The effects of elastic misfit stresses on coarsening: Kinetics and microstructural evolution

The effects of elastic misfit stresses on coarsening kinetics and on microstructural evolution during coarsening were studied in γ-γ ′ alloys. Unlike most previous work in this field, methods were employed which did not require any a priori assumptions regarding the particle morphologies or spatial distributions.; The coarsening of γ′-particles in Ni-Al alloys was quantified by measuring the γ-γ′ interfacial area per volume, S_v, instead of a characteristic average particle size. It was found that the interfacial area per volume follows a t−1/3 rate law with no change in the exponent or the rate constant, even at long coarsening times when elastic energy is a significant contribution to the total energy of the system and the microstructure is not self-similar. The coarsening rate constant was found to vary with volume fraction as predicted by theory in the absence of elastic stress, in contrast to previous experimental results which quantify coarsening kinetics using an average particle radius.; We investigated the three-dimensional structure of a model γ-γ ′ alloy using serial sectioning and digital reconstruction. The γ′-particles were found to align in parallel two-dimensional sheets along the <100> crystallographic directions. The sheets in different regions of the microstructure are aligned along different <100> crystallographic directions. No evidence of particle coalescence was found despite the small interparticle separation distances within sheets.; A variety of particle morphologies were observed. These morphologies are not determined solely by particle size, indicating a significant departure from single-particle equilibrium morphologies. Surprisingly, smaller particles tended to be more non-equiaxed than larger particles. Among the non-equiaxed particles no particular particle morphology is strongly favored. The particle morphologies appear to be determined by a combination of kinetics and interparticle elastic interactions both within sheets of particles and at the intersections of sheets. The γ′-particle size distribution was computed, and was found to be somewhat broader and to have a lower maximum peak height than the theoretical prediction in the absence of elastic stress.