Surface layering-induced crystallization of Ni–Si alloy drops

The crystallization of Ni90Si10 drops has been studied using molecular dynamics simulations. The atoms in the surface monolayer of the drop prematurely pack into a long-range ordered structure prior to volume crystallization, whereas diffusion dynamics maintains characteristic liquid-like features until the onset of volume crystallization. This surface crystallization process arises from strong density layering in the direction perpendicular to the drop surface. Due to density oscillations induced by the surface layering a high density state is produced in the drop surface, which is analogous to the effect of high pressure and locally increases the crystallization temperature of the surface layer, ultimately initiating crystallization from the surface. Such a layered structure causes pressure oscillation near the surface that produce a decrease in surface tension with decreasing temperature. The high density crystalline nature of the surface does not thermodynamically favor volume crystallization, which occurs separately via homogeneous nucleation in the interior with further decreasing temperature.