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Kinking is a deformation mechanism ubiquitous to layered systems, ranging from the nanometer scale in layered crystalline solids, to the kilometer scale in geological formations. Herein, we demonstrate its origins in the former through multiscale experiments and atomistic simulations. When compressively loaded parallel to their basal planes, layered crystalline solids first buckle elastically, then nucleate atomic-scale, highly stressed ripplocation boundaries - a process driven by redistributing strain from energetically expensive in-plane bonds to cheaper out-of-plane bonds. The consequences are far reaching as the unique mechanical properties of layered crystalline solids are highly dependent upon their ability to deform by kinking. Moreover, the compressive strength of numerous natural and engineered layered systems depends upon the ease of kinking or lack there of. Funding Agencies|CoorsTek Graduate Fellowship Program at Colorado School of Mines; ARO [W911NF1910389]; U.S. National Science Foundation through the DMREF program [1729335, 1729350]; NSF through CMMI program [1728041]; Swedish Research CouncilSwedish Research CouncilEuropean Commission [2016-04412]; Knut and Alice Wallenbergs FoundationKnut & Alice Wallenberg Foundation; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009 00971]; [KAW 2015.0043]

The maximum strength of polycrystalline metals/alloys has been suggested to occur at nanoscale grain sizes where the governing deformation mechanism transitions from dislocation plasticity to grain boundary mediated deformation. Despite tremendous progress recently uncovering links between transitions in nanoscale mechanisms and peak strength, the scientific literature is mostly devoid of any quantitative support, owing to the difficulty in measuring the resolved contribution of individual mechanisms to microstructural strain accommodation. In this study, the contribution of individual nanoscale mechanisms to the overall deformation of nanocrystalline Ni is calculated from atomistic simulations leveraging continuum-based kinematic metrics to compute mechanistic contributions to microstructural strain. By employing such a quantitative approach to analyze deformation behavior, it is shown that the realization of maximum strength in nanocrystalline metals corresponds to a grain size regime where the operative nanoscale mechanisms transition, and are thus equally competing to accommodate strain. However, the transition occurs between intergranular and intragranular mediated mechanisms, as it is found that dislocation plasticity alone is not the governing mechanism at all grain sizes above the peak strength regime.

We present a new solid-state material phase which is a disordered solid solution but offers many ordered line-compound features. The emergent physical phenomena are rooted in the perfect short-range order which conserves the local octet rule. We model the dual-sublattice-mixed semiconductor alloy $${\mathrm{(ZnSnN}}_{\mathrm{2}}{\mathrm{)}}_{1 - x}{\mathrm{(ZnO)}}_{2x}$$ (ZnSnN 2 ) 1 − x (ZnO) 2 x using first-principles calculations, Monte-Carlo simulations with a model Hamiltonian, and an extension of the regular solution model by incorporating short-range order. We demonstrate that this unique solid solution, occurring at a “magic” composition, can provide an electronically pristine character without disorder-induced charge localization and, therefore, a superior carrier transport similar to ordered phases. Interestingly, this phase shows singularities that are absent in the conventional solid-solution models, such as the regular solution and band-gap bowing model. Thermodynamically, this alloy phase has a sharply reduced enthalpy at its composition (like a line compound), but it still requires the entropy from long-range disorder to be stabilized at experimentally accessible temperatures.

Interfacial solute segregation behavior of P in nanocrystalline (NC) Ni alloys is investigated at the atomic scale using cross-correlative PED-APT measurements and atomistic simulations. Inhomogeneous P-segregation in grain boundaries (GBs) of NC Ni is observed from both experimental measurements and simulation calculations. Interfacial excess (IE) of the solute within GBs is further studied as function of GB misorientation. Significant scatter is detected in the IE of boundaries with similar misorientation angles and even within special boundaries with identical Σ values. From atomistic simulations, a general trend of increasing IE with initial GB energy and volume is observed but with poor correlation, indicating that the segregation behavior cannot be captured with a single average measure of the entire GB character. Rather the results suggest that the extent of interfacial solute segregation correlates better with fraction of high energy atomic sites in the boundary. By comparing IE values with thermodynamic model predictions, it is shown that the interfacial segregation behavior is strongly influenced by the energy distribution of atomic sites in the GBs. However, in order to predict the extent of solute segregation, the evolution of the atomic energy landscape in the GB with continuous solute segregation must be considered.

