Theory of surface effects in binary alloys. II. Segregating alloys

A theory of surface effects of segregating alloys is presented. It is based on a model consisting of pairwise interactions between nearest-neighbor atoms only. The entropy is calculated in the pair approximation following Kikuchi's method. An example of a (111) surface of a face-centered-cubic alloy is given. It is found that at temperatures lower than a segregation temperature, the equilibrium state consists of one phase in the interior, the second phase close to the surface, and a further enrichment of the surface layer in one of the component species. Molar fractions of the segregated phases and short-range - and long-range - order parameters as functions of temperature and nominal alloy concentration are shown.