In-situ SEM-EBSD investigation of the low-cycle fatigue deformation behavior of Inconel 718 at grain-scale

The low-cycle fatigue (LCF) deformation behavior of the Inconel 718 (IN718) specimen with micro-notch was investigated by an in-situ fatigue test inside a scanning electron microscope (SEM) combined with electron backscattered diffraction (EBSD). The initiation condition and propagation path of cracks were observed by SEM. The essential reasons for crack initiation, propagation and fatigue fracture are elaborately characterized by crystallographic information obtained from EBSD. Meanwhile, the strain evolution during fatigue deformation simulated by the crystal plastic finite element (CPFE) method is consistent with the experimental content, which further verifies and supplements the experimental results. Two LCF deformation mechanisms (location-related) of the IN718 specimen are proposed at grain-scale based on the direct observation of crack characteristics and in-depth analysis. Near the edge micro-notch region, the deformation mechanism is mainly related to crack initiation and propagation: the plastic strain localization along the most easily activated slip band leads to the initiation of fatigue microcracks inside the grain. The crack propagation is carried out by microcrack coalescence in different grains. Away from the edge micro-notch region, the deformation mechanism is mainly related to the instantaneous fatigue fracture: fatigue fracture comes from the intergranular plastic strain accumulation and heterogenous deformation at grain-scale. The result also deeply explains the fatigue crack propagation in metals (two consecutive modes).