Controlling site disorder in ternary and multinary compounds enables tuning optical and electronic properties at fixed lattice constants and stoichiometries, moving beyond many of the challenges fa...

While the advantageous mechanical properties of nanocrystalline (NC) materials have stimulated numerous studies over the past decade, fewer studies have looked at the onset of yielding and deformation and the role of grain boundaries (GBs) prior to plastic flow. In this computational study, Ni microstructures with grain size between 6 nm and 20 nm were studied to elucidate the role of GBs during yielding. Residual strain was quantified by relaxing to zero stress during loading and calculating the resulting evolution of strain accommodation. The results of this work reveal the accumulation of plastic strain in GBs and how microplasticity changes with grain size, leading to the initiation of macroscopic yielding. Microplasticity accumulates homogeneously within large grain microstructures, then becomes localized with decreasing grain size. The resulting microstructural strain accommodation helps understand changes in the onset of dislocation plasticity and yielding behavior in NC materials and may have broader implications for their continual plastic deformation.

Cation site disorder provides a degree of freedom in the growth of ternary nitrides for tuning the technologically relevant properties of a material system. For example, the band gap of ZnGeN$_2$ changes when the ordering of the structure deviates from that of its ground state. By combining the perspectives of carrier localization and defect states, we analyze the impact of different degrees of disordering on electronic properties in ZnGeN$_2$, addressing a gap in current studies which focus on dilute or fully disordered systems. The present study demonstrates changes in the density of states and localization of carriers in ZnGeN$_2$ calculated using band gap-corrected density functional theory and hybrid calculations on partially disordered supercells generated using the Monte Carlo method. We use localization and density of states to discuss the ill-defined nature of a band gap in a disordered material, comparing multiple definitions of the energy gap in the context of theory and experiment. Decreasing the order parameter results in a large reduction of the band gap in disordered cases. The reduction in band gap is due in part to isolated, localized states that form above the valence band continuum and are associated with nitrogen coordinated by more zinc than germanium. The prevalence of defect states in all but the perfectly ordered structure creates challenges for incorporating disordered ZnGeN$_2$ into optical devices, but the localization associated with these defects provides insight into mechanisms of electron/hole recombination in the material.

Recently, we have established that, when loaded in compression, edge-on, atomic layers in layered solid can fail by buckling. The resulting structure is termed a ripplocation. When more than one layer buckles, they outline standing waves with boundaries that we labeled ripplocation boundaries that are nearly fully recoverable. In this paper, we examine buckling of layers at the centimeter level to explore continuum buckling theory and its applicability to atomic layers. Specifically, we examine buckling by confining and cyclically loading thin steel sheets, edge-on, determining that increasing confining pressure, sheet thickness, and/or decreasing the number of layers increases the buckling load. Concomitantly, the resulting wavelengths and amplitudes are reduced. A nonlinear, folding mechanics model, which accounts for frictional bending and foundation energies, is adapted and verified on our experimental results. We also demonstrate that Coulombic friction between the layers can account for the energy dissipated per cycle. The predicted values of buckling nucleation loads and number of modes from the model are in good agreement---at low levels of confinement---with continuum and atomistic scale results. The wavelength estimates from the model correlate surprisingly well with the continuum buckling results; however, likely due to the complex mechanics at the lower length scales and limiting theoretical assumptions in the derivation, the accuracy decreases at the atomistic scale and at higher confining pressures.

