Crack initiation mechanisms and life prediction of GH4169 superalloy in the high cycle and very high cycle fatigue regime

In this study, the high cycle and very high cycle fatigue behavior of the GH4169 superalloy was investigated. The results of the fatigue tests demonstrated that failures in the high cycle fatigue regime were initiated from the material surface, whereas failures in the very high cycle fatigue regime had two competing modes of crack initiation: surface and internal. The fatigue fracture analysis revealed a quasi-cleavage fracture mode with facets for internal crack initiation. Additionally, the electron backscatter diffraction (EBSD) results indicated that the dislocations located at the internal twin boundaries of the material were preferentially activated under fatigue cyclic loading, resulting in significant strain localization at the twin boundaries. The {111} <110> slip systems with high Schmid factors had a higher probability of activation. The stress intensity factor (SIF) estimated based on rough area (RA) and facet sizes correspond to the fatigue long crack and short crack growth thresholds of the material, respectively. Finally, a modified FIP fatigue life prediction model was proposed based on the microstructure of crystallographic facets, and the results from experiment and the model prediction were found to be in good agreement.