Intrinsic stage i crack growth of directionally solidified Ni-Base superalloys during low-cycle fatigue at elevated temp
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I.
INTRODUCTION
T H E use of single-crystal and directionally solidified Ni-base superalloys for turbine blade and vane applications has enabled significantly increased turbine inlet temperatures, t~-9] This has been due, in part, to the improved creep rupture properties of these materials, which lack the weakening grain boundaries of polycrystalline castings. Because a superior fatigue strength is also required for turbine blade and other applications, many studies on high- and low-cycle fatigue fracture behavior have also been carded out. [1-91 Summarizing the fatigue fracture characteristics in single and directionally solidified Ni-base superalloys, the crystallographic slip plays an important role, and they exhibit the typical features of stage I fatigue fracture, t~-71With respect to fatigue crack growth in Ni-base superalloys, the effects of temperature, [2'3'6-9] f r e q u e n c y , (2'8'9l stress multiaxiality, tSJ and crystallographic orientations I41have been investigated from the viewpoint of fracture mechanics. However, most of the above research deals with the propagation of physically long cracks by using the compact tension specimen as an example, and there is little systematic information on the process from crack initiation to intrinsic small fatigue crack growth, which can be obtained only by an elaborate investigation in smooth specimens without notch. On the other hand, in recent years, it has been apparent that small fatigue crack growth behavior in polycrystalline engineering materials may not conform to that measured conventionally with long cracks, namely, when the length of cracks is small or they approach the size of the microstructurally important features, such as grain boundary, t~~ In fact, it is well known that small fatigue crack growth is notably affected by grain boundMASAKAZU OKAZAKI, Associate Professor, and TETSUO TABATA, Graduate Student, are with the Department of Mechanical Engineering, Nagaoka University of Technology, Tomioka, Nagaoka, Japan. SHINICHIROH NOHMI, Senior Engineer, is with Kawasaki Heavy Industry Ltd., Kawasaki-Cho, Akashi, Japan. Manuscript submitted January 2, 1990. METALLURGICALTRANSACTIONSA
aries. However, kinetic data on intrinsic stage I fatigue crack growth in polycrystalline materials are scarce, t~5] and micromechanisms for crack growth have not been well elucidated. ]~6,17]For the purpose of quantitative investigation on such a stage I fatigue crack growth, especially on the interaction with grain boundaries, directionally solidified alloys having columnar grain boundaries have some useful advantages because of their metallurgical homogeneity and large grain size. Summarizing the above, the study on the intrinsic small fatigue crack growth at elevated temperatures by using smooth specimens of directionally solidified Ni-base superalloys has the following advantages from the viewpoints of science and engineering: (1) it can make clear the crack initiation and the following small crack growth behavior in single-crystal and directionally solidified N
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