The effect of hydrostatic pressure on the shear deformation behavior of 10 Cu symmetric tilt interfaces has been investigated in the athermal deformation regime (10 K) using molecular dynamics simulations. Cu bicrystals were deformed under shear with a superimposed hydrostatic stress, until initial yield. For most interfaces, the shear strength increased with increasing compressive hydrostatic pressure. However, for the Σ 9 { 221 } and Σ 5 { 210 } interfaces, this trend was reversed. Neither the sign nor the magnitude of the pressure-induced elevation in shear strength was found to correlate with interface structure or particular deformation mechanism(s). The findings suggest that in general, metrics of interfacial excess volume and dilatation cannot reliably predict the pressure-sensitivity of shear strength; however, for interfaces containing the E structural unit, the annihilation of free volume appears to correlate well with the observed pressure-sensitivity.

Since their discovery in 2011, interest in MXenes, two-dimensional transition metal carbides and/or nitrides, has greatly expanded due to their promising functional properties and facile synthesis methods, and their development for emerging technologies is propitiously progressing. Despite promising advancements, there still remains a lack of understanding with regards to their fundamental mechanical properties. Here, nanoindentation of the Tin+1CnTx MXenes was studied via atomistic simulations utilizing a parametrization of the ReaxFF interatomic potential, to understand the influence of point defects. From force-displacement curves, the Young’s moduli of pristine Ti3C2O2 and Ti2CO2 were calculated to be 466 GPa and 983 GPa, respectively. The influence of both titanium and carbon vacancies (VTi and VC) on Ti3C2O2 were also quantified using simulated nanoindentation of a set of samples containing both 1% VTi and 10% VC, resulting in a reduction of the calculated Young’s modulus to 386 ± 31 GPa. Of particular importance, is that these results mirror recent experimental findings indicating the fundamental role of defects in the mechanical behavior of MXenes. The calculated modulus in this work for the defect-containing Ti3C2O2 surpasses that of graphene oxide, establishing it as a new benchmark in strength for solution-processed, 2D materials. Results here also indicate improvements can be made in current MXene processing methods to better approach the theoretical strength of pristine 2D materials.

${\mathrm{ZnGeN}}_{2}$ is sought as a semiconductor with comparable lattice constant to GaN and tunable band gap for integration in optoelectronic devices. Configurational disorder on the cation sublattice of ${\mathrm{ZnGeN}}_{2}$ can strongly modify the electronic structure compared to the ordered material, and both ordered and disordered forms of ${\mathrm{ZnGeN}}_{2}$ are candidates for light-emitting diodes and other emitters. The nonisovalent character of the disordered species (${\mathrm{Zn}}^{2+}$ and ${\mathrm{Ge}}^{4+}$) subjects the cation ordering to strong short-range order effects. To model these effects, we use Monte Carlo (MC) simulations utilizing a cluster expansion to approximate formation enthalpy. Representative disordered configurations in 1024-atom supercells are relaxed in density functional theory calculations. From the MC structures, we extract a short-range order parameter (the N-cation coordination motif), and two long-range order parameters (Bragg-Williams and stretching parameters), and examine their correlations. We perform a thermodynamic integration to determine the mixing entropy and free energy. ${\mathrm{ZnGeN}}_{2}$ exhibits a first-order phase transition with pronounced discontinuities in enthalpy and entropy, as well as in the structural order parameters. We discuss the relationship between the effective temperature used in the MC simulation and the growth temperatures in experiment in relation to the crossover from the nonequilibrium to the equilibrium growth regime. This work expands on current models of site disorder in ${\mathrm{ZnGeN}}_{2}$ and provides atomic structure models with a systematic variation of the degree of cation disorder.

