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

Two alloys of Mg-5wt%Ca-6wt%Zn (MCZ) and Mg-5wt%Ca6wt%Zn-2wt%Si (MCZS) were cast into a steel mold and then exposed to 160degreesC for up to 40 days. The dependences of microstructure, thermal behavior, microhardness, and hardness on the exposure time were determined. This enabled us to monitor the thermal stability of the cast alloys. The as-cast structure of MCZ alloy is composed mainly of alpha-Mg solid solution, while in the grain boundaries large (2mum dia.) elliptical precipitates of CaMg2 and a eutectic structure of Mg and Ca2Mg6Zn3 were found. CaMg2 precipitates do not appear in as-cast MCZS, whereas coarse CaMgSi grains are scattered between the matrix grains. The a-Mg grain size in both alloys was approximately 10mum. The structure of both alloys did not change during exposure to the elevated temperature, although a small increase of the amount of the inter-granular phases was observed. As-cast MCZ alloy contains fewer inter-granular precipitates, but the relative change in their amount during exposure to elevated temperature is higher than in MCZS. Exposure to 160degreesC resulted in a decrease of micro-hardness of alpha-Mg grains in both alloys, but no change in the overall hardness of MCZ alloy was observed during this time, in contrast to the age-hardening behavior of MCZS alloy. MCZ alloy is harder than MCZS alloy after all exposure times. Based on thermodynamic calculations utilizing the Thermo-Calc software package the variations in composition-microstructure-hardness were studied, and the effect of Si content in MCZS alloy on the microstructure was evaluated.