Thermodynamic analysis of the Al-Mg-Zn ternary system

In the present study, the Al-Mg-Zn ternary system, which is essential as the basic system of 7000 Al-based alloys and Mg-based casting alloys, was analyzed thermodynamically using the first-principles method and the CALPHAD approach. The (Al,Zn)₁₇Mg₂₁-type sublattice model was applied to the ternary compound φ phase based on the most recent experimental results. Furthermore, the Gibbs free energy for Al₁₇Mg₂₁ and Zn₁₇Mg₂₁ which are end members of the structures, the terminal compounds Al₂Mg and Zn₂Mg of Laves-C14 phase, as well as Mg₂Zn₁₁ phase were evaluated by first-principles calculations considering lattice vibration and volume expansion. These results were analyzed by the CALPHAD method together with experimental values on phase boundaries and thermodynamic properties, and the phase equilibrium of all compositional regions of this alloy system was clarified by calculations. The temperature change of the precipitation phase based on the analysis was found that the AlMg-γ phase mainly formed at the high temperature in the commercial AZ alloys. On the other hand, when the ratio of Zn:Al was changed, the precipitated phase altered to the τ phase or the φ phase, and therefore, it can be inferred that the mechanical properties in this alloy system may be expected to be improved by the structure change due to the composition change.