Influence of Micro-alloying on Oxidation Behavior of TiAl
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Influence of Micro-alloying on Oxidation Behavior of TiAl Michiko Yoshihara 1 and Shigeji Taniguchi 2 1. Department of Mechanical Engineering and Materials Science, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama, Kanagawa 240-8501, Japan 2. Department of Materials Science and Processing, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan ABSTRACT The influence of a wide range of elements on oxidation behavior of TiAl was investigated by micro-alloying using ion implantation with ion doses of 1019 to 1022 m-2 and at acceleration voltages of 40 to 340kV. The oxidation resistance was assessed by a cyclic oxidation test at 1200K in a flow of purified oxygen under atmospheric pressure. The implanted elements can be classified into several groups according to their effect and mechanism. The mechanisms by which the oxidation resistance is improved are as follows: (1) Formation of a protective Al2O3 layer through β-phase formation, which was confirmed by TEM observations, in the modified surface layer by the implantation. (2) Reduction of TiO2 growth rate due to doping effect of the implanted element. (3) Protective Al2O3 layer formation through migration of volatile halide. (4) Enrichment of oxide of the implanted element in the scale. On the other hand, the oxidation resistance is decreased by (1) enhanced TiO2 growth due to doping effect, (2) lattice defects induced by the implantation, and (3) decreased scale strength and enhanced scale spallation. INTRODUCTION TiAl-based alloys have attractive properties as light weight heat-resisting material. Numerous engineering alloys have been developed for structural applications in automotive and aerospace components [1]. The alloys thus developed contain at least a few alloying elements that may optimize thermally stable microstructures and/or improved high temperature mechanical properties. Their oxidation resistance is not sufficient at application temperatures higher than 800oC [2, 3], because TiAl alloys do not form a continuous and protective Al2O3 layer in spite of their high Al content. The influence of alloying element on their oxidation behavior has not been well clarified yet, and it is an important factor for further alloy development. Since there seems to be no single alloying element which can improve mechanical properties and oxidation resistance simultaneously, the surface treatment is a possible way for their compatibility. In this sense, the microalloying using ion-implantation technique is not only a useful way for such a surface treatment but also an excellent research tool to investigate the influence of the additional element on
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oxidation behavior [4-7]. This method can be applied for almost all the elements, including those which cannot be alloyed by the conventional ingot metallurgy. In the present study, 22 elements were implanted, and their influence on the oxidation behavior of TiAl and the mechanisms concerned are discussed. The oxidation was carried out in pure oxygen to avoid nitride formation, which
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