Dissolution kinetics of grain boundary allotriomorphs

Abstract

Observations have been made of the dissolution kinetics of individual grain boundary allotriomorphs in the Al-Cu and Al-Ag systems using the electron probe microanalyzer. It is found that the length and width of the precipitates decrease exponentially with time. This behavior is explained on the basis of a model in which it is considered that impingement of the diffusion fields from adjacent precipitates takes place early in the dissolution process. The diffusion field in the solid solution surrounding an allotriomorph is thus characteristic of a finite system. The model gives values of the diffusion coefficient that are in adequate agreement with literature values. Experimental observations show that the axial ratio of the precipitates generally changes by less than 30% during dissolution. At high homologous temperatures, there is an increase in axial ratio, whilst at low temperatures the axial ratio remains essentially constant or decreases slightly. These variations in axial ratio depend only on the homologous temperature and are independent of the degree of supersaturation. At high homologous temperatures, it is proposed that impingement of the diffusion fields attenuates the point effect of diffusion at the tip of the precipitate. This attenuation causes a decrease in the dissolution rate of the tip and thus increases the axial ratio of the precipitate. At low homologous temperatures, it is proposed that grain boundary diffusion always allows the allotriomorph tip to dissolve preferentially, thus maintaining the axial ratio.