MXenes are emerging two-dimensional (2D) materials for energy-storage applications and supercapacitors. Their surface chemistry, which determines critical properties, varies due to different synthesis conditions. In this work, we synthesized TiVC solid-solution MXenes by two different synthesis methods and investigated their surface functional groups. We performed etching of the TiVAlC MAX phase using two different solutions, a highly concentrated HF (50 wt % ≈ 29 M) and a mixture of LiF and HCl (1.9 M LiF/12 M HCl). Large-scale delamination of TiVCT

Virtual X-ray diffraction (XRD) and atomistic simulations are used to probe the relationship between XRD phenomena and the strain accommodation methods associated with deformation. Twinned and untwinned nanocrystalline (NC) samples loaded in tension reveal distinct XRD responses. In initially untwinned samples, peak splitting occurs precisely as dislocation mediated deformation mechanisms initiate at approximately 2.9% strain. However, initially pre-twinned samples reveal less dislocation mediated deformation and no observable XRD peak splitting. XRD responses from control sets of ideal defect structures representing bulk and unloaded NC samples are analyzed. This study shows that the peak splitting during deformation of the initially untwinned NC sample can be traced to both the high density of planar defects and the complex internal strain state present under external load.

It has long been assumed that basal dislocations were responsible for the deformation of layered, crystalline solids. Herein we make the case that, with the notable exception of some metals that kink, ripplocations - not basal dislocations - are the operative micromechanism. The reasons are: i) clear evidence for c-axis strain at multiple length scales including in transmission electron microscopy images; ii) strong influence of confining pressure on the compressive strengths of poly-, and especially single crystals; iii) ripplocations are a topological imperative if the layers are to move relative to each other, without breaking the in-plane bonds.

Many methods used to produce nanocrystalline (NC) materials leave behind non-equilibrium grain boundaries (GBs) containing excess free volume and higher energy than their equilibrium counterparts with identical 5 degrees of freedom. Since non-equilibrium GBs have increased amounts of both strain and free volume, these boundaries may act as more efficient sinks for the excess interstitials and vacancies produced in a material under irradiation as compared to equilibrium GBs. The relative sink strengths of equilibrium and non-equilibrium GBs were explored by comparing the behavior of annealed (equilibrium) and as-deposited (non-equilibrium) NC iron films on irradiation. These results were coupled with atomistic simulations to better reveal the underlying processes occurring on timescales too short to capture using in situ TEM. After irradiation, NC iron with non-equilibrium GBs contains both a smaller number density of defect clusters and a smaller average defect cluster size. Simulations showed that excess free volume contribute to a decreased survival rate of point defects in cascades occurring adjacent to the GB and that these boundaries undergo less dramatic changes in structure upon irradiation. These results suggest that non-equilibrium GBs act as more efficient sinks for defects and could be utilized to create more radiation tolerant materials in future.

The mechanisms of stress-assisted grain growth are explored using molecular dynamics simulations of nanoindentation in nanocrystalline Ni and Ni-1 at.% P as a function of grain size and deformation temperature. Grain coalescence is primarily confined to the high stress region beneath the simulated indentation zone in nanocrystalline Ni with a grain size of 3 nm. Grain orientation and atomic displacement vector mapping demonstrates that coalescence transpires through grain rotation and grain boundary migration, which are manifested in the grain interior and grain boundary components of the average microrotation. A doubling of the grain size to 6 nm and addition of 1 at.% P eliminates stress-assisted grain growth in Ni. In the absence of grain coalescence, deformation is accommodated by grain boundary-mediated dislocation plasticity and thermally activated in pure nanocrystalline Ni. By adding solute to the grain boundaries, the temperature-dependent deformation behavior observed in both the lattice and grain boundaries inverts, indicating that the individual processes of dislocation and grain boundary plasticity will exhibit different activity based on boundary chemistry and deformation temperature.

A method for the generation of atomistic realizations of polycrystalline aggregates from a phase field grain growth model is presented. Topologies of computational microstructures constructed from the proposed method as well as conventional Poisson Voronoi tessellation are quantitatively compared. While little difference is exhibited in the macroscale mechanical response, substantial differences in the resolved contribution to strain of deformation mechanisms of the structures under uniaxial tension are uncovered using post-processing kinematic metrics. These differences in the fundamental strain accommodation processes suggest that grain topology and grain boundary character significantly affect local responses of polycrystals in molecular dynamics simulations and that significant attention should be paid to the chosen starting microstructure.

