Phase Equilibria in the Ti-Rich Part of the Ti-Al-Nb System—Part I: Low-Temperature Phase Equilibria Between 700 and 900 °C

Abstract

Precise knowledge of the phase equilibria in the Ti-Al-Nb system between 700 and 900 °C is of crucial importance for the urgently needed improvement of TiAl-based turbine materials already in industrial use to achieve further energy savings. As a result of the occurrence of the two ternary intermetallic phases ωo (“Ti4NbAl3”) and O (“Ti2NbAl”), which form in the solid state just in the range of the application-relevant temperatures, the phase relations are very complex and not well studied. In the present investigation, isothermal sections of the Ti-rich part of the Ti-Al-Nb system at 700, 800, and 900 °C were determined by a systematic study of 15 ternary alloys, one solid-solid diffusion couple, and three liquid-solid diffusion couples. Using scanning electron microscopy, electron probe microanalysis (EPMA), x-ray diffraction (XRD), high-energy XRD (HEXRD), differential thermal analysis (DTA), and transmission electron microscopy (TEM) investigations, type and composition of phases as well as phase transitions were determined. With these results, the phase equilibria were established. A focus of the investigations is on the homogeneity ranges of the two ternary phases ωo and O, which both are stable up to temperatures above 900 °C. Based on the compositions measured for the ωo phase and its crystal structure type, a new formula (Ti,Nb)2Al is suggested. The results also indicate that the phase field of the ωo phase is split into two parts at 900 °C because of the growing phase field of the ordered (βTi,Nb)o phase.

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

“…x o ). [34] Such an isolated (bTi,Nb) o phase field has been previously reported by Hellwig et al [12,13] , Li et al [11] , and Eckert et al [14] at 1000 °C. All authors report the same three tie-triangles phase field is found matches in most cases.…”

“…% measured at 900 °C for alloy B1. [34] This assumption is supported by the observation that alloy B1 at 1000 °C no longer contains the (aTi) phase, which is more and more displaced by the growing (bTi,Nb) phase field as the temperature increases. For the isothermal sections at 1000 and 1100 °C in Fig.…”

“…Overall and phase compositions were determined as described in Ref. 34. Furthermore, the experimental details for phase identification by room temperature XRD and HEXRD, in situ HEXRD as well as the measurement of phase transformation temperatures investigated by DTA were carried out as described in Ref.…”

“…At 1000 °C, Ti 3 Al is stable in a composition range of Ti- (23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36)(37) Al-(0-15)Nb with the maximum Nb content reached in the three-phase equilibrium with (bTi,Nb) and Nb 2 Al (Fig. 13).…”

“…Phase equilibria at lower temperatures from 700 to 900 °C were already discussed and presented in Part I of this study. [34] Based on the combined experimental results which thus cover the temperature range from 700 to 1300 °C and considering some information from the literature especially regarding still higher temperatures, a complete reaction scheme of the Ti-Al-Nb system is presented in the last section.…”