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Ni-based superalloys for the aircraft engines of the future. The stakes are very high. It is still too soon to know what type of material will be used, but it is expected that more than one candidate material will be developed for selected applications. The number of these applications will grow with time as the material is improved and costs are reduced. This has been the trend, for example, with single-crystal superalloys. Alternate Materials to Nickel-Based Superalloys A high melting temperature is the first and most obvious requirement for the new generation of materials since the melting temperature must (obviously) exceed the operating temperature and the creep resistance increases with increasing melting temperature for materials in general. Low density is also very important since the weight of the propulsion system as a whole decreases rapidly with decreasing weight of the rotating components as discussed by Darolia.2 Oxidation resistance at high temperatures is also critical as are cost considerations. Cost constraints eliminate the use of precious metals or other intrinsically expensive materials where costs cannot be reduced by economies of scale. In addition the material must possess some intrinsic plasticity and toughness, the required levels of which are not yet totally clear. However, some deformability at room temperature is required for fit-up of the components during assembly, and a level of toughness substantially above the Griffith value is required at both low and operating temperatures as is discussed in the section on Ductility and Fracture-Toughness Requirements.

Intermetallic Compounds Intermetallic compounds have the requisite combination of properties to satisfy some of the list of requirements just described. For example Fleischer3 has compiled an extensive survey of the density and melting points of various classes of intermetallics, and showed that the necessary combination of low density and high melting temperature are difficult to achieve by utilizing the simple fcc-based Ll 2 or bcc-based B2 structures. More complex structures, such as A15, C15, or Cll b , may be required. This however poses a serious dilemma. Ductility at low temperatures is generally observed only in those intermetallics with the Ll2 or B2 crystal structures, of which singlecrystal Ni3Al is perhaps the best example. As the crystal structure becomes more complex, due either to a low-symmetry crystal structure or many atoms in the unit cell, the ductile-to-brittle transition temperature (DBTT) tends to increase rapidly. This was shown in the "Development Potential of Advanced Intermetallic Materials" by Anton and Shah4 who measured the DBTT, using bend tests, of a number of intermetallics with various crystal structures (see Figure 1). In this program, only those intermetallics with melting temperatures above approximately 1600°C were considered. Most of these materials show a DBTT near 1000°C or above, and NiAl shows the lowest DBTT. The inescapable conclusion to be drawn from the study of Anton and Shah4 is th