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The interface between cathode and electrolyte is a significant source of large interfacial resistance in solid-state batteries (SSBs). Spark plasma sintering (SPS) allows densifying electrolyte and electrodes in one step, which can improve the interfacial contact in SSBs and significantly shorten the processing time. In this work, we proposed a two-step joining process to prepare cathode (LiCoO2, LCO)/electrolyte (Li0.33La0.57TiO3, LLTO) half cells via SPS. Interdiffusion between Ti4+/Co3+ was observed at the interface by SEM/STEM, resulting in the formation of the Li−Ti−La−Co−O and Li−Ti−Co−O phases in LLTO and the Li−Co−Ti−O phase in LCO. Computational modeling was performed to verify that the Li−Ti−Co−O phase has a LiTi2O4 host lattice. In a study of interfacial electrical properties, the resistance of this interdiffusion layer was found to be 105 Ω, which is 40 times higher than the resistance of the individual LLTO phase. The formation of an interdiffusion layer is identified as the origin of the high interface resistance in the LLTO/LCO half-cell.

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.

A hybrid atomistic-continuum method can model the microstructure evolution of metals subjected to laser irradiation. This method combines classical molecular dynamics (MD) simulations with the two-temperature model (TTM) to account for the laser energy absorption and heat diffusion behavior. Accurate prediction of the temperature evolution in the combined MD-TTM method requires reliable accuracy in electron heat capacity, electron thermal conductivity, and electron–phonon coupling factor across the temperatures generated. This study uses the electronic density of states (DOS) obtained from first-principle calculations. The calculated electron temperature-dependent parameters are used in MD-TTM simulations to study the laser metal interactions in FCC and BCC metals and the phenomenon of laser shock loading and melting. This study uses FCC Al and BCC Ta as model systems to demonstrate this capability. When subjected to short pulsed laser shocks, the dynamic failure behavior predicted using temperature-dependent parameters is compared with the experimentally reported single-crystal and nanocrystalline Al and Ta systems. The MD-TTM simulations also investigate laser ablation and melting behavior of Ta to compare with the ablation threshold reported experimentally. This manuscript demonstrates that integrating the temperature-dependent parameters into MD-TTM simulations leads to the accurate modeling of the laser–metal interaction and allows the prediction of the kinetics of the solid–liquid interface.

Quasi-coarse-grained dynamics (QCGD) simulations are carried out to investigate the mesoscale deformation behavior during the impact of a 20 µm pure aluminum particle onto a substrate of pure aluminum at time and length scales relevant to cold spray deposition. A rigorous analysis of the evolution of pressure, temperature, strain, flow stress and microstructure is carried out to investigate the jetting mechanisms over a range of process parameters (impact velocity and particle temperature). The QCGD simulations identify a critical role of the pressure wave propagation in the initiation of a jet, i.e. outward flow of material at the particle/substrate interface periphery (edge). Jetting is observed to initiate when the shock wave interacts with the edge and results in localized softening of the metal in this region. This localized softening enables outward flow of the material and is accompanied by a release of the pressures in the particle and the substrate at the interface. Observations of final splat microstructures of systems that showed jetting revealed several new “small” grains in the range of 2-4 µm. These grains are mainly found at the interface, suggesting that recrystallization is favored in cold sprayed impacts of aluminum.

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.

The cold spray process utilizes a high-pressure gas jet to accelerate metallic particles to supersonic velocities, causing bonding to the substrate upon impact. The mechanical characterization of impacted particles is of great interest as their mechanical properties are strongly related to those of bulk cold-sprayed coatings. Here, in-situ micro-compression and tension results are presented from impacts in single-pass cold spray trials of as-atomized and heat-treated Al6061 powders. The effects of powder microstructure on the yield strength and ductility of individual impacts are discussed. Such observations contribute to a more fundamental understanding of the mechanical behavior of impacted metallic materials.

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.

Magnesium is widely regarded as an excellent structural material, primarily because it forms the basis for a range of light-weight high-strength alloys. Recently, high-strain rate deformation of magnesium has received a great deal of attention due to the complicated deformation modes that involve combinations of dislocation slip and deformation twinning. In this study, single crystal magnesium samples were shock-compressed along the c- and a-axis, then released back to ambient conditions. Post-mortem transmission electron microscopy revealed that extension twins developed for both c- and a-axis shock loading. Also, the nanoindentation hardness values for these shocked samples were compared to those for samples compressed under quasi-static conditions; it was found that the hardness decreased with increasing strain rate for both c- and a-axis loading. Molecular dynamics simulations were performed to elucidate the detailed mechanisms of deformation twinning in terms of inertial confinement of sample geometry and different stress relaxation speed between impact and lateral directions. The conversion from work-done to heat was discussed to explain the influence of shock-induced heating on the residual hardness. These results give new insights into the residual mechanical response in shock-compressed materials and may help to develop a more fundamental understanding of shock phenomena in metallic materials.

