Atomic structure insight into crystallization of undercooled liquid metal Zr during isothermal relaxation processes

The crystallization of undercooled liquid zirconium (Zr) was investigated via molecular dynamics simulations. The atomic structures were characterised and analysed by means of pair distribution function, angular distribution function, and the largest standard cluster analysis. At low temperature (T = 1000 K), it is found that the crystallization begins at the initial stage of relaxation and takes the pathway of undercooled liquid (UCL)-> BCC -> HCP instead of UCL -> HCP. The HCP-type medium-range orders (MROs) are formed from the inside of the precursors formed by the BCC-type MROs, and growing at the cost of the precursors, in agreement with the Ostwald's step rule. The number of MROs is found to play a key role in forming the crystal phases in this process. However, at high temperature (T = 1200 K) the time-scale to reach crystallization is two orders of magnitude bigger than that of T = 1000 K. No intermediate stage of crystallization is observed. The undercooled Zr melts are directly transformed into BCC crystal phases. The max size of BCC-MROs is intimately correlated with the crystallization.