosity model. The results point to a novel method of fabricating micropillars with different microstructures to study fundamental materials science of polycrystalline materials at micro to meso-length scales.

The increase in use of dual-phase (DP) steel grades by vehicle manufacturers to enhance crash resistance and reduce body car weight requires the development of a clear understanding of the effect of various microstructural parameters on the energy absorption in these materials. Accordingly, DP steelmakers are interested in predicting the effect of various microscopic factors as well as optimizing microstructural properties for application in crash-relevant components of vehicle bodies. This study presents a microstructure-based approach using a multiscale material and structure model. In this approach, Digimat and LS-DYNA software were coupled and employed to provide a full micro–macro multiscale material model, which is then used to simulate tensile tests. Microstructures with varied ferrite grain sizes, martensite volume fractions, and carbon content in DP steels were studied. The impact of these microstructural features at different strain rates on energy absorption characteristics of DP steels is investigated numerically using an elasto-viscoplastic constitutive model. The model is implemented in a multiscale finite-element framework. A comprehensive statistical parametric study using response surface methodology is performed to determine the optimum microstructural features for a required tensile toughness at different strain rates. The simulation results are validated using experimental data found in the literature. The developed methodology proved to be effective for investigating the influence and interaction of key microscopic properties on the energy absorption characteristics of DP steels. Furthermore, it is shown that this method can be used to identify optimum microstructural conditions at different strain-rate conditions.

Mechanical properties that are considered to be deterministic in the macro-scale have been shown to be stochastic in the sub-micron length scale. The origin of such stochastic responses is not well understood. This work examines the potential influence of grain boundaries and grain orientations on the stochastic nature of pop-in and hardness measurement in annealed high purity polycrystalline Cu samples during low load nanoindentation. Statistical analysis on pop-in load and hardness showed that variations of these measurements depend on crystal orientations and is influenced by the indenter probe size. Analysis on the pop-in load statistics showed that pop-ins are likely initiate from an atomic sized precursor that leads to dislocation generation or expansion. Variation in hardness measurements near an arbitrary chosen grain boundary and the apparent grain boundary hardening effect observed may be related to the higher density of dislocations at and near the grain boundary.

Cu/Ni/Nb tri-layer and CuNi/Nb alloy multilayer films were annealed to examine the microstructural evolution and mechanical properties of three component laminated metallic nanostructures. Scanning transmission electron microscopy showed Ni x Nb y compounds forming on either side of the Nb layer in both tri-layer and alloy films. Post annealing nanoindentation showed all annealing conditions resulted in increased hardness, indicative of the Ni x Nb y intermetallic phase increasing the hardness of the films. The hardness of both films achieves a maximum hardness after annealing for 3 h at 300 °C. The hardness increase as the annealing temperature increases corresponds to a thicker Ni x Nb y layer at the incoherent boundary. This strengthening technique could be implemented in a variety of different multilayer systems to achieve age-hardenable multilayer metallic nanostructures.

In this work, the mechanical behaviour and texture evolution of AZ31B magnesium alloy under uniaxial tensile testing are investigated at different strain rates and temperatures. A crystal plasticity model is developed and calibrated to predict the mechanical response of the AZ31B at different temperatures and strain rates. The model results show that the relative activity of the pyramidal slip increases with increasing temperature, reaching a maximum activity at 200 °C. In order to achieve the continuous increase in the relative activity of the pyramidal slip as reported in the literature, a grain boundary sliding mechanism is implemented in the crystal plasticity framework. The incorporation of the grain boundary sliding at elevated temperatures results in considerable improvement in the model’s capabilities for prediction of yielding, hardening and texture evolution.

Since the introduction of the term “nanolaminate” in the mid-1990s, considerable research activities on metallic/ceramic nanolaminates (MCN) have been conducted. Incorporating ceramics with high hardness and high melting point together with high ductile metals can improve their thermomechanical behavior in corrosive environments. A great number of researchers have reported that MCNs exhibit outstanding thermomechanical properties compared with the constituent layers and bulk material, which is attributed to the atomic structure and high density of the interfaces. This article provides a review of recent advances in modeling of the mechanical behavior of MCN composites, with focus on Nb/NbC and Ti/TiN multilayer composites. The main strengthening mechanisms of MCNs, based on the layer thickness, the interface structure, and the interaction of threading dislocations with the interface as well as dislocations nucleation from the interface, are reviewed, and recently, obtained results from molecular dynamics simulations, along with these findings, are presented. Moreover, MD-based flow surfaces for use in large-scale continuum models are reviewed in connection with results from MD of MCNs under various mechanical loading conditions, including uniaxial and biaxial loadings.

