INFLUENCE OF OXIDATION PROTECTIVE COATINGS ON THE DUCTILITY OF A γ-TiAl BASED ALLOY M. Moser, P. H. Mayrhofer, H. Clemens Dept. of Physical Metallurgy and Materials Testing, Montanuniversität Leoben, Franz-Josef Straße 18, A-8700 Leoben, Austria Keywords: γ-TiAl, coating, oxidation, embrittlement, CrAlYN, Al 2 Au Abstract Using γ-TiAl based alloys in high temperature applications require an effective protection against oxidation and concomitant loss of plasticity. Therefore, two coatings, Al 2 Au and Cr 0.45 Al 0.53 Y 0.02 N (CrAlYN), are tested for their oxidation protection and influence on the mechanical properties of a γ-TiAl based alloy (Ti-47Al-2Cr-0.2Si). This TiAl alloy was selected based on the high ductility at room temperature (RT) but low oxidation resistance. Hence, the influence of the coatings can be investigated in detail. Thermal exposure in air reveals that both coatings significantly improve the oxidation resistance of γ-TiAl. The specific mass gain after oxidation for 672 h at 800 °C is 0.12 mg/cm 2 for CrAlYN coated and 0.79 mg/cm 2 for Al 2 Au coated, but 8.9 mg/cm 2 for the uncoated γ-TiAl. The plastic strain in the γ-TiAl outer fiber during RT four-point-bending tests is reduced from 0.52 % to 0.31 and 0.20 % by the deposition with Al 2 Au and CrAlYN, respectively. Already, after 168 h oxidation at 800 °C uncoated and Al 2 Au coated γ-TiAl crack completely without plastic deformation due to the formed oxide layers and interdiffusion zones formed. Contrary, CrAlYN deposited γ-TiAl still exhibits plastic strain of 0.097 and 0.122 % after oxidation at 800 °C for 168 and 672 h, as only a thin oxide layer is formed and the diffusion zone is limited to 7 μm. Based on our results we can conclude that CrAlYN can effectively retard oxidation and interdiffusion of γ-TiAl. Hence, the commonly observed loss of mechanical properties by oxidation and interdiffusion can be avoided. Introduction Within the Ti-Al system, the 40-50 at% Al containing alloys –so called γ-TiAl based alloys– exhibit the most attractive properties such as low density, high specific stiffness, high yield strength and good creep resistance up to high temperatures. Consequently, γ-TiAl alloys have the potential to partly replace heavy steels and Ni-base alloys presently used in high-temperature automotive, aerospace and power-generation applications [1-4]. However, the long-term use of γ-TiAl base alloys in oxidative environments is limited to temperatures below 800 °C because of their poor oxidation resistance [1-7]. Consequently, the latter has to be improved to utilize the full potential of γ-TiAl. Alloying the material with e.g., Nb was shown to be an effective method in terms of oxidation resistance, but also influences its structural and mechanical properties [5, 7, 8]. Improving the oxidation resistance by surface techniques allows the development of γ-TiAl with application optimized mechanical properties. Hence, in recent years the effect of halogen ion implantation [9, 10] or magnetron sputter deposited intermetallic coatings on the oxidation of γ- TiAl has been studied [11-14]. Also, coating systems known for their good performance on Ni- base superalloys [15, 16] or for tool protection [17, 18] have been tested. However, due to the relatively low fracture toughness of γ-TiAl base alloys as compared to Ni- base superalloys the influence of the used surface modification on mechanical properties and