Understanding the phase transformation mechanisms that affect the dynamic response of Fe-based microstructures at the atomic scales

Large-scale molecular dynamics (MD) simulations were carried out to investigate the shock-induced evolution of microstructure in Fe-based systems comprising single-crystal and layered Cu/Fe alloys with a distribution of interfaces. The shock compression of pure single-crystal Fe oriented along [110] above a threshold pressure results in a BCC (alpha) & RARR; HCP (e) phase transformation behavior that generates a distribution of epsilon phase variants in the phase transformed region of the microstructure behind the shock front. The propagation of the release wave through a phase transformed epsilon phase causes a reverse e & RARR; alpha phase transformation and renders a distribution of twins for the [110] oriented Fe that serve as void nucleation sites during spall failure. The simulations reveal that the alpha & RARR; e & RARR; alpha transformation-induced twinning for shock loading along the [110] direction is due to a dominant e phase variant formed during compression that rotates on the arrival of the release wave followed by a reverse phase transformation to twins in the alpha phase. The modifications in the evolution of the e phase variants and twins in Fe behavior are also studied for Cu-Fe layered microstructures due to the shock wave interactions with the Cu/Fe interfaces using a newly constructed Cu-Fe alloy potential. The MD simulations suggest that interfaces affect the observed variants during shock compression and, hence, distributions of twins during shock release that affects the void nucleation stresses in the Fe phase of Cu/Fe microstructures.