The effect of exposure to elevated temperatures on the microstructure and hardness of Mg-Ca-Zn alloy

The die cast Mg-5 wt.% Ca-6 wt.% Zn ternary alloy was exposed to 160 °C for different times of up to 40 days. Microstructural analysis and microhardness and hardness measurements of the samples after different exposure times enabled the thermal stability of the cast alloy to be monitored. The as-cast structure is composed mainly of α-Mg solid solution, elliptical grain boundary particles of CaMg2, and an intergranular eutectics of Mg and Ca2Mg6Zn3. The grain size of α-Mg and the structure of the alloy did not change during the treatment, but the amount of the intergranular phases increased slightly. Exposure to 160 °C resulted in a decrease in micro-hardness of the α-Mg grains, but no change in the overall hardness of the samples was observed. Microstructural analyses indicated diffusion of the solute elements from the α-Mg grains to the grain boundaries, resulting in a decrease in hardness of the α-Mg which is, however, compensated by an increase in hardness related to intergranular precipitates. Using a simple diffusion model, the diffusion coefficient of Ca atoms in Mg at 160 °C was estimated to be 2.1 · 10 -17 m2 s- 1, which is greater than that of Zn by two orders of magnitude.