## 4 Results and Discussion  
### Evolution of Cyclic Hardening Potential and Hardness During TMF Loading  
The TMF-tests demonstrate, in spite of slightly larger stress range and mean stresses (91 MPa for CroFer(r)22 H and 75 MPa for P91), a higher lifetime for CroFer(r)22 H compared to P91 (see Figure 3). While P91 exhibits a strong degradation of its initial martensitic lath structure, called \"polygonization\",  
Figure 4: (a) Stereoscopic micrograph with indentation points at crack edges and crack tip; schematic description of (b) indentation force \(F\)-indentation depth \(h\) hysteresis, as well as plastic indentation depth amplitude h\({}_{\text{a,p}}\) and (c) h\({}_{\text{a,p}}\).-N curves with h\({}_{\text{a,p II}}\)[13]. the ferritic CroFer(r)22 H shows an increase of the Laves phase particle fraction, caused by TMF-loading and leading to a higher stress range and maximum stress in relation to P91 (see Figure 3 and [5]). These microstructural phenomena described in Reference [5] are dependent on the number of TMF-cycles \(N_{\text{TMF}}\) and influence the material's damage tolerance. To determine the evolution of damage tolerance during TMF-loading, CITs were performed at different \(N_{\text{TMF}}\). Although the TMF-tests were performed at variable, elevated temperatures, and thus, at different conditions compared to the CITs performed at ambient temperature, the CITs enable the determination of the evolution of the material's mechanical properties, i.e., hardness and cyclic hardening potential, resulting from TMF-loading. Note that cyclic hardening potential, represented by \(\text{e}_{\text{II}}\), strongly correlates with the material's damage tolerance [17; 18]. Comparing the materials in their initial states (IS), Crofer(r)22 H has lower hardness, but significantly higher cyclic hardening potential, compared to P91 (see Figure 5)--which correlates to stronger damage tolerance indicated in Reference [5]. Moreover, Crofer(r)22 H shows lower \(\text{h}_{\text{a,p}}\) values in relation to P91, due to extensively more pronounced cyclic hardening during cyclic indentation (see Figure 6). For P91 a pronounced decrease of \(|\text{e}_{\text{II}}|\) can be observed during the first 250 TMF-cycles, which is accompanied by a hardness decrease, correlating with the cyclic softening observed in TMF-experiments (comp. Figure 4, Figure 5 and [5]). However, from the 250 th TMF-cycle on, P91 exhibits a stabilization of both hardness and \(|\text{e}_{\text{II}}|\), which also correlates to the cyclic deformation curves shown in Figure 3, as well as the \(\text{h}_{\text{a,p}}\)-N curves in Figure 6. The degradation of martensite laths discussed in Reference [5] is most pronounced in the regime after the 250 th cycle, and consequently, seems to have no significant impact on the mechanical properties determined in CITs, and thus, cyclic hardening potential. However, crack propagation can be influenced by this \"polygonization\", which cannot be detected in CITs and has, hence, to be considered by evaluating the TMF-behavior. Figure 5: Cyclic hardening exponent\({}_{\text{CHT}}\)\(|\text{e}_{\text{II}}|\) and martens hardness _HM_ of the initial state (IS) and subject to TMF-cycles for (**a**) P91 and (**b**) Crofer®22 H.  
In CITs, Crofer(r)22 H exhibit a significant increase of hardness and decrease of \(|\mathrm{e_{II}}|\) in the first 20 TMF-cycles, which is accompanied by a pronounced cyclic hardening indicated by increasing stress range in Figure 3. With increasing \(N_{TMF}\), in spite of decreasing stress range (see Figure 3), the increase in Martens hardness and decrease in cyclic hardening potential continues, leading to a maximum of hardness and minimum of \(|\mathrm{e_{II}}|\) at \(N_{TMF}=1000\). These extensive changes in the material properties are accompanied by a pronounced shift of h\({}_{\mathrm{a,p}}\)-N curves to higher values at \(N_{TMF}=250\) (see Figure 6), resulting in decreasing hardening potential in this condition.