Pre-existing dislocations (PED) are ubiquitous inside crystalline lattices which in turn affect the yield stress and the process of plastic deformation. Hence, understanding the onset of dislocations motion and interaction is critical in modifying or designing new materials with advanced properties so that their mechanical behavior approach realistic conditions. One such family of new materials is the ceramic/metallic nanolaminates. In this work, we have investigated the effect of pre-existing dislocations on the mechanical behavior of NbC/Nb nanolaminates using molecular dynamics simulations. Upon unloading at different strains from stress-strain curve of 3 nm NbC/7 nm Nb sample, we were able to generate structures with various pre-existing dislocation densities inside the layers. Uniaxial loadings parallel to the interface at two different temperatures (10 K and 300 K) were performed on each structure. Also, the yield locus was determined at 300 K by applying biaxial in-plane loading and fitted with a general flow potential to be used in macroscale analysis. Finally, the tension-compression asymmetry (TCA) was investigated for the structures with pre-existing dislocations along two different in-plane loading directions.

Traditionally, modeling the effect of grain size on the mechanical behavior of crystalline materials is based on assuming an equivalent homogenous microstructure with strength being dependent on the average grain size, for example the well-known Hall-Petch relation. However, assuming an equivalent homogenized microstructure for a highly heterogeneous microstructure can lead to inaccurate prediction of strength and ductility, especially when the gradients in the spatial heterogeneity are severe. In this work, we employ a multiscale dislocation-based model combined with a strain/stress-gradient theory to investigate the effect of spatial heterogeneity of the microstructure on strength and ductility. We concentrate on understanding the effect of various grain size spatial distributions on the mechanical properties of interstitial free (IF)-steel. The results show that by controlling some parameters in the spatial distribution of the microstructure with regions composed of micro-grains and nano-grains one can achieve improved strength and ductility. Based on these results, it is suggested that the mechanical properties of gradient materials can be described by phenomenological relations that include two structural parameters, grain size and grain-size gradient, in contrast to Hall-Petch relation for homogenous materials where only grains size appears in the equation.

Metal-ceramic multilayers have been reported to show high strength, measurable plasticity, and a high strain-hardening rate when the crystallographic layers are a few nanometers thick. In this work, large-scale molecular dynamics simulations are carried out in order to understand the deformation mechanisms of the Ti/TiN multilayer subjected to multiaxial loading. The yield behavior of the Ti/TiN multilayer is thoroughly explored by constructing the yield surface in the interface plane. The strong dependency of the yielding stresses on the loading direction highlights the anisotropic behavior of the structure. The Ti/TiN multilayer structure shows high strength and ductility under uniform compression loading. However, low strength and ductility are observed under tensile loading, which favors crack initiation and propagation. Unlike typical metal stress-strain curves, metal/ceramic multilayers show two main yield points. Furthermore, the Ti/TiN multilayer structure shows three distinctive peak points for compressive loading normal and parallel to the interface. Different slip planes are activated depending on loading directions. Two main mechanisms are found to control the plasticity of the Ti/TiN multilayer: (1) interface strengthening, in which, when the metal-ceramic multilayers are under compressive loading, the interface acts as a barrier and induces repulsive forces against the slip transmission from the Ti layer into the TiN layer; (2) interface softening, in which, when applying tensile loading on the metal-ceramic multilayer structure, the interfacial misfit dislocations act as sources for the emission of dislocations into the TiN layer or promote slip transmission from the Ti to the TiN layer.

The mechanical behavior of nanoscale ceramic–metallic (NbC/Nb) multilayer composites with different thickness ratios is investigated using molecular dynamics (MD) simulations. Based on the obtained stress–strain behavior and its dependence on temperature, strain rate, and loading path, the flow stress for the onset of plasticity is identified and modeled based on the nucleation theory, and the in-plane yield loci for different layer thicknesses are constructed. The results are used to establish the plastic flow potential for developing a continuum viscoplastic constitutive model for potential use in large-scale applications.

The promising mechanical, physical and chemical properties of nano-scale metal/ceramic multilayers (MCMs) are of high interest for extreme environment applications. Understanding the plastic deformation mechanisms and the variables affecting those properties is therefore essential. The interface characteristics and the plastic deformation mechanisms under compressive loading in a Ti/TiN multilayer with a semi-coherent interface are numerically investigated. The interface structure of the Ti/TiN interface and the interface misfit dislocation were characterized using molecular dynamic simulations combined with atomically informed Frank-Bilby method. Three possible atomic stacking interface structures are identified according to the crystallographic analysis of the interface. Upon relaxation, large interface areas are occupied with the energetically stable configuration. Furthermore, the higher energy stacking are transformed into misfit dislocations or dislocation nodes. The molecular dynamic compressive stress strain response of the Ti/TiN multilayers exhibited three distinctive peaks. The first peak was generated by the dislocation dissociation of perfect dislocation into pairs of partials dislocation around extended nodes region at the interface. Upon further compression the second peak, identified as the first yielding, resulted from the activation of pyramidal slip planes in the Ti layer. Finally, a third peak identified as the second yielding, occurred when dislocation nucleated/transmitted in/into the TiN layer.

