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Transition metal dichalcogenides (TMDs) are a class of 2D materials demonstrating promising properties, such as high capacities and cycling stabilities, making them strong candidates to replace graphitic anodes in lithium-ion batteries. However, certain TMDs, for instance, MoS2, undergo a phase transformation from 2H to 1T during intercalation that can affect the mobility of the intercalating ions, the anode voltage, and the reversible capacity. In contrast, select TMDs, for instance, NbS2 and VS2, resist this type of phase transformation during Li-ion intercalation. This manuscript uses density functional theory simulations to investigate the phase transformation of TMD heterostructures during Li-, Na-, and K-ion intercalation. The simulations suggest that while stacking MoS2 layers with NbS2 layers is unable to limit this 2H → 1T transformation in MoS2 during Li-ion intercalation, the interfaces effectively stabilize the 2H phase of MoS2 during Na- and K-ion intercalation. However, stacking MoS2 layers with VS2 is able to suppress the 2H → 1T transformation of MoS2 during the intercalation of Li, Na, and K-ions. The creation of TMD heterostructures by stacking MoS2 with layers of non-transforming TMDs also renders theoretical capacities and electrical conductivities that are higher than that of bulk MoS2.

Damage evolution in metals during laser-shock loading (spallation) is a complex phenomenon accompanied by extremely high temperatures, pressures, and strain rates that affect the void nucleation/growth mechanisms. The current modeling efforts at the atomic scales to investigate the evolution of microstructure undergoing the spall failure at the atomic scales are limited to a hybrid atomistic–continuum method that combines the two-temperature model (TTM) with the molecular dynamics (MD) simulations. This manuscript demonstrates this capability by investigating the mechanisms of nucleation/evolution of voids for a nanocrystalline Al system experiencing an ultrafast laser pulse. This capability, however, is unable to model the laser shock response of experimental systems (with grain sizes greater than 100 nm and thicknesses in the microns) as well as the post-spall behavior (damage growth or recompression behavior). This work combines the TTM with the quasi-coarse-grained dynamics (QCGD) method to extend MD-TTM simulations to the mesoscales. The hybrid QCGD-TTM approach retains the laser energy absorption, heat generation/transfer, and microstructure evolution (melting, defects, and damage) behavior predicted by MD-TTM simulations. The QCGD-TTM simulations allow the investigation of the wave propagation behavior, the evolution of microstructure (defects and damage), temperature, and pressure at the time and length scales of laser-shock experiments. The QCGD-TTM simulations reported here investigate the nucleation and post-spall damage evolution behavior during spall failure of sc-Al and 0.5 µm grain-sized pc-Al films with a thickness of up to 2 µm. The QCGD-TTM-predicted damage evolution behavior captures the post-spall behavior observed experimentally and retains the atomistic characteristics of void nucleation and void collapse.

The predictive modeling of the experimentally observed behavior of metallic materials under shock loading conditions (wave structures, spall strengths) is a critical challenge toward the design of next-generation structural materials. This challenge is due to the lack of computational methods that can predict microstructural evolution at the mesoscales under dynamic loading conditions. While classical molecular dynamics simulations have been able to provide atomic-scale insights in the defect and damage nucleation/evolution mechanisms, the capability to have a direct comparison with experimental data at the same time and length scales is still a challenge. The current computational approaches require that several approximations be made either for the loading conditions or for the micromechanisms related to defect evolution and interaction at the mesoscales. This viewpoint discusses the insights obtained from molecular dynamics simulations of shock deformation and spall failure of heterogeneous metallic microstructures. An example Al–Ni microstructure is used to identify the critical atomic-scale phenomena that need to be addressed by the mesoscale methods when considering shock deformation and failure (spallation) at the mesoscales.

During the various stages of shock loading, many transient modes of deformation can activate and deactivate to affect the final state of a material. In order to fundamentally understand and optimize a shock response, researchers seek the ability to probe these modes in real-time and measure the microstructural evolutions with nanoscale resolution. Neither post-mortem analysis on recovered samples nor continuum-based methods during shock testing meet both requirements. High-speed diffraction offers a solution, but the interpretation of diffractograms suffers numerous debates and uncertainties. By atomistically simulating the shock, X-ray diffraction, and electron diffraction of three representative BCC and FCC metallic systems, we systematically isolated the characteristic fingerprints of salient deformation modes, such as dislocation slip (stacking faults), deformation twinning, and phase transformation as observed in experimental diffractograms. This study demonstrates how to use simulated diffractograms to connect the contributions from concurrent deformation modes to the evolutions of both 1D line profiles and 2D patterns for diffractograms from single crystals. Harnessing these fingerprints alongside information on local pressures and plasticity contributions facilitate the interpretation of shock experiments with cutting-edge resolution in both space and time.

