Role of Coherency Strain in Microstructural Development

Abstract

With a purely dilatational misfit strain, an elastically-soft, coherent precipitate imbedded in an infinite matrix tends to have a plate-like equilibrium morphology, whereas a hard particle tends to take on a round shape of high symmetry. Shape evolution proceeds through dynamic activities of coherency-induced interfacial waves. These interfacial waves seem to be responsible for the protrusions often observed along elastically hard directions in coherent particles of nickel-based superalloys. Soft particles with a positive misfit strain become plates perpendicular to an applied tensile stress, while hard particles elongate along the stress direction. If the elastic interaction between the applied stress and the coherency strain is strong enough, soft precipitates often split into smaller particles and then follow coarsening. If the applied stress increases further, coherent particles tend to dissolve into the matrix - in agreement with the theory of coherent phase equilibria. As expected, a coherent particle with a positive misfit strain migrates to the tension region of an edge dislocation, whereas a particle with a negative strain diffuses to the region of compression. Morphological change is, however, caused by the dislocation as the particle tries to capitalize on the dislocation stress field. The results are analyzed by means of a discrete atom method (DAM), which is predicated upon Hookean atomic interactions and Monte Carlo diffusion under the condition of a plane strain, a pureiy dilatational misfit strain, and no dislocation climbing.