SURFACE SEGREGATION AND BULK ORDER-DISORDER TRANSITION IN FCC ALLOYS WITH A (100) SURFACE

From a theoretical point of view, bulk phase transitions in ordering alloys are discussed by investigating the concentration and the long-range order parameter on each layer parallel to the surface in detail. As an example, we took fee binary alloys with a (100) surface. Surface segregation and layer order-disorder transitions are investigated for all bulk phases found in fee alloys. With decreasing the temperature above a first-order phase transition temperature, an ordered structure approximately identical to the one found in the lower temperature phase appears in a surface region consisting of the first several layers, and just at the transition temperature, the lower temperature phase is realized near the surface and the higher temperature phase remains deep inside the bulk crystal. This means that the system has become a mixture of the lower and the higher temperature phases. Our theory is developed within the Bragg-Williams approximation by taking into account the first and the second neighbor pairwise interactions.