Precipitation behavior and mechanical performances of A356.2 alloy treated by Al–Sr–La composite refinement-modification agent

In this work, precipitation behavior and mechanical performances of A356.2 alloy (treated by Al–6Sr–7La composite refinement-modification agent) were studied for the first time. Artificial aging treatment (T6) and long-time natural aging treatment (>10,000 h, T4) were applied to clarify the precipitation and growth behavior of Mg5Si6 phase. The results show that vacancies and dislocations formed during solution treatment promote the nucleation and growth of Mg5Si6 phase in the eutectic area during the subsequent aging process. During the T6 treatment, the growth of Mg5Si6 phase is promoted due to the interaction of atomic diffusion channel caused by higher Si concentration gradients and dislocations, leading to a large number of uniform distributed spherical Mg5Si6 phase in eutectic area. While, for the T4 treated alloy, atomic clusters with short rod-like Mg5Si6 phase exists inside the alloy. In addition, T6 treatment resulted in significant improvement of tensile strength (UTS), yield strength (YS) and vickers hardness (HV) compared with the T4 treated alloy and the reasons were discussed.