Effect of high pressure on the formation and evolution of clusters during the rapid solidification of zirconium melts

The formation and evolution of clusters during the rapid solidification of liquid zirconium (Zr) under the pressure of 0-80 GPa were examined through molecular dynamics simulations. Local atomic structures were analyzed using pair-distribution function, Honeycutt-Anderson bond-pair index method, and developed cluster-type index method. Results revealed that Zr melts can be completely vitrified into a glassy state under 30-60 GPa. (13 1-1441 10-1551 2-1661) Kasper clusters, instead of (12 12-1551) icosahedra, dominate the formation of Zr metallic glasses. The numbers of Kasper clusters increase with increasing pressure. Under <= 20 GPa, the coexistence structures of the stable hexagonal close-packed (HCP) with meta-stable body-centered cubic (BCC) configurations are obtained with phase separation in the final solids. The fraction of BCC structures increases with increasing pressure. The evolution of basic clusters in the crystallization of Zr melts always takes the path of Kasper clusters -> (14 6-1441 8-1661) BCC -> (12 6-1421 6-1422) HCP. Crystallization under >= 70 GPa lacks an intermediate stage. Instead, the supercooled liquid (scl) Zr is directly transformed into A15 phase (scl -> A15), which is composed of (12 12-1551) icosahedra and (14 12-1551 2-1661) Kasper clusters. The A15 structure is perfected with increasing pressure. Competition between densification and atomic diffusion contributes to the nonmonotonic effect of high pressures on the rapid cooling of Zr melts. (C) 2017 Elsevier B.V. All rights reserved.