Low Cycle Fatigue Behavior of Polycrystalline NiAl at 1000 K
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LOW CYCLE FATIGUE BEHAVIOR OF POLYCRYSTALLINE NiAI AT 1000 K R.D. Noebe and B.A. Lerch NASA Lewis Research Center, M.S. 49-3, Cleveland, OH, 44135. ABSTRACT The low cycle fatigue response of polycrystalline NiAI 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 NiAI 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 NiAI. 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 NiA1 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 NiA! was superior to conventional superalloys on a plastic strain range basis partly because of its high ductility and low flow stress but NiAI was not competitive on a stress range basis. The results indicate that binary NiAI has excellent plastic strain cycling capabilities and would be an acceptable material for the matrix phase of intermetallic matrix composites. INTRODUCTION In response to the need for high temperature structural materials, ordered intermetallic alloys and intermetallic matrix composites have been earnestly studied for the past decade. Of the many materials under consideration, NiAI is one of the few systems that has emerged as a promising candidate for further development as either a single crystal alloy or the matrix phase for intermetallic matrix composites. This is due to a number of property advantages including low density, high melting point, high thermal conductivity, excellent environmental resistance and the potential for significantly improving creep resistance through alloying [1,2]. While a reasonable understanding of the monotonic flow and fracture behavior of polycrystalline NiAI has been achieved over the years [2-4], cyclic properties have been neglected until recently. Room temperature fatigue tests have been performed on powder processed NiAI material and NiAI/A12 0 3 composites under displacement control between set stress
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