Novel protection solutions against environmental attack for light weight high temperature materials
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Novel protection solutions against environmental attack for light weight high temperature materials Alexander Donchev, Michael Schütze DECHEMA Forschungsinstitut, Frankfurt am Main/Germany ABSTRACT The use of light weight structural materials such as titanium in transport systems like aero planes leads to a significant reduction in fuel consumption. However, titanium and its alloys cannot be used at elevated temperatures above 500°C for several reasons. Today aero engine compressors are made of a mixture of light Ti- and heavy Ni-alloys. The improvement of Ti-alloys to withstand the conditions in the high pressure compressor i.e. temperatures above 500°C would enable the manufacturing of a compressor from titanium as a whole with all its associated benefits. Intermetallic TiAl-alloys are another class of light weight materials for several high temperature applications. The use of TiAl as low pressure turbine (LPT) blades in the last sections of a large jet engine could save up to 150 kg of weight. In the last sections of the LPT the temperature is quite moderate (max. 650°C). The improvement of the high temperature capability of TiAl would allow its use in hotter sections of the engine with additional weight reduction. Similarly, the response performance of TiAl-turbocharger rotors in automotive engines would be much faster compared to the heavy Ni-based alloys used today. Furthermore higher rotation speeds are possible. Due to the novel so called fluorine effect the oxidation mechanism of TiAl can be altered. Fluorine-treated TiAl-components are protected by an alumina layer formed during high temperature exposure in oxidizing environments. This effect can be transferred to Ti-base materials if they are enriched with aluminum in a thin surface zone. The concepts and the results of high temperature exposure experiments of treated Ti- and TiAlspecimens are presented in this paper. They are discussed in the view of a use for real components. INTRODUCTION The increasing demands for reducing fuel consumption and hence the influence on the environment resulted in a search for more efficient automotive engines or jet turbines. The replacement of the relatively heavy-weight Ni- or Co-based superalloys currently applied by lighter materials would be an important step in this direction. Light weight intermetallic TiAl-alloys and optimized Ti-alloys could be used to reduce weight. Titanium is widely used as a structural material but not at elevated temperatures above 600°C [1]. Its use in hotter sections of e.g. a jet engine is not possible due to reduced oxidation resistance at high temperatures in oxidizing environments. Rutile is not protective anymore under these conditions in contrast to its passivation properties at low temperatures [2]. TiAl-alloys suffer from the same problem at temperatures above 750°C. They do not form a protective alumina scale during exposure in air like other aluminides (e.g. NiAl) due to similar thermodynamic stabilities of Ti- and Al-oxide [3]. A nonprotective mixed TiO2/Al2O3/TiN-scale is form
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