A combined CALPHAD/first-principles remodeling of the thermodynamics of Al–Sr: unsuspected ground state energies by “rounding up the (un)usual suspects” Original Research Article

Despite numerous investigations, all previous efforts on thermodynamic modeling of Al–Sr have suffered from inaccurate energetics of either the solid-state compounds or of the liquid alloy. Here, we demonstrate a method yielding simultaneously, accurate solid-state and liquid energetics, as given by first-principles density functional calculations and experimental measurements, respectively. Via first-principles methods, we have investigated the T=0 K energetics of not only the reported ground state compounds in Al–Sr (“the usual suspects”), but also of a wealth of other possible crystal structures observed in isoelectronic alloy systems (somewhat more “unusual suspects”). We find: (i) LDA calculations surprisingly show that Al2Sr in the C15 structure is slightly lower in energy than the observed CeCu2-type structure. However, GGA predicts the opposite order, consistent with the observed CeCu2-type/C15 stability. (ii) An as-yet-unreported Al5Sr4 compound (observed in Al–Ba) is found to be on the T=0 K ground state hull. (iii) An Al3Sr8 phase, isostructural with the recently discovered Al3Ca8 compound, is predicted to lie above the ground state hull and is not a T=0 K ground state. Using the first-principles formation enthalpies along with experimental thermodynamic and phase stability information, we have performed a new CALPHAD modeling of Al–Sr, including the three observed intermediate compounds as well as a hypothetical compound Al3Sr8. Two different models of the liquid phase were considered: an associate and a random solution model. The descriptions resulting from the two liquid models are critically evaluated with respect to experimental data in the literature and the present first-principles results.