Magnesium

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.