A thermodynamic study of the D03-ordered intermetallic compound Fe3Al

Thermodynamic activities of aluminum in the iron-aluminum system were determined between about 720 and 850 K for the composition range between 23 and 33 at.% Al which includes the D0(3)-ordered nonstoichiometric intermetallic compound Fe3Al For this purpose an electromotive force method with a single-crystal CaF2 electrolyte was used, and the following cell arrangement was employed: (-)Pt vertical bar Ir vertical bar Al0.85Sn0.15, Na3AlF(6)vertical bar CaF2 vertical bar Al1 +/- xFe3 +/- x, Na3AlF6 vertical bar Ir vertical bar Pt(+). Activities of iron were obtained by means of a Gibbs-Duhem integration for the same composition range. The results of the activity measurements were interpreted in terms of a statistical-thermodynamic model for nonstoichiometric phases with the D0(3)-superstructure based on defect formation energies from the literature. It was found that non-stoichiometry in Fe3Al is caused by anti-structure atoms on two of the three possible sublattices. (c) 2007 Elsevier B.V. All rights reserved.