Atomistic simulations of crystallographic anisotropy and defects in shock loading

The impact of crystallographic orientation, select grain boundaries, and vacancies on the shock response of aluminum was investigated using molecular dynamics simulations. Shock loading in the [001], [011], and [111] directions was explored, revealing anisotropic behavior in shock speed, generated dislocation density, and melting. The Hugoniot elastic limit (HEL) in the [100], [110], and [111] directions were calculated as 18.7 GPa, 17.8 GPa, and 22.5 GPa, respectively. These results were found to be an order of magnitude larger than the uniaxial compressive yield strengths computed at high strain rate using an affine loading scheme. Metastable melting in the [011] and [111] directions occurred around a pressure of 100 GPa and roughly 1000 K below the observed metastable melting [001] direction and the equilibrium melt curve. The role of select twist and tilt grain boundaries was assessed. Differences in the wave speed profile were observed for most grain boundaries, but only for piston velocities [Formula: see text]1.5 km/s. Additionally, the presence of vacancies, both randomly distributed and clustered, led to a decrease in shock speed with a larger decrease in shock speed recorded at higher vacancy concentrations. The change in shock speed, relative to a defect free cell, exhibited a dependence on the configuration of the defects. Melting was also found to occur at lower pressures for increasing vacancy concentrations. These results highlight key trends in the role of defects and crystallographic orientation in the behavior of FCC metals, such as aluminum, under extreme loading conditions.