Effect of Yttrium Alloying on Intermediate to High-Temperature Oxidation Behavior of Mo-Si-B Alloys
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INTRODUCTION
DESIGN and development of new materials suitable for service for application temperatures beyond those of nickel-based super alloys (>1423 K) have been the key focus of interest for energy and aero-space industries. Molybdenum and niobium silicide-based composites are considered among the most promising candidates due to their favorable mechanical properties and oxidation resistance at higher temperatures.[1–6] The key technological challenge is to design a material which possesses good creep properties, adequate oxidation resistance beyond 1473 K (1200 C), and improved manufacturability. Alloys within the ternary Mo-Si-B system have shown attractive properties with respect to high-temperature creep and oxidation resistance.[7–11] A suitable alloy is composed of a tough and potentially ductile molybdenum solid solution (Moss) matrix with uniform distribution of Mo5SiB2 (T2) and Mo3Si (A15) intermediate phases. The volume fraction of these three phases depends upon the alloy composition, and Mo-9Si-8B (at. pct) shows a reasonable combination of phases, leading to balanced mechanical and oxidation properties.[7,8] The detailed oxidation behavior of Mo-Si-B alloys in different S. MAJUMDAR, Scientific Officer, is with the Materials Processing Division, Bhabha Atomic Research Centre, Mumbai 400085, India, and also Visiting Researcher with the Institut fu¨r Werkstofftechnik, Universita¨t Siegen, Paul-Bonatz-Str. 9-11, 57068 Siegen, Germany. Contact e-mail: [email protected] D. SCHLIEPHAKE, Ph.D. Candidate, and M. HEILMAIER, Professor, are with Institut fu¨r Angewandte Materialien-Werkstoffkunde (IAM-WK), Karlsruhe Institute of Technology (KIT), Engelbert-Arnold-Str. 4, 76131 Karlsruhe, Germany. B. GORR, Research Group Leader, and H.-J. CHRIST, Professor, are with the Institut fu¨r Werkstofftechnik, Universita¨t Siegen. Manuscript submitted September 3, 2012. Article published online January 4, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
temperature regimes has been studied and is well documented in the literature.[12–17] The alloy possesses excellent oxidation resistance in the range of 1373 K to 1573 K (1100 C to 1300 C) due to the formation of an amorphous and low viscosity borosilicate, which flows over the surface producing a protective scale. However, the alloy shows an inferior oxidation behavior at intermediate temperatures starting from 923 K to 1223 K (650 C to 950 C). During the oxidation of Mo-Si-B in 923 K to 1023 K (650 C to 750 C), a porous scale forms and MoO3 vapor flows out through the pores/channels of the oxide layer. Beyond 1073 K (800 C), the viscosity of the developed oxide scale is reduced and it is composed of SiB-Mo-O. An optimum concentration of B2O3 is required to produce a protective borosilicate scale.[13] The B2O3 to SiO2 ratio is high until 1223 K (950 C), and a low viscosity scale is formed in this intermediate temperature regime. The B-rich low viscosity scale allows MoO3 to permeate by bubbling through the scale and inward diffusion of oxygen is enhanced, which leads
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