Li-ion batteries function by Li intercalating into and through the layered electrode materials. Intercalation is a solid-state interaction resulting in the formation of new phases. The new observations presented here reveal that at the nanoscale the intercalation mechanism is fundamentally different from the existing models and is actually driven by nonuniform phase distributions rather than the localized Li concentration: the lithiation process is a ‘distribution-dependent’ phenomena. Direct structure imaging of 2H and 1T dual-phase microstructures in lithiated MoS2 and WS2 along with the localized chemical segregation has been demonstrated in the current study. Li, a perennial challenge for the TEM, is detected and imaged using a low-dose, direct-electron detection camera on an aberration-corrected TEM and confirmed by image simulation. This study shows the presence of fully lithiated nanoscale domains of 2D host matrix in the vicinity of Li-lean regions. This confirms the nanoscale phase formation followed by Oswald ripening, where the less-stable smaller domains dissolves at the expense of the larger and more stable phases.

Gas-atomized Al 6061 powder was studied in as-atomized, heat-treated, and homogenized conditions, and low-coverage cold spray deposits were produced from these powders to allow individual splats to be investigated. The microstructures were investigated by electron microscopy, and the mechanical behavior of the particle/splat interiors was evaluated by the compression of micropillars milled from the samples. The as-atomized and heat-treated powders exhibit cell-like solidification microstructures with Mg-Si and Fe-Al-Si secondary phases around the cell boundaries. These powders have flow stresses of ≈ 185 MPa with pronounced strain bursts due to rapid, stochastic dislocation motion confined within the cells. In the homogenized powders, the Mg-Si phase has dissolved, the Fe-Al-Si has coarsened, and the grain boundaries have unpinned. The stress–strain curves are smoother due to the dissolution of cell boundary obstacles, but the flow stress is about 30 MPa higher due to the increased solute content giving more sluggish dislocation motion. The microstructures of the splat interiors resemble those of the powders closely, but the flow stresses measured are all much higher (280-290 MPa) with little effect of powder microstructure or deposition pressure. This is consistent with the high dislocation density reaching a saturation value in all of the splats studied.

Recent capability of chemical vapor deposition (CVD) to grow large-area and high-quality monolayer and few-layered molybdenum disulfide (MoS2) structures renders intrinsic defects such as vacancies that alter the electronic properties of these structures. As a result, density functional theory (DFT) calculations are carried out to investigate the electronic structure of various types of CVD-grown vacancy defects and the role on the strain relaxation behavior of bilayer MoS2. DFT calculations suggest that additional charge states are activated in the gap between the valence band and conduction band for the atoms neighboring the defects in the layer and in the layer above the defects. In addition, the DFT results indicate that the presence of local defects lower energy barriers for strain relaxation of bilayer MoS2 attributed to sliding between the layers. These results demonstrate the modifications of the electronic properties of 2D structures and the strain due to the presence of defects.

Molecular dynamics (MD) simulations are used to investigate the role of size and distribution of nanoscale Cu/Ta interfaces on the nucleation and evolution of defects during shock loading and spall failure of nanocrystalline (nc) Cu/Ta alloys. Cu/Ta interfaces are introduced through the embedding of Ta clusters in nc-Cu matrix. The phase stability of the embedded Ta clusters either as FCC or BCC clusters is first investigated and reveals that the FCC Ta clusters have a lower energy for diameters less than 4 nm, whereas the BCC Ta clusters have a lower energy for the larger diameters. The shock simulations are then carried out for Ta clusters with an average diameter of 1 and 3 nm and concentrations of 3.0, 6.3 and 10.0% to investigate the role of size and distribution of Cu/Ta interfaces (due to presence of clusters) on the nucleation and evolution of dislocations as well as the spall strength of the alloy. The MD simulations indicate that the Cu/Ta interfaces reduce the capability of nc-Cu to accommodate plasticity through nucleation of dislocations and create void nucleation sites during spallation. The MD simulations further reveal that the impact strengthening effects due to the presence of nanoscale Cu/Ta interfaces are strongly dependent upon the size and distribution of Ta clusters, as well as the grain size of Cu matrix. Smaller size of interfaces (cluster size), higher concentration of Ta (smaller spacing between interfaces) and larger matrix grain size render higher spall strengths of nc-Cu/Ta microstructures.

