Isentropic compression of single-crystal aluminum: Extending bcc-persistence limit

Abstract

Aluminum (Al), a key material in many high-energy density systems, is known to undergo phase transition (PT) from face-centered cubic (fcc) to hexagonal close-packed (hcp) and body-centered cubic (bcc) structures under static compression. Recently, through extensive non-equilibrium molecular dynamics simulations, we showed that shock-induced fcc to hcp and fcc to bcc structural PT in single-crystal (sc) Al occurs at pressures nearly an order of magnitude lower than those required for static compression. With the aim of capturing these transient phases experimentally, we undertake a detailed investigation of solid–solid PT in sc-Al subjected to isentropic compression with varying ramp times, reminiscent of impact by different thickness graded density impactors or functionally graded materials. The bcc phase fraction is found to increase with increasing ramp pressure, reaching an optimal value at 160 GPa. Furthermore, the pressure for persistence of bcc crystalline phase, PbccMax, can be extended beyond 600 GPa under ramp compression through suitable tailoring of piston velocity profile and target thickness, representing a substantial enhancement compared to 110 GPa limit observed for single-shock compression. A systematic investigation of the influence of different peak piston velocities and its rise time on bcc phase fraction and disorder atoms is presented. Finally, phase distributions arising from simulations using five different interatomic potentials are compared for selected piston velocities.