Failure mechanism and life correlation of Inconel 718 in high and very high cycle fatigue regimes

High and very high cycle fatigue (HCF and VHCF) behaviours and failure mechanism of Inconel 718 were investigated via ultrasonic fatigue tests and microscopic characterization techniques. It is shown that the S-N curve continuously decreases in the HCF regime, while forming a plateau for VHCF. Cracks initiate from a single site on the surface or subsurface. Site-specific focused ion beam sampling and transmission electron microscopy characterization reveal that the crack initiation is governed by slip-induced localized plastic deformation. Microcracks initiate near non-metallic inclusions or slip band-grain boundary site. Plus, a life correlation model considering the size of micro-defect was proposed to better correlate the fatigue life.