The ‘quasi-coarse-grained dynamics’ (QCGD) method is extended to model the thermodynamic behavior and the shock response of HCP Ti systems at the mesoscales by coarse-graining the atomistic microstructure using representative atoms (R-atoms) and scaled interatomic potentials. To demonstrate the capability of the QCGD method, the melting behavior of a single-crystal slab of HCP Ti and the dynamic failure (spallation) behavior of nanocrystalline systems under shock loading conditions are first investigated using molecular dynamics (MD) simulations using an embedded atom method interatomic potential for Ti. The melting simulation suggests an interplay between the nucleation and propagation of the surface-induced heterogeneous melting and the nucleation and propagation of bulk homogeneous melting of the system. In addition, the spall strengths calculated using MD at strain rates of 1010 s−1 allow the development of improved models for the strain rate dependence of the spall strength determined experimentally at 105 s−1. The QCGD method is observed to be capable of reproducing the MD-predicted kinetics of melting and the shock response and spall failure of nanocrystalline Ti systems using a coarse-grained microstructure comprising of representative atoms (R-atoms). The QCGD simulations demonstrate the ability to model the mesoscale behavior of Ti systems by modeling the shock deformation and failure due to spallation of a 1 µm × 1 µm × 2 µm sized system at strain rates of 108 s−1 to bridge the gap between MD simulations and experiments.

Molecular dynamics (MD) simulations are carried out to investigate the effects of the type and spacing of FCC/BCC interfaces on the deformation and spall behavior. The simulations are carried out using model Cu/Ta multilayers with six different types of interfaces. The results suggest that interface type can significantly affect the structure and intensity of the incoming shock wave, change the activated slip systems, alter dislocation slip and twinning behavior, affect where and how voids are nucleated during spallation and the resulting spall strength. Moreover, the above aspects are significantly affected by the interface spacing. A transition from homogeneous to heterogeneous dislocation nucleation occurs as the interface spacing is decreased to 6 nm. Depending on interface type and spacing, damage (voids) nucleation and spall failure is observed to occur not only at the Cu/Ta interfaces, but also in the weaker Cu layer interior, or even in the stronger Ta layer interior, although different mechanisms underlie each of these three distinct failure modes. These findings point to the fact that, depending on the combination of interface type and spacing, interfaces can lead to both strengthening and weakening of the Cu/Ta multilayered microstructures.

Large scale molecular dynamics (MD) simulations are carried out to investigate the twinning behavior as well as the atomic scale micromechanisms of growth of tension and compression twins in polycrystalline Mg microstructures at high strain rates. A new defect characterization algorithm (extended-common neighbor analysis (E-CNA)) is developed that allows for an efficient identification of various types of twins in HCP microstructures. Unlike other local orientation analysis methods, the E-CNA method allows for atomic scale characterization of the structure of different types of twin boundaries in HCP microstructures. The MD simulations suggest that the local orientation of individual grains with the loading axis plays a critical role in determining the ability of grains to nucleate either compression twins or tension twins. The twinning behavior is observed through nucleation of a pair of planar faults and lateral growth of the twins occurs through nucleation of steps along the planar faults. The kinetics of migration of steps that determine the rate of growth of twins are investigated at the atomic scales. The twin tip velocity computed at high strain rates compares well with the experimentally reported values in the literature.

Density functional theory (DFT) calculations are carried out to investigate the role of grain boundaries on the energetics related to the oxygen and hydrogen segregation-induced embrittlement in polycrystalline Ni systems. Four model grain boundary (GB) systems for nickel are chosen to investigate this effect. These model GBs are the Σ5 (012) GB, the Σ5 (013) GB, the Σ11 (113) GB, and the Σ3 (111) coherent twin boundary (CTB). The chosen GBs enable the investigation of the role of the CTB in the embrittlement and decohesion mechanisms in comparison with the other GBs. The embrittling mechanism considered here is based on the investigation of the energetics related to (a) the segregation of atoms of embrittling species (oxygen, hydrogen) at the GB; (b) the formation of vacancies due to the segregation of embrittling species at the GB; and (c) the energetics related to decohesion at the GB as a function of concentration/accumulation of the embrittling species at the GB. DFT calculations suggest that the segregation of the embrittling species and the embrittling effect are closely related to the local atomic structure of the GB and the associated excess free volume. In particular, it is found that the Σ3 (111) CTB is less prone to segregation of oxygen and hydrogen based on the binding energetics of the embrittling species. However, among all the GBs considered, the Σ3 (111) CTB is found to be most susceptible to GB decohesion and crack formation in the presence of small amounts of segregated oxygen atoms. This dual behavior of the Σ3 (111) CTB is also confirmed for the case of hydrogen as the embrittling species using DFT simulations. Thus, the segregation-resistant Σ3 (111) CTB is observed to be the most susceptible to crack formation in the presence of small amounts of segregated embrittling atoms. The energetics of segregation of the embrittling species and the effect of segregation on the vacancy formation energies and GB decohesion are discussed.

