Experimental study and thermodynamic modelling of the ternary Al-Ta-Ti system

In the present paper a thermodynamic description of the entire ternary Al-Ta-Ti system is proposed, being obtained by CALPHAD modelling. Dedicated experiments were performed in order to complement literature data on phase equilibria reported mainly for the temperature interval from 1273 to 1723 K. As-cast and annealed samples were investigated by means of DTA, pyrometry after Pirani-Alterthum, SEM/EDS, SEM/EBSD and XRD techniques in order to prove the phase relations in ranges of invariant equilibria at melting, α2 + γ alloys and low-temperature ternary phases. The experiments revealed that two ternary phases exist in the Al-Ta-Ti system: the orthorhombic O-phase (Ti2.17Ta0.77Al1.06, Pearson symbol oC16) and the ordered B82-phase (Ti3.00Ta0.89Al2.11, Pearson symbol hP6) are thermodynamically stable below 1123 K and 1171 K, respectively. The obtained and literature data were employed throughout the optimization work, using the PARROT module of Thermo-Calc. The elaborated thermodynamic description was applied to calculate selected phase equilibria as to provide a comparison between calculated and experimental data. The calculations are shown to reproduce correctly experimental data. They revealed the presence of a miscibility gap for the ternary (βTi,Ta,Al) solid solution in the temperature interval from 1461 to 813 K.