SEGREGATION BEHAVIOR AT MOW(001) ALLOY SURFACES

The tight-binding Ising model has been used to determine equilibrium s egregation profiles at the (001) surface of MoW alloys. The descriptio n of the thermodynamic properties of such alloy surfaces is based on a cluster expansion in terms of correlation functions. The expansion co efficients are known as Effective Cluster Interactions and have been c omputed from their formal definition by means of the Direct Configurat ional Averaging technique. As observed experimentally strong Mo-segreg ation occurs at the surface, followed by a depletion of Mo in the seco nd layer. This alloy displays very remarkable behavior: the concentrat ion profile is found to be oscillatory whereas the bulk phase diagram of MoW exhibits phase separation.