Binary alloy surface compositions from bulk alloy thermodynamic data

Abstract

The surface composition of binary alloys is determined by minimizing alloy surface free energy with respect to atom exchange between the surface and the bulk. The theory is developed using a pairwise bound model of the solid with a broken bond surface. Parametric predictions for the atom fraction in the first four atomic layers for ideal and regular alloy solutions are presented. The predictions are based on pure element vaporization enthalpies and alloy solution activity coefficients. The surface atom fraction is different from the bulk atom fraction for all bulk compositions. The alloy element with the lower heat of vaporization is enriched with respect to its bulk atom fraction. The effects of chemisorption of a foreign species and alloy particle size on surface composition are also quantified. The theory is in good agreement with existing data on nickel-copper surface compositions.