Published online by Cambridge University Press: 01 January 1992
The low cycle fatigue response of polycrystalline NiAl above the brittle-to-ductile transition temperature (BDTT) was investigated. Samples of nominally stoichiometric NiAl were prepared from extruded ingots and from hot isostatically pressed (HIP'ed) prealloyed powders. The fatigue samples were cycled in a fully reversible fashion at plastic strain ranges between 0.06 and 1.0% in air. The HIP'ed NiAl material was also tested in vacuum at 1000 K. Both processing route and environment were found to have an effect on fatigue life. The lives of the powder samples were about a factor of three less than the cast and extruded material, which was attributed to the lower flow stress of the wrought NiAl. An environmental effect was noted for the HIP'ed material with a factor of 2-3 increase in fatigue life when samples were tested in vacuum compared to samples tested in air. In general, fatigue behavior for NiAl at high plastic strain ranges was typical of most metals, exhibiting a Coffin-Manson strain life behavior with a slope of -0.7. However, fatigue life of the HIP'ed powder material at low plastic strain ranges was controlled by intergranular cavitation and creep processes leading to a change in the slope of the fatigue life curve. Both materials exhibited cyclic softening over the majority of their fatigue lives with fatigue crack propagation occurring predominantly by intergranular mechanisms and final fracture by tensile overload occurring in a transgranular manner. Overall, the 1000 K fatigue life of NiAl was superior to conventional superalloys on a plastic strain range basis partly because of its high ductility and low flow stress but NiAl was not competitive on a stress range basis. The results indicate that binary NiAl has excellent plastic strain cycling capabilities and would be an acceptable material for the matrix phase of intermetallic matrix composites.