Additive manufacturing (AM) methods such as Aerosol Jet (AJ) printing allow the fabrication of structures via sintering of micro and/or nanoparticles, leading to microstructures that consist of various combinations of pore and grain sizes. It has been reported that AJ printed and sintered silver micropillars show an unusual behavior of high stiffness and high strain-to-failure for structures with high porosity and vice versa (Saleh et al. 2018 [1]). This behavior, however, is accompanied by the stiffer structures having smaller grain sizes and softer structures having larger grain sizes. To explain the physics of this behavior where a trade-off between hardening caused by size effects (grain refinement and gradients) and softening caused by porosity is expected to play a critical role, a multi-scale modeling approach is proposed in this paper. The model formulation consists of a continuum dislocation dynamics (CDD) framework, coupled with continuum plasticity and finite element analysis. The dislocation dynamics formulation is introduced into a user material subroutine and coupled with a finite element commercial solver, in this case, LS-DYNA, to solve the model in three-dimensional scale with the same size as the AM micropillars. The results from the model capture the general trends observed in compression tests of AM micropillars. In particular, it is shown that the grain size and dislocation density have a disproportionately higher influence over the mechanical deformation of metallic structures when compared to the porosity. These results show that the behavior of AM structures in the plastic regime is dominated by grain size effects rather than porosity. Some limitations of the model and possible future refinements are discussed. The paper provides an important analytical framework to model the mechanical behavior of AM structures with internal porosity in the plastic regime.

Nano-scale multilayer composite thin films are potential candidates for coating applications at harsh environments due to their promising mechanical and thermal properties. In this study, a viscoplasticity continuum model based on the plastic flow potential of metal/ceramic nanolayer composites, obtained from molecular dynamics (MD) simulations, is developed to build up a multiscale model bridges atomistic simulation with continuum models for the thin film composites. The model adopts a power law hardening considering confined layer slip (CLS) mechanism and accounts for the evolution of dislocation density based on the statistically stored dislocations and geometrically necessary dislocations. It is then implemented into a finite element code (ls-dyna) to investigate the deformation behavior of nanolayer composites at the macroscale. The deformation behavior of a high strength steel coated with Nb/NbC multilayer is also examined.

Molecular dynamics (MD) simulations were performed to explore the effect of interface structure on the inception of plastic deformation in Nb/NbC nanolayered composites. Using the atomistically informed Frank–Bilby method and disregistry analysis, we characterized the structure of the Nb/NbC interface, including misfit dislocations, dislocation nodes and three coherent interface structures. According to the crystallographic analysis of the interface, four possible coherent interface structures were identified. However, study of the interface energy showed that only three of these are energetically stable. After the relaxation of the interface, the unstable coherent region, which features Nb atoms in the Nb layer on the top of the Nb atoms in the NbC layer, evolves into a condensed interface dislocation node. Three stable coherent interface regions are retained in association with the formation, glide and reaction of interface misfit dislocation loops. Disregistry analysis of the Nb/NbC interface revealed that (i) all misfit dislocations are edge type, and (ii) misfit dislocations enclosing the coherent extended nodes have Burgers vectors along 〈1 1 0〉 Nb . The role of the interface structure in the plastic deformation of Nb/NbC nanolayered composites was studied under two loading conditions, i.e. uniform compression and nanoindentation. Under uniform compression, lattice dislocations primarily nucleate from the condensed nodal regions where the local strains are the highest. Dislocations propagate in two {1 1 0} slip planes with the same Schmid factors. Under nanoindentation, by proper positioning of the indenter, lattice dislocations nucleate from the segments of the misfit dislocations and propagate in {1 1 2} slip planes. MD simulations also show cross-slip of lattice dislocations from {1 1 2} planes into {1 1 0} planes and the formation of vacancies as a result of climb of dislocation jogs.

Strain-hardening in tri-component nano-scale metallic multilayers was investigated using nanoindentation and micro-pillar compression. Cu/Ni/Nb films were made in tri-layer structures as well as bi-layers consisting of an alloy of Cu–Ni/Nb. Strain-hardening increases as the layer thickness decreases, with 5 nm layers exhibiting higher strengths and hardening coefficients than 30 nm layers. The experimental evidence is described in light of the confined layer slip model, and supports the hypothesis that coherent interfaces with a modulus mismatch in the tri-layer system are responsible for additional deformation mechanisms that can lead to hardening in excess of that found in bi-layer systems.

A tri-layer nano-scale metallic multilayer (Cu/Ni/Nb) system with a mixture of incoherent and coherent interfaces was investigated to determine the effect of elevated temperature conditions on the strength at temperature and after annealing. Elevated temperature nanoindentation showed a reduction in the temperature sensitivity of hardness as individual layer thickness decreases (i.e. thinner layers retain strength better at elevated temperatures). This is explained using the confined layer slip model which suggests the drop in stress is due to both changes in the shear modulus of the film as well as dislocation/interface interactions. Molecular dynamic simulations of Cu/Nb bi-layers are presented in support of the concept that dislocation interactions at incoherent interfaces are less temperature sensitive than dislocation–dislocation interactions within the layers, supporting the experimental results.

