Thermodynamic analysis of the Al-Cu-Mg ternary system

A thermodynamic assessment of the Al-Cu-Mg ternary system was attempted using the first-principles calculation and CALPHAD approach. For Laves phase to which polymorphic structures are known, the Gibbs free energies of formation were evaluated by applying the cluster expansion and cluster variation method to each structure of C14, C15 and C36. Furthermore, concerning the free energies of formation of Al₂Mg and Cu2Mg as the end-members of these structures, entropy of the lattice vibration was considered. This procedure results in more accurate evaluation of the free energy of formation for the Laves phase. As for the xi;(Al₃₀Mg₂₃) phase and CuMg₂ phase that the thermodynamic properties were not well clarified, formation energies were also evaluated. Thermodynamic analysis was accomplished by an introduction of the result based on these first-principles approach as well as some experimental data in the framework of the CALPHAD method. According to the calculated result, it is suggested that the structural transformation in the Laves phase occurs in the Cu rich side than the previously reported composition region. Solidification process of a commercial Al alloy was discussed using the Scheil-Gulliver simulation.