Phase transition lowering in shock compressed single-crystal aluminum: Atomistic insights

Investigating the shock-induced structural phase transition (PT) in Al is of paramount interest due to its extensive use in numerous dynamic compression experiments. In this paper, we report nonequilibrium molecular dynamics simulation of shock compression in single-crystal Al to unfold structural PT at the atomic scale. Study along three major crystallographic orientations reveals that shock-induced $\text{fcc}\ensuremath{\rightarrow}\text{hcp}$ and $\text{fcc}\ensuremath{\rightarrow}\text{bcc}$ transitions are initiated at 18.8 and 30 GPa for the [001] direction and 32.2 (38) and 44 (46.1) GPa for the [110] ([111]) directions, respectively. Interestingly, these transition pressures are significantly lower than the values reported in the static compression experiment of Akahama et al. [Phys. Rev. Lett. 96, 045505 (2006)] and the ramp compression experiment of Polsin et al. [Phys. Rev. Lett. 119, 175702 (2017)]. Signatures of PT are identified from radial distribution function and virtual x-ray diffraction patterns. Domains of different phases are manifested in the pressure-temperature diagram of the principal Hugoniot.