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

摘要

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 (L1_2+Cr_2Al) and Al-37Ti-12Cr (gamma+TiAlCr). In this study, cyclic oxidation tests at 1000 deg C and 1200 deg C were performed for the specimens coated with both materials of 10 im thickness. Furthermore, breakaway oxidation caused by the formation of a rutile TiO_2 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 Al_2O_3 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 deg C and 1200 deg C 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.