This introductory article provides the background for the September 2003 issue of MRS Bulletin on Ultrahigh-Temperature Materials for Jet Engines. It covers the need for these materials, the history of their development, and current challenges driving continued research and development. The individual articles that follow review achievements in four different material classes (three in situ composites—based on molybdenum silicide, niobium silicide, and silicon carbide, respectively—and high-melting-point platinum-group-metal alloys), as well as advances in coating systems developed both for oxidation protection and as thermal barriers. The articles serve as a benchmark to illustrate the progress made to date and the challenges ahead for ultrahigh-temperature jet-engine materials.

Superalloys based on platinum-group metals are being developed for high-temperature applications. These alloys have two-phase structures comprising either ordered precipitates in a matrix analogous to the nickel-based superalloys or a fine dispersion of oxide particles in a matrix analogous to oxide-dispersion-strengthened nickel-based alloys. Currently, alloys based on iridium, rhodium, and platinum have been obtained. This article reviews the rationale of developments and the progress made in this area. Oxidation and compression tests as well as characterization with scanning electron microscopy and transmission electron microscopy were undertaken. These tests showed encouraging results, and further work is being done on new alloying additions and tensile testing.

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 (∼700°C) 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.

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 1350°C. 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 room-temperature 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.

SiC-matrix composites consist of ceramic fibers embedded in a silicon carbide matrix produced by gas-, liquid-, or solid-phase routes, yielding materials that differ in matrix crystallinity, residual porosity, and thermal properties. These composites can be highly engineered in terms of the nature of the reinforcement, the interphase used to control the fiber-matrix bonding, the matrix, and the seal coating used. SiC-matrix composites are refractory ceramics displaying outstanding mechanical and thermal properties at high temperature. Their durability in oxidizing atmospheres and under load exceeds 1000 h at temperatures of up to ∼1200°C. They have been used to fabricate different components of the hot zone of jet engines with significant weight savings and an increase in performance. This article reviews the state of the art in the processing, materials design, and properties of these composites as well as their applications in advanced jet engines.

Surface engineering is now a key materials technology in the design of future advanced gas-turbine engines. This article focuses on coating systems for hot-gas-path components, which can vary from low-cost aluminide diffusion coatings to the more exotic, and therefore expensive, thermal-barrier coatings. Available coating systems and their relative benefits are reviewed in terms of performance against manufacturing complexity and cost. Future trends in the design of environmental- and thermal-protection coatings are discussed, including the addition of multiple reactive elements, modified aluminide coatings, diffusion-barrier concepts, the design of “smart” corrosion-resistant coatings, and the development of structurally modified, low-thermal-conductivity thermal-barrier coatings.