Surface segregation near the temperature of bulk phase separation: Incomplete wetting in Cu(Ag) alloys.

Abstract

Surface segregation in alloys should present drastic modifications near a bulk phase transition. In particular, when the phase diagram exhibits a miscibility gap, one can wonder to what extent surface segregation could be viewed as the first step towards bulk phase separation. We show, in the particular case of very dilute Cu(Ag) alloys, that the actual situation is even more complex. More precisely, using simultaneously an energetic model based on the electronic structure (tight-binding Ising model) and a mean-field approximation formulated as an area-preserving map, we found evidence of incomplete wetting, i.e., a finite succession of layering transitions from almost pure Cu to almost pure Ag planes when the bulk Ag concentration approaches the solubility limit. This theoretical result compares satisfactorily with the experimental surface segregation isotherms derived from kinetics studies using Auger-electron spectroscopy, which indeed exhibit, at least, the first (surface) layering transition. Moreover, the experimentally observed hysteresis between the segregation and dissolution isotherms can be interpreted by taking into account the dependence of the size effect with respect to the surface concentration.