Oxidation Behavior of Nb-20Mo-15Si-25Cr and Nb-20Mo-15Si-25Cr-5B Alloys
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DUCTION
RESEARCH efforts are focusing on filling the need for materials systems capable of operating in aggressive environments at temperatures exceeding the maximum nickel super alloy service temperature [T < 1423 K (1150 °C)] in the aerospace sector. A variety of advances in thermal barrier coatings, design features, and manufacturing techniques have made it possible for current nickel-based super alloys to operate within 200 K of their melting points.[1] The development of alloys capable of operating at 1573 K (1300 °C) without additional protection would provide next-generation jet engines up to a 50 pct increases in the output power.[2] Niobium- and molybdenum-based alloys offer high melting points and creep resistance when alloyed with silicon and various oxidation resistance enhancing elements such as B, Cr, Ti, and Al, to name a few. Mo5Si3 and Mo2Si are two silicides of interest in the Mo-Si system; Mo5Si3, because of its complex crystal structure, offers high creep resistance, which is an order of magnitude better than MoSi2. However, it suffers from catastrophic oxidation and mass loss at elevated temperatures [T > 1073 K (800 °C)]. MoSi2 offers oxidation resistance up to temperatures as high as 1973 K (1700 °C), but it suffers from pest oxidation at intermediate temperatures and from low creep strength at elevated temperatures.[3] Additions of boron were found[4,5] to increase the oxidation resistance of molybdenum silicides, resulting in a large research effort focused on studying the Mo-Si-B system. Mo5SixBx type phases have been examined and found to offer high oxidation resistance. A layer of borosilicate glass BENEDICT I. PORTILLIO, Student, and SHAILENDRA K. VARMA, Professor, are with the Department of Metallurgical and Materials Engineering, The University of Texas at El Paso, El Paso, TX 79968-0520. Contact e-mail: [email protected] Manuscript submitted April 9, 2010. Article published online August 30, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
forms on the sample surface protecting it from further attack[6–10] during the oxidation. Additions of ductile phases, typically molybdenum solid solution,[11] are required to enhance the fracture toughness but result in unacceptable material recession typically caused by the formation and subsequent volatilization of MoO3 at T > 973 K (700 °C).[12] An attempt is being made to focus on the study of oxidation behavior of the alloys from the Nb-Si-B system to determine whether Nb5SixBx phases may offer similar oxidation resistance to their molybdenum counterparts. The oxidation resistance of Nb5Si3B2 has been found to be superior to undoped Nb5Si3 as reported by Murakami et al.[13] However, unlike the Mo5SixBx type silicides that suffer from the volatilization of MoO3 leading to the development of a continuous borosilicate glass layer, the Nb5Si3B2 type silicides develop nonvolatile Nb2O5. Another niobium-based system includes the Nb-Cr-Si in which a microstructure containing the silicide (Nb5Si3) and laves phase (NbCr2) has been observed. Chan[14] determined the
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