Indirect Evaluation of the Long-Term Oxidation Properties of Al-21Ti-23Cr and Al-37Ti-12Cr Coating Materials for TiAl Alloy

The most pertinent coating materials in the Al-Ti-Cr alloy system to improve the high temperature oxidation resistance of a TiAl alloy, with respect to oxidation properties, resistance to thermal stress, and chemical compatibility, are the two-phase alloys of Al-21Ti-23Cr (L12+Cr2Al) and Al-37Ti-12Cr (γ+TiAlCr). In this study, cyclic oxidation tests at 1000℃ and 1200℃ were performed for the specimens coated with both materials of 10 im thickness. Furthermore, breakaway oxidation caused by the formation of a rutile TiO2 scale was observed, though both bulk alloys showed very stable oxidation behavior. This phenomenon was resulted from the depleted Al content in the coating layer due to Al2O3 oxide growth and interdiffusion with the substrate. Considering the decrease of Al content due to oxide growth, the Al-21Ti-23Cr coating with the initial higher Al content was more effective for long-term oxidation protection of the TiAl alloy. On the other hand, when the Al content changes due to the interdiffusion with the substrate, the Al-37Ti-12Cr coating with a smaller compositional gradient with the TiAl substrate was more effective than the Al-21Ti-23Cr coating. Cyclic oxidation tests at 1000℃ and 1200℃ confirmed that for the longer lifetime of coating materials the initial Al content was more important than the smaller compositional gradient with the substrate. Consequently, the Al-21Ti-23Cr coating was considered as more effective one for the long-term oxidation resistance of TiAl alloys.