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Titanium aluminide alloys based on gamma-TiAl have a substantial potential for high temperature application. However, there are still problems concerning the high temperature oxidation behavior. Ion implantation is a promising tool to improve the oxidation resistance without disturbing the mechanical properties. A systematic investigation of the microstructure and phase development of silicon and molybdenum implanted gamma-TiAl is presented. For Si, the fluence is varied from 2.5·1016 cm-2 to 8·1017 cm-2 at an implantation energy of 1 MeV resulting in a local con-centration of Si between 1 at% and 35 at% at a projected range of 1 µm measured by AES depth profiling. Grazing incidence XRD and transmission electron microscopy show the formation of a buried Ti5Si3 enriched layer at 650 °C, which acts as a diffusion barrier for oxygen. Long term TGA oxidation tests at 900 °C in air show a positive effect in the beginning of oxi-dation for the fluence of 8·1017 cm-2. After a few hours the oxidation kinetic is similar to unimplanted Ti50Al, but the mass gain after 100 h is still 30 % smaller. The influence of the local Si concentration on the oxidation behavior is discussed. Implantation of Mo at 180 keV (2·1016 cm-2 and 1·1017 cm-2) has only minor influence on the oxidation at 900 °C. After implantation, Mo precipitates are found for the high fluence.

The effect of ion beam implantation and plasma immersion ion implantation of chlorine on the high temperature oxidation of titanium aluminides above 800 °C in air was investigated. Thermogravimetric oxidation tests (TGA) were performed to examine the long term protection. Depth profiling with Auger electron spectroscopy (AES) was used to investigate Cl diffusion and oxide formation during the first stage of oxidation. A microscopic model of the chlorine effect will be discussed. A systematic variation of the implantation energy and fluence shows that there is a narrow regime of the Cl concentration for optimum protective effect. The time to form a protective Al2O3 layer depends on the local Cl concentration. The oxidation rate after this incubation time is reduced by about 2 orders of magnitude compared to untreated Ti50Al and is nearly independent of the fluence. The implantation energy is not a sensitive parameter, because the implanted chlorine profile changes very quickly during high temperature oxidation.

The TiAl intermetallic compound is very promising for high temperature applications, because of its good high temperature strength and its low density. At temperatures exceeding 800°C, the low oxidation resistance is a limiting factor. It is known, that Cl doping reduces the oxidation strongly even in very low concentrations of about 500 ppm ("microalloy"). In the present investigation ion beam implantation is used to dope the material close to the surface quantitatively. The well-defined depth profile obtained after implantation provided a means to monitor the diffusion of additives during oxidation. Implantation of Cl+ ions (1 MeV, 1015 cm-2 - 1017 cm-2) results in a systematic reduction of the oxidation at 900°C in air for doses ≥ 1016 cm-2. AES measurements were performed to investigate the diffusion process during oxidation. A microscopic model will be proposed for the enhanced oxidation resistance. For beneficial effects of Silicon a higher concentration is required ("macroalloy"). Therefore high-dose implantations were carried out (up to 8·1017 cm-2). The change in phase composition, microstructure and the oxidation behaviour will be discussed. PACS: 81.65M, 81.40, 61.10

The influence of microalloying on the oxidation behavior of gamma-TiAl based alloys was studied. The microalloying elements were added by ion implantation. Oxidation tests at 1173 K in air showed that the addition of chlorine into TiAl improves the oxidation resistance resulting in a decrease of the oxidation rate by about 2 orders of magnitude compared to unalloyed TiAl. Microstructural investigations revealed that the formation of an protective alumina layer on top of Cl-implanted TiAl is the cause for the decrease in the oxidation rate. AES measurements in the initial stage of oxidation showed that chlorine is located under the alumina layer in the metal phase. Thermodynamic calculations, investigations on the temperature dependence of the chlorine effect and the oxidation kinetics of preoxidized Cl-implanted samples support the model of a selective Al-transport via AlCl. Furthermore, the influence of small additions (in the ppm range) of P, B, C and Br on the oxidation kinetics of g-TiAl-based alloys has been investigated. P, B and C implanted TiAl showed a different oxidation behaviour and oxide scale microstructure compared to Cl and Br microalloyed TiAl. Especially the P implanted sample revealed an extensive nitride formation connected with a breakaway oxidation after 100 h.

The TiAl intermetallic compound is very promising for high temperature applications, because of its good high temperature strength and its low density. At temperatures exceeding 800°C, the low oxidation resistance is a limiting factor. It is known, that Cl doping reduces the oxidation strongly even in very low concentrations of about 500 ppm ("microalloy"). In the present investigation ion beam implantation is used to dope the material close to the surface quantitatively. The well-defined depth profile obtained after implantation provided a means to monitor the diffusion of additives during oxidation. Implantation of Cl+ ions (1 MeV, 1015 cm-2 - 1017 cm-2) results in a systematic reduction of the oxidation at 900°C in air for doses ≥ 1016 cm-2. AES measurements were performed to investigate the diffusion process during oxidation. A microscopic model will be proposed for the enhanced oxidation resistance. For beneficial effects of Silicon a higher concentration is required ("macroalloy"). Therefore high-dose implantations were carried out (up to 8·1017 cm-2). The change in phase composition, microstructure and the oxidation behaviour will be discussed. PACS: 81.65M, 81.40, 61.10

The articles in this volume were written to commemorate Reinhold Remmert's 60th birthday in June, 1990. They are surveys, meant to facilitate access to some of the many aspects of the theory of complex manifolds, and demonstrate the interplay between complex analysis and many other branches of mathematics, algebraic geometry, differential topology, representations of Lie groups, and mathematical physics being only the most obvious of these branches. Each of these articles should serve not only to describe the particular circle of ideas in complex analysis with which it deals but also as a guide to the many mathematical ideas related to its theme.