AZ91 is a series of the most important die casting Mg alloys. The typical microstructures of the solidified AZ91 alloys usually consist of (Mg) grains and mainly the Al12Mg17 phase at the grain boundaries [Kabirian and Mahmudi, Metall Mater Trans A 40, 2190–2201 (2009) and Wang et al, Acta Mater. 54, 689–699 (2006)]. Similar microstructures are observed in other AZ series alloys [Wu et al, Trans. Nonferrous Met. Soc. China. 21, 784–789 (2011)].
The Scheil Solidification Simulation Calculator in Thermo-Calc can predict the phases formed during solidification under both rapid cooling (non-equilibrium) and slow cooling (equilibrium) conditions. The figure shows the total fraction of all solid phases as a function of temperature for AZ91, where the solid line corresponds to the non-equilibrium Scheil calculation and the dashed line to equilibrium. As can be seen, the calculation predicts the formation of three Al-Mn compounds, Al8Mn5, Al11Mn4, and Al4Mn, in addition to the experimentally observed phases, hcp_A3-(Mg) and Al12Mg17.