Thermodynamic assessment of the system Al-Co

The optimization of the thermodynamic description of the phases of the binary system Al-Co was performed using the program Parrot (27). All experimental phase diagrams and thermodynamic data available for this system were assessed. The excess Gibbs energy of the fee, hcp and liquid phases was modelled using Redlich-Kister equation. The CsCl-type compound, AlCo, presents a large deviation from ifs stoichiometric composition. Accordingly with its physical defects, its thermodynamic behaviour was modelled with the sublattice model (Al,Co)(Co,square) where square represents vacancy. The compounds Al9Co2, Al13Co4, Al3C, and Al5Co2 were considered stoichiometric. Three different phases with an approximate composition Al13Co4 were reported, but the phase relationship obtained by Godecke and Ellner (36) and Grushko et al. (37, 38) are inconsistent Only one phase with the stoichiometry Al13Co4 was thus modelled However, more experimental information relative to these phases are desirable. A set of thermodynamic parameters is presented. It allows a satisfactory agreement between calculated and most of the experimental results except for the measured chemical potential of aluminium in the two field (Co)+AlCo. This could be explained by the large variation of solubility in those two phases which may require a long annealing time to achieve thermodynamic equilibrium.