Ultrahigh-Temperature Nb-Silicide-Based Composites
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Ultrahigh-
Temperature Nb-Silicide-Based Composites
B.P. Bewlay, M.R. Jackson, J.-C. Zhao, P.R. Subramanian, M.G. Mendiratta, and J.J. Lewandowski Abstract This article reviews the most recent progress in the development of Nb-silicide-based in situ composites for potential applications in turbine engines with service temperatures of up to 1350C. These composites contain high-strength Nb silicides that are toughened by a ductile Nb solid solution. Preliminary composites were derived from binary Nb-Si alloys, while more recent systems are complex and are alloyed with Ti, Hf, W, B, Ge, Cr, and Al. Alloying schemes have been developed to achieve an excellent balance of roomtemperature toughness, fatigue-crack-growth behavior, high-temperature creep performance, and oxidation resistance over a broad range of temperatures. Nb-silicide-based composites are described with emphasis on processing, microstructure, and performance. Nb silicide composites have been produced using a range of processing routes, including induction skull melting, investment casting, hot extrusion, and powder metallurgy methods. Nb silicide composite properties are also compared with those of Ni-based superalloys. Keywords: casting, composites, creep, extrusion, fatigue, jet engines, Nb silicide, niobium, oxidation, superalloys, turbines, ultrahigh-temperature materials.
Introduction High-temperature materials have made substantial contributions to performance improvements of both aircraft engines and land-based gas turbines.1 Advances in Ni-based superalloy airfoils have enabled metal surface temperatures of advanced turbine-engine airfoils to approach 1150C. However, increases in the temperature capability of Ni-based superalloys beyond that of third-generation singlecrystal alloys (3GSXs) will be very difficult to achieve because most advanced superalloys soften above 1150C and melt at 1350C. High-temperature airfoil materials that can operate above the present temperature limit are desired for advanced gas turbines. Designers can generally exploit an increase in airfoil material temperature capability 646
in several ways, including life extension, higher combustion temperature, or a combination of the two. Turbines for either air or surface applications have a range of performance requirements, such as fuel efficiency in producing power or thrust, weight, and reliability. Reducing the amount of cooling air increases efficiency because less of the work done in compressing the air for combustion is lost to cooling. If the airfoil mass can be reduced significantly, the centrifugal stress on the rotor is reduced, and a smaller, lighter rotor can be employed. Consideration of the Nb-silicide-based composites for high-temperature structural applications began about 10 years ago.1–5 The Nb-silicide-based composites showed early promise, based on a good
balance of high-temperature strength and low-temperature damage tolerance. The lower density of Nb-silicide-based systems (7 g/cm3, as compared with 9.2 g/cm3 for Ni-based superalloys)5–9
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