Development of microstructure in cast Mg-Al-rare earth alloys

The microstructures and age hardening behaviours of a series of Mg-Al-rare earth (RE) alloys that had been either pressure die cast or permanent mould cast were investigated by SEM and analytical TEM. Two types of phases, AlMM and AlMg, were found in the as cast alloys and no pseudoternary Mg-Al-RE phases were present. The AlMM phase was thermally stable during solution treatment at temperatures as high as 500°C, whereas AlMg partially dissolved in the α-Mg matrix during solution treatment at 420°C. No rare earth containing precipitates formed during heat treatment of the investigated alloys but two types of AlMg precipitation took place. Colonies of discontinuous precipitation containing alternate lamellae of α-Mg and AlMg formed preferentially in regions of α-Mg with high aluminium content. Spheroidisation and coarsening of the discontinuous precipitates occurred after aging at 200°C. Continuous precipitation of AlMg also occurred and these precipitates had a rodlike morphology and grew in preferred crystallographic directions. MST/338