Nonisothermal and Isothermal Oxidation Behavior of Nb-Si-Mo Alloys
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IN recent years, significant research has been directed toward the development of high-temperature materials for aerospace structural applications with operating temperatures exceeding that for most of the advanced Ni-base superalloys. The refractory Nb-Si–based alloys with high melting temperatures are of interest as candidate materials for aero-engine applications.[1–15] The microstructure of the Nb-Si–based alloys consists of hard and brittle intermetallic Nb5Si3 and relatively ductile Nb-solid solution (Nbss) phases. The Nb5Si3 exhibits excellent high-temperature strength retention and creep resistance,[1] but poor ductility and fracture toughness.[4] The resistance to cracking in these alloys (with microstructure showing a mixture of Nbss and Nb5Si3) is imparted by the ductile phase through intrinsic toughening mechanisms, such as crack arrest and bridging.[8,15] Therefore, an optimum volume fraction of Nbss and Nb5Si3 is required to achieve a desirable balance of mechanical properties involving room temperature fracture toughness, as well as hightemperature yield and creep strength, supplemented by appropriate oxidation resistance for high-temperature applications. K. CHATTOPADHYAY, Research Scholar, R. MITRA, Associate Professor, and K.K. RAY, Professor, are with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur 721 302, India. Contact e-mail: rahul@metal. iitkgp.ernet.in Manuscript submitted June 18, 2007. Article published online January 17, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
One of the major limitations of the Nb-based or Nb-Si–based alloys is their poor oxidation resistance at elevated temperatures. The molar volume of monoclinic b-Nb2O5 is 58.3 cm3, while that of Nb is 10.9 cm3,[16] suggesting that volume expansion on oxidation of Nb is large. Such a large volume expansion naturally causes high biaxial residual stress at the metal-oxide interface, leading to spallation. In addition, Nb5Si3 undergoes accelerated pest disintegration in the temperature range of 700 C to 1000 C, forming Nb2O5.[17] Complete disintegration of the Nb5Si3 has been observed on exposure at 1000 C for 1 to 3 hours. The susceptibility of pest disintegration of Nb-silicides is commonly attributed to the mismatch in the coefficient of thermal expansion (CTE) of the oxidation products, along with the volume expansion accompanying the oxide growth. The CTE of Nb2O5 is one fourth that of Nb5Si3. The lower CTE of the oxide scale and the volume expansion during its formation leads to compressive residual stresses inside the scale and its spallation. The alloys with Nbss-Nb5Si3 mixture in the microstructure, thus suffer catastrophic oxidation upon exposure to air at temperatures above 500 C, as the oxygen anions diffuse rapidly through the loose and porous oxide scales, which spall off easily.[18] Research efforts directed at improvement of oxidation resistance of the binary Nb-Si alloys have involved the addition of different alloying elements, such as Ti, Al, and Cr.[19
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