Self-healing glass, a recent advancement in the class of smart sealing materials, has attracted great attention from both research and industrial communities because of its unique capability of repairing itself at elevated temperatures. However, further development and optimization of this material rely on a more fundamental and thorough understanding of its essential thermo-mechanical response characteristics, which is also pivotal in predicting the coupling and interactions between the nonlinear stress and temperature dependent damage and healing behaviors. In the current study, a continuum three-dimensional thermo-inelastic damage–healing constitutive framework has been developed for the compliant self-healing glass material with different damage mechanisms, i.e. micro-cracks and micro-pores, taken into account. The important feature of the present model is that different physically-driven evolution kinetics have been unified to represent the distinct inelastic, damage, and healing behaviors associated with the mechanical degradation processes. Coupled with the micro-crack and micro-void models reported in the literature, a continuum description of the healing behavior has been established based on the lower-length scale kinetic Monte Carlo simulations to characterize the local thermal–diffusional bond re-formation process across the fracture interface. The proposed formulations are implemented into finite element analyses and the effects of various loading conditions and material properties on the material’s mechanical resistance are investigated.

The viscoplastic behaviour of magnesium alloys at high temperatures leads to highly temperature-dependent mechanical properties. While at high strain rates a notable strain hardening response is observed, at low strain rates the material shows a smooth plastic response with negligible amount of hardening. This complicated behaviour is due to different deformation mechanisms that are active at different strain rate regimes, resulting in different strain rate sensitivity parameters. In this study we show, by utilizing both numerical simulations and experiments, that this behaviour can be predicted by a model that combines two deformation mechanisms, grain boundary sliding mechanism and dislocation glide mechanism. We discuss the importance of each deformation mechanism at different strain rate regimes based on the findings of modelling and experimental results for AZ3 magnesium alloy. By developing a model that includes the above-mentioned two deformation mechanism, the prediction of flow properties is expa...

Metal/ceramic multilayers combine high hardness of the ceramic layer and the high ductility of the metallic layer, enabling the design of novel composite coatings with high hardness and measurable ductility when the layer thickness reduces to a few nanometers. In this article, we review recent work with a focus on plastic deformation of metal/ceramic nanolayered composites from three aspects: experiment, theory, and atomistic modeling, and we propose several research directions in this topic.

Experimental studies show that metal–ceramic multilayers can have high strength, high strain hardening and measurable plasticity when the ceramic layer is a few nanometers thick. Using molecular dynamics simulations we studied deformation mechanisms in metal–ceramic multilayers and the role of interface structure and layer thickness on mechanical behavior. NbC/Nb multilayers were investigated numerically using the molecular dynamics (MD) method with empirical interatomic potentials. The interface dislocation structure was characterized by combining MD simulations and atomically informed Frank–Bilby theory. Two sets of pure edge misfit dislocations have been identified. Plastic deformation in NbC/Nb multilayers commences first in the metal layers by nucleation and glide of lattice dislocations initiating from interface misfit dislocations. These dislocations glide in the Nb layer and are deposited at the interface. The deposited dislocations facilitate slip transmission from the Nb layer to the NbC layer. The critical strain corresponding to dislocation nucleation is insensitive to layer thickness but depends on interface dislocation structure. The strain hardening and the peak flow strength of NbC/Nb multilayers are associated with the slip transmission from Nb to NbC, and are correlated to the interfacial dislocations, Nb layer thickness, and NbC layer thickness. The flow strength decreases with increasing Nb layer thickness and decreasing the NbC layer thickness.

Deformation twinning, a common and important plastic deformation mechanism, is the key contributor to the excellent combination of strength and ductility in twinning-induced plasticity (TWIP) steel. In the open literature, a significant amount of research has been reported on the microstructural characteristics of deformation twinning and its influence on the overall deformation behavior of TWIP steel. In this study, we examine the feasibility of a mechanism-based crystal plasticity model in simulating the microstructural level deformation characteristics of TWIP steel. To this end, a model considering both double-slip and double-twin is developed to investigate the stress–strain behavior and local microstructural features related to the formation and growth of micro-twins in low stacking fault energy (SFE) TWIP steel. The twin systems are described as pseudo-slips that can be activated when their resolved shear stress reaches the corresponding critical value. A hardening law that accounts for the interaction among the slip and twin systems is also developed. Numerical simulations for different mesh sizes and single crystal patch tests under different loading modes are carried out to verify the modeling procedure. Our simulation results reveal that, despite its simple nature, the double-slip/double-twin model can capture the key deformation features of TWIP steel, including twin volume fraction evolution, continuous strain hardening, and the final fracture in the form of strain localization.

