Phase equilibria of the Mg–Gd–Zn system at 500 °C

Highlights: • The isothermal section at 500 °C over the composition range of less than 50 at% Gd was constructed. • Nine stable ternary phases, τ1 to τ8 and W, were observed and their compositon ranges well determined. • The ternary phases τ1, τ2, τ3 and τ5 correspond to the X (14H), Z, I and F in the literature, respectively. • The third element solubility in binary phases was well measured. -- Abstract: The phase equilibria of the Mg–Gd–Zn system at 500 °C in the region below 50 at% Gd were investigated with equilibrated alloys, by means of the electron probe microanalysis and X-ray diffraction. Nine stable ternary phases, τ₁ to τ₈ and W, were found and their composition ranges determined. The solubility of the third elements in binary phases was well determined. W was found to be isostructural to the binary GdMg₃, while a very narrow two-phase field between GdMg₃ and W was determined. The τ₁ phase, with a composition range of Gd_5.5–6.2Mg_89.2–90.0Zn_4.3–4.7, is identical to the X-phase (LPSO-14 H). The τ₂ phase, with a composition range of Gd_6.4–8.6Mg_24.0–28.5Zn_63.1–69.3, was identified to be the hexagonal Gd_7.7Mg_28.6Zn_63.8 phase or the Z-phase in literature. The τ₃ and τ₅ phases were measured to have composition ranges of Gd_10.4–10.7Mg_24.0–24.5Zn_64.8–65.5 and of Gd_13.9–15.3Mg_18.7–24.4Zn_61.1–67.1, respectively, corresponding to the I-phase and the F-phase. A partial isothermal section in the region below 50 at% Gd was constructed.