Quantifying the Influences of Carbides and Porosities on the Fatigue Crack Evolution of a Ni-Based Single-Crystal Superalloy using X-ray Tomography

The detrimental effects of carbides and porosity on the fatigue crack initiation and propagation of nickel-based single-crystal superalloys have been reported by many previous studies. However, few studies have quantitatively compared the fatigue damaging effects of carbides and pores on the fatigue crack evolution. In this study, a high-resolution X-ray computed tomography (XCT) characterization of a DD5 nickel-based single-crystal superalloy during fatigue test was performed. The evolution of carbides, pores and cracks at all stages was observed and tracked. In order to quantify the 3D microstructures, a new damage factor that correlates the morphology of fracture surface with crack evolution behaviors was proposed. It was found that porosity was more detrimental than carbides in crack initiation and propagation during fatigue tests. Furthermore, pore spacing has been found to be the most significant factor among all controlling pore characteristics in the crack initiation stage and sphericity is the most critical pore characteristic in the crack propagation stage. Therefore, by statistically analyzing the evolution of carbides and pores during fatigue tests in this study, the underlying fatigue cracking mechanism of nickel-based superalloys is revealed.