We developed a plane-strain crystal plasticity finite element (CPFE) numerical model to predict the tensile behavior of twinning-induced plasticity (TWIP) steel with both slip and mechanical twinning as the main deformation modes. Our CPFE model may not only predict well the tensile stress versus strain (S-S) curve but also capture the variation in the volume fraction of twins with a reasonable accuracy. The nucleation of mechanical twin is obviously controlled by the stress concentration. At the same time, the growth of twin may either lead to a stress relaxation in the matrix or cause a local stress concentration around twin, which depends on the deformation condition.

Precipitation strengthening in nanostructured metallic multilayer (NMM) films of Cr/Cu–Cr was studied using nanoindentation and electron microscopy. Magnetron-sputtered NMM films having layer thicknesses of 10, 20 and 30 nm were prepared at room temperature (RT) and 100 °C. Some of the RT-deposited films were annealed at 100 °C for 30 min. Cr was introduced in the Cu–Cr layers by using a Cu–Cr target (95 at.% – 5 at.%) target. A significant increase in nanoindentation hardness was observed in the Cr/Cu–Cr. A reduction of hardness dependence on layer thickness was also observed in the Cr/Cu–Cr, such that sample having a layer thickness of 30 nm provides the equivalent strength of a 10/10 nm Cr/Cu. Uniformly distributed Cr particles in the Cu–Cr layers are key strengthening features in these new Cr/Cu–Cr NMM films. A single dislocation-based model was used to correlate the observed mechanical behaviours and microstructure. The model predicts similar trend observed from the experimental results, suggesting t...

Nanoscale metallic multilayer (NMM) composites represent a class of advanced engineering materials that are shown to exhibit high structural stability, mechanical strength, high ductility, toughness and resistance to fracture and fatigue. This paper addresses the question of the effect of the interface imperfections on the strengthening of NMMs with incoherent interfaces, by performing molecular dynamics simulations. Two types of interfaces are considered, a perfect one, and one with discontinuities (steps and ledges). Our simulations demonstrate that the result of the interaction between dislocations and interfaces is the creation of interfacial disconnections made of a dislocation that spreads within the interface, and a step entrapped at the interface. The energy calculations show that these steps increase the total energy of the system and enhance the strengthening effect of the interface by adding extra barriers to slip transmission, thus improving the mechanical properties of the structure.

The mechanical behavior of Cu–Ni–Nb- based nanoscale metallic multilayers (NMM) under high strain rate loadings is investigated in this work using molecular dynamics simulations. The simulations of NMMs with various individual layer thicknesses under uniaxial tensile strains at two different controlled strain rates (109/s and 1010/s) are performed. This type of loading condition generates a stress state necessary for void nucleation commonly observed under shock loading. The mechanisms for void nucleation in the NMMs are examined and identified; the void nucleation strengths (VNS) of the NMMs and their variations with respect to increasing individual layer thickness as well as available nucleation sites (affected by addition of interfacial disconnections) are obtained and explained. It is discovered that the void always nucleate from within the Cu layers, where the partial dislocations intersect with each other or with existing stacking faults. The void nucleation strength of the NMMs is closely related to the density of available sites for void nucleation. By introducing interfacial steps into the incoherent interfaces of the NMMs the abundance of dislocation sources is changed, thus more (less) sites for void nucleation are produced which decrease (increase) the void nucleation strength of the NMMs.

Pure platinum was probed with a nanoindenter fitted with a Berkovich tip to various depths. The indent pattern was made on the as-polished specimen prior to heat treating, after heat treating at 500 °C for 30 min, and again after further heat treating at 1000 °C for 30 min. The variability in the measured hardness decreased as the indentation depth increased from 50 to 300 nm. When the sampled was annealed, the hardness variation was also greater. Increasing hardness variation with decreasing dislocation density and sampling volume indicates that dislocation density plays a critical role in the observed variation, beyond solely instrumentation uncertainty, and supports a defect-based explanation for the stochastic behavior. It appears that the stochastic behavior occurs when multiple dislocations are present in the sampled volume rather than sampling only a single dislocation.

The strengthening effect of precipitates in metals is investigated within a multiscale approach that utilizes models of various length scales; namely, molecular mechanics (MM), discrete dislocation dynamics (DD), and an equivalent inclusion method (EIM). In particular, precipitates are modeled as particles whose stress fields interact with dislocations. The stress field resulting from the elastic mismatch between the particles and the matrix is accounted for by using the EIM, whereas the MM method is employed to develop rules for the DD method for short range interactions between a single dislocation and an inclusion. The DD method is used to predict the strength of the composite structure resulting from the interaction between ensembles of dislocations and particles. As an application to this method, the mechanical behavior of advanced high strength steel is investigated and the results are compared to the experimental data published in previous studies. The results show that the finely dispersiv...

