Atomistic investigation of structural phase transformation in single-crystal aluminum under double-shock compression and release

Aluminum (Al) is widely used in high energy density systems due to its structural stability and well-characterized equation of state. Although solid-to-solid phase transitions (PTs) in ambient fcc-Al under hydrostatic compression have been extensively studied, their response to dynamic compression remains less explored. Recent large-scale non-equilibrium molecular dynamics simulations in single-crystal Al show that shock loading can induce hcp and bcc phases at pressures nearly an order of magnitude lower than those required under static compression. For [001]-oriented Al, single-shock (SS) compression limits bcc persistence to 110 GPa, whereas isentropic compression extends it beyond 600 GPa. Here, we investigate solid-to-solid PT under double-shock (DS) loading, providing an intermediate and tunable pressure–temperature compression path. Two DS combinations are considered based on the relative strengths of plastic waves: EP-P, with the first shock in the elastic–plastic two-wave regime and the second in pure-plastic region, and P-P, with both shocks in pure-plastic regime. It is shown that EP-P DS loading enhances bcc phase fraction, while P-P DS extends the bcc-persistence limit beyond 200 GPa, substantially exceeding SS limit. Combined with our previous SS and isentropic-compression studies, DS results demonstrate a strong loading-path dependence of bcc persistence, with the maximum pressure for bcc formation increasing progressively from SS to DS and ramp compression. The broader pressure range for bcc transition in DS approach arises from compression with minimum additional heating, thereby suppressing amorphization. Further, our study demonstrates that both SS and DS loading of [110]-Al induce fcc-to-bcc PT during shock release, with DS enhancing the bcc transformation.