Large scale molecular dynamics (MD) simulations are carried out to investigate the failure response of nanocrystalline Mg using the EAM potential under conditions of uniaxial tensile stress and uniaxial tensile strain loading. The MD simulations are carried out at a strain rate of 109s-1 for grain sizes in the range of 10 nm to 30 nm. The effect of grain size on the strength of the metal is investigated and the critical grain size for transition to inverse Hall-Petch regime is identified.

The dynamic evolution and interaction of defects under the conditions of shock loading in nanocrystalline Al with an average grain size of 20 nm is investigated using molecular dynamics simulations for an impact velocity of 1 km/s. Four stages of defect evolution are identified during shock deformation and failure that correspond to the initial shock compression (I), the propagation of the compression wave (II), the propagation and interaction of the reflected tensile waves (III), and the nucleation, growth, and coalescence of voids (IV). The results suggest that the spall strength of the nanocrystalline Al system is attributed to a high density of Shockley partials and a slightly lower density of twinning partials (twins) in the material experiencing the peak tensile pressures.

A critical challenge in the predictive capability of materials deformation behavior under extreme environments is the availability of computational methods to model the microstructural evolution at the mesoscale. The capability of the recently-developed quasi-coarse-grained dynamics (QCGD) method to model mesoscale behavior is demonstrated for the phenomenon of supersonic impact of 20 µm sized Al particles on to an Al substrate at various impact velocities and over time and length scales relevant to cold spray deposition. The QCGD simulations are able to model the kinetics related to heat generation and dissipation, and the pressure evolution and propagation, during single particle impact over the time and length scales that are important experimentally. These simulations are able to unravel the roles of particle and substrate deformation behavior that lead to an outward/upward flow of both the particle and the substrate, which is a likely precursor for the experimentally observed jetting and bonding of the particles during cold spray impact.

The chemical vapor deposition (CVD)-grown two-dimensional molybdenum disulfide (MoS2) structures comprise of flakes of few layers with different dimensions. The top layers are relatively smaller in size than the bottom layers, resulting in the formation of edges/steps across adjacent layers. The strain response of such few-layer terraced structures is therefore likely to be different from exfoliated few-layered structures with similar dimensions without any terraces. In this study, the strain response of CVD-grown few-layered MoS2 terraced structures is investigated at the atomic scales using classic molecular dynamics (MD) simulations. MD simulations suggest that the strain relaxation of CVD-grown triangular terraced structures is observed in the vertical displacement of the atoms across the layers that results in the formation of Moiré patterns. The Moiré islands are observed to nucleate at the corners or edges of the few-layered structure and propagate inwards under both tensile and compressive strains. The nucleation of these islands is observed to happen at tensile strains of ~ 2% and at compressive strains of ~2.5%. The vertical displacements of the atoms and the dimensions of the Moiré islands predicted using the MD simulation are in excellent agreement with that observed experimentally.

Large-scale molecular dynamics (MD) simulations are carried out using the Tersoff potential to understand the shock wave propagation behavior and the microstructural response of amorphous silica (a-SiO2) and α-quartz. The effect of shock pressure on the densification and phase transformation behavior is investigated using impact velocities of 0.5, 1.0, 1.5, and 2.0 km/s for a-SiO2 and using impact velocities of 2.0 and 3.0 km/s for α-quartz. MD simulations for a-SiO2 suggest that impact velocities of 1.5 km/s and higher result in average pressures that are greater than 9 GPa for the compressed material leading to permanent densification of the material behind the shock front. In addition, the high peak pressures render a phase transformation of the amorphous phase to the high-pressure stishovite phase, and the microstructure corresponds to a heterogeneous mixture of stishovite and liquid SiO2. Spall failure due to the interaction of the reflected tensile waves, however, is not observed for any of the velocities considered for amorphous silica as the peak tensile pressure generated is insufficient to nucleate cracks. This is verified through MD simulations of uniaxial expansion of fused silica to compute the spall strength at the strain rates generated during shock simulations (109 to 1010 s−1). The uniaxial expansion simulations suggest a brittle mode of failure for a-SiO2, as observed experimentally. In comparison, shock-induced densification and phase transformation behavior to the high-pressure stishovite phase are also observed for α-quartz for an impact velocity of 3.0 km/s. The threshold pressures for the densification and phase transformation behavior for amorphous silica and α-quartz compare very well with those observed experimentally.

