Low- cycle fatigue behavior of polycrystalline nial at 1000 k
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INTRODUCTION
T H E R E is considerable interest in developing new structural materials in which high use temperatures and strength, coupled with low density, are minimum requirements. The goal for these new materials is to provide operational capability well beyond the working range of conventional superalloys. In response to this challenge, ordered intermetallic alloys have been the subject of extensive investigations. Of the many intermetallic alloys under consideration, NiA1 is one of the few systems that has emerged as a promising candidate for further development, m This is due to a number of property advantages, including low density, high melting temperature, high thermal conductivity, excellent environmental resistance, a relatively low brittle-toductile transition temperature (BDTT), and the potential for significantly improving creep resistance through alloying. ~2j However, minimal ductility and low fracture toughness at lower temperatures remain limitations of NiA1. Consequently, recent investigations on polycrystalline material have emphasized the need to understand the basic flow and fracture behavior of NiAP 3 61 and also have emphasized the potential for increasing lowtemperature tensile ductility through alloying 17'8'91 and improved processing techniques, l~~ While ductility improvements through alloying have been unsuccessful to date, 17'8'91 efforts at improving the processing and cleanliness of stoichiometric binary NiA1 have resulted in limited, but at least consistent and reproducible, room-temperature tensile elongations on the order of 1 to 3 pct for cast plus extruded (C+E) material. 13-5,8,~~ B.A. LERCH and R.D. NOEBE, Research Engineers, are with NASA Lewis Research Center, Cleveland, OH 44135. Manuscript submitted May 7, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
The consistent tensile ductilities obtained for C + E NiAI at room temperature subsequently have led to the investigation of fully reversed, low-cycle fatigue loading of polycrystalline NiAI.I~3'I4] At room temperature, polycrystalline NiA1 work hardens continuously to failure, eventually reaching stresses 60 pct greater than the monotonic fracture strength of the material. The plastic strain range-fatigue life relationship for C + E NiAI at room temperature had a much shallower slope, - 0 . 1 4 , than the value of - 0 . 6 usually observed for metallic materials. This shallow slope is representative of the brittle fracture behavior of NiA1 at room temperature, with fracture initiating at prior defects once a critical stress level is achievedJ 15] At intermediate temperatures, tensile ductility is no longer a use-limiting problem, because extruded binary NiA1 undergoes a brittle-to-ductile transition between approximately 550 and 700 K. Dramatic increases in tensile elongation, fracture strength, 13"5,6'8] and fracture toughness 116,171 occur in this temperature range. While the mechanism responsible for these changes in behavior is not fully resolved, the BDTT is probably a result of the operation of additional defo
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