Towards high strength cast Mg-RE based alloys: Phase diagrams and strengthening mechanisms

Mg-rare earth (RE) based systems provide several important commercial alloys and many alloy development opportunities for high strength applications, especially in aerospace and defense industries. The phase diagrams, microstructure, and strengthening mechanisms of these multicomponent systems are very complex and often not well understood in literature. We have calculated phase diagrams of important binary, ternary, and multicomponent RE-containing alloy systems, using CALPHAD (CALculation of PHAse Diagrams). Based on these phase diagrams, this paper offers a critical overview on phase equilibria and strengthening mechanisms in these alloy systems, including precipitation, long period stacking order (LPSO), and other intermetallic phases. This review also summarized several promising Mg-RE based cast alloys in comparison with commercial WE54 and WE43 alloys; and explored new strategies for future alloy development for high strength applications. It is pointed out that the combination of precipitation and LPSO phases can lead to superior strength and ductility in Mg-RE based cast alloys. The precipitates and LPSO phases can form a complex three-dimensional network that effectively impedes dislocation motion on the basal and non-basal planes. The LPSO phases can also prevent the coarsening of precipitates when they interact, thus providing good thermal stability at elevated temperatures. Future research is needed to determine how the combination of these two types of phases can be used in alloy design and industrial scale applications.