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The exceptional strength of nanolaminates is attributed to the influence of their fine stratification on the movement of dislocations. Through atomistic simulations, the impact of interfacial structure on the dynamics of an edge dislocation, which is compelled to move within a nanoscale layer of a nanolaminate, is examined for three different nanolaminates. In this study, we model confined layer slip in three structures: nanolaminated Ag and two types of Ag/Cu nanolaminates. We find that the glide motion is jerky in the presence of incoherent interfaces characterized by distinct arrays of misfit dislocations. In addition, the glide planes exhibit varying levels of resistance to dislocation motion, where planes with intersection lines that coincide with misfit dislocation lines experience greater resistance than planes without such intersection lines. [ABSTRACT FROM AUTHOR]

Plastic deformation of refractory multi-principal element alloys (RMPEAs) is known to differ greatly from those of refractory pure metals. The fundamental cause is the different dislocation dynamics in the two types of metals. In this Letter, we use atomistic simulations to quantify dislocation glide in two RMPEAs: MoNbTi and NbTiZr. Edge and screw dislocations on the {110} and {112} slip planes are studied. A series of dislocation line lengths, ranging from 1 nm to 50 nm, are employed to elucidate the line-length-dependence. To serve as references, the same simulations are performed on pure metals. For the RMPEAs, the dependence of critical stresses on length becomes undetectable within the statistical dispersion for dislocations longer than 25 nm, as a result of the change in dislocation behavior. This length is in good agreement with those predicted by analytical models. Compared to the pure metals, the critical stress anisotropy among different slip planes and character angles is substantially reduced, providing an explanation for the homogeneous plasticity in RMPEAs observed in prior experiments. [ABSTRACT FROM AUTHOR]

In this work, we calculate the local slip resistances (LSRs) in equal-molar MoNbTi multi-principal element alloy via molecular static simulations. We consider dislocations of either screw or edge character gliding on four types of slip planes, {110}, {112}, {123}, and {134}, in either forward or backward sense of the 111 slip direction. As references, we also compute the Peierls stresses of the same dislocations in two natural reference metals, Mo and Nb, and a synthetic one, the mean-field, A -atom potential-based MoNbTi. Further, we compare the LSRs with the corresponding ideal shear strengths that do not account for the lattice distortions induced by dislocation cores. We show that for neither dislocation character is the LSR on the {110} plane the lowest in MoNbTi, in contrast to Mo and Nb. For edge dislocations, slip on the {134} plane is the easiest, but for the screw dislocations, it is the hardest. For screw dislocations, the {112} glide plane is the most favored, while for edge dislocations, it is the least favored. We also find that the screw-to-edge ratio in the slip resistance is reduced by one order of magnitude in MoNbTi compared to that of any pure reference metal for the same type of slip plane. These results suggest that, in contrast to pure body-centered cubic (BCC) metals, BCC MPEAs could deform by a multiplicity of slip modes due to the lower screw-to-edge ratios and the lower LSRs for edge dislocations on the three higher order planes. [ABSTRACT FROM AUTHOR]

Concurrent atomistic–continuum simulations are employed to study Si/Ge (111) semicoherent interfaces in terms of their responses to an in‐plane shear and interactions with lattice dislocations. Three types of Si/Ge interfaces, differing in interfacial structures and energy, are considered. Type I interface coincides with the shuffle‐set slip plane and contains a hexagonal network of edge dislocations. Type II and Type III interfaces both coincide with the glide‐set slip plane, yet they contain, respectively, a triangular and a hexagonal network of Shockley partial dislocations. The simulations show that among the three types of interfaces, 1) Type I interface is the least stable subject to an in‐plane shear and 2) Type III interface impedes the gliding of lattice dislocations the most significantly. [ABSTRACT FROM AUTHOR]

