Segregation to surfaces: Dilute alloys of the transition metals

In the context of this paper, the term segregation refers to the inhomogeneous distribution of the components of an alloy. Under conditions of temperature and pressure where the bulk phase diagram shows the alloy to be in a single-phase region, the inhomogeneites in composition are associated with external surfaces and crystalline defects such as dislocations and internal boundaries. With multiphase alloys. the components will generally be partitioned unequally among the various phases; this can be predicted from the bulk phase diagram and is not considered under the general classification of segregation. Segregation may, however, be associated with interphase boundaries. For an understanding of the data to be presented later in this review, it is thus important to distinguish segregation from precipitation. The situation is somewhat similar to that encountered in adsorption on a surface from a gas phase: detectable accumulation on the surface of gas atoms or molecules may occur under pressure and temperature conditions where no bulk condensed phase is stable.