Effects of heat treatment on the precipitates and mechanical properties of sand-cast Mg-4Y-2.3Nd-1Gd-0.6Zr magnesium alloy

The effects of single and two-stage ageing treatments, on the kind, size and density of precipitates of a sand-cast commercial Mg-4Y-2.3Nd-1Gd-0.6Zr (wt%) (WE43) alloy were investigated and the relationship between precipitates and mechanical properties was discussed. For the single-stage ageing at 175 °C, 200 °C, 250 °C and 300 °C, the precipitates in the microstructure of peak-aged alloy mainly consist of homogeneous fine β″ phase, β′ phase, β′ and β1 phase and heterogeneous coarse βe phase, respectively. As the ageing temperature increases, the precipitates in the peak-aged alloy grew large, leading to the decrease of their densities, but increase in their aspect ratios (width/thickness). The alloy showed remarkable age-hardening responses and improvement of yield strength and ultimate tensile strength at 175 °C, 200 °C and 250 °C, but poor hardening response at higher temperature of 300 °C. It was revealed that the kind, density of precipitate and its aspect ratio were critical to precipitation strengthening effect. The peak-aged alloy at low temperature of 175 °C further aged at high temperature of 250 °C (i.e. two-stage ageing treatment) leaded to that β″ phase dissolved into the Mg matrix rather than transformed directly to β′ phase, which resulted in poor enhanced strength as compared to that of the peak-aged alloy at 250 °C. However, when the peak-aged alloy at high temperature of 250 °C was further heat treated at low temperature of 175 °C, it showed a further strengthening but embrittlement due to the secondary precipitation of β″ and β′ phases.