SEGREGATION DRIVING FORCES ON THE (111)FACE AND (110)FACE OF A CUAG ALLOY

Using a tight-binding Ising model (TBIM), the segregation energy on th e (111) face of a Cu-Ag 0.45 at% alloy and on the (110) face of a Cu-A g 0.30 at% alloy as a function of surface coverage, is derived from eq uilibrium segregation experiments. It is shown that a surface size eff ect term which depends on the square of the surface concentration has to be introduced in the segregation energy of the (111) face in order to take into account the surface strain energy which occurs in the fir st layer when the silver atoms segregate at the surface. On the contra ry, on the (110) face the segregation energy expression reveals that t he surface size effect does not depend on the silver surface concentra tion. Also, the effective surface pair interaction beta(s) can be dete rmined and related to the effective bulk pair interaction beta(b) thro ugh the relation beta(s) = 1.37 beta(b) for the (111) face and beta(s) = 1.61 beta(b) for the (110) face, which is in good agreement with th eoretical predictions.