Alloy surfaces: segregation, reconstruction and phase transitions

Abstract Surface segregation in alloys, i.e., concentration modulation in the surface selvedge at thermodynamical equilibrium, can be viewed as resulting from two kinds of competition or synergy. The first one is between surface and bulk interactions, i.e., surface energy versus bulk alloying interactions, whereas the second one is between these chemical forces and the atomic size-mismatch. Modelling the phenomenon then requires to account for all these forces on the same level. This leads to use both realistic energetic models derived from electronic structure and efficient statistical tools, from mean–field approximation to Monte Carlo simulations. A particular attention must be payed to the possible multisolution character of the problem. Actually, the possible coexistence of stable and metastable solutions may be at the origin of superficial phase transitions, in which the surface acts as a precursor of bulk order–disorder transitions: layering transitions, concentration profile transitions, surface induced order or disorder. All these phenomena are illustrated here on some specific systems and analyzed with tools derived from tight-binding formalism. Finally, the strong coupling between surface segregation and atomic reconstructions is illustrated in the case of strong size-mismatch by means of molecular dynamics calculations.