Spinodal decomposition and pattern formation near a crystalline surface

The kinetics of spinodal decomposition and morphological evolution near a crystalline surface (an edge in two dimensions) were investigated by microscopic master equations in both the point and pair approximations and a second-neighbor interaction model in a two-dimensional model system. It is shown that, in the presence of a surface, spinodal decomposition initially involves surface segregation, followed by anisotropic decomposition in the near-surface region, with subsequent isotropic decomposition in the bulk. It is demonstrated that, due to segregation, a surface spinodal decomposition may take place for alloys whose overall average compositions are outside the bulk spinodal. It is found that the presence of a surface results in a dominant concentration wave, which produces interesting transient morphological patterns such as distorted hexagonal precipitate lattices for relatively low-volume fractions and straight stripes at high-volume fractions in the near-surface region. The effect of pair correlations on the kinetics of spinodal decomposition and morphologies was studied.