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(单晶合金、定向凝固合金和多晶合金)的高温疲劳小裂纹扩展行为。(1)镍基高温合金的疲劳小裂纹扩展行为对晶界、晶界、滑移面和滑移方向等微观组织以及伽马析出物的大小都很敏感。考虑了弹性柔度和裂纹闭合程度对温度的依赖关系，固有小裂纹扩展阻力随温度的升高而减小。环境效应对伽马析出物的溶解与上述温度依赖性密切相关。(2)长裂纹扩展的信息并不总是适用于小裂纹。即使在低于长裂纹门槛值的应力强度因子下，小裂纹也会扩展。类似的现象也经常出现在其他类型的材料中，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)单晶合金中长裂纹扩展速率与小裂纹扩展速率之间的差异被裂纹闭合现象所补偿，而在多晶合金中则不能。强度性能的不均匀性以及断裂方式的差异与长裂纹和小裂纹的上述差异密切相关。

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.