Atomistic simulation methods are appropriate tools for investigating the dynamics of dislocations and their interactions with obstacles in metallic materials. In particular, molecular dynamics (MD) simulations have been widely employed on these two topics in the past several decades. However, even for the same type of simulation, the results can vary. While some of the quantitative differences may be due to the choices of interatomic potential and simulation cell size, they could similarly be attributed to choice of model settings, which have also differed substantially to date. In this paper, we carry out systematic MD simulations to study the effects of a few key model settings on the dynamics of an edge dislocation and its interaction with a void in copper. For a fixed interatomic potential, three modeling parameters, including applied loading mode, boundary conditions, and thermostat, are considered and their influences on the stress–strain response, the dislocation velocity, and the critical stress for a dislocation to bypass a void are compared. For a few select cases, we further examine the influence of temperature, strain rate, and simulation cell size. The results show that (i) compared with flexible boundary conditions, rigid boundary conditions result in greater stress oscillations in simulation cells of certain sizes; (ii) compared with the cases of no thermostat and a full thermostat, a partial thermostat provides better temperature control and lower friction on the dislocation core, respectively; and (iii) for dislocation–void interactions, the critical dislocation bypassing stress in shear loading can be appropriately determined with either a constant applied strain rate or a constant applied stress although the strain rate cannot be controlled in the latter. This analysis reveals that these three settings greatly influence the accuracy and interpretation of the results for the same type of simulation. [ABSTRACT FROM AUTHOR]

In this work, selecting the equal-molar CoNiRu multi-principal element alloy (MPEA) as a model material, we study dislocation core structures starting from first principles. We begin by sifting through all possible configurations to find those corresponding to elastic stability and energetically favored face-centered cubic (fcc) phases and, then, for these configurations, employ a phase field-based model to predict the extent of dislocations lying within them. The main findings are that for the fcc phase, (i) large variations in atomic configuration for the same chemical composition can cause significant changes in the generalized stacking fault energy surface and (ii) the dispersion in defect fault energies are chiefly responsible for substantial variations in the intrinsic stacking fault (ISF) widths of screw and edge dislocations. For instance, positive the ISF energy can vary by 10 times, with the lower values correlated with entirely Ni and Ru atoms and higher values with only Co and Ru atoms across the slip plane. Variations in lattice parameter and stiffness tensor accompany local differences in atomic configuration are also taken into account but shown to play a lesser role. We find that the dislocation can experience profound variations (3–7-fold changes) in its associated ISF width along its line, with the screw dislocation experiencing a greater variation than the edge dislocation (6.02–43.22 Å for the screw dislocation, and 19.6–62.62 Å for the edge dislocation). We envision that the ab initio-informed phase-field modeling method developed here can be readily adapted to MPEAs with other chemical compositions. [ABSTRACT FROM AUTHOR]

In the continuum context, the displacements of atoms induced by a dislocation can be approximated by a continuum disregistry field. In this work, two phase-field (PF)-based approaches and their variants are employed to calculate the disregistry fields of static, extended dislocations of pure edge and pure screw character in two face-centred cubic metals: Au and Al, which have distinct stable stacking fault energy and elastic anisotropy. A new truncated Fourier series form is developed to approximate the generalised stacking fault energy (GSFE) surface, which shows significant improvement over the previously employed Fourier series form. By measuring the intrinsic stacking fault (ISF) width and partial dislocation core size in different ways, the PF-based disregistry fields are quantitatively compared against those predicted by molecular statics. In particular, two new measures for the ISF widths are proposed and shown to overcome drawbacks of the more commonly used standards in the literature. Our calculations also show that continuum formulation of the elastic energy and the GSFE for a homogeneous surface can successfully characterise the core structure. Last, our comparisons highlight the significance of including the gradient energy in the free energy formulation when an accurate description of the dislocation core structure is desired. [ABSTRACT FROM AUTHOR]

[Display omitted] With exposure to high-energy particle irradiation, structural metals develop a number of nanoscale defects, one of which is the nano-sized He bubble. Formed throughout the material, these bubbles can directly impair the movement of dislocations and dramatically alter mechanical performance. Propagating edge dislocations in face-centered cubic metals, driven under sufficient mechanical forces, are expected to cut through He bubbles, due to the higher energetic expense. Here, using atomistic simulations for a model Cu material, we find that when the He-vacancy ratio and bubble diameter become sufficiently large and the ratio of the bubble spacing to the diameter is reduced below a threshold, an extended edge dislocation engages in a multi-step-bypass (MSB) maneuver to change its glide plane and overcome the bubble at room temperature. This unanticipated MSB mechanism provides a more energetically favorable pathway compared to conventional bypass mechanisms under these severe conditions. [ABSTRACT FROM AUTHOR]

