Intermediate structural evolution preceding growing BCC crystal interface in deeply undercooled monatomic metallic liquids

We analyze the structural evolution at the arriving crystal/liquid interface in deeply undercooled elemental metallic liquids. Specifically, we use molecular dynamics simulations to monitor the intermediate dumbbell-interstitial-like pairs (DILPs) preceding the growing BCC crystal. This new defect-mediated approach has enabled us to examine in detail the population and interconnection, as well as the energetics and dynamics, of the DILPs before they join the crystal lattice. Under large undercooling the transition of the DILP motifs into the expanding crystal is found to require cooperative and collective rearrangement and hence thermal activation with an appreciable energy barrier. In comparison, the intermediate structural motifs in FCC-forming cases are primarily in the form of 1D chains, which can be incorporated into the growing lattice via easy slide. These differences in the transitional motifs and associated activation energy barrier offer a perspective from structural evolution standpoint to explain the very different crystal growth speed for BCC and FCC metals in deeply undercooled liquids. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Automated summary

This study analyzed the structural evolution at the crystal/liquid interface in deeply undercooled elemental metallic liquids, focusing on the transition of DILPs preceding the growing BCC crystal and the differences between BCC and FCC forming cases. It was found that under large undercooling, the transition of DILP motifs into the expanding crystal requires cooperative and collective rearrangement with an appreciable energy barrier, while the intermediate structural motifs in FCC-forming cases primarily exist in the form of 1D chains. These differences in transitional motifs and associated activation energy barrier provide insight into the varying crystal growth speeds of BCC and FCC metals in deeply undercooled liquids.