Phase equilibria of the Ni–Ti–Ta system at 927 °C

The isothermal section at 927 °C of the Ni–Ti–Ta system was established by combining the diffusion couple technique and equilibrated alloys. The microstructural observation and phase identification were carried out by means of optical microscopy, scanning electron microscopy, electron probe microanalysis, and X-ray diffraction analysis. A ternary compound was found, and temporally designated as τ. The following 7 three-phase equilibria are well determined: (1) TiNi3 + TaNi3 + TaNi2, (2) TiNi3 + TaNi2 + TiNi, (3) TaNi2 + τ + TaNi, (4) τ + TiNi + TaNi, (5) TiNi + TaNi + Ta2Ni, (6) TiNi + Ta2Ni + (Ta, Ti), and (7) TiNi + Ti2Ni + (Ta, Ti). The solubilities of Ti in TaNi2, TaNi, and Ta2Ni were measured to be 21.6, 18.1, and 17.5 at.%Ti, respectively; those of Ta in TiNi and Ti2Ni to be 11.1 and 5.8 at.%Ta, respectively.