Confined layer slip (CLS) is considered as the primary mechanism for plastic deformation in nanolaminates. It depicts the confined motion of a single dislocation between two parallel interfaces nanometers apart. Using atomistic simulations, we investigate the effects of interface spacing, or equivalently individual layer thickness, and dislocation distribution on the CLS of an edge dislocation in nanolaminated Nb with coherent twin boundaries and Nb nanofilm. We show that CLS motion transitions from smooth glide to staggered glide as the nanolayer thickness increases, while the line bows out, as theoretically pictured. For the same nanolayer thicknesses L , dislocation CLS in the nanofilm is smooth and the CLS resistance is 47% to 79% lower. For both the nanolaminate and film, the nanolayer size L effect on CLS resistance is found to follow the expected ln(L)/ L scaling with an additional constant that depends on the interface/dislocation interaction but is independent of L. When like-signed dislocation glide concurrently on the same glide plane in the adjacent layers, the dislocations move with almost no bow-out, and the CLS resistance is substantially reduced, even below that of the thin film bounded by two free surfaces. With an increase in the glide plane separation between like-signed dislocations in the neighboring layers, a higher flow stress is obtained. However, when the dislocations are oppositely signed, CLS motion is jerky and the stress fluctuates and overall increases three fold. • Molecular dynamics simulation is utilized to study confined layer slip (CLS) of a dislocation. • The dislocation glides in a Nb/Nb nanolaminate with coherent twin boundaries. • The effects of layer thickness and dislocation distribution across layers are studied. • Analytical CLS strength models are compared to the molecular dynamics results. • The modified CLS model is introduced that agrees with the simulation results on size effects. [ABSTRACT FROM AUTHOR]

• The local slip resistances (LSRs) are calculated for the {110}, {112}, and {123} planes in four MPEAs. • The two MPEAs with Cr have larger lattice distortion than without. • A larger lattice distortion results in increased LSR and reduced anisotropy in LSR. The design and development of structural materials that can survive under the extreme conditions of operation are critical to next generation aerospace and energy technologies. Selectively designed multi-principal element alloys (MPEAs), which are solid solution phases with three or more principal elements on simple underlying lattices, are expected to fulfill such requirements. The combination of refractory metals with elements known for enhancing oxidation resistance, high temperature strength, and thermal stability makes them ideal candidates for high temperature applications. Due to their unique microstructures and chemical compositions, MPEAs may exhibit excellent mechanical properties, such as high strengths at elevated temperatures and improved hardnesses. Improving the mechanical properties of MPEAs requires knowledge of their plastic deformation mechanisms, at the core of which is dislocation slip, which is intimately connected to the local slip resistances (LSRs). In this work, atomistic calculations are conducted to obtain LSRs of edge and screw dislocations on three slip planes – {110}, {112}, and {123} – in four refractory MPEAs, CrMoNbTa, CrNbTaW, MoNbTaV, and MoNbTaW.The goal of this work is to determine the LSR and the role that lattice distortion has. We find that the two MPEAs containing Cr bear an increased lattice distortion and achieve the highest LSR values and lowest anisotropy in LSR. It is also shown that the MPEAs possess much lower slip resistance anisotropy than pure metals. [ABSTRACT FROM AUTHOR]

Obstacles, such as voids and precipitates, are prevalent in crystalline materials. They strengthen crystals by serving as barriers to dislocation glide. In this work, we develop a phase-field dislocation dynamics (PFDD) technique for investigating the interactions between dislocations and second-phase obstacles, which can be either voids or precipitates. The PFDD technique is constructed to account for elastic heterogeneity, elastic anisotropy, dissociation of the dislocation, and dislocation transmission across bicrystalline interfaces. Within the framework, we present a model for "pseudo-voids", which are voids shearable by dislocations, in contrast to unphysical, unshearable voids in conventional phase-field dislocation formulations. We employ the PFDD technique to investigate the in-plane interactions between an edge dislocation and an array of nano-scale obstacles with different spacings. In this application, the interactions take place in glide planes of either a face-centered cubic (FCC) Cu or a body-centered cubic (BCC) Nb matrix, while the precipitates have a Cu 1 − x Nb x composition, with x varying from 0.1 to 0.9. Our atomistic simulations find that the alloy precipitates can have an FCC, an amorphous, or a BCC phase, depending on the compositional ratio between Cu and Nb, i.e., value of x. Among all types of obstacles, the critical stresses for dislocation bypass are the highest for unshearable amorphous precipitates, followed by shearable crystalline precipitates, and then the pseudo-voids. • A phase-field dislocation dynamics model for elastic heterogeneity is established. • Dislocation/obstacle interactions in a Cu or a Nb matrix are studied. • The obstacles are voids, crystalline precipitates, or amorphous precipitates. [ABSTRACT FROM AUTHOR]