Bond-order potentials have been developed for the ${\mathrm{Ti}}_{3}{\mathrm{AlC}}_{2}$ and ${\mathrm{Ti}}_{3}{\mathrm{SiC}}_{2}$ MAX phases within the Tersoff formalism. Parameters were determined by independently considering each interatomic interaction present in the system and fitting them to the relevant structural, elastic, and defect properties for a number of unary, binary, and ternary structures. A number of material properties, including those not used in the fitting procedure, are reproduced with a high degree of accuracy when compared to experiment and ab initio calculations. Additionally, well-documented MAX phase behaviors such as plastic anisotropy and kinking nonlinear elasticity are demonstrated to be captured by the potentials. As a first highly accurate atomistic model for MAX phases, these potentials provide the opportunity to study some of the fundamental mechanisms behind unique MAX phase properties. Additionally, the fitting procedure employed is highly transferable and should be applicable to numerous other MAX phases.

We report the stress and microstructural evolution for a series of Ni thin films sputter deposited over a range of rates (0.076 and 0.250 nm/s), pressures (0.27, 0.67 and 1.33 Pa), and substrate temperatures (ambient, 100, and 200 °C). In general, as the sputtering pressure increased, the stress-thickness product, measured by wafer curvature, became tensile and trended with an increase in pressure regardless of the deposition rate. However, at the lowest sputtering rate and highest substrate temperature, the films exhibited a compressive growth stress. The collective data was then fitted to a kinetic model that accounts for the stress generation at the grain boundaries, from grain growth, and from the creation of defects within the film. The model's predicted fitted parameters matched well to the experimental measurements except for films deposited at the lowest deposition pressure. These particular films exhibited a bimodal grain size distribution which could not be accommodated by the model's use of a singular grain diameter. Nevertheless, in monomodal grain sizes, the results do provide support for the kinetic model in helping to ascertain the various contributing factors for the intrinsic stress development and their microstructural relationship in context to the Thornton zonal morphology descriptors for thin films and coatings.

We describe an approach that enables the activation of two distinct nanocrystalline stability mechanisms in the same Ni(P) alloy by controlling the annealing temperature and chemical composition. Using a two-step heating process, it is shown that a low temperature first stage anneal (350 °C/1 h) drives the solute in a Ni-1at%P alloy to decorate the grain boundaries (GBs) in achieving thermodynamic stabilization at higher temperatures (550 °C). Atom probe tomography reveals a very low contamination level of species (

Molecular dynamics simulations are used to investigate lithium (Li) transport rates in symmetric tilt graphite grain boundaries (GBs). Experiments have quantified highly varied diffusion rates of L...

The influence of grain boundaries (GBs) on the mechanical behavior and deformation of crystalline materials is well documented. Specifically, intergranular failure within the microstructure often is due to GB fracture or decohesion under load. By analyzing the incipient failure response of individual GBs (e.g., decohesion), as a function of GB character, a more fundamental understanding of microstructure failure can be uncovered. Accordingly, this work leverages atomistic modeling and simulations of GBs in Al to ascertain the governing role of GB character on decohesion behavior. A range of GB characters are studied by using a newly developed GB structure database to develop trends and identify fundamental decohesion behaviors using macroscopic GB descriptors (e.g., energy, misorientation, and excess volume). However, we find that no clear correlations exist between GB decohesion and these descriptors, indicating that the origins of decohesion are missed by GB descriptors that ignore the underlying spectrum of atomic environments that compose a GB. Our results further indicate that GB decohesion behavior may be driven by atomic environments and/or dynamics specific to individual GBs, potentially independent of the macroscopic descriptions of GBs commonly employed for identification and model development.

