The effect of Al8Mn5 intermetallic particles on grain size of as-cast Mg–Al–Zn AZ91D alloy

Al8Mn5 intermetallic particles are usually present in most of commercial Mg–Al based alloys. In recent years, the ability for the Al8Mn5 particles to act as potent nucleation sites for the α-Mg grains during the solidification of the Mg–Al alloys has been the subject of debate. In this work, the effect of Al8Mn5 particles on grain size of an Mg–9Al–1Zn (AZ91D) alloy was assessed. The experimental result from two AZ91D alloys with different Mn concentrations, i.e., 0.02 wt.% and 0.22 wt.% respectively, indicated that the grain size did not show any notable change. Further, high resolution transmission electron microscopy (HRTEM) was conducted on samples with concentrated Al8Mn5 particles collected by pressurised filtration of the AZ91D alloy melt. The extensive TEM examination on the Al8Mn5/α-Mg interfaces revealed no crystallographic orientation relationship (OR) between the Al8Mn5 and α-Mg crystals. It is therefore concluded that Al8Mn5 particles are unlikely to act as effective nucleation sites for the α-Mg during solidification of Mg–Al based alloys.