The Peierls stress required for a single dislocation to glide in a perfect crystal is considered one of the most important properties controlling plastic deformation of metals. Values for the Peierls stress computed by atomistic models often differ even for the same dislocation and when using the same material interatomic potential, limiting their value in understanding material behavior. Here, using molecular static simulations, we study the effects of model configuration, model size, and loading mode on the Peierls stress of an edge dislocation on the {112} glide plane in body-centered cubic Nb in the twinning and anti-twinning senses of slip. The analysis includes six model configurations, a model size range spanning over an order of magnitude, and three loading modes, all repeated for two interatomic potentials for Nb. Results show that, in most cases studied, smaller model sizes artificially lead to lower Peierls stresses. The study also reveals a substantial effect of model configuration on whether the Peierls stress converges with size, the minimum size required for convergence, and the magnitude of the Peierls stress. Only one model configuration, the periodic array of dislocation (PAD) model, achieves a strictly converged value for the two interatomic potentials considered here. In addition, PAD model predicts the lowest Peierls stress values among all model configurations, provided that the model size is sufficiently large. We show that with the same interatomic potential, while one model configuration predicts a higher anti-twinning Peierls stress than twinning Peierls stress, another one may predict the opposite asymmetry. Unlike the stress-controlled loading mode, the shear-controlled and displacement-controlled loading modes provide similar results using both potentials. The findings here will be useful for selecting the appropriate model settings in research studies involving Peierls stress calculations via atomistic simulations. [ABSTRACT FROM AUTHOR]

Many elementary deformation processes in metals involve the motion of dislocations. The planes of glide and specific processes dislocations prefer depend heavily on their atomic core structures. Atomistic simulations are desirable for dislocation modeling but their application to even sub-micron scale problems is in general computationally costly. Accordingly, continuum-based approaches, such as the phase-field microelasticity, phase-field dislocation dynamics (PFDD), generalized Peierls–Nabarro (GPN) models, and the concurrent atomistic–continuum (CAC) method, have attracted increasing attention in the field of dislocation modeling because they well represent both short-range cores interactions and long-range stress fields of dislocations. To better understand their similarities and differences, it is useful to compare these methods in the context of benchmark simulations and predictions. In this paper, we apply the CAC method and different PFDD variants – one of them is equivalent to a GPN model – to simulate an extended (i.e., dissociated) dislocation in Al with initially pure edge or pure screw character in terms of the disregistry. CAC and discrete forms of PFDD are also employed to calculate the Peierls stress. By conducting comprehensive convergence studies, we quantify the dependence of these measures on time/grid resolution and simulation cell size. Several important but often overlooked differences between PFDD/GPN variants are clarified. Our work sheds light on the advantages and limitations of each method, as well as the path towards enabling them to effectively model complex dislocation processes at larger length scales. • Disregistry and Peierls stress of a static dislocation in Al are simulated. • A phase-field dislocation dynamics method and a concurrent atomistic-continuum method are used. • Different PFDD variants are employed and the results are compared. [ABSTRACT FROM AUTHOR]

• Elastic anisotropy is implemented into the phase-field dislocation dynamics method. • Dislocations with arbitrary character angel in eight transition metals are modeled. • The isotropic Voigt average may best predict the ISF width of the edge dislocation. • The isotropic Reuss average may best predict the ISF width of the screw dislocation. • At 60∘, elastic anisotropy and isotropic Hill average predict the same ISF width. In this study, we present a phase-field dislocation dynamics (PFDD) method that includes full anisotropic elasticity. We apply it to calculate the equilibrium core structures of dislocations with arbitrary character angle in eight face-centered cubic transition metals. The calculations investigate the effects of the gradient energy density in the total energy density and the choice of the averaging scheme to determine the isotropic equivalent elastic moduli (i.e., Voigt, Reuss, and Hill). We show that the addition of the gradient energy term increases the intrinsic stacking fault (ISF) widths for the edge and screw dislocations in most of the metals studied here, but decreases the ISF widths for the edge dislocations in four metals: Ir, Ni, Pd, and Rh. The analysis indicates that among the three isotropic averaging schemes, the Voigt isotropic equivalent modulus best predicts the ISF widths of the edge dislocations and the Reuss scheme for the ISF widths of the screw dislocations, compared to the full elastic anisotropy. Finally, a critical character angle (∼ 60∘) is revealed, at which the PFDD simulations with full elastic anisotropy and those with the isotropic Hill average predict the same ISF width. Our work advances the basic understanding of the elastic anisotropic effects on the equilibrium dislocation core structures and can help guide the choice of isotropic averaged moduli. [ABSTRACT FROM AUTHOR]