Formation of 14H-type long period stacking ordered structure in the as-cast and solid solution treated Mg-Gd-Zn-Zr alloys

The coherent fine lamellae consisting of the 2H-Mg and the 14H-type long period stacking ordered (LPSO) structure within alpha'-Mg matrix have been first observed in an as-cast Mg96.32Gd2.5Zn1Zr0.18 alloy. During subsequent solid solution heat treatment at 698-813 K, in addition to the lamellae within matrix, a novel lamellar X phase (Mg-8.37 +/- 1.0Zn-11.32 +/- 1.0Gd, at.%) with the 14H-type LPSO structure was transformed from the dendritical beta phase, and a corresponding time-temperature-transformation (TTT) diagram was established. The 14H-type LPSO structure existing in Mg-Gd-Zn-Zr alloys derives from two variant means: the formation of LPSO structure within a'-Mg matrix and the transformation of the dendritical beta phase to a lamellar X phase with the LPSO structure. The alloy solid solution treated at 773 K for 35 h exhibits higher tensile strength and better elongation than the nonheated alloy because of the lamellar X phase with the 14H-type LPSO structure and the 14H-type LPSO Structure within matrix.