Segregation and phase separation in nanocrystalline materials

Surface segregation has been calculated in a slab of thickness d. In ordering AB systems there is a competition between the surface segregation of A atoms and bulk ordering, and either a surface disturbed region or an antiphase boundary can be formed inside the slab. With decreasing d the average order can decrease well below the bulk value. In phase separating systems with strong segregation a phase separation from the surfaces develops and two layers of the A-rich phase cover the A-poor phase in the slab (lamellar structure) and there is an enrichment of A atoms in the near surface layers of the upper phase. For weakly segregating systems if T is below a certain temperature, T-h, A-rich and A-poor phases can be in equilibrium in a columnar structure. With decreasing d there is an increased as well as a decreased solubility in A-poor and in A-rich phases, respectively. It was shown that a segregation stabilisation of the nanostructure is possible and with increasing d there is a sharp surface phase transition from a large to a small coverage. (C) 1997 Acta Metallurgica Inc.