Experimental study of phase equilibria in the Nb-Ni-Ta system involving the liquid phase

The phase formation and liquidus projection of the ternary Ni-Nb-Ta system were investigated by combining thermodynamic calculations with differential thermal analysis (DTA) and microstructural characterization. Binary and ternary alloys were synthesized to study both the Ni-rich and Ni-poor composition ranges, with particular focus on invariant and monovariant reactions. The binary subsystems exhibit eutectic reactions involving (Ni) and Ni₃(Nb,Ta) on the Ni-rich side, and µ phase formation (Ta₇Ni₆, Nb₇Ni₆) on the Ni-poor side. Thermodynamic calculations predict a continuous solid solution between isomorphic binary phases, forming ternary extensions of the (Nb,Ta)Ni₃ phase and the µ phase (Nb,Ta)₇Ni₆. Experimental DTA data of ternary alloys reveal systematic deviations between experimental and calculated phase transformation temperatures, particularly along the monovariant lines and for the extension of the peritectic reaction. In contrast to the calculated results, which predict a melting minimum in the ternary system, a monotonic decrease of the monovariant eutectic lines is experimentally observed from the Ni-Ta side towards the Nb-Ni side. Microstructural analysis confirms primary phase formation of (Ni) or the µ phase, depending on the composition range, followed by a eutectic or quasi-peritectic reaction, depending on the alloy composition. In particular, the ternary eutectic and peritectic monovariant lines originating from the Ni-Ta system extend further into the ternary system composition space before merging in a quasi-peritectic reaction L + TaNi₂ → (Nb,Ta)Ni₃ + µ. These findings indicate a need for revision of the current Ni-Ta phase diagram and provide new insights into the phase equilibria and solidification paths in the Ni-Nb-Ta system. • Solidification behaviour of alloy compositions along the monovariant lines. • experimental results on ternary alloy compositions with Ni content less than 60 ma%. • calculated eutectic compositions are not in accordance to the actual compositions. • extension of peritectic formation further into the ternary system than predicted.