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 liqu id phases was modelled using Redlich-Kister equation. The CsCl-type co mpound, AlCo, presents a large deviation from ifs stoichiometric compo sition. Accordingly with its physical defects, its thermodynamic behav iour was modelled with the sublattice model (Al,Co)(Co,square) where s quare represents vacancy. The compounds Al9Co2, Al13Co4, Al3C, and Al5 Co2 were considered stoichiometric. Three different phases with an app roximate composition Al13Co4 were reported, but the phase relationship obtained by Godecke and Ellner (36) and Grushko et al. (37, 38) are i nconsistent Only one phase with the stoichiometry Al13Co4 was thus mod elled However, more experimental information relative to these phases are desirable. A set of thermodynamic parameters is presented. It allo ws a satisfactory agreement between calculated and most of the experim ental results except for the measured chemical potential of aluminium in the two field (Co)+AlCo. This could be explained by the large varia tion of solubility in those two phases which may require a long anneal ing time to achieve thermodynamic equilibrium.