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We investigated the effect of complexity on the mechanical behavior and recovery of metallic nanoarchitecture structures. We studied four different suggested geometries with various levels of complexity using molecular dynamics simulations. The structures exhibited multiple degrees of self-recovery under compressive loading conditions at three temperatures, 300 K, 400 K, and 500 K. A methodology to qualitatively measure the geometric complexity was used. The results revealed correlations between the complexity of the structures and their recovery ability and strength, and the geometric cell size and temperature. These findings can guide the design of novel nanoarchitecture geometries for specific applications with tailored properties.

Amongst the many adaptive features of a bivalve shell, valve shape has been hypothesised to increase structural stability and to aid in withstanding crushing predation. Using Finite Element Analysi...

In this study, a continuum multiscale model based on discrete dislocation dynamics and viscoplastic self-consistent theory is developed. The model is capable of describing the macroscopic deformation behavior of polycrystalline metals and alloys, maintaining at the same time a microscopic level resolution that captures the dislocation density patterns within each grain. The continuum dislocation dynamics model is based on a set of nonlinear partial differential equations of the dislocation densities inside each grain. Three different types of dislocations, two mobile (positive and negative) and one immobile, are considered. The shear strain within each grain is calculated by the total dislocation density at every time step, and then linked to the viscoplastic self-consistent framework to calculate the macroscopic behavior of the polycrystalline material. At the same time, the dislocation density pattering within each grain, calculated by the evolution equations, is recorded to provide a local view of the material at grain level. For the purpose of demonstration, the focus of this paper is on polycrystalline Aluminum and the numerical solutions are compared with experimental data.

Metallic nanofoams, cellular structures consisting of interlinked thin nanowires and empty pores, create low density, high surface area materials. These structures can suffer from macroscopically brittle behavior. In this work, we present a multiscale approach to study and explain the mechanical behavior of metallic nanofoams obtained by an electrospinning method. In this multiscale approach, atomistic simulations were first used to obtain the yield surfaces of different metallic nanofoam cell structures. Then, a continuum plasticity model using finite elements was used to predict the alloy nanofoam's overall strength in compression. The manufactured metallic nanofoams were produced by electrospinning a polymeric non-woven fabric containing metal precursors for alloys of copper–nickel and then thermally processing the fabric to create alloy metallic nanofoams. The nanofoams were tested with nanoindentation. The experimental results suggest that the addition of nickel increases the hardening of the nanofoams. The multiscale simulation modeling results agreed qualitatively with the experiments by suggesting that the addition of the alloying can be beneficial to the hardening behavior of the metallic nanofoams and helps to isolate the effects of alloying from morphological changes in the foam. This behavior was related to the addition of solute atoms that prevent the free dislocation movement and increase the strength of the structure.

The mechanical properties of core–shell bimetallic composite nanowires, forming the bases of nanoporous metallic foams, have been investigated and compared with pure metal nanowires using molecular dynamics simulations. In the current study, pure copper and gold nanowires under uniaxial loading were tested at room temperature and compared to composite nanowires of the same materials (core) with a nickel coating (shell). The core radius ranged from 1 to 15 nm, and the shell thickness ranged from 0.1 to 5 nm. The tension strain was performed along the [001] direction under room temperature. Both coherent and semi-coherent composite nanowires were studied, and the effect of coating layer thickness was investigated. The strengthening mechanisms of the core–shell structures due to the presence of the two different types of interfaces were investigated for various nickel thicknesses. The atomistic simulation results revealed that the addition of the nickel shell strengthens the structure when the layer thickness exceeds a critical value.

Pure metallic nanofoams in the form of interconnected networks have shown strong potentials over the past few years in areas such as catalysts, batteries and plasmonics. However, they are often fragile and difficult to integrate in engineering applications. In order to better understand their deformation mechanisms, a multiscale approach is required to simulate the mechanical behavior of the nanofoams, although these materials will operate at the macroscale, they will still be maintaining an atomistic ordering. Hence, in this work we combine molecular dynamics (MD) and finite element analysis (FEA) to study the mechanical behavior of copper (Cu) nanofoams. Molecular dynamics simulations were performed to study the yield surface of a representative cell structure. The nanofoam structure has been generated by spinodal decomposition of binary alloy using an atomistic approach. Then, the information obtained from the molecular dynamics simulations in the form of yield function is transferred to the finite element model to study the macroscopic behavior of the Cu nanofoams. The simulated mechanical behavior of Cu nanofoams is in good agreement of the real experiment results.

