The types and structures of the intermetallic phases in a cast Mg−4Al−15Gd−4Y−1Zn alloy

The types and structures of the intermetallic phases formed during solidification in an as-cast Mg−4Al−15Gd−4Y−1Zn alloy were systematically investigated in detail using transmission electron microscopy (TEM). The results indicate that both 14H- and 18R-long period stacking-ordered (LPSO) phases are clearly different from the conventional LPSO phases in the Mg−TM−RE (TM = transition metal) systems and with the order-disorder structures. Furthermore, these two LPSO phases occasionally coexisted with each other following an orientation relationship as (0014)14H about 70.6° from (0018)18R and [11¯00]14H//[11¯00]18R. In addition, another six intermetallic phases namely the large and the small blocky phases on grain boundary, the short and the long parallel plates near grain boundary, the large lamellae phase with irregular profile and the small lamellae phase coexisted on the 18R-LPSO phase were found in the studied alloy. The former three phases are Mg5RE phase and the later three phases are 14H-LPSO, Mg4.8Al12RE7.2 and Mg24RE5, respectively. Finally, the special 18R-LPSO phase in the studied alloy may be transformed from the Mg4.8Al12RE7.2 phase following an orientation relationship as (0018)18R//(111)Mg4.8Al12RE7.2 and [11¯00]18R//[01¯1]Mg4.8Al12RE7.2.