Increasing interest in the airline engine industry has accelerated studies on the efficiency, performance, and reliability of aero engine materials. Development of materials meeting such requirements made a major contribution to the performance enhancement of engines. TiAl alloy has been accepted as a suitable candidate for structural materials at elevated temperatures because of its low density compared with the currently used Ni-based superalloys. In addition, the superior mechanical properties of TiAl alloy at high temperatures increase the potential uses of TiAl alloy as a practical material. However, due to insufficient oxidation resistance, its practical application is limited below 800℃, lower than that permissible by its mechanical properties. There have been two main approaches to enhancing oxidation resistance above 800℃ : alloy design and surface coating. Many reports indicate that the improvement of oxidation resistance favors surface coating rather than alloy design. To protect the substrate effectively in the air, not only the oxidation properties but also the resistance to thermal stress and chemical compatibility with the substrate should be taken into account when selecting the coating material. This accelerates studies on $L1_{2}$ -based alloys in the Al-Ti-Cr alloys system as coating materials for TiAl alloy. However, $L1_{2}$ -based alloys encountered a limitation for the application as a coating material because the mechanical properties, in particular cracking resistance, of $L1_{2}$ -based alloy could be degraded due to poor phase stability of $L1_{2}$ phase around 800℃. Henceforth, it is paramount to overcome the poor phase stability of $L1_{2}$ phase. In this study, the most appropriate coating composition among $L1_{2}$ -based alloys will be selected to overcome the insufficient phase stability and to maximize the mechanical properties, in particular cracking resistance. And the $L1_{2}$ -based alloy with most suitable composition will be applied as a practical coating using RF magnetron sputtering and thermal spraying. Lastly, the long-term oxidation properties of the coating proposed in this study will be evaluated.
Compressive tests on Al-21Ti-23Cr ($L1_{2}$+$Cr_{2}Al), Al-21Ti-15Cr ($L1_{2}$+$Al_{17}Cr_{9}) and Al-30Ti-15Cr ($L1_{2}$+TiAl+TiAlCr) alloys were performed to investigate the effects of alloy compositions on the phase stability of $L1_{2}$-based alloys at temperatures below 1000℃. In Al-30Ti-15Cr alloy, yield strength increased significantly after exposure at 800℃ as compared to the other two alloys. Microstructure observation and phase identification confirmed that the Al-30Ti-15Cr alloy with Ti-rich composition exhibited complete decomposition of the $L1_2$ phase in contrast to partial decomposition of the $L1_{2}$ phase in Al-21Ti-23Cr and Al-21Ti-15Cr alloys under the same conditions. This results from the fact that the $L1_{2}$ phase field is shifted in the direction of the Al-rich corner with decreasing temperature in the Al-Ti-Cr phase diagram. The fracture toughness of Al-30Ti-15Cr alloy from three-point bend tests was more degraded after exposure at 800℃ than the other two alloys because the Al-30Ti-15Cr alloy exhibited poor phase stability due to its higher Ti content. Therefore, the $L1_{2}$ -based Al-21Ti-23Cr alloy would be supposed to be most appropriate as a coating material when considering phase stability coupled with fracture toughness and the ductility at room temperature.
Ti-48Al specimens were coated with Al-21Ti-23Cr film of about 10㎛ thickness at 200 W, 0.8 Pa and 300℃ using RF magnetron sputtering. The oxidation behavior of the coated specimens was investigated through isothermal and cyclic oxidation tests, and the effect of coating on the mechanical properties of TiAl alloy was, in particular, assessed by means of focusing on the tensile properties, which also provided the key to investigate the adhesion between TiAl substrate and Al-21Ti-23Cr coating and between Al-21Ti-23Cr coating and $Al_{2}O_{3}$ scale. The isothermal and cyclic oxidation curves showed that the Al-21Ti-23Cr coating was very effective in improving the oxidation resistance of Ti-48Al at 1000℃. This excellent oxidation resistance is attributable to the formation of a protective $Al_{2}O_{3}$ scale on the surface of the Al-21Ti-23Cr film. Although extensive cracking in a transverse direction was observed on the surface of the film, the delamination of the coating from the substrate and the spallation of the $Al_{2}O_{3}$ scale were not found. From the tensile test, it was confirmed that the Al-21Ti-23Cr coating enabled Ti-48Al to maintain its tensile properties regardless of exposure to oxidizing atmosphere. In addition, the result of the microhardness test indicated that the Al-21Ti-23Cr coating was very effective in suppressing the embrittlement of the Ti-48Al when exposed at 1000℃. Thus, it could be suggested that the Al-21Ti-23Cr coating provides the key not only to improve oxidation resistance but also to maintain the mechanical properties of TiAl alloy.
To improve the oxidation resistance of γ -based TiAl alloy, Al-21Ti-23Cr coatings of approximately 150㎛ thickness were formed on Ti-48Al alloy using air plasma spraying (APS) process. We used Al-Ti-Cr powders with the composition of Al-21Ti-23Cr (at. %). Isothermal and cyclic oxidation tests at 1000℃ in air were utilized to investigate the oxidation resistance of both Al-Ti-Cr coatings and of TiAl alloys with Al-Ti-Cr coatings. After exposure at 1000℃ in air, rutile $TiO_{2}$, which resulted from the oxidation of Ti during spraying, was always found in addition to protective α -$Al_{2}O_{3}$ in the oxide scale. However, isothermal and cyclic oxidation tests of a Ti-48Al alloy coated with an Al-Ti-Cr coating showed that this coating improved the oxidation resistance of TiAl alloy at 1000℃. Moreover, cyclic oxidation tests of Al-Ti-Cr coatings showed stable oxidation behavior at 1000℃ for up to 100 h because of good adhesion between the coating layer and the oxide scale although the growth rate of oxide was relatively high. Therefore, it could be suggested that the Al-21Ti-23Cr coating was effective in enhancing the oxidation resistance of the TiAl alloy at 1000℃, although the improvement of adhesion between the coating layer and the TiAl substrate was required.
The possibility for a practical application as an oxidation-resistant coating depends on the ability to protect the substrate effectively for a long time. Thus, long-term oxidation properties of Al-21Ti-23Cr ($L1_{2}$+$Cr_{2}Al$) coating, which were compared to that of Al-37Ti-12Cr (γ +TiAlCr) coating were discussed to investigate the possibility for a practical coating of TiAl alloy. The study on long-term oxidation properties of these coatings was performed under two conditions where one is assuming that there is crack free in the coating layer and the other is assuming that there is crack in the coating layer. When there is crack free until the coating will failed by oxidation process, it is expected that the Al-21Ti-23Cr coating will be more effective under long-term oxidation exposure because of its higher Al content. Additionally, it is considered that the possibility for crack occurrence in both coatings is similar, based on the mechanical properties of the two alloys (Al-21Ti-23Cr and Al-37Ti-12Cr alloys). Thus, the oxidation resistance of diffusion layer will be important in affecting the long-term oxidation properties when there is crack in the coating layer. The long-term oxidation properties of Al-21Ti-23Cr coating will be better because the diffusion layer between Al-21Ti-23Cr coating and TiAl substrate exhibits better oxidation resistance as compared to that between Al-37Ti-12Cr and TiAl substrate.
Henceforth, it is elucidated that Al-21Ti-23Cr alloy is most suitable coating material for TiAl alloy from the result on mechanical properties and phase stability in Al-Ti-Cr alloy system. In addition, it would be supposed that the Al-21Ti-23Cr coating is a practical coating for TiAl alloy from the study on the application of coating process and the investigation on long-term oxidation properties.
