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Accelerated Aging of Materials and Structures The Effects of Long-Term Elevated-Temperature Exposure (1996) / Chapter Skim
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3 CANDIDATE MATERIALS
Pages 11-16

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From page 11...
... Standard ingot alloys, such as those used for subsonic aircraft, typically have more favorable strength and toughness combinations than elevated-temperature aluminum alloys. For example, Figure 3-2 shows fracture toughness versus tensile yield strength for X8019 (Al-Pe-Ce)
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From page 12...
... , and aging to produce a substantial volume fraction of the hexagonal-close-packed alpha phase in a continuous beta matrix. Such alloys offer an excellent combination of high strength, modest density, and formability, as well as good corrosion and fatigue resistance.
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From page 13...
... Omega phase has been detected in the low-cost beta alloy after short-term aging below 400 °C (752 °F; Rack, 19941. 13 SUPERALLOYS Nickel-based superalloys are chemically complex alloys with a combination of high-temperature strength, resistance to environmental degradation, toughness, and fatigue resistance not available in any other current alloy system (Sims, 1986~.
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From page 14...
... The development of crystalline morphology requires reproducible control of the cool-down process to permit the necessary polymer mobility needed for crystalline growth. Phase boundaries in crystalline thermoplastics also represent stress discontinuities which may compound the failure process, particularly under hot and wet conditions where the amoIphous regions will have a higher affinity for moisture.
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From page 15...
... The combination of excellent creep resistance and high strength at elevated temperatures has not been achieved, but materials development efforts have shown sufficient promise that programs to develop composite fabrication methods are using Nicalon as a model fiber, assuming that an adequate polycrystalline SiC fiber will eventually be available. The required feature of the mechanical response for a ceramic-matrix composite is "graceful" or noncatastrophic failure which implies crack deflection at the fiber-matrix interface.
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From page 16...
... for SiC/SiC composites, it has been shown to be unacceptable for structural applications on oxidizing ambients where long lives are required (James and Lamicq, 1993~. New materials for the fiber-matrix interphase region must be developed.
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Key Terms

  • fracture toughness
  • candidate materials
  • ingot metallurgy
  • creep resistance
  • ingot alloys

  • tensile yield
  • fatigue resistance
  • metallurgy alloys
  • crystalline morphology
  • aluminum alloys

  • yield strength
  • omega phase
  • elastic modulus
  • mechanical properties
  • alpha phase

  • exhaust nozzle
  • advanced aircraft
  • toughness versus
  • standard ingot
  • elevated temperatures

  • aircraft structures
  • stress resistance
  • specific strength
  • prime candidate
  • subsonic aircraft

  • exposure temperature
  • thermal stress
  • service temperatures
  • versus tensile
  • temperatures exceed


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