Layered materials and formations are ubiquitous in Nature and span the gamut from individual graphene layers in graphite, to layered composites, to geologic formations. And while the similarities in the deformation of the latter two have been recognized, that the same physics applies at the atomic scale has not. Using atomistic simulations on graphite, and simple instrumented cylindrical indentation experiments on decks of cards and thin steel sheets the authors show that, in all cases, confined buckling leads to the nucleation of multiple ripplocations that rapidly propagate away from under the indenter in a wavelike manner. Upon unloading, they disappear, after dissipating considerable frictional energy. In short, Nature's solution for the deformation of all layered solids, g20 orders of magnitude in scale, is as simple as it is universal: buckling. To be able to shed light on how an earthquake propagates from studying the deformation of graphite and vice versa is quite astonishing and remarkable indeed.

The opportunity for enhanced functional properties in semiconductor solid solutions has attracted vast scientific interest for a variety of novel applications. However, the functional versatility originating from the additional degrees of freedom due to atomic composition and ordering comes along with new challenges in characterization and modeling. Developing predictive synthesis-structure-property relationships is prerequisite for effective materials design strategies. Here, a first-principles based model for property prediction in such complex semiconductor materials is presented. This framework incorporates nonequilibrium synthesis, dopants and defects, and the change of the electronic structure with composition and short range order. This approach is applied to ZnSnN

The mechanical behavior of nanocrystalline, nanoporous platinum (NP Pt) is investigated using a combination of experimental measurements and molecular dynamics (MD) simulations. The NP Pt considered in this work is characterized by an hierarchical internal structure with comparable characteristic strut thickness t and grain size D. The hardness of NP Pt with typical strut thickness t = 2–10 nm is measured by nanoindentation experiments to be Hf = 0.2–1.3 GPa. Using standard scaling assumptions, a characteristic individual strut strength can be estimated from the hardness measurements as σs = 0.3–2.5 GPa. These values compare reasonably well with the ones obtained from MD simulations of isolated struts, represented as nanowires with crystalline structure similar to the experimentally observed NP Pt. The findings suggest that for nanocrystalline NP structures with t ∼ D the strength of individual struts decreases gradually with decreasing strut size, so that the overall mechanical strength remains relatively high. The simulations provide insight into the underlying deformation mechanisms and behavior within the struts during mechanical loading. In particular, as struts decrease in size, they accommodate strain primarily through grain boundary plasticity, such as sliding. However, as strut size increases, dislocations begin to play a larger role in accommodating strain.

We present a new interatomic potential for solids and liquids called Spectral Neighbor Analysis Potential (SNAP). The SNAP potential has a very general form and uses machine-learning techniques to reproduce the energies, forces, and stress tensors of a large set of small configurations of atoms, which are obtained using high-accuracy quantum electronic structure (QM) calculations. The local environment of each atom is characterized by a set of bispectrum components of the local neighbor density projected onto a basis of hyperspherical harmonics in four dimensions. The bispectrum components are the same bond-orientational order parameters employed by the GAP potential 1]. The SNAP potential, unlike GAP, assumes a linear relationship between atom energy and bispectrum components. The linear SNAP coefficients are determined using weighted least-squares linear regression against the full QM training set. This allows the SNAP potential to be fit in a robust, automated manner to large QM data sets using many bispectrum components. The calculation of the bispectrum components and the SNAP potential are implemented in the LAMMPS parallel molecular dynamics code. We demonstrate that a previously unnoticed symmetry property can be exploited to reduce the computational cost of the force calculations by more than one order of magnitude. We present results for a SNAP potential for tantalum, showing that it accurately reproduces a range of commonly calculated properties of both the crystalline solid and the liquid phases. In addition, unlike simpler existing potentials, SNAP correctly predicts the energy barrier for screw dislocation migration in BCC tantalum.