A polycrystalline Cu foam with sub-micron ligament sizes was formed by creating a non-woven fabric via electrospinning with a homogeneous mixture of polyvinyl alcohol(PVA)-and copper acetate(Cu(Ac)2). Thermogravimetric measurement of the electrospun fabric of the precursor solution is reported. Oxidizing the precursor fabric at 773K formed an oxide nano-foam; subsequent heating at 573K with a reducing gas transformed the CuO nano-foam to Cu with a similar ligament and meso-scale pore size morphology. A cross-section prepared by focused ion beam lift-out shows the polycrystalline structure with multi-scale porosity. The mechanical property of the Cu nano-foam is measured by nano-indentation. The load-depth curves and deduced mechanical properties suggest that additional intra-ligament pores lead to unique structure-property relations in this non-conventional form of metal.

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.

Nanoscale metallic material composites consisting of bilayer and trilayer systems of two and three different metallic alternating layers show significant gains in hardness over monolithic single phase films. One of the main applications of these composites can be as protective coatings to technical components to increase their lifespan acting as a mechanical barrier to the carriers of permanent deformation. In this work, we study the strength of bilayer structures made of alternating layers of niobium (Nb) and copper–nickel (Cu–Ni) alloys. The effect of the layer size and composition on strength and hardening as well as the effect of the metal–alloy interface on the dislocation motion is investigated. The simulations reveal a close relationship between the atomic composition of the alloy and the hardening of the film. The results are also compared with experimental findings on nanopillars made of similar structures, and strong similarities are revealed and discussed.

Exceptional mechanical and physical properties of transition metal carbides and nitrides make them good coating-material candidates for extreme corrosive environments such as oil and natural gas wells. However, existence of small pores, pinholes and columnar structures of these ceramics significantly affect their resistance to corrosion, as pore sites would accelerate the diffusion of corrosive media into the substrate. In this research, molecular dynamics atomistic simulations are employed to investigate the effects of the isolated vacancies and the columnar structure on the diffusion rate of H atoms in NbC single crystal at various temperatures. Diffusion coefficient (D) of H atoms in NbC increased with C vacancy concentration. At elevated temperatures, the trapping effect of Nb vacancies is less effective when C vacancies are also present, as H atoms gain enough energy to jump back and forth between the C vacancies. Atomistic simulations also showed a jump in diffusion coefficient for cylindrical pore size of larger than 3 A radius. Furthermore, D increased monotonically with temperature up to 1000 K in the presence of cylindrical pores. Further increase in temperature resulted in a drop in the diffusion coefficient for small pores while the large pores only showed a lower increasing trend in diffusion coefficient with the temperature.

In this work, we demonstrate that the phase field crystal (PFC) method can be applied to identify and predict the microstructure evolution during the solidification of the BCC metals in additive manufacturing. The results reveal the columnar structure of the grains, which matches the grain growth observed in real samples produced with the additive manufacturing technique. The effect of ambient temperature, seed-seed distance and seed-seed misorientation on the grain growth has been investigated. In addition, the formation of crystal defects during the process is recorded and the resulted long-range stresses are calculated using the eigenstresses theory.

This chapter, outlines a multiscale dislocation-based plasticity framework coupling discrete dislocation dynamics (DDD) with continuum dislocation-based plasticity. In this framework, and guided by DDD, a continuum dislocation dynamics (CDD) plasticity model involving a set of spatio-temporal evolution equations for dislocation densities representing mobile and immobile species is developed. The evolution laws consist of a set of components each corresponding to a physical mechanism that can be explicitly evaluated and quantified from DDD analyses. In this framework, stochastic events such as cross-slip of screw dislocations and uncertainties associated with initial microstructural conditions are explicitly incorporated in the continuum theory based on probability distribution functions defined by activation energy and activation volumes. The result is a multiscale dislocation-based plasticity model which can predict not only the macroscopic material mechanical behavior but also the corresponding microscale deformation and the evolution of dislocation patterns, size and gradient-dependent deformation phenomena, and related material instabilities at various length and time scales.

