The α-Mg solvus and isothermal section of Mg-rich corner in the Mg-Zn-Al ternary system at 320 deg. C

The phase equilibria and compositions in Mg-rich corner at 320 deg. C have been investigated in detail in the Mg-Zn-Al ternary system through the equilibrated alloy method by using X-ray diffraction (XRD), scanning electron microscopy (SEM) assisted with energy dispersive spectroscopy of X-ray (EDS). Besides the binary compounds Mg17Al12 (γ) and MgZn in equilibrium with α-Mg solid solution, there exist two ternary compounds Al5Mg11Zn4 (φ) and Mg32(Al, Zn)49 (τ). A new two-phase region (α-Mg + τ) has been experimentally identified. The whole composition region of the φ phase has been also obtained at 320 deg. C, i.e., ∼53.7-57.0 at.% Mg, ∼14.7-27.4 at.% Al, ∼17.7-30.5 at.% Zn. The solubility of Al in the α-Mg solid solution in equilibrium with the γ phase decreases due to the dissolution of Zn, while the solubility of Zn increases for the addition of Al when the MgZn/α-Mg phases are in equilibrium, compared to the Mg-Al and Mg-Zn binary systems, respectively. The maximum solubility of Zn in the γ phase and that of Al in the MgZn phase are both about 8.3 at.% in the Mg-rich corner. Moreover, the element Al mainly substitutes the Zn in the MgZn phase while the Mg content hardly changes.