Microscopic dynamics of structural transition in iron with a nanovoid under shock loading.

The shock-induced bcc (body-centered cubic) to hcp (hexagonal-closed packing) transition in iron containing a nanovoid was investigated by molecular dynamics simulations with a shock-front absorbing boundary condition. The results demonstrate the transition time induced by a nanovoid reduces exponentially with increasing shock pressure, which indicates a similar law to the recent experimental observations. Micromorphology evolution of hcp nuclei is presented by the atomic centrosymmetry parameter. A flaky growth pattern along the {111} planes is observed, while the system finally forms into a laminar structure along the {110} planes. Furthermore, the atomic mechanical path through the transition is analyzed in detail. It is found that the transformed atoms do cross a shear pressure barrier and then show an over-relaxation of pressure, while their potential increases to a much higher value than bcc atoms.