Low cycle fatigue behavior of polycrystalline Ni 3 Al alloys at ambient and elevated temperatures
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INTRODUCTION
SINCE the discovery of boron ductilization of polycrystalline Ni3AI, t~J significant efforts have been made to develop ordered materials for use in high-temperature structural components. Previous investigations of the fatigue [2,3'41 and fatigue crack propagation r4-sJ behavior of these materials have indicated that they may be wellsuited for use in fatigue-critical applications. Unfortunately, very little is understood about the microstructural response of these alloys during LCF. The majority of previous LCF studies on Ni3A1 alloys have been made primarily on monocyrstals, t9-~j with little reported about polycrystalline behavior. 13a61 Furthermore, there has been no attempt to evaluate the effect of Cr additions on LCF resistance at either ambient or elevated temperature. This addition is an important consideration, as additions of approximately 8 at. pet Cr are used to enhance the high-temperature ductility of polycrystalline Ni3A1 by reducing grain boundary dynamic embrittlement3 ~71. *Dynamic embrittlement results in intergranular crack propagation from the application of normal stresses in the presence of gaseous oxygen at elevated temperature.
Such information is crucial for guiding the further development of these alloys for use in fatigue-critical applications. Low cycle fatigue studies of single phase L12 alloys GRAHAM WEBB, formerly Postdoctoral Research Fellow, School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA, is Materials Engineer, AlliedSignal Engines, Phoenix, AZ. STEPHEN D. ANTOLOVICH, Chairman and Professor, is with the Department of Mechanical and Materials Engineering, Washington State University, Pullman, WA. Manuscript submitted December 28, 1992. METALLURGICALAND MATERIALS TRANSACTIONS A
have all demonstrated various degrees of cyclic hardening, the exact characteristics of which are dependent upon the temperature, crystal orientation, and applied strain range.t3,9J 1-14]Cyclic hardening was observed until failure, except for high temperatures or for low cyclic strain amplitudes, for which a saturation stress was achieved prior to failure. For polycrystalline materials, significant hardening is observed at ambient temperature, 131but not at elevated temperature (600 ~ For these materials, hardening is followed by extensive softening, t~3j Transmission electron microscope (TEM) investigations t~~ of cyclically deformed monocrystals revealed that cyclic deformation results from the motion of (110) screw dislocations within slip bands. In these investigations, cyclic hardening was correlated to the accumulation of various forms of dislocation debris (jogs and dipoles) as a result of the intersection of primary glide dislocations during cyclic deformation, t1~ Previous investigations have indicated that fatigue crack initiation during LCF is dependent upon both temperature and environment, t~~ Fatigue cracks in single crystals have been observed to initiate at PSBs oriented parallel to the primary {111} slip planes for temperatures up
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