Microstructures and mechanical properties of a high pressure die-cast Mg–4Al−4Gd−0.3Mn alloy

A new high pressure die-cast (HPDC) magnesium (Mg) alloy with high performance, namely Mg–4Al–4Gd–0.3Mn (AGd44), was developed by using the heavy rare earth element Gd. The microstructures under as-cast and peak-aged states were investigated in detail, especially by transmission electron microscopy (TEM). The studied alloy has finer cells although clearly less intermetallic phases than the conventional Mg–Al–RE based alloys. However, the structures of intermetallic phases in AGd44 alloys are fairly complicated. Most of the blocky phases are Al2Gd while minor of them are Al10Gd2Mn7 and Al8GdMn4, and the petaloid and the lamellae phases are Al2Gd and Mg17Al12, respectively. Peak-aging treatment can further enhance the yield strength of the studied alloy, which is mainly attributed to more coarse lamellae Mg17Al12 phase and fine lath-shaped Mg17Al12 precipitate appearing at cell boundaries and in α-Mg cells, respectively. Furthermore, the relatively smaller cells as well as the novel intermetallic phase component contribute to the more excellent strength-ductility balance of the studied alloy than the commercial/experimental HPDC Mg–Al-based alloys. The results provide new insight into developing HPDC Mg alloys with high mechanical properties.