Enhanced microstructural stability and softening resistance in AZ61–0.7Si alloy with Bi additions

The ultimate shear strength (USS), determined using the hot shear punch test (SPT), together with the hot indentation hardness ( H v) of the as-cast AZ61–0.7Si–(0.0, 0.25, 0.5, 1.0) wt% Bi alloys were comprehensively examined over a temperature range from 298 to 523 K. The thermal stability and resistance to softening of the alloys were assessed by the transition temperature and softening coefficient concepts, which were measured from the variations in the shear strength and hardness with testing temperature. The experimental findings indicated that the Bi-containing alloys exhibited superior thermal stability, softening resistance and high-temperature mechanical performance compared to the base alloy. This was evidenced by enhanced shear strength and hardness at elevated temperatures, the respective higher transition temperatures of 5.4 and 7.1%, together with the reduced softening coefficients of about 29% for the shear strength and hardness. The observed enhancements were attributed to the influence of Bi on refining the microstructural constituents and the creation of thermally stable Mg 3 Bi 2 intermetallic particles. These particles effectively stabilized the microstructure through hindering dislocation motion, thereby contributing to improved thermal stability and resistant to softening during deformation at high temperatures.