SPINODAL DECOMPOSITION AND PATTERN-FORMATION NEAR A CRYSTALLINE SURFACE

The kinetics of spinodal decomposition and morphological evolution nea r a crystalline surface (an edge in two dimensions) were investigated by microscopic master equations in both the point and pair approximati ons and a second-neighbor interaction model in a two-dimensional model system. It is shown that, in the presence of a surface, spinodal deco mposition initially involves surface segregation, followed by anisotro pic decomposition in the near-surface region, with subsequent isotropi c decomposition in the bulk. It is demonstrated that, due to segregati on, a surface spinodal decomposition may take place for alloys whose o verall average compositions are outside the bulk spinodal. It is found that the presence of a surface results in a dominant concentration wa ve, which produces interesting transient morphological patterns such a s distorted hexagonal precipitate lattices for relatively low-volume f ractions and straight stripes at high-volume fractions in the near-sur face region. The effect of pair correlations on the kinetics of spinod al decomposition and morphologies was studied.