Thermodynamics, kinetics, and morphological evolution of phase separation in nanoscale thin films

The thermodynamics, kinetics, and morphological evolution of phase separation in nanoscale thin films are discussed and compared to those in the corresponding bulk systems. Cluster Variation Method (CVM) in the pair approximation is employed to determine the dependence of the miscibility gap as a function of film thickness in a model FCC binary alloy with nearest neighbor interactions. For a fixed temperature and under certain microstructural constraint, the mutual solubility of a binary alloy increase as the film thickness decreases, and it becomes totally miscible for every thin films. For studying the kinetics of phase separation and morphological evolution, a model two-dimensional square lattice with a second-neighbor interaction model is considered and microscopic master equations in the point or pair approximations are employed. It is found that the presence of surfaces in nanoscale thin films plays a dominant role in the kinetics of spinodal decomposition and the development of morphologies. It is shown that there is always a dominant concentration wave perpendicular to the surfaces, which produces anisotropic morphological patterns, such as distorted hexagonal precipitate lattices at relatively low-volume fractions and straight stripes parallel to a surface at high volume fractions, even though the interphase boundary energy is nearly isotropic and the effect of elastic strain energy is not considered.