## 4 Results and Discussion
### Evolution of Cyclic Hardening Potential and Hardness During TMF Loading
Investigation of the Local Cyclic Hardening Potential and Hardness in the Vicinity of a Fatigue Crack  
As shown in Reference [5] and the results of the CITs given above, the damage tolerance of Crofer(r)22 H is significantly higher, and TMF-induced changes are more pronounced compared to P91. Moreover, the discussion in Section 4.1 demonstrates, that the evolution of cyclic hardening potential of Crofer(r)22 H is strongly dependent on dislocation density and Laves phase particles, which relate to the amount of plastic deformation, being significantly higher at the crack tip. Therefore, CITs were performed at the crack edges, as well as the plastically deformed zone in front of the crack tip of a secondary fatigue crack observed at the Crofer(r)22 H specimen loaded for \(N_{TMF}=4342\) (see Figure 4a). These locally performed CITs enable a detailed analysis of the evolution of damage tolerance during fatigue crack propagation. The results obtained with \(F_{max}=1000\) mN shown on the left-hand side of Figure 7, exhibit slightly higher hardness and a significantly reduced cyclic hardening potential in front of the crack tip in relation to the material's volume in the gauge length. This is assumed to be caused by more pronounced plastic deformation in front of the fatigue crack. In correspondence to that, the crack edges also show decreased cyclic hardening potential. Due to higher amounts of plastic deformation, Laves phase particle formation is assumed to be more pronounced at the crack tip and edges (see Reference [5]), which also influences hardness, as well as \(|\)e\({}_{\rm II}\)\(|\) and is objective of further investigation. To analyze the differences between the crack tip and crack edge, CITs with \(F_{max}=100\) mN were conducted, leading to a higher spatial resolution. Due to the indentation size effect (see References [16; 22]), the CITs with \(F_{max}=100\) mN yield a higher hardness compared to \(F_{max}=1000\) mN (see Figure 7). By using the lower \(F_{max}\), a higher cyclic hardening potential at the crack edges in relation to the plastically deformed volume at the crack tip can be observed. This correlates to sub-grain formation at the crack edges caused by plastic deformation during crack propagation, which is less pronounced in the plastic zone around the crack tip (see Figure 8). However, this cannot explain the decreased hardness in the vicinity of crack edges compared to the crack tip. Moreover, it has to be considered, that the areas at the crack edges were plastically deformed in an earlier state of the TMF-test. This increases the possibility of the Laves phase particle formation at dislocations, which probably also influences cyclic hardening potential and hardness. This will be the focus of the ongoing work. Conclusively, the presented investigations at the fatigue crack demonstrate, that CITs can be used to determine local differences in the material's cyclic properties, which correlates to microstructural changes observed. The evolution of the complex microstructure of Crofer(r)22 H in the fatigue crack vicinity, and its influence on the results obtained in CITs will be investigated intensively in future work.