High-temperature oxidation of Ti3Al-based titanium aluminides in oxygen

The oxidation behavior of Ti-25Al, Ti-24Al-15Nb, and Ti-25Al-11Nb (at. pct) titanium aluminides was studied in dry oxygen at atmospheric pressure in the temperature range 1000 to 1300 K for 4 to 6 hours by thermogravimetry. The oxidation products were characterized by X-ray diffraction (XRD) and scanning electron microscopy with energy dispersive X-ray (EDX) analysis. Although some departures from the parabolic rate law were found, the analysis of data revealed that the parabolic rate law was a more reliable basis for interpretation of results as compared to the empirical power law. Parabolic rate constants (k _p ) for Ti-24Al-15Nb and Ti-25Al-11Nb were almost the same at the same temperature. However, k _p values for Ti-25Al were 2 to 8 times larger than niobium-containing alloys except at 1000 K, where k _p values for all three alloys were approximately the same. The effective activation energy (Q _eff) was 289 kJ/mole for Ti-25Al in the range of 1000 to 1300 K, while in the case of Ti-24Al-15Nb and Ti-25Al-11Nb,Q _eff values were 329 and 330 kJ/mole, respectively, in the range of 1100 to 1300 K. In general, the scales on Ti-25Al were porous and exhibited significant spallation above 1100 K. The scales were thinner, compact, and adherent for niobium-containing alloys. The scales formed on these alloys were predominantly composed of TiO₂, while Al₂O₃ was also present as a minor constituent. The superior oxidation resistance of the niobium-containing alloys has been attributed to the doping effect of niobium in TiO₂.