Oxidation Behavior of β+γ TiAl Alloys with an Addition of Cr at Elevated Temperatures

Abstract

Oxidation behavior of β+γ TiAl alloys with an addition of Cr was evaluated by cyclic oxidation tests at temperatures ranging from 1073 to 1173K under the atmospheric environment, comparing with that of a binary Ti–49Al alloy. The scales formed on the Cr added alloys didn't spall in the oxidation test at 1073K, even though that of Ti–49Al alloy spalled markedly. Ti–44Al–8Cr and Ti–45Al–9Cr with high Al and high Cr contents showed little scale spallation up to 1123K and the mass gains were found to be much smaller. To the contrary, the mass gains of Ti–40Al–7Cr and Ti–42Al–6Cr with low Al and low Cr contents were significantly large, similar to that of Ti–49Al. The scales formed during the oxidation test were composed predominately of five layers from outside to the TiAl substrate, i.e., TiO₂/Al₂O₃/TiO₂+Al₂O₃/Al₂O₃/α₂-Ti₃Al. Even in the Cr added alloys, Cr₂O₃ wasn't detected by X-ray diffraction analysis and scanning electron microscopy observation. It is found in Ti–44Al–8Cr and Ti–45Al–9Cr that the oxide layers of Al₂O₃ and α₂-Ti₃Al, which were formed in the vicinity of the scale/substrate interface, are significantly thin compared with those of the other specimens. This appears to influence the excellent oxidation behavior in the Cr added alloys with high Al and high Cr contents.