Experimental investigation and thermodynamic assessment of the Mg-Gd-Ag system focused on Mg-rich region

Phase equilibria of the ternary Mg-Gd-Ag system focused on Mg-rich region at 400 °C and 450 °C are experimentally studied through evaluation of six selected compositions under the long annealed states. Analysis techniques, such as SEM/BSE, TEM and EPMA are employed for phase identification and chemical investigation. A previously unreported ternary compound, designated as X, is detected in the microstructures of all the samples. The crystal structure of this phase is determined as diamond-cubic structure with the lattice parameter of a=1.59±0.01 nm. Extensive solubility of the Mg5Gd phase along the direction parallel to the Gd-Ag binary in the ternary field is observed. Based on the thermodynamic descriptions of the three constituent binaries and the ternary experimental data obtained in the present study, a thermodynamic description for the Mg-Gd-Ag system is developed by using CALPHAD technique. Reasonable agreement between the calculated and experimental results is achieved.