（1. School of Electronic and Electrical Engineering, Zhaoqing University, Zhaoqing 526061, China） (2. School of Materials Science and Engineering, Central South University, Changsha 410083, China)
Al-5.5Mg-0.4Mn-0.25Sc(wt%) alloy was casted by water-cooled copper die. The hardness variation of the as-cast alloy annealed at different temperatures was studied. The existence modality and formation mechanism of Al3Sc particles in the alloy were investigated by metallographic microscope and transmission electron microscope. The results show that 025wt% Sc does not contribute much to the refinement of as-cast grain structure, and the grain refinement comes from the effect of rapid solidification technology. At high cooling rate, it is difficult to find the presence of primary Al3Sc particles in the alloy. The annealing temperature has a significant effect on the hardness of the alloy. At low annealing temperatures, the hardness of the alloy changes slowly, and it takes a long time to appear the hardness peak. At higher annealing temperatures, the peak platform appears faster, but after this peak the hardness decreases rapidly as the annealing continues. Among these temperature tests, the hardness curve of 300 ℃ is outstanding, which has a higher hardness peak platform and maintains a long time without decreasing. The change of hardness during annealing is closely related to the precipitation of secondary Al3Sc particles. When the annealing temperature is low, the precipitation of secondary Al3Sc particles is insufficient and the particle size is small, and the pinning effect on grain boundaries, sub-crystalline boundaries and dislocations is weak; while the temperature is too high, Al3Sc particles coarsen and result in poor properties.