Abstract: The phase transformation and phase equilibria related with the ternary compound XB in the lowTi side of NbNiTi system were systemically investigated through the equilibrated alloy method by using the scanning electron microscopy(SEM)assisted with energy dispersive spectroscopy of Xray(EDS)， electron probe microanalysis(EPMA)， Xray diffraction(XRD)and differential scanning calorimetry(DSC). It has been shown that the threephase field XB+(Nb)+TiNi exists at both 1000℃ and 1100℃ isothermal sections of the phase diagram in the NbNiTi system. The compound XB could be in solidstate equilibrium with the Nbbased solid solution(Nb)and the TiNi phase till about 1133℃. There exists the eutectic phase transformation LTiNi+XB at 1133℃ in the NbNiTi system. As the liquid forms， the compound XB could not be in equilibrium with the Nbbased solid solution and the TiNi phase. The threephase field XB+(Nb)+L exists at 1150℃ isothermal sections of the phase diagram in the NbNiTi system， XB keeps phase equilibrium with the liquid and the Nbbased solid solution.

Key words: NbNiTi system, compound, eutectic transformation, phase equilibrium, phase diagram