Computer simulation of the effect of coherency strain on cluster growth kinetics

To understand clustering behavior under the influence of a coherency strain, Monte Carlo simulations were carried out for both two-dimensional (2-D) square and three-dimensional (3-D) simple cubic lattices. In the Monte Carlo model, each solute was assumed to exert coherency stress owing to a tetragonal misfit strain and to have surface energy when in contact with solvent atoms. To account for the coherency strain of a cluster whose morphology continuously changes during aging, exact expressions for both the self-strain energy and elastic interaction term for rectangular parallelepipeds were derived. Strong elastic interactions among platelike clusters are shown to develop a stabilized structure with a tendency for bridging the clusters at a right angle. In the early stage of evolution, solute atoms were found to diffuse into regions of stress concentration. Morphological changes revealed step movements on the edge of a cluster, and some steps were moving in the direction of dissolution (rather than growth) for the cluster, thus displaying a dynamic nature of step movement. When an initial shape was an elastically unstable one, a large cluster was found to dissolve into its solid solution, while, in the same environment, a cluster of the same size with a stable morphology sustained growth. During dynamic evolution, some clusters showed concave, instead of convex, surfaces, even though the former are highly nonequilibrium shapes. This paper is based on a presentation made in the symposium “The Role of Ledges in Phase Transformations” presented as part of the 1989 Fall Meeting of TMS-MSD, October 1–5, 1989, in Indianapolis, IN, under the auspices of the Phase Transformations Committee of the Materials Science Division, ASM INTERNATIONAL.