Nanoporous gold (NP-Au) exhibits microscale plasticity, but macroscopically fails in a relatively brittle manner. This current study suggests that a core-shell structure can increase both ductility and strength of NP-Au. A core Au foam structure was created using conventional dealloying methods with average ligament size of 60 nm. Nickel was then electroplated on to the NP-Au with layer thicknesses ranging from 2.5 nm to 25 nm. Nanoindentation demonstrated a significant increase in the hardness of the coated Np-Au, to about five times of that of the pure Np-Au, and a decrease in creep by increasing the thickness of the coated Ni layer. Molecular dynamics simulations of Au–Ni ligaments show the same trend of strengthening behavior with increasing Ni thickness suggesting that the strengthening mechanisms of the Np-Au are comparable to those for fcc nano ligaments. The simulations demonstrate two different strengthening mechanisms with the increased activity of the twins in plated Au–Ni ligaments, which lead...

Nanostructured metallic material (NMM) composites are a new class of materials that 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. Among them, is the question of the strengthening of NMM using second phase particles and if the addition of precipitates will strengthen the structures in the same manner as in bulk crystalline solids. This issue is addressed in this work by performing molecular dynamics simulations on NMM with precipitates of various sizes and comparing the results with the same structure without precipitates. In this view, Cu/Nb bilayer thin films with spherical Nb particles inside the Cu layer were examined using molecular dynamics simulations and show a significant i...

The use of self-healing glass as hermetic seals is a recent advancement in sealing technology development for the planar solid oxide fuel cells (SOFCs). Because of its capability to restore mechanical properties at elevated temperatures, the self-healing glass seal is expected to provide high reliability in maintaining the long-term structural integrity and functionality of SOFCs. To accommodate the design and evaluate the effectiveness of these engineered seals under various thermomechanical operating conditions, a computational modeling framework must be developed to accurately capture and predict the healing behavior of the glass material. In the present work, a mechanistic-based, two-stage model was developed to study the stress and temperature-dependent crack healing of the self-healing glass materials. The model initially was first calibrated by experimental measurements combined with kinetic Monte Carlo (kMC) simulation results and then implemented into finite element analysis (FEA). The effects of various factors, e.g., stress, temperature, and crack morphology, on the healing behavior of the glass were investigated and discussed.

High energy particle irradiation of structural polycrystalline materials usually produces irradiation hardening and embrittlement. The development of predictive capability for the influence of irradiation on mechanical behavior is very important in materials design for next-generation reactors. A multiscale approach was implemented in this work to predict irradiation hardening of iron based structural materials. In the microscale, dislocation dynamics models were used to predict the critical resolved shear stress from the evolution of local dislocation and defects. In the macroscale, a viscoplastic self-consistent model was applied to predict the irradiation hardening in samples with changes in texture. The effects of defect density and texture were investigated. Simulated evolution of yield strength with irradiation agrees well with the experimental data of irradiation strengthening of stainless steel 304L, 316L and T91. This multiscale modeling can provide a guidance tool in performance evaluation of structural materials for next-generation nuclear reactors.

The effect of layer thickness on the hardness of nanometallic material composites with both coherent and incoherent interfaces was investigated using nanoindentation. Then, atomistic simulations were performed to identify the critical deformation mechanisms and explain the macroscopic behavior of the materials under investigation. Nanocomposites of different individual layer thicknesses, ranging from 1–30 nm, were manufactured and tested in nanoindentation. The findings were compared to the stress–strain curves obtained by atomistic simulations. The results reveal the role of the individual layer thickness as the thicker structures exhibit somehow different behavior than the thinner ones. This difference is attributed to the motion of the dislocations inside the layers. However, in all cases the hybrid structure was the strongest, implying that a particular improvement to the mechanical properties of the coherent nanocomposites can be achieved by adding a body-centered cubic layer on top of a face-centered cubic bilayer.

Strain hardening in metallic multilayers is shown experimentally to be greater in trilayer films of Cu–Ni–Nb than bilayer films of Cu–Ni or Cu–Nb using both direct measurements of flow strength at different effective strains and an analysis of the out-of-plane deformation around nanoindentation impressions. The mechanism that accounts for increased strain hardening in the trilayer is a proposed super-threader dislocation and cross-slip mechanism, modeled using three-dimensional dislocation dynamics simulations.

Using molecular dynamics simulations, the deformation behavior of two types of nanocomposite metallic materials (nano-layered thin films and composite nanowires) is investigated and compared with that of the bulk materials. The first structure is a hybrid nano-layered metallic composite formed by alternating layers of Cu, Ni, Cu, and Nb layers. The nanocomposite has a pre-existing dislocation structure inside it, generated by initially loading a perfect structure to a high strain to nucleate dislocations, then completely unloading it, and loading it again. Four different structures are considered all having the same Cu and Ni layers thickness and varying Nb thickness. Comparison of the deformation behavior between the different structures revealed that the addition of Nb layer makes the material stronger. However, this behavior has a critical limit below which the strength of the material decreases. This is attributed to the extended shearing of the interface that results from the accumulation of dislocations in the Cu/Nb interface. The second structure discussed is a composite nanowire made of a Ni layer sandwiched between two Cu layers. We show that the natural development of coherency stresses at the interfaces between the two layers increases the ability of the wire to deform by a twinning process under tensile loading. This process results in the reorientation of the composite nanowire that, under unloading, forces the nanowire to completely recover the straining, leading to a pronounced pseudoelastic behavior.

