Crystallographic Fracture Mechanics Study on Creep-Fatigue Small Crack Propagation in Ni-base Superalloyas at High Temperatures

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Small fatigue crack growth behavior was investigated in three kinds of Ni-base superalloys at high temperatues ; a single crystal alloy, a directionally-solidified alloy and a pollycrystalline alloy. The effects of temperature and microstructures were also studies base on the muasurement of the crack openig and closing behavior.The main conclusions obtained are summarized as follows :(1) The small fatigue crack growth behavior in Ni-base superalloys are sensitive to the microstructures, such as grain boundaries, crystallographic slip plane and slip direction, and the size of gamma'precipitates. The intrinsic small fatigue crack growth resistance, in which the temperature dependence of elastic compliance and that of crack closure level are taken into account, decreases with temperature. The dissolution of gamma'precipitates by environmental effect closely contributes to the above temperature dependence.(2) The information on the long crack growth are not always applied to thesmall crack. The small cracks propagate even at lower stress intensity factor than the long crack threshold level. The similar phenomena are often found in other kinds of materials, such as the metal-matrix composites materials. These experimental evidences indicate the importance of the investigation of small crack growth behavior.(3) The diffecence between the long and small crack growth rates in the single crystal alloy are compensated by the crack closure phenomenon, but not in the polycrystalline alloy. The inhomogenity of strength properties as well as the difference of the fracture mode are closely related to the above difference between the long and small cracks.