PHASE-FORMATION IN ION-BOMBARDED METALLIC-FILMS

The interplay of several events, ranging from production, migration and interaction of defects, to irradiation enhanced atomic diffusion and chemical mixing, is responsible for phase formation in surface layers of ion bombarded metallic alloys. The problem is so complicated that even the interpretation and the prediction of extreme cases such as the attainment of a crystalline, or a glassy product are presently beyond the possibilities of first principle approaches, and empirical criteria have been proposed to this end. In this work we limit ourselves to the very beginning of phase formation, i.e. the nucleation stage, in the frame of an atomistic model. In a binary alloy, after formation of collision cascades, the relaxation to metastable equilibrium of the locally altered compositional profile due to preferential migration to the cascade-matrix interface of one alloy component, is schematized by charge transfer events As a result, dimers of an effective alloy are formed. Conditions specific of glass and respectively crystal formation are extracted from an analysis of surface and thermochemical properties of starting and effective alloys.