## 4 Results and Discussion
### Evolution of Cyclic Hardening Potential and Hardness During TMF Loading
However, after 2000 TMF cycles, Crofer(r)22 H exhibits a decrease in hardness, but an increasing \(|\mathrm{e_{II}}|\). Consequently, the damage tolerance of Crofer(r)22 H seems to be improved within the 2\({}^{\mathrm{nd}}\) half of TMF-lifetime, i.e., \(N_{TMF}>2000\). The increase of cyclic hardening potential, and hence, damage tolerance, of Crofer(r)22 H in this state corresponds to the prolonged damage phase in TMF-test reported in Reference [5]. To explain the results observed in CITs at Crofer(r)22 H, different microstructural phenomena have to be considered. Because of the massive plastic deformation at the beginning of the TMF-experiments, an increase of dislocation density, leading to smaller \(|\mathrm{e_{II}}|\) and higher hardness (see Reference [16]), is assumed. In addition to that, the results in Reference [5] demonstrate an accelerated Laves phase formation caused by TMF-loading. Note that higher dislocation density would be expected to be non-permanent in TMF-condition, but because Laves phase precipitates pin the dislocations, it is assumed to be quasi-permanent [5]. The number of the Laves phase particles significantly increases in the first 2000 cycles, and afterwards, decreases until failure, which at least roughly correlates with the hardness maximum observed in CITs, but also with the respective minimum in cyclic hardening potential (comp. Figure 5 and Reference [5]). Besides a decrease in a number of the Laves phase particles, a pronounced coarsening of the Laves phase particles can be observed at \(N_{TMF}>2000\), which obviously leads to an increase of \(|\mathrm{e_{II}}|\) and lower hardness. In this context, the results of Reference [13] for precipitation hardened Cu alloyed steels show, that the hardness, as well as the cyclic hardening potential, depend on the number and size, as well as the coherency state of Cu precipitates. Consequently, the Laves phase evolution has to be considered to explain the results observed in CITs. It is assumed that the combination of the Laves phase particle evolution and cyclic plastic deformation in TMF-tests leads to the observed changes in cyclic hardening potential, as well as hardness. Future research aims to investigate the interrelation between dislocation density, Laves phase particles, and mechanical properties. Figure 6: Comparison of the h\({}_{\mathrm{a,p}}\)-N curves of specimens made of P91, as well as Crofer®22 H and loaded for a different number of TMF-cycles \(N_{TMF}\). Comparing the different materials, Crofer(r)22 H shows more pronounced changes in hardness and \(|\)e\({}_{\rm II}\)\(|\), due to TMF-loading (see Figure 5), which further can be seen in the h\({}_{\rm a,p}\)-N curves (see Figure 6). While P91 has a higher hardness and smaller \(|\)e\({}_{\rm II}\)\(|\) in the initial state, an oop TMF-loading of approximately 1000 cycles yields higher hardness and lower cyclic hardening potential for Crofer(r)22 H. However, at \(N_{TMF}>2000\), Crofer(r)22 H shows increasing cyclic hardening potential, whereas P91 exhibits a stabilized state, underlining the higher damage tolerance of Crofer(r)22 H described in Reference [5]. Furthermore, the h\({}_{\rm a,p}\) values obtained for Crofer(r)22 H are smaller compared to the ones of P91 (see Figure 6), which is less pronounced at higher \(N_{TMF}\), demonstrating the more extensive microstructural changes of Crofer(r)22 H during TMF-loading. As a summary, the results observed in CITs correspond to the cyclic deformation behavior obtained in TMF-tests. Moreover, for Crofer(r)22 H, microstructural changes correlate with changes in cyclic hardening potential and hardness obtained in CITs. Furthermore, the results of CITs underline the assumption, that Crofer(r)22 H has a significantly higher damage tolerance. Additionally, Crofer(r)22 H shows in relation to P91 more pronounced changes in damage tolerance during TMF-loading, i.e., an increasing cyclic hardening potential during TMF-test, leading to a longer lifetime.