Phase relationship in the Mg-rich corner of the Mg-Zn-Gd ternary system at 400–475 °C

Phase relationship in the Mg-rich corner at 400–475 °C of the Mg-Zn-Gd ternary system has been investigated in detail by using electron probe microanalysis (EPMA), X-ray diffraction (XRD), transmission electron microscopy (TEM), and differential scanning calorimetry (DSC) measurements. At 400 °C, the Z, I, F, and W phases are all in equilibrium with α-Mg. When the temperature rises to 440 °C, the Z and I phases are no longer in equilibrium with α-Mg. Only the W phase can coexist with α-Mg when the temperature reaches 475 °C. The four-phase peritectic eutectic transformations F+liquid → α-Mg+I and W+liquid → α-Mg+F occur at 427.5 °C and 448 °C, respectively. The compositions of the Z, I, F, and W phases are Mg-(64.2–64.3 at%)Zn-(7.4–8.0 at%)Gd, Mg-(57.9–58.7 at%)Zn-(8.7–9.1 at%)Gd, Mg-(59.8–63.4 at%)Zn-(13.8–14.4 at%)Gd, and Mg-(49.7–50.7 at%)Zn-(24.2–25.5 at%)Gd, respectively. The solubility of Gd in the α-Mg solid solution is dramatically decreased by the addition of Zn when the Z, I, F, W/α-Mg phases are in equilibrium, quite different from the case of Mg-Gd binary system. •At 400 °C, the Z, I, F, and W phases are all in equilibrium with α-Mg.•The Z and I phases are no longer in equilibrium with α-Mg at 440 °C.•Only the W phase can coexist with α-Mg when the temperature reaches 475 °C.•The perieutectic eutectic transformation W+ liquid → α-Mg+ F occurs at 448 °C.•Gd is almost insoluble in the α-Mg solid solution in equilibrium with the Z, I, F, W phases.