Thermodynamic modeling of the Al-Ta system

Consistent sets of thermodynamic functions for the Al-Ta system are obtained by a computer-operated least squares method applied to all of the experimental phase diagram and thermodynamic data from the literature. Special attention is paid to the sigma phase, AlTa2, which is described by a sublattice model, (Al,Ta)(10)Ta-4(Ta,Al)(16), in a final treatment. The other phases are modeled with the Redlich-Kister formula [liquid, fcc (Al), and bcc (Ta)] or as stoichiometric compounds (Al3Ta, Al69Ta39, Al3Ta2, Al7Ta5, and AlTa). Four treatments are performed. In the first and second treatments, which differ by whether or not the experimental partial Gibbs energy data of Al in bcc (Ta) are used, AlTa2 is assumed to be a stoichiometric compound; in the third and final ones, it is described by sublattice models (Al,Ta)(1)Ta-2 and (Al,Ta)(10)Ta-4(Ta,Al)(16), respectively. Data on the partial Gibbs energy of Al in bcc (Ta) are not consistent with solid solubility data of Al in bcc (Ta), In the second, third, and final treatments, this step-by-step modeling procedure provides reliable estimates and useful starting values for the parameters at each of the higher levels, In addition, the complete and consistent thermodynamic functions of the second or third treatments may be useful for a simplified description of the Al-Ta system if less accuracy for the a phase is required. A comparison with other a phase descriptions in the literature suggests some benefits of the model used in the final treatment. It covers a broad composition range. It is supported by the crystal structure, and it uses a small number of adjustable parameters.