Molecular dynamics study on microstructural evolution during crystallization of rapidly supercooled zirconium melts

YF Mo and ZA Tian and RS Liu and ZY Hou and LL Zhou and P Peng and HT Zhang and YC Liang, JOURNAL OF ALLOYS AND COMPOUNDS, 688, 654-665 (2016).

DOI: 10.1016/j.jallcom.2016.07.221

Molecular dynamics simulations have been performed to explore the crystallization mechanism under rapid cooling for Zirconium that has body-centred cubic (bcc) and hexagonal close-packed (hcp) phases separated at 1136 K. The structural evolution was analysed in terms of the system energy, the pair (and angular) distribution function, and the largest standard cluster analysis. It is found that the critical cooling rate (gamma(c)) for vitrification is about 5.0 x 10(13) K/s, and the Ostwald's step rule is applied to Zr. Crystallization always takes the pathway of supercooled-liquid (scl) -> bcc -> hcp, determined by the competition between thermodynamics and kinetics at different conditions. Particularly with gamma decrease both the onset temperature of scl -> bcc and the maximal number of bcc atoms increase gamma while both the onset temperature of the bcc -> hcp transition and the rate of different crystalline atoms in the final solids are randomly. A rather perfect bcc phase can be obtained at gamma < 1.0 x 10(12) K/s, however, it is unstable and always transforms to the stable hcp phase under all conditions examined here. In the final solids, most of face-centred cubic (fcc) structures act as the hcp crystalline boundaries not comprises individual fcc phase. These findings are useful for understanding the crystallization of metals, especially for those that have a high-temperature-stabled phase. (C) 2016 Elsevier B.V. All rights reserved.

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