Different structure transitions of rapidly supercooled Tantalum melts under pressures

Molecular dynamics (MD) simulations have been performed to study the effect of pressure (P) on the crystallization of Tantalum (Ta) under [0, 100] GPa. The average potential energy of atoms in the system, the pair distribution function and the largest standard cluster analysis (LSCA) have been employed to analyze the structural evolution. It is found that the solidified state at 100 K changes from the complex crystal (β-Ta) through the body-centered cubic (bcc) crystal (α-Ta) to the hexagonal close-packed (hcp) crystal with pressure increase. At P ≤ 3 GPa the favorable state is β-Ta that is composed of Z12, Z14 and Z15 atoms, and crystallization starts at the same temperature of crystallization (Tc=1897 K), whereas it is a stochastic relationship between the crystallinity and pressure. At P∈(3, 57.5] GPa, the melt is always crystalized into a rather perfect α-Ta, and Tc is nearly linear with pressure. While when P>57.5 GPa, a quite perfect bcc crystal firstly formed, then transform into a hcp crystal through a solid-solid (bcc-hcp) phase transition. If the new hcp atoms formed in the bcc stage are arranged in regular grains, the bcc-hcp transition will take a multiple­intermediate­state pathway; otherwise, a single-intermediate­state pathway. In addition, the parameter δs can well reflect the crystallinity of β-Ta and, the small the value of δs, the lower the potential of the β-Ta. Finally, during the bcc-hcp transition under high pressure, the volume reduction is due to the rearrangement of the atoms rather than the reduction of the atomic radius; a slight increase in the number of nearest neighbor pairs results in a significant increase in system energy.