Atomistic studies of material dynamics: from petascale to exascale

Computational materials science has provided great insight into the response of materials under extreme conditions that are difficult to probe experimentally. For example, shock-induced plasticity and phase transformation processes in single-crystal and nanocrystalline metals have been widely studied via large-scale molecular dynamics simulations, and many of these predictions are beginning to be tested at advanced fourth generation light sources such as Argonne’s Advanced Photon Source and SLAC’s Linac Coherent Light Source. I will give two examples from our work on the mechanical response of metals to shock loading: (i) copper and iron single crystals, probed via ultrafast in situ X-ray diffraction; and (ii) grain boundaries in copper, and deformation processes probed at an atomistic scale with post situ high-resolution TEM. I will then discuss outstanding challenges in modeling the response of materials to extreme mechanical and radiation environments, and our efforts to tackle these as part of the multiinstitutional, multidisciplinary Exascale Codesign Center for Materials in Extreme Environments (ExMatEx). As we look ahead from the current petascale (10–15 operations per second) era towards the exascale (10–18 operations per second) platforms expected to be deployed by the end of this decade, multiscale, or scale-bridging, techniques are particularly promising. ExMatEx is an effort to do this by initiating an early and extensive collaboration between computational materials scientists, computer scientists, and hardware manufacturers

Strain Rate and Orientation Dependencies of the Strength of Single Crystalline Copper under Compression

Molecular dynamics (MD) simulations are used to model the compression under uniaxial strain of copper single crystals of different orientations at various temperatures and strain rates. Uniaxial strain is used because of the close resemblance of the resulting stress state with the one behind a shock front, while allowing a control of parameters such as strain rate and temperature to better understand the behavior under complex dynamic shock conditions. Our simulations show that for most orientations, the yield strength of the sample is increased with increasing strain rate. This yield strength is also dependent on the orientation of the sample, but less dependent on temperature. We find three regimes for the atomistic behavior around the yield: homogeneous dislocation nucleation, appearance of disordered atoms followed by dislocation nucleation, and amorphization. Finally, we show that a criterion solely based on a critical resolved shear and normal stress is insufficient at these strain rates to determine slip on a system

Shock wave loading and spallation of copper bicrystals with asymmetric Σ3〈110〉tilt grain boundaries

We investigate the effect of asymmetric grain boundaries (GBs) on the shock response of Cu bicrystals with molecular dynamics simulations. We choose a representative Σ3〈110〉tilt GB type, (110)_1/(114)_2, and a grain size of about 15 nm. The shock loading directions lie on the GB plane and are along [001] and [221] for the two constituent crystals. The bicrystal is characterized in terms of local structure, shear strain, displacement, stress and temperature during shock compression, and subsequent release and tension. The shock response of the bicrystal manifests pronounced deviation from planar loading as well as strong stress and strain concentrations, due to GBs and the strong anisotropy in elasticity and plasticity. We explore incipient to full spallation. Voids nucleate either at GBs or on GB-initiated shear planes, and the spall damage also depends on grain orientation

Molecular Dynamics Simulations of Detonation Instability

After making modifications to the Reactive Empirical Bond Order potential for Molecular Dynamics (MD) of Brenner et al. in order to make the model behave in a more conventional manner, we discover that the new model exhibits detonation instability, a first for MD. The instability is analyzed in terms of the accepted theory.Comment: 7 pages, 6 figures. Submitted to Phys. Rev. E Minor edits. Removed parenthetical statement about P^\nu from conclusion

Anisotropic shock response of columnar nanocrystalline Cu

We perform molecular dynamics simulations to investigate the shock response of idealized hexagonal columnar nanocrystalline Cu, including plasticity, local shear, and spall damage during dynamic compression, release, and tension. Shock loading (one-dimensional strain) is applied along three principal directions of the columnar Cu sample, one longitudinal (along the column axis) and two transverse directions, exhibiting a strong anisotropy in the response to shock loading and release. Grain boundaries (GBs) serve as the nucleation sites for crystal plasticity and voids, due to the GB weakening effect as well as stress and shear concentrations. Stress gradients induce GB sliding which is pronounced for the transverse loading. The flow stress and GB sliding are the lowest but the spall strength is the highest, for longitudinal loading. For the grain size and loading conditions explored, void nucleation occurs at the peak shear deformation sites (GBs, and particularly triple junctions); spall damage is entirely intergranular for the transverse loading, while it may extend into grain interiors for the longitudinal loading. Crystal plasticity assists the void growth at the early stage but the growth is mainly achieved via GB separation at later stages for the transverse loading. Our simulations reveal such deformation mechanisms as GB sliding, stress, and shear concentration, GB-initiated crystal plasticity, and GB separation in nanocrystalline solids under shock wave loading

Shock response of granular Ni/Al nanocomposites

Intermolecular reactive composites find diverse applications in defense, microelectronics and medicine, where strong, localized sources of heat are required. However, fundamental questions of the initiation and propagation mechanisms on the nanoscale remain to be addressed, which is a roadblock to their widespread application. The performance and response of these materials is predominantly influenced by their nanostructure, and the complex interplay of mechanical, thermal, and chemical processes that occur at very short time scales. Motivated by experimental work which has shown that high-energy ball milling (which leads to the formation of granular composites of Ni/Al) can significantly improve the reactivity as well as the ease of ignition of Ni/Al intermetallic composites, we present large scale (~41 million atom) molecular dynamics simulations of the shock response of granular Ni/Al composites, which are designed to mimic the microstructure that is obtained post mechanical milling. The shock response of granular composite materials is not well understood, and much less so for reactive nano-composites. Fully atomistic simulations such as these provide a unique insight into the subgrain response of granular media. Shock propagation in these porous, lamellar materials is observed to be extremely diffuse at low impact velocities, leading to large inhomogeneity in the local stress states of the material; whereas at higher impact velocities, the shock front is observed to be much sharper. We relate this transition in the nature of the shock, to the mechanism of void collapse, with plastic deformation dominant at slow impacts but jetting into the voids dominant at higher impact velocities