Development of microstructure in cast Mg–AI–rare earth alloys

Abstract

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, 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 Al12Mg17 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 Al12Mg17 precipitation took place. Colonies of discontinuous precipitation containing alternate lamellae of α-Mg and Al12Mg17 formed preferentially in regions α-Mg with high aluminium content. Spheroidisation and coarsening of the discontinuous precipitates occurred after aging at 200°C. Continuous precipitation of Al12Mg17 also occurred and these precipitates had a rodlike morphology and grew in preferred crystallographic directions.MST/3382