Microstructure and mechanical properties of die-cast AZ91D magnesium alloy by Pr additions

Abstract

Mg-9Al- xPr ( x=0.4, 0.8 and 1.2, mass fraction, %) magnesium alloys were prepared by high-pressure die-casting technique. The effects of Pr on the microstructures of die-cast Mg-9Al based alloy were investigated by XRD and SEM. Needle-like Al 11Pr 3 phase and polygon Al 6Mn 6Pr phase are found in the microstructure. With 0.4% Pr addition, fine needle-like Al 11Pr 3 phase and a small amount of polygon Al 6Mn 6Pr phase near the grain boundary are found in the microstructure. Increasing Pr addition to 0.8%, lots of coarse needle-like Al 11Pr 3 phase within grain and polygon Al 6Mn 6Pr phase on grain boundary are observed. Further increasing Pr addition, the size of needle-like Al 11Pr 3 phase decreases, while the size of polygon Al 6Mn 6Pr relatively increases. The mass fraction of Pr at around 0.8% is considered to be suitable to obtain the optimal mechanical properties. The optimal mechanical properties are mainly resulted from grain boundary strengthening obtained by precipitates and solid solution.