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.

Dynamic properties of structural transition in iron under uniaxial compression.

By using molecular dynamics simulations, we have successfully simulated the bcc [Formula: see text] hcp structural transition in single-crystal iron under isothermal compression along the [001] direction. The results reveal a distinct softening of C(33) and a hardening of C(31) (or C(32)) prior to the transition and an over-relaxation of the stress after transition. Above the critical stress the morphology evolution of structural transition is analyzed, which can be divided into four stages: hcp homogeneously nucleated, columnar grains formed, nuclei competed and merged, and a laminar structure formed along {110} planes. Besides, our simulations demonstrate that in mixed phases the hcp phase has negative shear stress and the potential of the hcp phase is higher than the bcc phase, and the shear stress of the system keeps a linear decrease with hcp mass fraction. The effect of temperature on the structural transition is also discussed.