Phase equilibria of the Co–Ni–Ta system at 1100 °C

The phase equilibria of the Co–Ni–Ta system at 1100 °C are investigated using diffusion couples and five equilibrated alloys. The samples are examined by means of optical microscopy, scanning electron microscopy, and electron probe microanalysis. The experimental results show no existence of ternary compounds at 1100 °C. The following 4 three-phase equilibria are observed: (1) Ta(Co, Ni) + β-TaCo2 + TaNi3; (2) Ta(Co, Ni) + TaNi2 + TaNi3; (3) β-TaCo2 + TaNi3 + (Co, Ni); and (4) TaNi3 + TaNi8 + (Co, Ni). The compounds Ta2Co and Ta2Ni form continuous solid solutions. It is also the case for TaCo and TaNi. The solubilities of Co in TaNi3, and Ni in β-TaCo2 were determined to be up to 70 at.% Co and 30 at.% Ni, respectively.