## 3 Results and Discussion
### Damage Initiation and Crack Propagation  
Damage, assessed from deviation of the fatigue curve from a linear approximation to the stable curve section, appears at 1961 cycles in P91 (Figure 3a, Table 2). A site of preferential crack initiation and propagation is hard to identify because of the martensite lath degradation, but typically crack branching at triple junctions (Figure 4e) is not encountered. Consequently, P91 exhibits a mixed inter-/intragranular, not the expected fatigue typical, exclusively transgranular [43] crack propagation. The same was encountered in the case of long crack propagation under creep fatigue conditions at 600 \({}^{\circ}\)C in X20 steel [44; 45], where a similar type of polygonization of the martensite lath microstructure took place because of plastic strain agglomeration in front of crack tips. Based on the results it seems plausible to utilize such pronounced microstructural changes for the assessment of fatigue damage susceptibility of AFM steels. Nevertheless, further detailed examination on the relevance of cycle parameters to polygonization, the relation of the extent of polygonization to macroscopic crack initiation and extension to other AFM steels is necessary for secure application in damage and remaining life assessment. Furthermore, creep strength determination of such pre-fatigued microstructures appears to be imperative.  
Figure 7: Scanning electron micrographs (post-processed, binarized and inverted for better particle visibility) of intragranular Laves phase precipitate evolution in ferritic Crofer®22 H steel during TMF cycling: 10 (**a**), 20 (**b**), 35 (**c**), 250 (**d**), 2000 (**e**), 3000 (**f**) and after failure at 4342 cycles (**g**).  
In Crofer(r)22 H, damage is induced at 1993 cycles (Figure 3a, Table 2). Crack nucleation and propagation are typically transgranular (Figures 9 and 10). Formation and migration of sub-grain boundaries in the grain interiors is hindered by the intragranular Laves phase precipitates. Polygonization phenomena for this reason are constricted to areas of large plastic distortions, like slender zones along fatigue crack edges (\(\sim\)50-60 \(\upmu\)m, cf. Figure 9b) and PFZs, in which dislocation pinning by precipitates is not effective enough along high-angle grain boundaries (Figure 9b). Crack progression is furthermore obstructed by precipitates in front of cracks, which prevent the formation of sharp crack tips (Figures 9 and 10) and in this way contribute to the described decrease of stress range degression per fatigue cycle in comparison to martensitic P91. The higher cyclic hardening potential and increased damage tolerance of Crofer(r)22 H is analyzed in detail in [46].  
Figure 8: Evolution of Laves phase particle population in ferritic Crofer®22 H steel during TMF cycling: Size classification by equivalent circle diameter (ECD) (**a**) total particle area and number (**b**).  
Figure 9: Limited sub-grain formation in close vicinity of a crack tip in ferritic Crofer®22 H after TMF failure: SEM image (**a**) and EBSD band contrast image with fatigue-induced sub-grain boundaries (misorientation definition angle: \(>\)3\({}^{\circ}\), step size: 0.438 \(\upmu\)m) added (**b**). HAGB denotes pre-existing high angle grain boundaries.  
Particle-covered high-angle grain boundaries and sub-grain boundaries, condensing from cyclically induced dislocations, in front of and around the edges of cracks as well as within PFZs, often lead to crack branching (Figure 10). This dissipates energy away from the main crack path and constitutes another mechanism causing damage to progress and final failure to appear less rapid in ferritic Crofer(r)22 H steel.  
Enhanced, deformation-induced precipitation, achieved by optimized alloy composition, may potentially lead to a kind of active crack obstruction by particles nucleating in front of propagating crack tips. By this, further increase of fatigue strength may be achievable.