Mo-Si-B Alloys: Developing a Revolutionary Turbine-Engine Material
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Mo-Si-B Alloys:
Developing a Revolutionary Turbine-Engine Material
Dennis M. Dimiduk and John H. Perepezko Abstract This article discusses three main points. First, ultrahigh-temperature materials offer a large return to the propulsion community and to the economy, but replacing Ni-based alloys is a very difficult problem for both technical and financial reasons. Second, the oxidation resistance of selected Mo-Si-B alloys and the prospects for combining this with useful structural properties are remarkable, given the nature of conventional Mo alloys; however, there are nagging issues associated with their low-temperature (700C) behavior. Third, further advances in the processing of such materials, together with assessments of their affordability and reliability, are vital for achieving a successful engineering technology. Keywords: aerospace, high-temperature materials, intermetallic alloys, materials development, oxidation, phase equilibria, powder metallurgy, structural materials, turbine engines.
Introduction The invention and discovery of new high-temperature structural metals is one of the truly hard problems in materials science and engineering. There is no parallel in nature to the remarkable synthetic achievements in this field, and the potential impact of these materials on the economy, our lifestyle, and our future is enormous. More than 50 years ago, the paradigm for these materials was established within Ni alloys or superalloys,1 and, as the articles in this issue of MRS Bulletin suggest, there are many reasons for attempting to establish a new paradigm that may not only sustain many more years of development and growth of gas turbines, but also spur new technologies enabling hypersonic flight and ready access to space. A look at the historical data regarding the use of Ni alloy technologies in gasturbine engines reveals the magnitude of this endeavor. As shown in Figure 1, one MRS BULLETIN/SEPTEMBER 2003
measure of both material and engine performance is the amount of horsepower produced as a function of the turbine rotor inlet temperature, the T4 temperature of an engine. This relationship is known for an ideal gas-turbine engine and is shown in the figure as an analytical expression and by the green line. Blue bullets in Figure 1 represent several Pratt and Whitney engines that were developed from the 1940s through the early 1990s. In each case, the actual engine performance falls short of the ideal line because of inefficiencies in the engine. Over the years, structural and aerothermal design advances minimized leakage in the pressurized systems, optimized aerothermal flows of the engine, and took advantage of highly refined cooling and coating schemes to prevent materials from melting in the high-pressure turbine (see the fine cooling-hole configuration on the airfoil inset in Figure 1). The 1
blue shading on the lower-left region of Figure 1 depicts the incremental gain in service temperatures from such design refinements. The orange and purple shaded regions of the figure depict the advan
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