Effect of heat treatment on microstructure of thixoformed AZ91D magnesium alloy

Microstructure and mechanical properties of a thixoformed AZ91D magnesium alloy in as-cast and heat treatment conditions were characterised by means of optical microscope (OM), scanning electron microscopy (SEM), XRD and hardness tests. The results show that the thixoformed AZ91D magnesium alloy exhibits a faster β phase dissolution rate than that in the high pressure die-cast alloy under solution treatment at 415 °C. Al-rich halos are observed at the early stage of solution treatment because Al diffuses from the β phase to the primary α-Mg phase. During aging at 216 °C, the thixoformed AZ91D magnesium alloy reaches its peak hardness at about 5 h while the high pressure die-cast alloy at 12 h. Both discontinuous and continuous precipitates of the β phase in the thixoformed alloy are observed.