Phase Equilibria in the Ti-Rich Part of the Ti-Al-Nb System—Part II: High-Temperature Phase Equilibria Between 1000 and 1300 °C

The knowledge of phase equilibria in the Ti-Al-Nb system above 1000 °C is of importance for the manufacturing of TiAl-based parts for high-temperature structural applications. Especially the extended homogeneity range of the cubic (βTi,Nb) phase, which is determined by its Al solubility, and the position and extension of the high-temperature (αTi) phase is of crucial importance for the hot-workability and microstructure control of these alloys. However, the phase diagrams reported in the literature are very contradicting especially regarding these aspects. For this reason, a systematic reinvestigation of the phase equilibria in this part of the system was carried out. A total of 17 ternary alloys were synthesized, heat-treated at 1000-1300 °C, and analyzed by electron probe microanalysis (EPMA), x-ray diffraction (XRD), high-energy XRD (HEXRD), and differential thermal analysis (DTA) to determine composition and type of equilibrium phases as well as transition temperatures. With this information, isothermal sections of the Ti-rich part of the Ti-Al-Nb system at 1000, 1100, 1200, and 1300 °C were established. An isolated (βTi,Nb)o phase field is found to be stable at 1000 and 1100 °C. Furthermore, the formation and homogeneity range of (αTi) at high temperatures as well as the presence of Ti3Al at 1200 °C is experimentally investigated and discussed. Based on the observed phase equilibria and transition temperatures, an improved reaction scheme for the entire Ti-Al-Nb system is proposed.