Finite-size effects on the phase diagrams of binary alloy films

The thermodynamics of ordering binary alloys with film geometry is studied for the case of symmetry-preserving surfaces. In particular, we analyze the dependence of the phase diagrams on film thickness for the case of vanishing surface fields (ordinary transitions). We focus on the interplay between ordering and segregation and its effects on the phase equilibria of b.c.c.(1 1 0) films as well as on the low-temperature behavior of the critical curve. At T = 0 the critical curve of a b.c.c. ordering binaryalloy film ends at a finite concentration. It is shown that it is useful to interpret the zero temperature transitions as percolation-like transitions. Calculations by the cluster variation method in the pair and tetrahedron approximations are presented.