The fatigue failure occurred at the NZ was obtained only in the VHCF range as plotted in Fig. 4. In this case, it was found that the fatigue crack initiation site of one specimen was located at the surface and another one at the subsurface. The fracture morphology of the specimen with subsurface initiation site is shown in Fig. 7. It can be seen that the fracture surface at the NZ is more flat than that at the TMAZ because of the generation of fine and equiaxed recrystallized grains. At the center of the initiation zone, a particle can be detected with a length of 12 μm and this is very similar to the failure mechanism of high-strength steels in the VHCF regime [21], [22] and [23], which the crack initiation mostly takes place at inclusions inside the specimens and is accompanied by the formation of a ring-like fracture surface, the so-called fish-eye.