The η-Al5Fe2 phase in the Al–Fe system: The issue with the sublattice model

The thermodynamic descriptions of a non-stoichiometric crystalline phase used within the framework of the CALPHAD approach has to include an appropriate model to estimate the configurational entropy, e.g. within the compound energy formalism. Thereby, typically a sublattice model based on the known crystal structure data of the considered phase is employed. In the present work a revised thermodynamic description of the Al−Fe system is presented with focus on the sublattice model for the η-Al5Fe2 phase. In view of its channel structure containing Al+Fe as channel atoms with largely composition-independent atomic density (and not Al atoms and vacancies as believed previously), a new sublattice model Al4Fe2(Fe,Al)m with m = 1.5 is proposed. In contrast to previously proposed sublattice models, the new model is well able to reproduce the experimentally observed homogeneity range of the η-Al5Fe2 phase in the course of thermodynamic modelling. Thereby, in order to derive the new model, it was necessary to transfer the crystallographic model used during structure refinement specifically to a sublattice model for the thermodynamic modelling, in order to naturally represent the otherwise easily overestimated the configurational entropy.