Experimental study and thermodynamic modelling of the ternary Al–Fe–Ta system

In the present article, a thermodynamic description of the ternary Al–Fe–Ta system is proposed, being obtained by CALPHAD modelling based on assessed experimental information from literature and our own experimental results. Dedicated experiments were performed to complement literature data on phase equilibria and thermodynamic properties. As-cast and annealed samples were prepared and analysed by means of SEM/EDS, XRD, DTA and DSC techniques, as well as isoperibolic dissolution calorimetry. The experiments revealed that three binary phases have large ternary extensions: the C14 Laves phase based on TaFe₂ dissolves up to about 56 at.% Al, the μ or D8₅ phase dissolves up to about 39 at.% Al and the σ phase based on Ta₂Al dissolves up to 12 at.% Fe. The ternary ordered Heusler phase L2₁ (the MnCu₂Al crystal structure type) is modelled as a stable phase such that it forms through a second-order ordering process at 1122 K from the ordered B2 phase of composition Ta_0.04Fe_0.50Al_0.46 involving the substitution of Al with Ta atoms and reaches TaFe₂Al stoichiometry at ambient temperatures. The elaborated thermodynamic description was applied to calculate selected phase equilibria as to provide a comparison between calculated and experimental results. The calculations are shown to reproduce reasonably the experimental data.