Behavior of excess vacancies during the nucleation of precipitates in aluminum-silicon alloys

Abstract

Precipitation processes of vacancies and of silicon atoms were investigated by means of resistometric measurement and transmission electron microscopy, and the mechanism of vacancy enhanced nucleation of silicon precipitates was discussed. When a specimen quenched from 580°C and preaged to enhance the nucleation of silicon precipitates is aged at near 200°C, the resistivity decreases in four stages. In the first stage, dislocation loops are formed by condensation of vacancies. The loops disappear and silicon atom clusters become visible in the second stage where the resistivity decreases rather slowly. The third and fourth stages are due to precipitation of silicon atoms in a stable form. Nucleation of silicon atom clusters is considered to take place during the first stage. Growth and shrinkage of dislocation loops due to condensation and evaporation of vacancies during the initial stage of aging are considered important in the nucleation of silicon clusters. The possibility for silicon atoms to flow to dislocation loops together with excess vacancies is also discussed.