Revisiting the intermetallic phases in high-pressure die-cast Mg–4Al–4Ce and Mg–4Al–4La alloys

Mg–Al–RE based alloys such as AE42 (Mg–4Al–2RE) and AE44 (Mg–4Al–4RE) are commonly known to contain A111RE3 and Al2RE intermetallic phases, with the former being predominant. However, it was found recently that the predominant intermetallic phase in a gravity-cast Mg–4.5Al–6.3La alloy is not A111La3 but a new ternary phase with the composition of (Al,Mg)3La. In this work, the intermetallic phases in high-pressure die-cast Mg–4Al–4Ce and Mg–4Al–4La alloys are thus re-examined by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It is shown that the predominant intermetallic phase is A111Ce3 in the die-cast Mg–4Al–4Ce alloy but (Al,Mg)3La in the die-cast Mg–4Al–4La alloy. Both A111Ce3 and (Al,Mg)3La remain thermally stable upon annealing at 500 °C for 100 h and are most likely equilibrium phases in each alloy. This study provides strong evidence that the formation of intermetallic phases in Mg–Al–RE alloys is affected by the choice of individual RE elements.