Cluster-method study of surface effects in fcc binary alloys

Surface phenomena in binary alloys with face-centered-cubic structure undergoing a bulk first-order transition are studied within the tetrahedron approximation of the cluster-variation method. It is found that for $A_3$B (L$1_2$) ordered alloys with nearest-neighbor interactions, the (100) surface is weakly coupled to the bulk, disordering, in most cases, in a continuous manner. Results for the surface phase diagram and for the temperature dependence of the concentration and order parameters in the surface region are presented. The theory, as applied to ${\mathrm{Cu}}_3$Au, is found to be in very good agreement with recent experimental data.