Temperature dependence of the intrinsic small fatigue crack growth behavior in ni-base superalloys based on measurement
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INTRODUCTION
NICKEL-BASE superalloys have been used with high reliability for blades and vane applications in gas turbines in recent decadesJ H3J Recently, the application of directionally solidified and single crystal alloys has become common in advanced gas turbines. On these materials, several kinds of studies, such as on the effects of temperature,C~m loading frequency,c~,4,6J stress multiaxiality,m environment,E3] microstmcture,V 3] and crystallographic orientation~sJ on the crack growth during fatigue, creep, and creep fatigue, have been performed. Referring to some of the works which dealt with the temperature dependence in fatigue crack propagation resistance at high temperature, the following trends are reported. Some of them have confirmed the decreasing~3,41and/or insensitivd2~property with temperature, respectively. However, others have indicated increasing fatigue crack propagation resistance with temperature, tSj The reason that several kinds of tendencies even on such a basic problem are reported seems to be closely related not only to the materials but also to the lack of consideration of crack closure phenomenon,tZ0] which plays an important role in fatigue crack propagation due to the difficulty of the measurement at high temperatures. Furthermore, most of the preceding studies dealt with the propagation of physically long cracks using the compact tension specimens, and there is little information on the small fatigue crack growth process, v~ The importance of studying small fatigue crack growth behavior is briefly summarized from the viewpoints of engineering and aca-
M. OKAZAKI, Associate Professor, and H. YAMADA, Graduate Student, are with the Department of Mechanical Engineering, Nagaoka University of Technology, Nagaoka 940-21, Japan. S. NOHMI, Senior Engineer, is with Kawasaki Heavy Industries Co. Ltd., Kawasaki-cho 11, Akashi, Japan. Manuscript submitted June 24, 1994. METALLURGICAL AND MATERIALSTRANSACTIONS A
demic interest as follows:El~ (1) for the most part, fatigue life is dominated by the small crack growth process; (2) small crack growth behavior may not conform to that measured conventionally with physically long cracks (i.e., the lack of similitude in propagation rates between small and long cracks, which provides dangerous information on reliability in some cases); and (3) small crack growth must be notably affected by microstructures, such as grain boundary and strengthening precipitates, and by environment. In this work, the effect of temperature on physically small fatigue crack growth behavior of directionally solidified and single crystal Ni-base superalloys was investigated at temperatures between 873 and 1123 K. The small crack growth behavior thus obtained was compared with the physically long crack propagation behavior. The factors which lead to the lack of similitude law in propagation rate between small and long cracks were also discussed, based on the measurement of crack closure levels and the chemical analysis near the crack tip using the electron probe microa
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