Atomistic modeling of phase transformations: Point-defect concentrations and the time-scale problem

Abstract

The time scale of diffusive phase transformations in alloys depends on point-defect concentrations, which evolve with the microstructure. We present a simple method that provides a physical time scale for atomistic simulations of such transformations, even when performed with constant point-defect numbers. It also gives an on-the-fly evaluation of the real point-defect concentration, when equilibrium conditions are fulfilled. The method is applied to kinetic Monte Carlo simulations of precipitation in binary alloys occurring by vacancy diffusion. The vacancy concentration is found to be very dependent on the difference in formation energy between the matrix and the precipitates, and therefore on the composition and volume fraction of these two phases. The effect of the interface curvature, through a Gibbs-Thomson effect, is revealed. A mean-field approximation is also developed for computing the point-defect concentrations. Contrary to previous models, it takes into account the short range order in nonideal and concentrated solutions. Atomistic simulations and mean-field simulations are validated by direct comparisons.