Behavior of Mg–15Gd–5Y–0.5Zr alloy during solution heat treatment from 500 to 540 °C

The development of the microstructure in Mg–15Gd–5Y–0.5Zr alloys was investigated both in the as-cast condition and after solution heat treatment from 500 to 540 °C in the time range 0.5–72 h. In the as-cast specimen, the main eutectic phase is Mg5(GdY). After the solution heat treatment, the continuous network-like Mg5(GdY) phase was fused into disconnected and fragmentized island morphology. With the temperature increased and the time prolonged, almost all the Mg5(GdY) phase were spheroidized after 24 h at 500 °C. After 12 h solution treatment at 525 °C, the Mg5(GdY) phase was completely dissolved, and some cuboid-shaped compound with composition of Mg2Y3Gd2 comes into being. The evolution of the microstructure from as-cast to cast-T4 involves α-Mg solid solution + Mg5(GdY) eutectic phase → α-Mg solid solution + spheroidized Mg5(GdY) phase → α-Mg supersaturated solid solution + cuboid-shaped compound (Mg2Y3Gd2).