Pure metal nanofoams in the form of interconnected ligament networks have shown strong potential over the last few years in areas such as catalysts, batteries, and optics. However, they are often fragile and therefore difficult to integrate into engineering applications. For these reasons, a new class of materials, composite metallic nanofoams made of ligaments coated with thin metallic layers, have been proposed to solve these issue. The mechanical properties of these nanofoams depend on their relative density and their internal geometrical structure. In this work, we combine molecular dynamics (MD) and finite element method (FEM) to investigate the compressive behavior of nanofoams made of copper ligaments coated with nickel. We also investigate how this behavior relates to their microstructures. For that purpose, we built different types of representative cell structures, and atomistic simulations of multiaxial compression tests were performed to produce yield surfaces. Then the generated yield surfaces were curve fitted into a normalized model to obtain the shape parameters. Last, a plasticity model was introduced to study and compare their mechanical behavior under compression. The results suggest that the coating of the ligaments can improve the mechanical behavior of the nanofoams. They also reveal the importance and the limitations of the microstructural geometry on the strengthening of these structures. These findings can be used for the design of metallic nanofoams with tailored mechanical properties.

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.

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.

The treatment of coherent interfaces and cracks is discussed in the framework of dislocation dynamics (DD). In the case of interfaces, we use DD to study dislocation interactions in nanoscale bimetallic laminates, and to predict their structure after relaxation and during loading. In agreement with experimental observations, our discrete dynamics simulations show that dislocation structure develops only at the interface between coherent layers leaving layers’ interior dislocation-free. The main dislocation mechanism at this length scale is Oworan bowing of threading dislocations confined to their respective layers by the sign-alternating coherency stress field in the layers. Slip transmission across the interfaces marks the end of the confined slip regime, hence, the breakdown of the interfaces and macroscopic yielding of these structures. In the case of crack, its long-range and singular stress field is determined by modeling the crack as continuous distribution of dislocation loops. The traction boundary condition to be satisfied at the crack surface, results into a singular integral equation of the first kind that is solved numerically. The model is integrated with the DD technique to investigate the behavior of a specimen containing cracks of different shapes under fatigue. The results are compared with the behavior of an uncracked specimen and conclusions are extracted. Extension of this crack treatment methodology to account for their presence at interfaces, all within the frame dislocations dynamics, opens the door for a more realistic approach to a wide range of interfaces-related problems.

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.

Mapping of residual stresses at the mesoscale is increasingly practical thanks to technological developments in electron backscatter diffraction (EBSD) and X-ray microdiffraction using high brilliance synchrotron sources. An analysis is presented of a Cu single crystal deformed in compression to about 10% macroscopic strain. Local orientation measurements were made on sectioned and polished specimens using EBSD and X-ray microdiffraction. In broad strokes, the results are similar to each other with orientations being observed that are on the order of 5° misoriented from that of the original crystallite. At the fine scale it is apparent that the X-ray technique can distinguish features in the structure that are much finer in detail than those observed using EBSD even though the spatial resolution of EBSD is superior to that of X-ray diffraction by approximately two orders of magnitude. The results are explained by the sensitivity of the EBSD technique to the specimen surface condition. Dislocation dynamics...

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.

Electron backscatter diffraction (EBSD) techniques have been used to measure the dislocation density tensor for various materials. Orientation data are typically obtained over a planar array of measurement positions and the minimum dislocation content required to produce the observed lattice curvature is calculated as the geometrically necessary (or excess) dislocation density. The present work shows a comparison of measurements in two-dimensions and three-dimensions using a dual beam instrument (focused ion beam, electron beam) to obtain the data. The two-dimensional estimate is obviously lower than that obtained from three-dimensional data since the 2D analysis necessarily assumes that the third dimension has no curvature in the lattice. Effects of the free-surface on EBSD measurements are discussed in conjunction with comparisons against X-ray microdiffraction experiments and a discrete dislocation dynamics model. It is observed that the EBSD measurements are sensitive to free-surface effects that may yield dislocation density observations that are not consistent with that of the bulk material.

