Small fatigue crack propagation rate and behaviours in a powder metallurgy superalloy: Role of stress ratio and local microstructure

The role of stress ratio and local microstructure on small crack propagation (0.065–1.0 mm) behaviours in superalloy FGH4096 were investigated using the Photomicroscopy method. ΔK correlated the growth rates of small cracks with different stress ratios well, which is attributed to the low crack closure level. Statistical and crystallographic analysis revealed that grain boundaries resulted in crack temporary arrests, which is independent of loading conditions and crack length. Temporarily arrested cracks propagated further by connecting to the secondary crack that initiated on the slip plane that parallels to the twin boundary or in the grain with a local highest Taylor factor.