Transient growth of second phases during solution treatment

Abstract

This study shows experimentally that, in the initial stages of a solution treatment process, an unstable phase may grow prior to dissolving if the diffusion flux in the unstable phase is larger than that in the stable matrix. The effect was demonstrated using diffusion couples fabricated from thin, multiple layers of α and β brass such that the mean compositions of the couples relative to the α-β phase equilibrium would result in solution of the β phase. Since at the diffusion temperature of 870°C the interdiffusion coefficient of the β phase is about twenty times that of the α phase, the initial β phase flux was much greater than that of the α phase. Metallographic observations of two sets of diffusion couples, each having a different mean composition, as a function of time of interdiffusion revealed that approximately 22 pct growth of the β phase preceded its ultimate dissolution. The analysis of this phenomenon was verified by reasonably good agreement between measured phase thickness values as a function of time and calculated values based upon a numerical solution of the appropriate multiphase diffusion problem.