Microscopic master equation approach to diffusional transformations in inhomogeneous systems-single-site approximation and direct exchange mechanism

A computer simulation technique based on microscopic master equations is developed for modeling the dynamics of morphological evolution during diffusional phase transformations in binary solid solutions including barrierless nucleation of ordered domains, subsequent domain growth and coalescence, coarsening of antiphase domains, compositional phase separation, Ostwald ripening, and kinetics of simultaneous ordering and phase separation. Assuming a direct exchange mechanism for atomic diffusion and using the single-site approximation, the kinetic equations produce equilibrium states closer to the Bethe approximation than the Bragg-Williams approximation. Computer simulation examples of microstructural evolution during ordering, spinodal decomposition, and simultaneous ordering and phase separation in a binary solid solution are presented using a second-neighbor interaction model.