Over the past decade, numerous efforts have sought to understand the influence of twin boundaries on the behavior of polycrystalline materials. Early results suggested that twin boundaries within nanocrystalline face-centered cubic metals have a considerable effect on material behavior by altering the activated deformation mechanisms. In this work, we employ molecular dynamics simulations to elucidate the role of twin boundaries on the deformation of 〈 1 0 0 〉 columnar nanocrystalline copper at room temperature under uniaxial strain. We leverage non-local kinematic metrics, formulated from continuum mechanics theory, to compute atomically-resolved rotational and strain fields during plastic deformation. These results are then utilized to compute the distribution of various nanoscale mechanisms during straining, and quantitatively resolve their contribution to the total strain accommodation within the microstructure, highlighting the fundamental role of twin boundaries. Our results show that nanoscale twins influence nanocrystalline copper by altering the cooperation of fundamental deformation mechanisms and their contributed role in strain accommodation, and we present new methods for extracting useful information from atomistic simulations. The simulation results suggest a tension–compression asymmetry in the distribution of deformation mechanisms and strain accommodation by either dislocations or twin boundary mechanisms. In highly twinned microstructures, twin boundary migration can become a significant deformation mode, in comparison to lattice dislocation plasticity in non-twinned columnar microstructures, especially during compression.

We have performed direct molecular dynamics (MD) simulations of heteroepitaxial vapor deposition of $\mathrm{I}{\mathrm{n}}_{x}\mathrm{G}{\mathrm{a}}_{1\ensuremath{-}x}\mathrm{N}$ films on nonpolar $(11\overline{2}0)$ wurtzite-GaN surfaces to investigate strain relaxation by misfit-dislocation formation. The simulated growth is conducted on an atypically large scale by sequentially injecting nearly a million individual vapor-phase atoms towards a fixed GaN substrate. We apply time-and-position-dependent boundary constraints to affect the appropriate environments for the vapor phase, the near-surface solid phase, and the bulklike regions of the growing layer. The simulations employ a newly optimized Stillinger-Weber In-Ga-N system interatomic potential wherein multiple binary and ternary structures are included in the underlying density-functional theory and experimental training sets to improve the treatment of the In-Ga-N related interactions. To examine the effect of growth conditions, we study a matrix of 63 different MD-growth simulations spanning seven $\mathrm{I}{\mathrm{n}}_{x}\mathrm{G}{\mathrm{a}}_{1\ensuremath{-}x}\mathrm{N}$-alloy compositions ranging from $x=0.0$ to $x=0.8$ and nine growth temperatures above half the simulated melt temperature. We found a composition dependent temperature range where all kinetically trapped defects were eliminated, leaving only quasiequilibrium misfit and threading dislocations present in the simulated films. Based on the MD results obtained in this temperature range, we observe the formation of interfacial misfit and threading dislocation arrays with morphologies strikingly close to those seen in experiments. In addition, we compare the MD-observed thickness-dependent onset of misfit-dislocation formation to continuum-elasticity-theory models of the critical thickness and find reasonably good agreement. Finally, we use the three-dimensional atomistic details uniquely available in the MD-growth histories to directly observe the nucleation of dislocations at surface pits in the evolving free surface.

In this perspective, recent advances and current research challenges concerning the mechanical properties of stabilized nanocrystalline face-centered cubic (FCC) metals are discussed. First, a brief review of key experiments and modeling efforts over the last two decades is provided, with a focus on elucidating the mechanisms associated with plastic yield, hardening, and microstructure stabilization in nanocrystalline metals. This prior work has provided an understanding of the transition between dislocation-based and grain boundary-mediated mechanisms in plasticity and has identified several strategies to mitigate temperature or stress driven grain growth. Yet, the consequence of various stabilization methods on mechanical properties is not well understood. Future research challenges are presented in order to address this scientific gap, most critically the need to include grain boundary chemistry or grain boundary phases resulting from stabilization methods in new mechanistic theories for mechanical properties of nanocrystalline FCC metals.