Shape changes during dissolution of θ -CuAl₂

Abstract

A theoretical study has been made of the shape change of precipitates dissolving under volume diffusion control. It is shown that square and similarly shaped precipitates become circular during dissolution at a rate controlled by the saturation conditions. Shape change is little affected by diffusion fields from adjacent precipitates. Experimental measurements were made of dissolution of the θ phase in aluminum-copper alloys. These had shapes that could be approximated to distorted spheres. It was found that the precipitates dissolved at a rate controlled by volume diffusion and that the precipitates became spherical by the time their radius had decreased 50 pct as indicated by the theory. Both surface and bulk precipitates were studied. There was a much higher density of surface precipitates due to preferential nucleation. However, their dissolution characteristics were the same as bulk precipitates.