Effect of Bi, Sb, and Ca additions on the hot hardness and microstructure of cast Mg–5Sn alloy

The effects of 1–3 wt.% Bi, 0.15–0.70 wt.% Sb, and 0.7–2 wt.% Ca additions on the high temperature hardness of a cast Mg–5Sn alloy were studied in the temperature range 298–523 K. Results show that small amounts of Bi, Sb, and Ca additions increase the hardness of Mg–5Sn alloy at both room and elevated temperature. Compared to the base alloy, the ternary alloys can better retain their hardness at elevated temperatures, due to their more stable microstructures. This is attributed to the formation of thermally stable second phase particles, such as Mg3Bi2, Mg3Sb2, and CaMgSn in the Bi-, Sb-, and Ca-containing alloys, respectively. These particles strengthen both matrix and grain boundaries during high temperature deformation. It is shown that the Mg–5Sn–2Ca alloy exhibits the highest hot hardness values among all tested materials, due to a refined microstructure and the high volume fraction of the thermally stable CaMgSn phase. Hot hardness data were analyzed to calculate the transition temperature, and softening coefficient for different materials. The variation of these parameters with alloying element content was investigated and the optimum level of each element for achievement of the highest hot hardness was evaluated.