Phase Equilibria at Ti–Al Interface Under Low Oxygen Pressure

Abstract

In this paper, thirty-nine phases in the Ti-Al-O system at high temperatures have been analyzed through thermodynamic calculations. A Ti-Al-O isothermal stability diagram was obtained from the thermochemical data. The diagrams described the equations for Ti/Al intermetallic compounds at Al2O3/TixAly and Al2O3/TiAl2O5 interfaces, and their corresponding regions. Four univariant equilibria points and ten bivariant equilibria lines at 1000 K were obtained. The equations for the coexistence points and interface lines were also obtained. A three-domain diagram of Ti-Al-O phase arrangement at temperatures between 1000 K and 1200 K was shown. Thermodynamic calculations confirmed that the formation of titanium aluminate spinel (TiAl2O5) requires a threshold TiO2 activity and a partial pressure of oxygen. In the Ti-Al-O system, when aTiO2 < 0.121 and log PO2 < –47.2 atm, TiAl2O5, TiO2, and Al2O3 compounds are not formed at 1000 K; while at the Ti/Al interfaces, compounds are formed in the order of Ti3Al(s) → TiAl(s) → TiAl2(s) → TiAl3(s).