Effect of heat treatment on microstructure and hardness of Mg-5Sn-1Si alloy

Mg-5Sn-1Si (mass fraction,%) magnesium alloy was prepared by casting under argon atmosphere protection in electric-resistance furnace. The as-cast microstructure, the precipitated phases evolution and the relation between microstructure and hardness of the Mg-5Sn-1Si alloy heat-treated were investigated, the heat treatment processes were soild solution at 480°C, and then aging at 180°C and 280°C, respectively. The results show that the as-cast Mg-5Sn-1Si alloy mainly consists of dendrites α-Mg, eutectic Mg₂Sn phase and eutectic Mg₂Si phase. The Mg₂Sn phase is completely redissolved and the large block Mg₂Si phases get slightly globularity during the solid solution treatment at 480°C for 14 h. However, the Mg₂Si phases get globularity in the process of aging treatment. No Mg₂Sn phase precipitation is observed in the alloy aged at 180°C, the Mg-5Sn-1Si alloy exhibits the obvious aging hardening behavior, but showing lower hardness, it is only 24.1 HV when aging at 180°C for 24 h. On the contrary, the hardness increases obviously with Mg₂Sn phase dispersive precipitation when aging at 280°C, and the hardness peak value is 47.6 HV for the alloy aged at 280°C for 18 h, and there is over-aging with extension of aging treatment time. Precipitation free zones (PFZ) form at early stage of aging at 280°C, and the PFZ disappears with extension of aging time.