The interplay of surface segregation and atomic order in alloys

Abstract

Surface segregation and related phenomena in alloys with long-range order (LRO) or short-range order (SRO) are reviewed with emphasis on prominent deviations from the well-known behavior of random solid-solutions. Issues concerning the competition between segregation and ordering tendencies, such as segregation suppression manifested in peaked equilibrium–segregation vs. temperature curves, on the one hand, and the disruption of near-surface LRO, on the other hand, are addressed in some detail. The pertinent roles of segregation entropy and segregation energy are considered for the cases of endothermic and exothermic processes. Besides surface-induced disorder (SID), effects of segregation on surface order–disorder transitions that can promote surface-induced order (SIO) are discussed. Several theoretical treatments based on the Ising model Hamiltonian energetics and on a number of statistical-mechanical approximations, including the Bragg–Williams, the cluster variation and the free-energy expansion methods, as well as Monte-Carlo simulations, are introduced and compared to experimental results reported mainly during the last decade. Segregation/ordering phenomena in thin films and nano-particles are addressed as well.