Ultrathin MoS2 sheets and nanostructures are promising materials for electronic and optoelectronic devices as well as chemical catalysts. To expand their potential in applications, a fundamental understanding is needed of the electronic structure and carrier mobility as a function of strain. In this paper, the effect of strain on electronic properties of monolayer MoS2 is investigated using ab initio simulations based on density functional theory. Our calculations are performed in both infinitely large two-dimensional (2D) sheets and one-dimensional (1D) nanoribbons which are theoretically cut from the sheets with semiconducting $$ [\bar{1}100] $$ (armchair) edges. The 2D crystal is studied under biaxial strain, uniaxial strain, and uniaxial stress conditions, while the 1D nanoribbon is studied under a uniaxial stress condition. Our results suggest that the electronic bandgap of the 2D sheet experiences a direct-indirect transition under both tensile and compressive strains. Its bandgap energy (E g) decreases under tensile strain/stress conditions, while for an in-plane compression, E g is initially raised by a small amount and then decreased as the strain varies from 0 to −6 %. On the other hand, E g at the semiconducting edges of monolayer MoS2 nanoribbons is relatively invariant under uniaxial stretches or compressions. The effective masses of electrons at the conduction band minimum (CBM) and holes at the valence band maximum (VBM) are generally decreased as the in-plane extensions or compressions become stronger, but abrupt changes occur when CBM or VBM shifts between different k-points in the first Brillouin zone.

The potential of the applicability of two-dimensional molybdenum disulfide (MoS2) structures, in various electronics, optoelectronics, and flexible devices requires a fundamental understanding of the effects of strain on the electronic, magnetic and optical properties. Particularly important is the recent capability to grow large flakes of few-layered structures using chemical vapor deposition (CVD) wherein the top layers are relatively smaller in size than the bottom layers, resulting in the presence of edges/steps across adjacent layers. This paper investigates the strain response of such suspended few-layered structures at the atomic scales using classic molecular dynamics (MD) simulations. MD simulations suggest that the suspended CVD-grown structures are able to relax the applied in-plane strain through the nucleation of ripples under both tensile and compressive loading conditions. The presence of terraced edges in these structures is the cause for the nucleation of ripples at the edges that grow towards the center of the structure under applied in-plane strains. The peak amplitudes of ripples observed are in excellent agreement with the experimental observations. The study provides critical insights into the mechanisms of strain relaxation of suspended few-layered MoS2 structures that determine the interplay between the mechanical response and the electronic properties of CVD-grown structures.

Large-scale molecular dynamics (MD) simulations are used to understand the macroscopic yield behavior of nanocrystalline Cu with an average grain size of 6 nm at high strain rates. The MD simulations at strain rates varying from 109 s−1 to 8 × 109 s−1 suggest an asymmetry in the flow stress values in tension and compression, with the nanocrystalline metal being stronger in compression than in tension. The tension-compression strength asymmetry is very small at 109 s−1, but increases with increasing strain rate. The calculated yield stresses and flow stresses under combined biaxial loading conditions (X-Y) gives a locus of points that can be described with a traditional ellipse. An asymmetry parameter is introduced that allows for the incorporation of the small tension-compression asymmetry. The biaxial yield surface (X-Y) is calculated for different values of stress in the Z direction, the superposition of which gives a full three-dimensional (3-D) yield surface. The 3-D yield surface shows a cylinder that is symmetric around the hydrostatic axis. These results suggest that a von Mises-type yield criterion can be used to understand the macroscopic deformation behavior of nanocrystalline Cu with a grain size in the inverse Hall–Petch regime at high strain rates.

The mechanisms of the misfit strain relaxation in heteroepitaxial islands are investigated in two-dimensional molecular dynamics simulations. Stress distributions are analyzed for coherent and dislocated islands. Thermally-activated nucleation of misfit dislocations upon annealing at an elevated temperature and their motion from the edges of the islands towards the positions corresponding to the maximum strain relief is observed and related to the corresponding decrease of the total strain energy of the system. Differences between the predictions of the energy balance and force balance criteria for the appearance of misfit dislocations is discussed. Simulations of an island located at different distances form the edge of a mesa indicate that the energy of the system decreases sharply as the island position shifts toward the edge. These results suggest that there may be a region near the edge of a mesa where nucleation and growth of ordered arrays of islands is favored.