Anisotropic atomic motion at undercooled crystal/melt interfaces

Dynamics of crystal growth in pure metals is investigated as a function of undercooling using molecular dynamics computer simulations. For growth on (100) in fcc and (100) in bcc metals, we observe that the atomic mobility of atoms at the interface far exceeds that in the bulk liquid and that this difference grows with increasing undercooling. The higher mobility is associated with a small fraction of atoms undergoing long jumps. These long jumps, moreover, are anisotropic, showing enhancement along closed packed directions in the crystal. The lengths of the long jumps, however, are considerably smaller than interatomic distances. The results are interpreted using a defect model of crystallization.