Deformation Behavior of Polycrystalline Nial Cyclicly Deformed near the Brittle-to-Ductile Transition Temperature
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DEFORMATION BEHAVIOR OF POLYCRYSTALLINE NIAL CYCLICLY DEFORMED NEAR THE BRITILE-TO-DUCTILE TRANSITION TEMPERATURE Cheryl L. Cullers and Stephen D. Antolovich', Georgia Institute of Technology, Atlanta, GA; "presently at Washington State University, Pullman, WA Ronald D. Noebe, NASA Lewis Research Center, Cleveland, OH. ABSTRACT Low cycle fatigue (LCF) behavior of polycrystalline NiAI was investigated near the monotonic brittle-to-ductile transition temperature (BDTT) at plastic strain ranges of 0.5 and 1.0%. Between 600 and 700 K, NiAI exhibited rapid hardening for the first few cycles followed by a stress plateau and a subsequent return to hardening. Slip traces were observed on the gage surfaces of most LCF specimens using scanning electron microscopy (SEM). The fatigue properties in this intermediate temperature range (600 to 700 K) were found to be a logical transition between previously reported ambient and elevated temperature properties. Transmission electron microscopy (TEM) confirmed that the dislocations had typical Burgers vectors. A cellular dislocation structure began developing before saturation was achieved. This structure transformed at longer lives to elongated cells and eventually to veins of dislocation tangles. The resulting dislocation morphology did not change from 600 to 700 K, but the dislocation density decreased noticeably. INTRODUCTION Stoichiometric NiAl possesses excellent oxidation resistance, low density, and high melting temperature. Despite these advantages, binary NiAl exhibits poor high-temperature strength and low ambient-temperature ductility and fracture toughness. However, NiAl experiences a sharp transformation from brittle to ductile behavior. This change occurs at a low temperature compared to other intermetallics and is an important phenomenon in the behavior of NiAI. Above about 600 K, fracture strength and tensile ductility increase markedly and the percentage of transgranular fracture increases [1]. While agreement on the mechanism responsible for this change in behavior has not been reached, several studies [1-5] have suggested that thermally activated dislocation climb provides the additional independent deformation mechanisms necessary to accommodate deformation near the grain boundaries. Fatigue properties of NiAl alloys have been investigated only to a limited extent. Bain, et al. [6] have examined the cyclic behavior of single crystal Ni-49.9 Al-0.1 Mo at room temperature and 1033 K. Preliminary results by Smith, et al. [7] of room temperature fatigue properties for oriented NiAl single crystals have also been reported. Lerch and Noebe have performed LCF tests on polycrystalline NiAI at both room temperature [8] and 1000 K [9]. Some general fatigue behavior trends can be drawn from the previous NiAl fatigue studies. Both single and polycrystalline NiAI cyclicly hardened and fractured with little or no stable crack growth when tested at room temperature . In fact, the polycrystalline samples cyclicly hardened to stresses -60% greater than the monotonic tensile frac
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