Experimental investigation and thermodynamic assessment of the Mg-Zn-Gd system focused on Mg-rich corner

The phase equilibria in the Mg-rich corner of Mg–Zn–Gd ternary system at 673 K were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS), and electron probe microanalysis (EPMA). Three ternary phases, X-(Mg12ZnGd), W-(Mg3Zn3Gd2) and I-(Mg3Zn6Gd1), have been identified, which are in equilibrium with Mg solid solution. A thermodynamic modeling and optimization of Zn–Gd and Mg–Zn–Gd systems has been carried out for the first time using the CALPHAD method. The sublattice model was used to describe the thermodynamic functions of both solution phases and intermetallic phases presented in these systems. In particular, order/disorder transition between BCC_B2 and BCC_A2 has been taken into account, and their Gibbs energies were expressed with identical function. The thermodynamic database was applied to case studies of experimentally observed microstructures and demonstrated that it is a valuable basis for alloy design.