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 degrees C, 200 degrees C, 250 degrees C and 300 degrees C, the precipitates in the microstructure of peak-aged alloy mainly consist of homogeneous fine beta '' phase, beta' phase, beta' and beta(1) phase and heterogeneous coarse beta(e) 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 degrees C, 200 degrees C and 250 degrees C, but poor hardening response at higher temperature of 300 degrees 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 degrees C further aged at high temperature of 250 degrees C (i.e. two-stage ageing treatment) leaded to that beta '' phase dissolved into the Mg matrix rather than transformed directly to beta' phase, which resulted in poor enhanced strength as compared to that of the peak-aged alloy at 250 degrees C. However, when the peak-aged alloy at high temperature of 250 degrees C was further heat treated at low temperature of 175 degrees C, it showed a further strengthening but embrittlement due to the secondary precipitation of beta '' and beta' phases. (C) 2015 Elsevier B.V. All rights reserved.