The cyclic oxidation resistance in air was investigated mainly at 900 and 950°C for stoichiometric TiAl intermetallics alloyed with up to 5 at%Nb and Ti-Al-Nb ternary alloys with 50 at%Al and 0-20 at%Nb.
The mass gains of most of the Nb-added alloys due to oxidation are significantly less than those of the binary TiAl, indicating that the oxidation resistance is remarkably improved by Nb addition. The alloys containing more than 15 at%Nb show a relatively larger mass gain owing to the TiO2·Nb2O5 formation. Oxide scales formed during oxidation are similar in morphology to those formed on TiAl, become thinner as the Nb content increased and the Al2O3 layer in the scales becomes denser and continuous. The improvement of oxidation resistance is attributed to both the suppression of TiO2 growth and the promotion of protective Al2O3 layer formation. Substitution of Nb+5 into the TiO2 is expected to decrease the oxygen vacancy concentration and to reduce the growth rate of TiO2. The content of Nb which is just enough to eliminate oxygen vacancies is estimated to be 2 at%, while Ti-Al-Nb containing about 10 at%Nb shows the best oxidation resistance. Hence, it is suggested that Nb also affects the diffusion behaviour of Al so as to enhance the formation of Al2O3. The alloys containing a few percent Nb, when heat-treated under a low partial pressure oxygen atmosphere, show better oxidation resistance than binary TiAl with the same heat treatment. It is presumed that an Al2O3 layer formed on the alloys by the treatment is thicker than that formed on TiAl, which supports the above suggestion. The heat treatment, however, is not effective in improving the oxidation resistance of the alloys with more than 10 at%Nb. It is probably because that the β phase appeares in Al-depleted region which forms as an Al2O3 grows, thus resulting in the formation of thick internal oxidation layer.
