  Molecular dynamics simulations are performed to investigate the structural
phase transition in body-centered cubic (bcc) single crystal iron under high
strain rate loading. We study the nucleation and growth of the
hexagonal-close-packed (hcp) and face-centered-cubic (fcc) phases, and their
crystal orientation dependence. Results reveal that the transition pressures
are less dependent on the crystal orientations ($\mathtt{\sim}$14 GPa for
loading along [001], [011], and [111] directions). However, the pressure
interval of mixed phase for [011] loading is much shorter than loading along
other orientations. And the temperature increased amplitude for [001] loading
is evidently lower than other orientations. The hcp/fcc nucleation process is
presented by the topological medium-range-order analysis. For loading along
[001] direction, we find that the hcp structure occurs firstly and grows into
laminar morphology in the (011)$_{\text{bcc}}$ planes with a little fcc atoms
as intermediate structure. For loading along [011] and [111] directions, both
the hcp and fcc structures nucleation and growth along the
\{110\}$_{\text{bcc}}$ planes are observed, whose morphology is also discussed.
