In the present study, an Al-0.5%Mg2Si alloy was fabricated, and the precipitation sequence was examined through a comparison between a wrought material subjected to solution treatment and aging after processing and a cast material aged directly after casting. Since the Al-7%Si-0.3%Mg alloy contains as much as 7% Si, the excess Si is expected to exert a significant influence on the precipitation sequence. In this study, the Al-0.5%Mg2Si alloy was used as the base composition, and the effect of increasing Si content on precipitation behavior was examined. Age-hardening curves indicated that the hardness increased with aging time and decreased after peak aging. In the Al-0.5%Mg2Si alloy, all precipitates were identified as the β′ phase, whereas β″ phase precipitation occurred with increasing Si content. In Si-added alloys, precipitates formed on dislocations exhibited a characteristic bright-spot arrangement similar to that observed in deformed Al-Mg-Si alloys. The growth of these dislocation-related precipitates was suppressed, and at the peak-aged condition, they were obscured by β″ phases formed in the matrix. The presence of Si particles was found to be associated with the observation of coarser β″ phases in the proximity of the particles, as compared to their distribution within the matrix.
TSUCHIYA et al. (Sun,) studied this question.