THERMODYNAMICALLY-CONTROLLED PROCESSES ON MULTICOMPONENT ALLOY SURFACES

The chemical composition of alloy surfaces is subject of segregation p rocesses at elevated temperatures. In multicomponent alloy systems the joint surface enrichment of different solute atoms (cosegregation) is possible. Under thermodynamic control, i.e. when the temperature is s ufficiently high to allow rapid bulk diffusion of the components, cose gregation may cause the formation of thermodynamically stable two-dime nsional chemical compounds (surface compounds), which are epitaxially arranged on alloy surfaces of suitable structure and orientation. The cosegregation-induced surface confined formation of two-dimensional ch emical compounds must be distinguished from the precipitation of three -dimensional compounds at surfaces (surface precipitates). According t o the Gibbs phase rule, three-dimensional precipitates can be formed u pon oversaturation in coexistence with the saturated solid solution. I n this contribution the cosegregation-induced growth of two- and three -dimensional surface compounds and surface precipitates on several mul ticomponent alloy single crystals will be described and discussed.