Small fatigue crack initiation and propagation behaviors at room temperature of a directly aged nickel-based superalloy GH4169 were studied by in-situ scanning electron microscope (SEM) fatigue test. The results showed that under tension-tension fatigue loading with R=0.1

small fatigue the crack initiation period was only about 20% of the total fatigue life. Small fatigue cracks initiated at surface inclusions and grew as semi-elliptical surface cracks. The surface crack broke through one side of the specimen and subsequently became a corner crack with fast growth rates

leading to the final fracture. The early growth of small fatigue cracks was strongly affected by the local microstructure of materials

and the scattering of small fatigue crack growth rate was large.