This paper presents the development of a methodology for microstructure analysis and modeling of hot mix asphalt (HMA). This methodology relies on using effective local material properties and strain gradient theory in the finite element analysis of HMA microstructure. The effective local properties are calculated using an analytical micromechanical model that captures the influence of percent of particles on the microscopic response of HMA. Strain gradient elasticity is used in order to account for the effect of particle size in the finite element analysis. The autocorrelation function and the moving window technique are used to determine the microstructure characteristic length scales that are used in strain gradient elasticity. A number of asphalt mixes with different aggregate types and size distributions are analyzed in this paper.

A microstructure-based viscoplastic continuum model is developed for the permanent deformation of asphalt concrete (AC). The model accounts for several phenomena that influence the permanent deformation of AC at high temperatures. These phenomena include strain rate dependency, confining pressure dependency, dilation, aggregate friction, anisotropy, and damage. The material anisotropy was included in the model by replacing the stress invariants in the yield function with invariants of both the stress and a microstructure tensor, which describes the aggregate orientation distribution. The components of the microstructure tensor were determined using image analysis techniques (IAT) conducted on digital images taken from two-dimensional cut sections of AC. Furthermore, damage was included in the model based on the effective stress theory to account for crack and air void growth that significantly reduces the load-carrying capacity of the material. Experimental data from triaxial compressive strength tests conducted at five strain rates and three confining pressures were used to develop a methodology to determine the material parameters that characterize AC permanent deformation. The model predictions were in a good agreement with the experimental measurements.

Permanent deformation is one of the major distresses in asphalt pavements. It is caused mainly by high traffic loads associated with high field temperatures. An anisotropic viscoplastic continuum damage model is developed in this study to describe permanent deformation of asphalt pavements. The model is based on Perzyna's formulation with Drucker-Prager yield function modified to account for material anisotropy and microstructure damage. The material anisotropy is captured through microstructural analysis of aggregate distribution on two-dimensional sections of hot mix asphalt. A damage parameter is included in the model to quantify the nucleation of cracks and growth of air voids and cracks. A parametric study was conducted to demonstrate the sensitivity of the model to strain rate, aggregate distribution, and microstructure damage. Triaxial strength and static creep measurements obtained from the Federal Highway Administration Accelerated Loading Facility were used to determine the model parameters.

Realization of high temperature physical measurement sensors, which are needed in many of the current and emerging technologies, is challenging due to the degradation of their electrical stability by drift currents, material oxidation, thermal strain, and creep. In this paper, for the first time, we demonstrate that 3D printed sensors show a metamaterial-like behavior, resulting in superior performance such as high sensitivity, low thermal strain, and enhanced thermal stability. The sensors were fabricated using silver (Ag) nanoparticles (NPs), using an advanced Aerosol Jet based additive printing method followed by thermal sintering. The sensors were tested under cyclic strain up to a temperature of 500 °C and showed a gauge factor of 3.15 ± 0.086, which is about 57% higher than that of those available commercially. The sensor thermal strain was also an order of magnitude lower than that of commercial gages for operation up to a temperature of 500 °C. An analytical model was developed to account for the ...

The cold pilger metal forming technique is known to produce round titanium alloy tubing with mechanical properties that may be significantly anisotropic. These mechanical properties are of interest to both the manufacturers and consumers for defining initial manufacturing limitations and defining the final product design limitations. This study focuses on experimentally characterizing the yield locus development of Ti-3Al-2.5V seamless tubing during cold pilgering and a subsequent thermal stress relieving process. The materials are experimentally characterized using a biaxial testing apparatus, which subjects the specimen tubes to combined axial load and internal pressure. The Hill yield criterion is subsequently fit to the experimental results producing continuous yield loci. Each specimen is also experimentally characterized using X-ray diffraction to gain insight into the material textures that accompany the macroscopic properties. All work is focused on one particular pilger pass at two different production rates. A second experimental variable is introduced to the study by using two significantly different input materials, as characterized by X-ray diffraction. This study also investigates the nature of the plastic deformation of the tubing developed during cold pilgering via finite element analysis and discusses the relationship between the finite element predictions and the mechanical anisotropy.

