Elevated-temperature oxidation behavior of titanium silicide and titanium silicide-based alloy and composite
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INTRODUCTION
In recent years, there has been a drive to develop ordered intermetallic compounds for high-temperature applications. Titanium silicide (Ti5Si3) is a candidate material for hightemperature applications because of its high melting point (2130 7C), lower density, and capacity to retain high strength up to and beyond 1200 7C.[1] Coarse-grained Ti5Si3 has been reported to have a very low room-temperature fracture toughness value of 2.1 MPa=m.[2,3] This can be improved to some extent by reducing the grain size[3,4,5] and by introducing a second phase in the microstructure by alloying or by reinforcement with ceramic particles.[5] Alloying with Al and reinforcing with TiC phase helps improve the room-temperature fracture toughness and might also help reduce the ductile-to-brittle transition temperature.[6,7] TiC is reported to have chemical compatability with Ti5Si3.[1] For high-temperature applications, it is essential to have a thorough understanding of the oxidation behavior of the material. Outstanding oxidation resistance can also lead to applications as high-temperature coatings. It is a wellknown fact that molybdenum disilicide (MoSi2), also possessing a high melting point and mechanical properties similar to that of Ti5Si3, has an outstanding oxidation resistance. Ti5Si3 has an advantage over MoSi2 in that it has a lower density. Boron- and germanium-doped titanium silicide coatings grown on some of the titanium alloys have provided excellent oxidation resistance at 1000 7C.[8] Most metallic alloys are protected from oxidation at elevated temperatures by the formation of a dense, continuous, and adherent film of Al2O3 or SiO2, which are very stable alloys. The oxidation resistance of silicides arises from the formation of an impervious and adherent SiO2 layer on the surface, which protects the interior from further oxidation. This mechanism is very effective in the case of MoSi2.[9] Formation of a continuous layer of SiO2, however, requires that SiO2 be the most stable oxide at the temperature of oxidation, as compared to other possible oxides in the M-Si-O system. Al alloying is reported to improve the highR. MITRA, Scientist ‘‘D,’’ and V.V. RAMA RAO, Scientist ‘‘C,’’ are with the Composites Group, Defence Metallurgical Research Laboratory, Kanchanbagh, Hyderabad 500 058, India. Manuscript submitted December 12, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
temperature oxidation resistance of MoSi2 further, due to an increase in the spallation resistance on mullite formation.[10] Not much work is reported in the literature on the oxidation of titanium silicides. The oxidation behavior of titanium silicide has been studied by Thom et al.[11] and Frommeyer and Rosenkranz.[12] Thom et al.[11] have examined the effect of processing and grain size on the oxidation of Ti5Si3 between 700 7C and 1000 7C, and have concluded that Ti5Si3 with a finer grain size has a greater tendency to oxidize than that of the coarser grain size. Frommeyer and Rosenkranz[12] have studied the oxidation behavior of Ti5Si
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