Studies on Synthesis and Characterization of Mo Based In Situ Composite by Silicothermy Co-reduction Process
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INTRODUCTION
THE design of material for ultra-high–temperature application has been a challenge for scientists and engineers for 10 years or so. An increasing demand of materials persists with high-temperature and load-bearing capabilities for improved performance in engine and structural applications. Refractory metal and alloys are known to possess high-temperature strength. Among the various refractory metal alloys, molybdenum-based alloys are considered the most attractive and promising because of their superior high-temperature properties, such as excellent creep and tensile strength at elevated temperature; adequate compatibility with molten metals such as Pb, Pb-Bi eutectic, and so on; and exceptionally high melting temperature.[1] These alloys are commonly used as structural materials for applications at high temperatures in which a high mechanical strength is required. However, a drawback of molybdenum-based alloys for versatile use is their poor high-temperature oxidation resistance. The major component Mo is easily oxidized in air above 823 K (550 C) to form MoO3. Subsequently, catastrophic failure of these materials is caused by rapid volatilization of MoO3 above 923 K (650 C).[2] Thus, a major barrier to the use of molybdenum-based alloys for high-temperature applications is their catastrophic behavior under oxidizing environments. In contrast, molybdenum silicides have excellent high-temperature oxidation resistance with a high BHASKAR PAUL, Scientific Officer-D, S.P. CHAKRABORTY, Scientific Officer-G, JUGAL KISHOR, Scientific Officer-SB, I.G. SHARMA, Scientific Officer-H, and A.K. SURI, Director, are with the Materials Group, Bhabha Atomic Research Centre, Mumbai 400 085 India. Contact e-mail: [email protected] Manuscript submitted October 5, 2010. Article published online April 22, 2011. 700—VOLUME 42B, AUGUST 2011
melting point. However, silicides in monolithic form have inadequate damage tolerance and extremely low fracture toughness at low temperatures, poor deformability at high temperature, and suitability for the practical applications of these materials as structural components; thus, they are hindered by these drawbacks.[3] It has been reported in some literature that the fracture toughness of silicides can be improved by incorporating a ductile Mo phase (i.e., ductile phase toughening or refractory metal–intermetallic composites [RMICs]). Ductile phase-toughened composites are a class of materials that use a ductile reinforcement, usually metallic, to improve the fracture resistance of a brittle matrix.[4–6] A high-temperature melting molybdenumchromium-silicon ternary alloy system is one such example of RMIC.[7,8] It is evident from the binary phase diagram of the Mo-Si system as shown in Figure 1(a) that, below 9 pct of Si, the microstructure consists of Mo and Mo3Si. The oxidation resistance of Mo3Si is enhanced by the addition of Cr because of the formation of a thermally stable impervious oxide layer of Cr2O3.[9,10] The multiphase approach has led to the study of systems that provide a high leve
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