AbstractThe 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, Al4MM and Al12Mg17, were found in the as cast alloys and no pseudoternary Mg–Al–RE phases were present. The Al4MM phase was thermally stable during solution treatment at temperatures as high as 500°C, whereas Al12Mg17partially dissolved in theα-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 Al12Mg17precipitation took place. Colonies of discontinuous precipitation containing alternate lamellae ofα-Mg and Al12Mg17formed preferentially in regionsα-Mg with high aluminium content. Spheroidisation and coarsening of the discontinuous precipitates occurred after aging at 200°C. Continuous precipitation of Al12Mg17also occurred and these precipitates had a rodlike morphology and grew in preferred crystallographic directions.MST/3382