  We study the influence of the liquid-vapor surface on the crystallization
kinetics of supercooled metal alloys. While a good glass former,
$\rm{Cu}_{50}\rm{Zr}_{50}$, shows no evidence of surface enhancement of
crystallization, $\rm{Ni}_{50}\rm{Al}_{50}$ 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 $\rm{Ni}_{50}\rm{Al}_{50}$
while the $\rm{Cu}_{50}\rm{Zr}_{50}$ 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.