Two-dimensional titanium carbide MXenes, Ti3C2Tx, possess high surface area coupled with metallic conductivity and potential for functionalization. These properties make them especially attractive ...

Electrocatalysis of water is a scalable and easily available source of the production of hydrogen (H2), the future energy carrier. This drive for clean energy inspired us to develop an inexpensive, readily producible, highly active, and stable catalyst to replace current state of the art platinum catalysts. Building on the promising hydrogen evolution reaction (HER) activity of many pyrites, their structural tuning by different metals and nonmetals has been found to be effective in several instances. We present here one such effort by partial surface selenization of mesoporous cobalt sulfide material, which displayed long-term operational stability (for at least 25 h) besides attaining a current density of 100 mA cm–2 at an overpotential of 160 mV versus the reversible hydrogen electrode (RHE) (in acidic media). A low Tafel slope (of 52 mV dec–1) and high exchange current density (j0) (of 70 μA cm–2) make our catalyst better to most existing systems. More importantly, using a variety of analytical techniq...

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.

The links between loading orientation of single crystal Al and the dynamic evolution of defects (dislocations, twins, stacking faults etc.) during spallation are investigated using molecular dynamics (MD) simulations. The microstructural evolution during the shock compression of single crystal Al is observed to be primarily guided by the nucleation and evolution of Shockley partials and twin partials. The shock response and spallation behavior of single crystal Al is observed to be anisotropic and is influenced by the density of various types of dislocations during shock compression and void nucleation at the spall plane. The capability of the “quasi-coarse-grained dynamics” (QCGD) method to reproduce the MD predicted nucleation, interaction and evolution of dislocations during shock compression and spallation of single crystal Al is discussed. The QCGD method retains the relative contribution of different types of dislocations during propagation of shock compression wave, interactions of the wave and spallation of single crystal Al as predicted by MD using a fraction of representative atoms allowing the modeling of of larger sized systems for larger times.

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.

Titanium alloys find extensive use in the aerospace and biomedical industries due to a unique combination of strength, density, and corrosion resistance. Decades of mostly experimental research has led to a large body of knowledge of the processing-microstructure-properties linkages. But much of the existing understanding of point defects that play a significant role in the mechanical properties of titanium is based on semi-empirical rules. In this work, we present the results of a detailed self-consistent first-principles study that was developed to determine formation energies of intrinsic point defects including vacancies, self-interstitials, and extrinsic point defects, such as, interstitial and substitutional impurities/dopants. We find that most elements, regardless of size, prefer substitutional positions, but highly electronegative elements, such as C, N, O, F, S, and Cl, some of which are common impurities in Ti, occupy interstitial positions. First principles, not semi-empirical rules, can systematically predict how point defects form because of impurities in titanium. A team led by Sanjeev Nayak and Rainer Herbert at the University of Connecticut, USA, used density functional theory to model the effect of more than twenty impurities in titanium, including the first twenty chemical elements as well as transition metals commonly added to titanium. Electronegative elements such as carbon and oxygen were stable at octahedral interstitial positions in the titanium lattice, in an identical manner to interstitial titanium. In contrast, metallic impurities such as vanadium preferred substitutional sites. Finally, vacancies in the lattice only persisted if they were far enough from their interstitial titanium atom. Research into point defect formation and stability may help us control materials for advanced manufacturing processes such as 3D printing.

A hybrid atomic-scale and continuum-modeling framework is used to study the microstructural evolution during the laser-induced shock deformation and failure (spallation) of copper microstructures. A continuum two-temperature model (TTM) is used to account for the interaction of Cu atoms with a laser in molecular dynamics (MD) simulations. The MD-TTM simulations study the effect of laser-loading conditions (laser fluence) on the microstructure (defects) evolution during various stages of shock wave propagation, reflection, and interaction in single-crystal (sc) Cu systems. In addition, the role of the microstructure is investigated by comparing the defect evolution and spall response of sc-Cu and nanocrystalline Cu systems. The defect (stacking faults and twin faults) evolution behavior in the metal at various times is further characterized using virtual in situ selected area electron diffraction and x-ray diffraction during various stages of evolution of microstructure. The simulations elucidate the uncertain relation between spall strength and strain-rate and the much stronger relation between the spall strength and the temperatures generated due to laser shock loading for the small Cu sample dimensions considered here.

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.

Designing materials with microstructural features like multiphase interfaces have shown significant promise for usage in the next generation defense and nuclear applications. The dynamic response of these interfaces in extreme environments is observed to vary with the deformability of individual phases, which could subsequently alter the favored damage nucleation sites related to spall failure. This study investigates the role of spacing of interfaces in a nanocrystalline layered Aluminum-Silicon system on the shock wave propagation behavior, and microstructural and defect evolution using classical molecular dynamics simulations. The simulations are carried out with different hypothetical Al/Si microstructures including variations in the distribution of Si grains as layers in a nanocrystalline Al matrix. The molecular dynamics simulations suggest that spall failure is preferentially initiated at Al/Si interfaces. In addition, for the same system volume and same concentration of Si, a higher number of layers is marked by an increase in shock wave velocity and reduced resistance to spall failure.

