Atomistic simulations of shock-induced transformations and their orientation dependence in bcc Fe single crystals

Abstract

By multimillion-atom classical molecular dynamics simulations employing an embedded atom method potential, we investigate shock-induced phase transformations in body-centered cubic Fe single crystals caused by shock loading along ${[001]}_{\mathrm{bcc}}$, ${[011]}_{\mathrm{bcc}}$, and ${[111]}_{\mathrm{bcc}}$ directions. Significant dependence of the developing microstructure on the crystallographic shock direction is evident, but we see only a weak dependence of the transition pressures for the body-$\text{centered}\ensuremath{\rightarrow}\text{close}$-packed and $\text{solid}\ensuremath{\rightarrow}\text{melt}$ transitions on the shock direction. The Hugoniots obtained by simulations of samples with lengths approaching one micrometer are compared to experimental work for pressures and temperatures above shock-induced melting. Crystallographic relationships between the parent and product phase found in the simulations are common for martensitic transformations. We discuss the influence of different embedded atom method potentials on the dynamics of the transformation. We see solitary waves ahead of the shock front. The velocities of these waves decrease in time, such that they are absorbed into the shock front within a distance of propagation of one $\ensuremath{\mu}\mathrm{m}$ or less.