Phase equilibria and crystal structures of ternary compounds in the Mg–Gd–Ag system over a composition range of 0–50 at.% Gd at 450 and 500 °C

Abstract

Alloying with Gd and Ag can significantly enhance the comprehensive properties of magnesium alloys, and accurate phase equilibria are a necessity for advanced alloy design. However, literature review reveals limited information on the phase equilibria in the ternary Mg–Gd–Ag system. Thus, in this paper, the phase equilibria of the ternary Mg–Gd–Ag system in the region of 0–50 at.% Gd at 450 °C and 500 °C were investigated by combining the electron probe microanalysis and X-ray diffraction of totally 66 equilibrated alloys, with two isothermal sections at 450 °C and 500 °C established accordingly; and relatively high solid solubility of Ag in GdMg₃ was characterized. Moreover, seven ternary compounds (denoted as τ₁ to τ₇) were found, and their crystal structures were refined by using Rietveld method. The τ₁ was identical to the previously reported X phase with a diamond-cubic structure, while the remaining six ternary compounds (τ₂ to τ₇) were newly found. The seven ternary compounds (τ₁ to τ₇) are among the space groups of Fd$\overline{3}$$\overline{3}$m (τ₁), P4/nmm (τ₂), P6₃mc (τ₃), P6₃mc (τ₄), Pmn2₁ (τ₅), P$\overline{6}$$\overline{6}$2 m (τ₆) and Pc (τ₇). Their homogeneity ranges and lattice parameters were carefully determined. The solubilities of the third elements in the binary compounds of the three subsystems were also well measured. It is anticipated that the presently obtained phase equilibria as well as the crystal structures of ternary compounds in the Mg–Gd–Ag system would serve as a foundation for developing thermodynamic database and alloy design in the near future.