Fatigue crack initiation and failure analysis of IN718 alloy after push–pull loading at room temperature and at 700 °C

The paper deals with the effect of applied fatigue loading with cycle asymmetry parameter R = −1(a symmetric push–pull, σ_m = 0 MPa), test temperature and loading frequency on the initiation of fatigue cracks in superalloy IN718. Two sets of experimental specimens were fabricated, for fatigue tests at ambient temperature and fatigue tests at 700 ± 5 °C. The loading frequency was f ≈ 20 150 Hz at ambient temperature and f ≈ 58 Hz at 700 ± 5 °C. The number of cycles to fracture, N_f = 2.10⁷, was determined as the so-called “run-out”. SEM fractography analysis was performed on all specimens to define the fatigue crack initiation mechanism. Fatigue crack initiation was always performed on the surface of the samples, most often by the slip mechanism (PSB′s − Persistent Slip Bands) − tests at ambient temperature, in the case of tests at 700 ± 5 °C, initiation was observed on the oxide phases on the surface or just below the surface. The initiation of oxide phases (mostly of NiO or FeO type) on the surface at 700 ± 5 °C is related to the creep-fatigue load interaction. After fatigue tests, S-N fatigue life curves were plotted with emphasis on determining the N_i − critical number of cycles required to initiate a fatigue crack with approximate length “a” ≈ 25 μm. Based on the tests performed, it can be concluded that the number of cycles required to initiate a crack of critical dimensions increases with decreasing load amplitude σ_a. In general, the fatigue crack initiation (depending on the microstructure and the δ-phase fraction) in the case of superalloy IN718 represents approximately 80–90 % of the total fatigue life.