Experimental study and thermodynamic re-assessment of the binary Fe–Ta system

Phase relations and thermodynamic properties in the binary Fe–Ta system were re-assessed taking into account available and new experimental data. Seven alloys with different chemical composition were prepared. Both, as-cast and annealed samples were analyzed by means of DTA, SEM/EDX and XRD techniques in order to experimentally verify the phase relations in the system. The experimental results showed congruent melting of the Laves phase (λ), two eutectic reactions L ↔ (δFe) + λ, L ↔ (Ta) + μ and one peritectic reaction L + λ ↔ μ. Samples with 45.1, 52.3 and 56.2 at. % Ta seem to contain small amounts of the metastable Ta3Fe2 phase, the fractions being too small to be identified by conventional X-ray diffraction.The enthalpies of formation of solid Fe–Ta alloys referred to bcc-Fe and bcc-Ta at 298 K were determined by isoperibolic dissolution calorimetry using liquid Ni at 1773 K as solvent.A new thermodynamic description of the binary Fe–Ta system was obtained by CALPHAD modeling of the Gibbs energy of all individual phases, taking into account the above experimental data as well as assessed published data. Calculations of the phase diagram and of thermodynamic properties were performed with the Thermo-Calc software. They are shown to properly reproduce experimental data.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Experimental analysis of 7 distinct alloys in both, as-cast and annealed conditions using XRD, SEM/EDX and DTA techniques for determination of microstructure constituents, composition of coexisting phases and phase transition temperatures. ► Determination of the standard enthalpies of formation of solid alloys at 298 K by means of isoperibolic dissolution calorimetry. ► Thermodynamic modeling by the CALPHAD method, taking into account assessed experimental information from literature and own experimental results. ► Thermodynamic equilibrium calculations of phase equilibria and selected thermodynamic properties in comparison to experimental data.