A dislocation density based crystal plasticity finite element model (CPFEM) is developed for aluminum in which dislocation densities evolve on all octahedral slip systems. Based upon the kinematics of crystal deformation and dislocation interaction laws, dislocation generation and annihilation are modeled. The CPFEM model is calibrated for pure aluminum using experimental stress-strain curves of pure aluminum single crystal from literature. Crystallographic texture predictions in plane-strain compression of aluminum are validated against experimental observations in the literature. The framework is implemented in ABAQUS with user interface UMAT subroutine. Dislocation densities evolve and are tracked as state variables in the model, leading to spatially inhomogeneous dislocation densities that show patterning in the dislocation structures.

A dislocation-density-based crystal plasticity finite-element model (CPFEM) is developed in which different dislocation densities evolve. Based upon the kinematics of crystal deformation and dislocation interaction laws, dislocation generation and annihilation are modeled. The CPFEM model is calibrated for pure aluminum using experimental stress–strain curves of pure aluminum single crystal from the literature. Crystallographic texture predictions in plane-strain compression of aluminum are validated against experimental observations in the literature. The framework is implemented in ABAQUS with user interface UMAT subroutine.

In this article, the effect of different shape cracks on fatigue behavior in metals is investigated with the help of the discrete dislocation dynamics technique. The cracks are represented as distributions of infinitesimal dislocation loops. The distribution is determined by an integral equation satisfying stress-free boundary conditions and containing a singular kernel of the third type. The stress field in the cracked domain is calculated using a superposition principle coupled with an iterative technique. The derived stress fields describe the interaction between the cracks and the dislocations into the framework of dislocation dynamics. The simulation results provide an insight on the three-dimensional character of the dislocation structure around the cracks and its relation to the crack shapes. The effect of the cracks on the macroscopic yield stress is also determined for various crack sizes and shapes.

The interaction between a penny-shape crack and a dislocation in crystalline materials is investigated within the framework of dislocation dynamics. The long-range and singular stress field resulting from the crack is determined by modeling the crack as continuous distribution of dislocation loops. This distribution is determined by satisfying the traction boundary condition at the crack face, resulting into a singular integral equation of the first kind that is solved numerically. This crack model is integrated with the dislocation dynamics simulation technique to yield the stress field of the combine system of crack and different types of dislocations situated at different positions in a three dimensional space. The integrated system is then used to investigate the dislocation behavior and its influence on the crack opening displacement and the characteristic of the stress field near the crack tip. It is shown that, depending on the relative position of the dislocation and its character, the dislocation may result in reduction in the stress amplitude at the crack tip and in some cases in closure of the crack tip. These analyses yield shielding and amplification zones near the crack providing an insight of the dislocation influence on the crack. The full dislocation dynamic analysis reveals the nature of the crack dislocation interaction and the manner in which the dislocation morphology changes as it is attracted to the crack surfaces, as well as the changes it causes to the crack profile.

The dynamical properties of dislocation interactions for a system of 220 parallel straight dislocations are investigated by a computer simulation method based on the stochastic integration of the Langevin equations of dislocation motion. Relaxation of an assembly of parallel edge dislocations in single and double slip configurations under cyclic deformation conditions is presented.

The de-adhesion of a thin film from a rigid substrate is studied. It is assumed, that a periodic array of micro-cracks exists along the film/substrate interface. During formation, the film expands, while being constrained by the substrate. This phenomenon leads to development of compressive stresses. Then buckling may occur and cause crack growth either along the interface or in the film towards the free surface. A finite element model has been developed, which simulates film buckling and subsequent interfacial crack growth, based on film/substrate adhesive constitutive relations. These relations have been motivated by atomistic calculations on bimaterial failure. The model does not require any facture criterion. Interfacial work of separation has a significant effect on damage growth ahead of the crack tip, along the interface. Also, a critical remote compressive stress exists, at which damage progresses without further loading of the film. The relation between the critical compressive stress for extensive damage and the interfacial work of separation can be used in combination with experiments for the quantitative characterization of the film/substrate interface.