Superplastic forming is a valuable metal working technique because of the extreme ductility that can be achieved. However, it is limited in application due to the presence of small voids that grow and coalesce during the forming process, often causing premature failure. In order to understand and control this phenomenon accurate constitutive models must be developed which account for void parameters that affect the macroscopic behavior of the material. This paper looks specifically at the effect of void size and spacing on the ductility and flow stress of viscoplastic materials. Based on the gradient-dependent theory of plasticity, a model is proposed that accounts for size effects by incorporating strain gradient terms into a continuum based constitutive equation. Both experimental testing and finite element (FE) modeling were performed on Pb–Sn, tensile specimens with small holes drilled in them in random patterns. The experimental tests indicate that a decrease in void size results in an increase in ductility. The FE results demonstrate that the gradient terms strengthen the material by diffusing the strain in areas of high strain concentration and delay failure by slowing void growth. In addition, the model predicted an increase in ductility and flow stress with decreasing void size.

Over the history of materials science there are many examples of materials discoveries that have made superlative materials; the strongest, lightest, or toughest material is almost always a goal when we invent new materials. However, often these have been a result of enormous trial and error approaches. A new methodology, one in which researchers design, from the atoms up, new ultra-strong materials for use in energy applications, is taking hold within the science and engineering community. This project focused on one particular new classification of materials; nanolaminate metallic composites. These materials, where two metallic materials are intimately bonded and layered over and over to form sheets or coatings, have been shown over the past decade to reach strengths over 10 times that of their constituents. However, they are not yet widely used in part because while extremely strong (they don’t permanently bend), they are also not particularly tough (they break relatively easily when notched). Our program took a coupled approach to investigating new materials systems within the laminate field. We used computational materials science to explore ways to institute new deformation mechanisms that occurred when a tri-layer, rather than the more common bi-layer system was created. Our predictions suggested thatmore » copper-nickel or copper-niobium composites (two very common bi-layer systems) with layer thicknesses on the order of 20 nm and then layered 100’s of times, would be less tough than a copper-nickel-niobium metallic composite of similar thicknesses. In particular, a particular mode of permanent deformation, cross-slip, could be activated only in the tri-layer system; the crystal structure of the other bi-layers would prohibit this particular mode of deformation. We then experimentally validated this predication using a wide range of tools. We utilized a DOE user facility, the Center for Integrated Nanotechnology (CINT), to fabricate, for the first time, these tri-layer composites. CINT formed nanolaminate composites were tested in tension, with bulge testing, using nanoindentation, and using micro-compression testing to demonstrate that the tri-layer films were indeed tougher and hardened more during deformation (they got stronger as we deformed them) than equivalent bi-layers. The seven graduate students, 4 post-docs and research faculty, and the two faculty co-PI’s were able to create a collaborated computational prediction and experimental validation team to demonstrate the benefits of this class of materials to the community. The computational work crossed from atomistic to bulk simulations, and the experiments coupled form nm-scale to the mm scale; closely matching the simulations. The simulations provided viable mechanisms that explained the observed results, and new experimental results were used to push the boundaries of the simulation tools. Over the life of the 7 years of this program we proved that tri-layer nanolaminate metallic composite systems exceeded the mechanical performance of bi-layer systems if the right materials were chosen, and that the mechanism responsible for this was tied to the cross slip of dislocations. With 30 journal publications resulting from this work we have broadly disseminated this family of results to the scientific community.« less

The superplastic deformation and cavitation damage characteristics of a modified aluminum alloy are investigated at a temperature range from 500 to 550°C. The baseline alloy is AA5083. Nominally this alloy contains about 4.5% Mg, 0.8% Mn, 0.2% Cr, 0.037% Si, 0.08% Fe and 0.025% Ti by weight. The experimental program consists of uniaxial tension tests and digital image analysis for measuring cavitation. The experiments reveal that evolution of damage is due to both nucleation and growth of voids. A viscoplastic model for describing deformation and damage in this alloy is developed based on a continuum mechanics framework. The model includes the effect of strain hardening, strain rate sensitivity, dynamic and static recovery, and nucleation and growth of voids. The model predictions compare well with the experimental results.

Substantial void growth in metals constitutes a problem in many industrial operations that utilize superplastic deformation. This is because of the likelihood of material failure due to such growth. Hence, there is a need to study void growth mechanisms in an effort to understand the parameters governing it. In this work, numerical and experimental studies of void growth, and the parameters that affect it, in a superplastically deforming (SPD) metal have been performed. In the numerical studies, using the finite-element method, a 1×2 sized thin plate (i.e. plane stress conditions) of a viscoplastic material with pre-existing holes has been subjected to a constant extension rate. The experimental studies were performed under similar conditions to the numerical ones and provided for qualitative comparison. The parameters affecting void growth in SPD are: m (the strain-rate sensitivity), void size (i.e. diameter) and the number (density) of existing voids. The results showed that increased m values produced strengthening and decreased the rate of void growth. In addition, larger initial void size (or, equivalently, a larger initial void fraction) had the effect of weakening the specimen through causing accelerated void growth. Finally, multiple holes had the effect of increasing the metal ductility by reducing the extent of necking and its onset. This was realized through diffusing the plastic deformation at the different hole sites and reducing the stress concentration. The numerical results were in good qualitative agreement with the experiment and suggested the need to refine existing phenomenological void growth models to include the dependence on the void fraction.