Oxidation Response and Coatings for Mo-Si-B Alloys
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Oxidation Response and Coatings for Mo-Si-B Alloys
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J. H. Perepezko1 and R. Sakidja1 University of Wisconsin-Madison, Department of Materials Science & Engineering 1509 University Avenue, Madison, WI 53706 USA
ABSTRACT Mo-Si-B alloys respond to high temperature oxidation in two distinct stages. First, there is a transient stage with an initial high recession rate that corresponds to the evaporation of volatile MoO3 due to the oxidation of the molybdenum rich phases. The steady state stage of the oxidation begins when a borosilica layer that initiated in the transient period becomes continuous and protects the alloy from further rapid oxidation. Then, the oxidation rate is limited by oxygen diffusion through the borosilicate layer. In order to improve the oxidation performance of the Mo-Si-B alloys, it is necessary to minimize the transient stage. The three phases, Mo (solid solution), Mo3Si (A15) and Mo5SiB2 (T2), composing the Mo-Si-B alloys play different roles in the transient stage. The interaction of the three phases with a reduced microstructure scale can reduce considerably the transient oxidation stage. As a further approach to inhibit the transient stage, a kinetic biasing strategy has been developed to capitalize on the reactions between different phases to develop useful reaction products and alloy compositions that evolve toward a steady state of a compatible system. In order to achieve a compatible interface coating together with enhanced oxidation resistance, a pack cementation process has been adopted to apply diffusion coatings. Two areas are highlighted for successful coating applications on Mo-Si-B alloys and robust high temperature oxidation resistance: development of metal-rich silicide + borosilicide high-temperature coating and in-situ thermal-barrier + borosilica coatings. INTRODUCTION There are severe material performance constraints in terms of melting point, oxidation resistance and structural functionality that must be satisfied in order to meet the challenges of a high temperature environment (T>1400° C) [1]. In spite of their remarkable success, it is now widely recognized that Ni-base superalloys have essentially reached their limit of high temperature operation [2]. If the turboengine metal operating temperature (without cooling) could be increased to 1400°C, the gain in efficiency and output power would be about 50% and represent a revolutionary jump in performance. The gain can not be achieved with Ni-base alloys. However, alloys in the Mo-Si-B system have demonstrated the potential to achieve the enhanced high temperature performance [3-7]. Mo-Si-B Alloy System For Mo-rich Mo-Si-B alloys the phase equilibria has been established as shown in figure 1 where the ternary intermetallic Mo5SiB2 (T2) phase is a key constituent in the multiphase equilibria [6]. Multiphase microstructures based upon (Mo + Mo3Si + T2) three phase alloys appear to offer an effective balance of high temperature performance [3-7]. It is noteworthy that
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the relative atomic packing density in the T2 pha
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