Crystal structure and stability of complex precipitate phases in Al–Cu–Mg–(Si) and Al–Zn–Mg alloys

Abstract We demonstrate how first-principles total energy calculations may be used to elucidate both the crystal structures and formation enthalpies of complex precipitates in multicomponent Al alloys. For the precipitates, S (Al–Cu–Mg), η ′ (Al–Zn–Mg), and Q (Al–Cu–Mg–Si), energetics were computed for each of the models of the crystal structures available in the literature allowing a critical assessment of the validity of the models. In all three systems, energetics were also calculated for solid solution phases as well as other key phases (e.g., equilibrium phases, GP zones) in each precipitation sequence. For both the S and η ′ phases, we find that recently proposed structures (based on electron microscopy) produce unreasonably high energies, and thus we suggest that these models should be re-evaluated. However, for all three precipitates, we find that structures based on X-ray diffraction refinements provide both reasonable energetics and structural parameters, and therefore the first-principles results lend support to these structural refinements. Further, we predict energy-lowering site occupations and stoichiometries of the precipitate phases, where experimental information is incomplete. This work suggests that first-principles total energy calculations can be used in the future as a complementary technique with diffraction or microscopy for studying precipitate structures and stabilities.