Microstructure Evolution in Isochronally Heat Treated Mg–Gd Alloys

High thermal stability and good mechanical properties are crucial for the wider future application of magnesium alloys. One of the most promising directions 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 been investigated after a solution heat treatment by isochronal annealing up to 500 °C using electrical resistivity and hardness measurements. The microstructural development during this treatment, responsible for the observed changes, was observed by transmission electron microscopy. The decomposition of α′-Mg supersaturated solid solution in Mg–14.55 wt% Gd with increasing heating temperature is as follows: β″ (D019) metastable phase → β′ (c-b.c.o.) metastable phase → β(Mg5Gd f.c.c.) stable. Peak hardening is achieved by a heat treatment resulting in precipitation of β′ phase in the shape of fine plates parallel to all three 21-1-0 planes of the α-Mg matrix. At higher temperatures (above 280 °C) coarsening occurs and only one orientation of β′ 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 β′ phase.