Microstructure evolution in isochronally heat treated Mg-Gd alloys

High thermal stability and good mechanical properties are crucial for the w ider future application of magnesium alloys. One of the most promising dire ctions is the alloying of Mg with heavy rare earth elements such as Gd, Dy, Tb, etc. Three squeeze cast Mg-Gd binary alloys (up to 15 wt% Gd) have bee n investigated after a solution heat treatment by isochronal annealing up t o 500 degrees C using electrical resistivity and hardness measurements. The microstructural development during this treatment, responsible for the obs erved changes, was observed by transmission electron microscopy. The decomp osition of alpha'-Mg supersaturated solid solution in Mg-14.55 wt% Gd with increasing heating temperature is as follows: beta " (D0(19)) metastable ph ase --> beta' (c-b.c.o.) metastable phase --> beta (Mg5Gd f.c.c.) stable. P eak hardening is achieved by a heat treatment resulting in precipitation of beta' phase in the shape of fine plates parallel to all three {2<(11)over bar>0} planes of the alpha-Mg matrix. At higher temperatures (above 280 deg rees C) coarsening occurs and only one orientation of beta' plates remains. The decomposition of Mg-4.47 wt% Gd and Mg-9.33 wt% Gd alloys differs from that of Mg-14.55 wt% Gd by the absence of the beta' phase.