Comparative study of crystallization process in metallic melts using ab initio molecular dynamics simulations.

ABSTRACT

The crystallization process of liquid metals is studied using ab initio molecular dynamics simulations. The evolution of short-range order during quenching in Pb and Zn liquids is compared with body-centered cubic (bcc) Nb and V, and hexagonal closed-packed (hcp) Mg. We found that the fraction and type of the short-range order depends on the system under consideration, in which the icosahedral symmetry seems to dominate in the body-centered cubic metals. Although the local atomic structures in stable liquids are similar, liquid hcp-like Zn, bcc-like Nb and V can be deeply supercooled far below its melting point before crystallization while the supercooled temperature range in liquid Pb is limited. Further investigations into the nucleation process reveal the process of polymorph selection. In the body-centered cubic systems, the polymorph selection occurs in the supercooled state before the nucleation is initiated, while in the closed-packed systems it starts at the time of onset of crystallization. Atoms with bcc-like lattices in all studied supercooled liquids are always detected before the polymorph selection. It is also found that the bond orientational ordering is strongly correlated with the crystallization process in supercooled Zn and Pb liquids.