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

研究了三种镍基高温合金的高温疲劳裂纹扩展行为；单晶合金、定向凝固合金和多晶合金。在对裂纹开闭行为进行测量的基础上，研究了温度和显微组织的影响。主要结论如下：(1)ni基高温合金的疲劳小裂纹扩展行为对晶界、晶体滑移面、滑移方向以及γ′析出相的大小等微观组织非常敏感。考虑弹性柔度和裂纹闭合程度对温度的依赖关系的固有小疲劳裂纹扩展阻力随温度的升高而降低。环境作用下γ′沉淀的溶解与上述温度依赖性密切相关。(2)长裂纹扩展信息并不总是适用于小裂纹。即使在较低的应力强度系数下，小裂纹也会扩展。在其他材料中也经常发现类似的现象，例如金属基复合材料。这些实验证据表明了研究小裂纹扩展行为的重要性。(3)在单晶合金中，长裂纹和小裂纹扩展速率的差异由裂纹闭合现象补偿，而在多晶合金中则没有。强度性能的不均匀性和断裂方式的差异与上述长裂纹和小裂纹的差异密切相关。

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.