Pressure waves induced by the bcc-hcp phase transition in dynamically loaded single crystal iron

Pressure contours in a space–time diagram extracted from the MD simulation of the ramp compression of a [001]-oriented iron single crystal, showing backward release and recompression waves (state 2) induced by the bcc-hcp phase transformation, pressure and velocity distributions at time 65 ps and snapshot of the atomic configuration at the same time with the hcp phase in pink. Molecular Dynamics simulations have been used to investigate the dynamic response of single crystal iron to ramp compression along the [001] crystallographic direction, with a focus on the coupling between the propagation and interaction of pressure waves and the phase transformation process. In particular, we report an original observation at the atomic level of release and recompression waves specifically induced by the phase transition. We provide a straightforward, physically-based explanation of the origin of these waves based on impedance mismatch between the parent, daughter, and mixed-phase region, and show how they depend directly on the kinetics of the transformation. This analysis may be generalized to other time-dependent phase transformations in other materials subjected to dynamic loading, and it is probably still valid at the much larger space and time scales encountered in experiments, which could have practical implications.