The effect of concentration of the alloying element silicon (Si) on the shock and spall response of aluminum (Al) is studied using classical molecular dynamics (MD) simulations. Silicon is distributed as individual grains and as grain boundaries and the dynamic response under shock is studied for an impact velocity of 1000 m/s. A general trend observed here is that the spallation is almost exclusively intergranular and limited to Al-Si interfaces. The spall strengths are observed to a) decrease with an increase in alloying of Si as grains and b) show little increase for up to 50% grain boundary segregation. An increase in grain size of the whole system showed negligible differences in spall strength, highlighting the significance of concentration of the alloying species rather than its grain size. These results provide an atomistic understanding of light-weight metallic systems under shock compression and pave the way for designing multiphase metal matrix alloys and composites for defense/armor applications.

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.

Large-scale molecular dynamics simulations are carried out to investigate the role of pre-existing dislocation loops on the shock-induced deformation and spall behavior of single-crystal Cu microstructures. This study investigates the role of loading orientation and initial density of pre-existing dislocations on the decay behavior of the Hugoniot elastic limit (HEL) as well as the damage nucleation and growth behavior during spall failure of single-crystal Cu systems. The results suggest that the presence of pre-existing dislocation loops results in a decrease of the shock wave velocity and a substantial decay of the HEL values. The increased decay behavior is attributed to the decrease in the density of Shockley partials at the shock front as the shock wave travels through the metal as compared to defect-free initial single-crystal microstructures. Similarly, the presence of pre-existing dislocations is observed to result in increased values for the spall strength as compared to defect-free initial single-crystal microstructures wherein a higher density of dislocations results in the nucleation of a larger number of smaller voids. The decay behavior of the HEL values is observed to have a power–law dependence on the shock propagation distance with the initial dislocation density as a parameter. Similarly, a power–law dependence is also proposed for the number of voids nucleated at the spall plane with a dependence on the size of the voids as well as the initial density of dislocations. The evolution of microstructure (dislocation densities and voids) for the various loading orientations and initial densities of dislocations is discussed.

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.

Surface oxides formed on powder feedstocks used for cold spray deposition can play an important role in the bonding of the particles and in the development of defects in the deposit. A combination of scanning transmission electron microscopy and x-ray photoelectron spectroscopy was used to investigate the oxides formed on gas-atomized Al 6061 alloy feedstock powders. The powders were studied in the as-atomized condition and after two different thermal exposures that correspond to typical feedstock pre-treatment conditions. The surface features and internal microstructures are consistent with those reported previously for these powders. The as-atomized powders have 5.2 nm thick amorphous oxide layers, with an outer Mg-rich sub-layer and an inner Mg-lean sub-layer. Powders heat-treated at 230 °C in air exhibit slightly thicker oxide layers with a crystalline MgAl2O4 spinel outer sub-layer and an amorphous aluminum oxide inner sub-layer. Powders homogenized under Ar at 400 and 530 °C have significantly thicker (8.9 nm) oxide layers with evidence for a defect inverse spinel Al(Mg,Al)2O4 inner sub-layer between the MgAl2O4 spinel outer sub-layer and the alloy. Differences between these observations and those reported previously for oxidation of bulk alloys are explained on the basis of Mg surface segregation during the gas atomization process.

Molecular dynamics simulations are carried out to investigate the deformation response during shock compression of nanocrystalline Al microstructures at the atomic scales. The shock response is investigated for various grain sizes (18 nm–100 nm) and impact velocities (700 m/s to 1500 m/s). The simulations suggest an increase in shock front width and a decay in the velocity and the amplitude of the elastic precursor wave (Hugoniot elastic limit) as the wave travels through the microstructure. For a limited sample depth of 500 nm of polycrystalline Al, the Hugoniot elastic limit (HEL) values are higher for larger grain sizes due to a lower density of grain boundaries and higher for higher piston velocities/shock pressures. Quasi-coarse-grained dynamics (QCGD) simulations are carried out to extend this investigation of the shock response of polycrystalline microstructures to the mesoscales. The capability of QCGD simulations to reproduce the atomic scale evolution of dislocation density fractions and shock wave structures is first demonstrated for a 100 nm grain-sized Al system. The evolution of shock front width, HEL, and dislocation densities are investigated for microstructures with grain sizes ranging from 100 nm to 800 nm and system lengths ranging from 600 nm to 9.6 μm. The MD and QCGD simulations indicate that the decay behavior is attributed to the capability of the shock wave to generate Shockley partial dislocations in the compressed microstructure. The simulations indicate that grain size and impact velocities affect the rates of generation of Shockley partials and hence affect the decay of the HEL. The MD and QCGD predicted values for HEL reported here show excellent agreement with the experimentally observed sample thickness dependence for Al. An empirical model is developed to predict the HEL of Al microstructures with grain size, shock pressure and sample depth as variables.

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.