Surface segregation in binary alloys: a simple theoretical model

Abstract

An embedded cluster model of surface segregation of binary alloys is presented in which the dilute limit of a single solute atom in a host metal is considered. The model was applied to the test case of a Cu impurity in a Ni host for which the results indicate a strong preference of the Cu impurity for a surface site rather than a bulk site. Application was also made to the case of a Ni impurity in a Cu host which resulted in a preference of the Ni atom for a bulk site, thereby indicating solvent enrichment of the surfae for Ni-Cu alloys. The presence of the surface potential is found to be of paramount importance in the explanation of these results. A simplified model consisting of a single muffin tin interacting with an infinite barrier model of the surface is then presented in order to aid interpretation. The important factors relating to the segregation phenomena are identified as (i) the difference in number of d-electrons per atom of the two constituents forming the alloy and (ii) the difference in sizes of the two atoms which enters the calculation through their respective distances from the barrier. The model is applied to a number of systems and is found to give rather good agreement with experiment.