Incoherent long period stacking ordered phases and age-hardening in Mg-Gd-Ga alloys

The intermetallic phases of as-cast and annealed Mg-5Gd-3.5Ga (at.%) alloys were characterized by means of electron microscopies. Coherent and incoherent 18R long period stacking ordered (LPSO) phases with α-Mg were observed in the alloys and the high diffusion coefficient of Ga was the key factor for the appearance of incoherent LPSO phases. Segregation and βF’ phases were observed in the peak-aged samples, which contributed to the large improvements of microhardness and compression properties after heat treatment. In addition, the GdMgGa phase with a hexagonal ZrNiAl-type structure was studied by transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). The incoherent LPSO phases provided a new way of strengthening the mechanical properties of Mg alloys instead of introducing dispersive LPSO phases only.