Chemical ordering and crystal nucleation at the liquid surface: A comparison of Cu50Zr50 and Ni50Al50 alloys.

We study the influence of the liquid-vapor surface on the crystallization kinetics of supercooled metal alloys. While a good glass former, Cu50Zr50, shows no evidence of surface enhancement of crystallization, Ni50Al50 exhibits an increased rate of crystallization due to heterogeneous nucleation at the free liquid surface. The difference in the compositional fluctuations at the interface is proposed as the explanation of the distinction between the two alloys. Specifically, we observe compositional ordering at the surface of Ni50Al50, while the Cu50Zr50 alloy only exhibits a diffuse adsorption of the Cu at the interface. We argue that the general difference in composition susceptibilities at planar surfaces represents an important factor in understanding the difference in the glass forming ability of the two alloys.