Influencing the shape of creep cavities in nickel aluminides by stress changes
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J. H. Schneibel Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6116 (Received 19 June 1989; accepted 26 July 1989)
A nickel aluminide of composition Ni-23.5Al-0.5Hf-0.2B (at. %) is found to develop intergranular cavities with a quasi-equilibrium shape when it is creep deformed at high temperatures and low stresses. When the temperature is lowered and the stress significantly increased (by a factor of 10), crack-like noses form at the tips of the original quasiequilibrium cavities. Although this observation is in general agreement with previously published computer simulations, certain complications are found as well. The experimental and theoretical relationships between growth rates and sizes of the crack-like noses are inconsistent, and growth rates vary from nose to nose. Possible reasons for these effects are discussed.
I. INTRODUCTION During the high-temperature creep of polycrystalline materials cavities are often formed at the grain boundaries.1 The growth and coalescence of these cavities ultimately cause intergranular fracture. Creep ductilities can be quite low, of the order of a few percent,2'3 and a thorough understanding of the nucleation and growth mechanisms responsible for creep fracture is of interest in both creep life predictions as well as in reliability assessments. Cavities can grow by several different mechanisms. They may grow by plastic deformation of the matrix surrounding them4'5 or by vacancy absorption, i.e., a diffusional process as originally described by Hull and Rimmer.6 Diffusional growth can occur in the quasi-equilibrium mode, in which surface diffusion is fast enough to maintain the equilibrium shape of the growing cavities.7'8 If, on the other hand, surface diffusion cannot maintain the equilibrium shape, crack-like (thin disk-shaped) cavities will form.9'10 As the cavity sizes increase during a creep test, the growth mechanism may change. Cavities growing initially in the quasi-equilibrium mode may switch over to the crack-like growth mode, or to the plastic growth mode. Such transitions will usually occur gradually and are difficult to analyze in detail in typical materials which exhibit broad cavity size distributions.11 Abrupt changes in the cavity growth mechanism can be induced by stress changes. For example, a substantial stress decrease might cause existing crack-like cavities to gradually adopt quasi-equilibrium shapes. If the applied stress is reduced to a negative value (i.e., compression or hot-pressing), existing disk-shaped cavities might become thinner until they disappear. A different behavior is expected for stress increases. At low stress levels, relatively large quasi-equilibrium cavities can be grown. If the stress is increased sufficiently J. Mater. Res., Vol. 4, No. 6, Nov/Dec 1989
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GRAIN BOUNDARY
FIG. 1. Computer simulation of crack-like noses forming at a quasiequilibrium cavity (after Nix and co-workers'3'14).
these cavities will now start to grow in a c
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