## 5 Discussions  
### Fracture Morphology  
Fracture surface was observed using the OM and SEM. Crack mechanism was disclosed through the feature of fracture morphology. It is necessary to point out that specimen did not break into two parts. In other words, the test device stopped before the crack propagated to all the test section. The cracked specimen was pulled by another tensile fixture. One, single initiation site existed in all the fractures, regardless of the heat treatment, loading level, or fatigue life. \"Fish eye\", which occurs in some VHCF cases [41; 42], did not appear in this study. Initiation was situated at the surface or close to the subsurface, as shown in Figure 9. The propagation duration distributed from several to tens of seconds depending on total life, while it was always less than 1% of total fatigue lifetime. SEM micrographs of the fracture surface of aged Inconel 718 are shown in Figures 10 and 11. It is widely noted that fatigue fracture can be generally divided into several typical areas (Figure 10a): initiation area (I) with a relatively flat surface, propagation area (II) with a distinctly rough area and with radial streaks along the crack propagation direction, and dimples' zone (III) due to overloading by another tensile grip. Figure 10: SEM micrographs of fracture surface of aged Inconel 718 (\(\sigma_{a}=600\) MPa, \(N_{f}=1.58\times 10^{6}\) cycles): (**a**) fatigue fracture, (**b**) initiation, (**c**) nucleation, (**d**) propagation. Fine granular area (FGA) [43], also called optical dark area (ODA) [4] and granular bright facet (BGF) [44], indicates the boundary of initiation area I, as marked in Figure 10b. Secondary electrons were excited and emitted from the sample, then entered into the receiver probe inclined to the fracture surface. Thus, the steep surface and radial edge appeared brighter than flat surface FGA (Figure 10b). [PERSON] [4] assumed that FGA zones are caused by hydrogen embrittlement, in which hydrogen is trapped. [PERSON] [45] assumed that they are related to the polygonization of microstructure followed by debonding, resulting in crack initiation. [PERSON] [46] proposed that a fine granular layer is observed only when the fatigue testing is conducted under negative load ratios, not under positive ratios. Crack was initiated at a subsurface smooth facet in the short life case (Figure 10c). The facet had a length of nearly 50 \(\upmu\)m and was located at 100 \(\upmu\)m from the specimen surface. According to the EDS analysis, the chemical composition of this facet was similar to the surrounding matrix. It was not a precipitation or inclusion. The formation of such facet has not been clearly explained up to now. Some authors [47; 48] explained that such a fracture surface pattern comes from the sliding plane, while another author [21; 37] proposed a twinning mechanism to explain the very smooth surface. It is supposed that the grain boundary or twin boundary that already existed before the test could explain the very smooth surface in this study. Propagation area (II) can be divided into several parts according to the difference in the crack growth rate (CGR): near threshold regime, Paris regime, and rapid propagation regime. Quasi-cleavage (or cleavage-like) circumferential beach marks and fine striations appeared (Figure 10d) with the increase in stress intensity factor (SIF), which indicated the stable crack propagation. For the long-life cases, there was no obvious inclusion or facet in the center of the initiation zone (Figure 11a). It is necessary to point out that secondary crack perpendicular to the main crack significantly improved the fatigue life (Figure 11b). However, the mechanism of the formation of such a secondary crack is still unclear. The presence of a secondary crack maybe the evidence of extensive plastic deformation ahead of the crack tip [49].