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′-Mg18Si10 and β″-Mg5Si6). 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.