Metallurgical and Materials Transactions A

, Volume 27, Issue 5, pp 1229–1240 | Cite as

High- temperature deformation properties of nial single crystals

  • K. R. Forbes
  • U. Glatzel
  • R. Darolia
  • W. D. Nix
Mechanical Behavior

Abstract

The high-temperature deformation properties of stoichiometric NiAl single crystals have been studied in the temperature range from 850 °C and 1200 °C. We have established a basic data set for and have explored the high-temperature deformation characteristics of this intermetallic compound. The results provide a basis for determining the controlling mechanisms of high-temperature deformation. Constant stress tension creep and constant stress or constant strain rate compression experiments were conducted on crystals oriented with loading axes along the “hard,” [001] orientation, where no driving force exists for glide ofb = (001) dislocations, and along various “soft” orientations, [223], [111], and [110], where deformation can occur by the glide of these dislocations. In addition to these monotonie tests, high-temperature deformation transients were studied using stress relaxation, strain rate change, and stress change experiments. These transient deformation experiments were conducted in an effort to further elucidate the mechanisms that control high-temperature deformation of this material. The steady-state deformation properties of these differently oriented single crystals can be characterized by creep activation energies that all coincide, within experimental error, with the activation energy for diffusion of Ni in NiAl, 308 ± 10 kJ/mol. The stress dependence of steady-state deformation can be characterized with stress exponents that range from about 9 at 850 °C to about 4 at 1200 °C. At all temperatures and stresses, the soft oriented crystals creep about two orders of magnitude faster than the hard oriented crystals at the same stresses and temperatures. Soft oriented crystals loaded along [223] and [111] axes tested in both tension creep and constant stress or constant strain rate compression are found to deform at the steady-state rate from the very beginning of the deformation experiment. Crystals with these orientations exhibit virtually no evidence of strain hard-ening. Transients associated with stress changes suggest that deformation is limited primarily by the mobility of dislocations and not by dislocation interactions. These characteristics of deformation are consistent with the operation of easyb = (001) glide processes in these crystals. Crystals loaded along [110] exhibit small deformation transients which indicate both sluggish dislocation motion and some substructure formation. We speculate that cross-slip of dislocations from {110} to {010} planes is responsible for this effect. Deformation in hard oriented crystals provides evidence for both mo-bility and substructure controlled deformation. Creep in hard oriented crystals is characterized by a dramatic sigmoidal transient suggesting very low dislocation mobility. However, the strain hardening observed in monotonic tests and the transient responses suggest that deformation is also limited by a dislocation substructure that forms during deformation. These findings support the conclusion, explored fully in a forthcoming article, that creep deformation in the hard orientation is controlled by the motion and interaction ofb = (101) dislocations.

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Copyright information

© The Minerals, Metals & Material Society and ASM International 1996

Authors and Affiliations

  • K. R. Forbes
    • 1
  • U. Glatzel
    • 2
  • R. Darolia
    • 3
  • W. D. Nix
    • 4
  1. 1.Plasma Etch DepartmentSEMATECHAustin
  2. 2.Institute for Materials Science, Technical University of BerlinBerlinGermany
  3. 3.Engineering Materials Technology Laboratories, GE Aircraft EnginesCincinnati
  4. 4.Department of Materials Science and EngineeringStanford UniversityStanford

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