The microstructure evolution with lamellar 14H-type LPSO structure in an Mg96.5Gd2.5Zn1 alloy during solid solution heat treatment at 773 K

In an as-cast Mg96.5Gd2.5Zn1 alloy, by optical microscope (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations, the as-cast microstructure was confirmed to be composed of the α′-Mg solid solution, and the coherent fine-lamellae and the eutectic, in which the second phase is the β-phase ((Mg, Zn)3Gd) with a fcc structure. While, the coherent fine-lamellae consist of the 2H-Mg and the 14H-type long period stacking ordered (LPSO) structure formed in α′-Mg matrix during solidification. The 14H-type LPSO structure has first been observed in as-cast Mg–Gd–Zn alloys. Moreover, during subsequent solid solution heat treatment at 773 K for 0.5–45 h, the microstructure evolutions, especially the lamellae with 14H-type LPSO structure and the β-phase were studied. It was concluded that a lamellar X-phase with a 14H-type LPSO structure can be transformed from the β-phase with the fcc structure at 773 K in addition to the 14H-type LPSO structure in the matrix. The lamellar X-phase with 14H-type LPSO structure has first been issued in Mg–Gd–Zn alloys.