Solvus boundaries of (meta)stable phases in the Al-Mg-Si system: First-principles phonon calculations and thermodynamic modeling

Using first-principles total energies and frozen phonon calculations, we predict the thermodynamic properties (enthalpies of formation and vibrational entropies) for three phases in the Al-Mg-Si system: the stable phase (β-Mg₂Si), and two metastable precipitate phases (β^′ - Mg₁₈ Si₁₀ and β^″ - Mg₅ Si₆). The stable fcc / β and the metastable fcc / β^′ and fcc / β^″ phase boundaries are obtained from a combination of the Gibbs energy of the compounds determined from first-principles and the free energy of the Al-rich solid solution (fcc phase) taken from the literature. Predicted phase boundaries show good agreement with available phase stability measurements. The present work demonstrates the capability of first-principles calculations in predicting Gibbs energies of stable and metastable phases.