To study the strain hardening in nanoscale multilayer metallic (NMM) composites, atomistic simulations of nanoindentation are performed on CuNi, CuNb, and CuNiNb multilayers. The load-depth data were converted to hardness-strain data that were then modeled using power law. The plastic deformation of the multilayers is closely examined. It is found that the strain hardening in the incoherent CuNb and NiNb interfaces is stronger than the coherent CuNi interface. The hardening parameters are discovered to be closely related to the density of the dislocations in the incoherent interfaces, which in turn is found to have power law dependence on two length scales: indentation depth and layer thickness. Based on these results, a constitutive law for extracting strain hardening in NMM from nanoindentation data is developed. [DOI: 10.1115/1.4023672]

In latent interactions of dislocations, junction formation is one of the most important phenomena that contribute to the evolution of strength. In this work, the latent hardening coefficients for pure aluminum are estimated using 3D multiscale dislocation dynamics program (MDDP). Three well-known junction configurations, namely, the Hirth lock, the glissile junction, and the Lomer lock, are studied using 3D discrete dislocation dynamics simulations. The evolution of strength is discussed as a function of the resolved shear stress (RSS) and the number of junctions for the three junctions investigated. Hirth lock and Lomer lock are found to be the weakest and strongest junctions, respectively. Collinear reaction of dislocations does not form a junction but causes a higher strength than a Lomer lock. Quantitative and qualitative results are compared with those found in the literature.

The deformation mechanisms in Cu-Ni-Cu composite nanowires subjected to uniaxial tensile loading are investigated using molecular-dynamics simulations. We particularly explore the coupled effects of geometry and coherent interface on the tendency of nanowires to deform via twins and show pseudoelastic behavior. It is found that the critical size to exhibit pseudoelasticity in composite nanowires is $5.6\ifmmode\times\else\texttimes\fi{}5.6\text{ }{\text{nm}}^{2}$, which is 6.5 times greater than single-crystalline Cu nanowires. Our results also show that the composite nanowires offer stiffness enhancement compared to the corresponding single-crystal Cu nanowires.

Structural materials in the new Generation IV reactors will operate in harsh radiation conditions coupled with high levels of hydrogen and helium production, thus experiencing severe degradation of mechanical properties. The development of structural materials for use in such a hostile environment is predicated on understanding the underlying physical mechanisms responsible for microstructural evolution along with corresponding dimensional instabilities and mechanical property changes. As the phenomena involved are very complex and span in several length scales, a multiscale approach is necessary in order to fully understand the degradation of materials in irradiated environments. The purpose of this work is to study the behavior of Fe systems (namely a-Fe, Fe-Cr and Fe-Ni) under irradiation using both Molecular Dynamics (MD) and Dislocation Dynamics (DD) simulations. Critical information is passed from the atomistic (MD) to the microscopic scale (DD) in order to study the degradation of the material under examination. In particular, information pertaining to the dislocation-defects (such as voids, helium bubbles and prismatic loops) interactions is obtained from MD simulations. Then this information is used by DD to simulate large systems with high dislocation and defect densities.

Although in recent years there has been an attempt to model crack — dislocation interactions [1, 2], the work has been limited to two dimensions and for certain special cases. In this work we address the problem within a three-dimensional discrete dislocation dynamics frame-work coupled with a dislocation density model for cracks.

We investigate the deformation behavior of bimetallic and trimetallic nanoscale multilayer metallic composites under biaxial loading using molecular dynamics. Three types of structures were studied: (a) Cu–Ni fcc/fcc bilayer, (b) Cu–Nb fcc/bcc bilayer, and (c) Ni–Cu–Nb fcc/fcc/bcc trilayer. A configuration with a dislocation structure inside is generated by initially loading a perfect structure to a high strain to nucleate dislocations, then completely unloading it and loading it again. The comparison between the deformation behavior of bilayer and trilayer structures revealed that the Cu–Ni is more ductile, the Cu–Nb is stronger, and the trilayer structure exhibits both high strength and ductility.

We investigate the pseudoelastic behavior at room temperature of composite nanowires using molecular dynamics simulations. The nanowires are composed of a nickel core surrounded by a copper shell, leading to high coherency stresses. The coherency and surface stresses cause the nanowires to undergo a lattice reorientation, by twinning, from ⟨001⟩ to ⟨110⟩ during relaxation. Nanowires of different cross-sectional areas (varying from 2.17×2.17 up to 2.9×2.9 nm2) were studied. In all cases, under tensile loading, the nanowires reorient to ⟨001⟩ and then under unloading reorient back to ⟨110⟩, thus exhibiting pseudoelastic behavior. This behavior is more pronounced in composite nanowires with a coherent interface than for single crystal nanowires.