The influence of additions of Nb and Cr on the aluminizing behavior of TiAl alloy

Abstract

The intermetallic compound TiAl is expected to be one of the candidates for high-temperature systems such as aircraft engines because it has a high specific strength at elevated temperatures.[1,2] However, there are two major drawbacks with the intermetallic compound TiAl as an engineering material: the first is its poor room-temperature ductility and the other is its poor oxidation resistance above 800 8C. The former has been generally overcome by modification of the microstructure using ternary element addition such as Nb, Si, or W. However, improvement in the oxidation resistance by addition of these alloying elements is not enough for practical application. Furthermore, the alloying elements may have a deleterious effect on the mechanical properties of the material. Therefore, surface treatments are thought to be more attractive. Fig. 4—Typical Cr31 photoluminescence spectra acquired from thermal Several studies on the surface treatments have been perbarrier coated (MCrAlY bond coat and EB-PVD YSZ ceramic coat) turbine formed to improve the oxidation resistance of TiAl. The blades before and after the engine test. surface treatments contain MCrAlY overlay coating,[3,4] aluminizing,[5,6] silicide/ceramics,[7,8] and preoxiding at low oxygen pressure.[9] However, none of them seems suffiapplication of Cr31 photoluminescence piezospectroscopy ciently successful. is currently being investigated as a nondestructive inspection Aluminide coatings are widely used for high-temperature technique for TBCs. materials because of their practical advantages over other coatings. The pack aluminizing of TiAl basically produces TiAl3 coating. However, TiAl3 is reported to be more brittle than TiAl. During cyclic oxidation under thermal stress, the The authors thank Drs. G. Wagner and D.B. Allen, SieTiAl3 layer is easily spalled off due to its brittle nature. The mens-Westinghouse Power Corporation, as well as the additions of Nb and Cr to TiAl and TiAl3 alloys have been technical staff at Renishaw, Inc., for helpful discussions shown to be beneficial for mechanical properties as well as and experimental collaborations. Financial support, profor oxidation resistance.[10] However, the effect of the alloyvided by the Department of Energy under the Advanced ing element contained in the TiAl substrate on the aluminizGas Turbine Systems Research Program (Grant No. ing behavior of TiAl alloy has not been extensively AGTSR 99-01-SR073), is gratefully acknowledged. studied yet. In this investigation, pack aluminizing is studied with the TiAl alloy. Emphasis is placed on the effect of alloying REFERENCES elements on the aluminizing behavior of TiAl alloy. To inves1. S.M. Meier, D.M. Nissley, K.D. Sheffler, and S. Bose: J. Eng. 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