Surface cosegregation phenomena on bcc(001) alloy surfaces: The effect of nearest and next nearest M–X neighbor interactions

Abstract

In the present work we introduce a three-dimensional lattice gas model for the description of surface cosegregation phenomena on (001) oriented surfaces of multicomponent bcc alloys. It is assumed that the lattice consists of two types of lattice sites M and X. The metal sites M form a body-centered cubic lattice, whose quasioctahedral interstices constitute the nonmetal sublattice. In most cases lattices of 64×64×64 sites of the bcc structure (interstitials plus metal sites) are used. The M sites are accessible to either MA or MB atoms, while the nonmetal sites are either occupied by X atoms or remain empty. Pairwise repulsive nearest and attractive next nearest neighbor interactions between MA–X and MB–X atoms are considered as well as nearest and next nearest neighbor X–X repulsions. The Monte Carlo simulations are performed in the grand canonical ensemble. The simulations indicate that the cosegregation induced formation of the surface compound MBX is basically due to preferential next nearest MB–X neighbor attractions, φMBX(2). Surface compound formation causes an increase of the excess Gibbs free energies of segregation ΔGxsX and ΔGxsM. The Gibbs free energies of segregation can be estimated roughly via a mean field approximation. Upon increasing the strength of the preferential next nearest MB–X attractions surface compound formation is accompanied by a first order phase transition at low temperatures. Depending on the relative magnitude of the nearest neighbor M–X repulsions we observe a strong MB subsurface depletion [φMBX(1)<φMAX(1)<0] or enrichment [φMAX(1)<φMBX(1)<0], the latter has been verified by x-ray photoelectron diffraction (XPD) for the CrN surface compound on Fe–15%Cr–N (001).