Study On Phase Equilibria Of The Nb-Ni-Ti Ternary System At High Temperature

Research on the Nb-Ni-Ti ternary phase diagram, which originated in the development of Ni-base super alloy, began with research of phase equilibria of the Ni-rich corner. Much research has been done the composition of the phase equilibrium with the development of the wide lagging shape memory alloy. However, the information of the part of the hyperthermia phase equilibrium is limited; and cannot meet the needs of the Nb-Ni-Ti of amorphous alloy and filtration membrane alloy hydrogen related alloy design. Due to lack of experimental data, the credibility of the system thermodynamics parameter is low.The isothermal section of the Nb-Ni-Ti ternary system of the Ni-rich corner at1250℃and the part of the Ti-Nb side eutectic reaction have been investigated, used metallography and the method of mainly by optical microstructure analysis and scanning electron microscopy, energy spectrum analysis, X-ray diffraction and transmission electron microscope diffraction structural analysis, and with the differential scanning calorimetry. The results of the study show that:(1) The ternary compounds XE and XC are also discovered in the Ni-rich corner at1250℃. The crystal structure of the XE-phase is p-hexagonal, the crystal parameters are a=0.5129nm, c=0.8366nm, it’s composition range from7.4～10.9at.%Ti,76.9～80.8at.%Ni and10.6～12.9at.%Nb. The Xc is also p-hexagonal, its crystal parameters are a=0.511nm, c=0.4209nm, it’s composition range from5.5～8.4at.%Ti,75.4～80.1at.%Ni and14.4～16.2at.%Nb.(2) At1250℃the highest solubility of Nb in the Ni solid solution is about9.8at.%, the highest solubility of Ti in the Ni solid solution is about11.9at.%, the solubility of (Ti+Nb) is higher than14.9at.%. In addition, the fCC lattice constant of the Ni solid solution can be increased from a=0.3524nm to a=0.3604nm. What’s more, lots of Ti and Nb respectively exist at Ni3Nb phase and TiNi3phase. The highest solubility of Ti in the Ni3Nb is about9.3at.%and the solubility of Nb and the Nb in the TiNi3is higher than5.4at.%.(3) In the Nb-Ni-Ti ternary phase diagram, the highest solubility of Nb in the Ti2Ni phase is about5.5at.%, which lead to the fusion temperature of the Ti2Ni phase increase increasing from942.0℃to950.5℃.(4) The three-phase equilibrium of (Ni)+Ni3Nb+XE exist in the Nb-Ni-Ti ternary phase diagram. In addition, XC and TiNi3phase exist equilibrium with the Ni solid solution.(5) The three-phase eutectic reaction exists in Nb-Ni-Ti ternary phase diagram:L→β(Nb, Ti)+TiNi. This eutectic reaction can be continuously happened at982.6℃～1070.9℃.