Influence of Y-addition on the oxidation behavior of Al-rich γ-TiAl alloys

Influence of Y-addition on the oxidation behavior of cyclic and isothermal oxidation of Al-rich γ-TiAl alloys containing 0.1–1.0 at.%Y with equiaxial gamma microstructure was studied in synthetic air with a flow of 200–250 ml/min at 800 °C and 900 °C. The oxidation rates, structures of scale as well as the scale spallation resistance were closely related to the concentrations of Y. Under isothermal oxidation at 900 °C, a small amount of Y-addition (0.1 and 0.3 at.%) was effective in improving the oxidation resistance, especially for 0.3 at.%Y-added sample, which showed suppression of the growth of TiO2 scale, and formation of a protective compact Al2O3 scale at the initial stage of oxidation. However, the alloys with higher Y contents (0.6 and 1.0 at.%) had a detrimental effect on the oxidation resistance after initial isothermal exposure. In the cyclic oxidation tests, all Y-containing alloys showed better oxidation resistance than Y-free alloy oxidized at 800 °C for 165 h; the Y-containing alloys showed a significantly low weight gain, especially for 0.3 at.%Y-added alloy. The constituents and structures of the multi-layered scale were modified with Y-addition; an additional Y-rich layer composed of fine-grained (Al,Y) O-type oxide was formed close to the TiAl substrate and the formation of continuous Al2O3 scale in the outer layer largely contributed to the improvement of oxidation resistance of TiAl-based alloys. No spallation and cracking were observed at the scale/substrate interface in the higher Y-containing alloys, while Y-free alloy showed severe spallation of the scale at 800 °C during cyclic exposure.