Microstructure observation in T5 treated Al-Si-Mg system cast alloys

Al-Si-Mg alloys have good castability and high strength can obtained by aging heat treatment. There are some reports about the age-hardening behavior of these alloys. However, the effect of additive elements on the age-hardening behavior with microstructure evolution in Al-Si-Mg alloys is not totally clarified yet. In the previous work, the effect of T5 heat treatment for age-hardening behavior and microstructure evolution in AC4C cast alloy were investigated. Fine rod-like precipitates which were reported in Al-Mg-Si alloy were observed near eutectic Si phase in AC4C alloy. Rod-like precipitates became coarser toward the center of α-Al dendrite. In this work, high purity Al- Si-Mg alloys were fabricated, and the microstructure of the alloys in T5 condition was investigated by transmission electron microscopy (TEM). In Al-7%Si-0.4%Mg (mass%) alloy aged at 473K for 7.2ks, lots of needle-like precipitates, which were identified as β phase, were observed near the eutectic Si phase. On the other hand, rod-like precipitates and granular precipitates were observed far from the eutectic Si phase. Based on SAED patterns obtained from the rod-like precipitates and granular precipitates, Those precipitates were identified as Type-A precipitates and Si phase, respectively.