Nonisothermal and Cyclic Oxidation Behavior of Mo-Si-B and Mo-Si-B-Al Alloys
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INTRODUCTION
STRUCTURAL intermetallics based on MoSi2, Mo5Si3, and Mo3Si as well as their composites have strong potential for elevated temperature applications due to their high melting points (>2000 C), impressive strength retention, and oxidation resistance.[1–5] Some of the recent studies[6–8] have shown that boron-doped Mo5Si3 and Mo5SiB2 possess significantly higher creep resistance than MoSi2. However, the major hurdles that stand in the way of their widespread application are their inherent brittleness and poor room-temperature (RT) fracture toughness. A compromise for the optimum combination of RT fracture toughness, hightemperature creep, and oxidation resistance can be found in the Mo-Si-B alloys,[3,9–12] which have a dispersion of ductile a-Mo phase in equilibrium with the brittle Mo3Si and Mo5SiB2. A study by Meyer and Akinc[13] has shown that pestlike accelerated oxidation of Mo5Si3 occurs with the formation and vaporization of MoO3 at ~800 C, and this can be avoided through doping with boron.[13] Protection against environmental degradation is provided in the case of B-doped Mo5Si3 and Mo-Si-B alloys SHARMA PASWAN, Research Scholar, and R. MITRA and S.K. ROY, Professors, are with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur 721302, West Bengal, India. Contact e-mail: [email protected]. ernet.in Manuscript submitted December 24, 2007. Article published online August 25, 2009 2644—VOLUME 40A, NOVEMBER 2009
by the formation of a borosilicate (B2O3-SiO2) scale.[14–17] Because the B2O3 has a low melting temperature of 450 C,[18] the oxidation resistance of B-doped Mo5Si3 is improved due to sintering of the viscous oxide scale, leading to the closure of its pores.[13] Isothermal oxidation studies[17] of the selected Mo-Si-B and Mo-Si-B-Al alloys have shown pestlike accelerated degradation in the temperature range of 700 C to 900 C, whereas a continuous and stable scale of either B2O3-SiO2 or B-doped silica could be formed during isothermal exposure at 1150 C for 24 hours. However, the stability of this protective scale under cyclic oxidation conditions has not been tested. Efforts to improve the oxidation resistance of Mo-silicide-based materials also include alloying with Al.[19–23] For example, the addition of Al as an alloying element to MoSi2[19–21] and Mo3Si[22] has been found to be effective in improving their resistance to pest oxidation and spallation. In another study by Yamauchi et al.,[23] it has been shown that the presence of Al in the 72.9Mo-8.7Si-17.4B-1Al (composition in mole percent) alloy leads to a significant improvement in the oxidation resistance in the range of 800 C to 1300 C by the formation of a dense Al-Si-O scale. However, a comparative study on the isothermal oxidation behavior of selected Mo-Si-B and Mo-Si-B-Al alloys in the range of 400 C to 800 C[24] and 700 C to 1300 C[17] has shown higher mass loss for the latter alloys, because a protective oxide scale is not formed on alloying with Al. Thus, the information
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