High-temperature deformation behavior of coarse- and fine-grained MoSi 2 with different silica contents
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ON Molybdenum disilicide (MoSi2) has evoked a great deal of interest in the research community for high-temperature structural applications above 1000 °C, because of its high melting point of as high as 2020 °C, outstanding oxidation resistance up to 1700 °C, and brittle-to-ductile transition temperature (BDTT) in the range of 1000 °C to 1300 °C. The added advantages over other oxidation-resistant ceramics are its limited ductility around and above 1000 °C to 1200 °C and its electrical conductivity. For its outstanding oxidation resistance and adequate electrical resistivity, it has been used in the heating element “superkanthal” (Bysakh and Company, Kolkata, India) in high-temperature air furnaces. In recent years, applications have been proposed or are being attempted in hot-end components of aerospace gas turbines, diesel engine glow plugs, and molten-metal lances.[1] Having being projected as a material for high-temperature structural applications, the thrust of research on MoSi2 has been on improving the high-temperature mechanical properties[2-12] and establishing a comprehensive understanding of deformation mechanisms.[11-25] Single-crystal MoSi2 has been found to plastically deform even at room temperature[13] in relatively “soft” orientations away from [001], such as [110], [201], or [0 15 1], while polycrystalline MoSi2 cannot R. MITRA, formerly Scientist “E,” Defence Metallurgical Research Laboratory, is Assistant Professor, Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur–721 302, West Bengal, India. Contact e-mail: [email protected] or [email protected] N. ESWARA PRASAD, Scientist “E,” SWEETY KUMARI, Scientist “B,” and A. VENUGOPAL RAO, Scientist “D,” are with the Defence Metallurgical Research Laboratory, Hyderabad–500 058, India. Manuscript submitted April 25, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
deform without cracking until 1000 °C, as only four independent slip systems have been found to be active at or below 1000 °C.[14] The five most active slip systems reported by Ito et al.[15] are {013}具331典, {110}具111典, {011}具100典, {010}具100典, and {023}具100典. Maloy et al.[14] have reported that {011}具100典 slip is active at all temperatures, is the primary slip system while {110} 1/2具111典 comprises the secondary slip system, and is activated at 600 °C. The critically resolved shear stress (CRSS) of the slip system {013} 1/2具331典 is very much dependent on crystal orientation and is very high for crystals with a stress axis near the [001] orientation at temperatures lower than 1300 °C.[13] The strong orientation dependence of the CRSS of the {013} 1/2具331典 slip system could be used to explain the high BDTT of polycrystalline MoSi2. But, there are other factors affecting the onset of plastic deformation. From the measurement of outer fiber strain in four-point bend specimens,[2] the BDTT in MoSi2 has been reported as 1300 °C, irrespective of SiO2 content. Some articles[3,4] have however, reported 1000 °C to 1200 °C as the BDTT in MoSi2.
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