在室温和650 ℃条件下，研究晶粒尺寸对GH4169合金疲劳小裂纹萌生机理和扩展行为的影响。通过在960 ℃、 1 010 ℃、1 020 ℃、1 050 ℃条件下，各保温1 h来改变试样晶粒尺寸。通过光学显微镜来观察不同热处理后的微观组织。利用覆膜法监测小裂纹扩展过程。结果表明，两种温度下疲劳小裂纹萌生机理和扩展模式在细晶组织和粗晶组织间均会发生转变。室温和650 ℃条件下，裂纹扩展速率在小裂纹阶段近似恒定。室温下，平均晶粒尺寸变化对小裂纹扩展速率几乎没有影响。650 ℃下，随着平均晶粒尺寸增加，小裂纹扩展速率增大。无论哪种温度下，一旦表面裂纹长度达到一定临界尺寸，小裂纹会加速扩展，进入长裂纹阶段。室温下不同平均晶粒尺寸试样临界裂纹长度基本相同，650 ℃下随着平均晶粒尺寸增加临界裂纹长度增加。

Addressing grain size effect on the initiation and propagation behavior of small fatigue crack of GH4169 alloy at room temperature and 650 ℃. Changing the grain size of specimen by solid solution heat treated at 960 ℃、1 010 ℃、1 020 ℃、1 050 ℃ about 1 h. The microstructures after different heat treatments are investigated by optical microscope(OM). The replica method is used to measure the crack length. Results show that there are transition of fatigue crack initiation mechanism and crack propagation mode between fine-grained material and coarse-grained material at both room temperature and 650 ℃. Crack propagation rates are almost constant in small crack stages at both room temperature and 650 ℃. The variations of average grain size have almost no effect on the small crack rates at room temperature. But at 650 ℃ small crack propagation rates increase with the increase of average grain size. Once the length of surface cracks reach a critical size,the cracks propagate quickly. At room temperature the critical sizes of surface crack of specimens with different average grain size are almost uniform. However the critical size of surface cracks increase with the increase of average grain size.