## 3 Methods  
Total strain-controlled out-of-phase (oop) thermomechanical fatigue (TMF-) tests were performed in accordance with the European Code-of-Practice [33]. Therefore, cylindrical specimens with a gauge length of 15 mm and a diameter of 7 mm, as well as servo-hydraulic fatigue testing systems, equipped with inductive specimen heating systems, were used. For temperature control, a type R ribbon thermocouple was applied in the center of the gauge length. The strain was directly measured using high-temperature extensometers. All experiments were conducted in a temperature range from 250 to 650 \({}^{\circ}\)C with heating, as well as the cooling rate of \(dT/dt=10\) Ks\({}^{-1}\). To reduce creep deformation \begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline
**Material** & C & N & Cr & Mn & Si & Nb & W & V & Al & Ni & Mo & La & Ti \\ \hline
**P91** & 0.1 & 0.051 & 8.1 & 0.46 & – & 0.07 & – & 0.18 & 0.34 & 0.33 & 0.92 & – & – \\ \hline
**Crofer®22 H** & \textless{}0.01 & \textless{}0.01 & 22.93 & 0.43 & 0.21 & 0.51 & 1.94 & – & – & – & – & 0.08 & 0.07 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical compositions in wt.% of P91 and Crofer®22 H. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Material** & **Yield Strength** & **Tensile Strength** & **Hardness** \\  & **(MPa)** & **(MPa)** & **(MPa)** \\ \hline P91 (tubes [31]) & \(\geq\)415 & 620–850 & \(\geq\)190 \\ \hline Crofer®22 H (Sheet and plate \(\leq\) 16 mm [32]) & 390 & 520 & 160–200 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Typical mechanical properties of P91 and Crofer®22 H at ambient temperature in the as-delivered state. Figure 2: The microstructure of the fully ferritic Crofer®22 H steel in the precipitated state. PFZ, particle-free zones; GB, grain boundaries. to a minimum, no holding time was applied. To achieve the specified cooling rate, the specimens were directly cooled with compressed air after reaching the maximum temperature. Moreover, the resulting thermal strain \(\varepsilon_{th}\) was suppressed by applying a mechanical strain of \(\varepsilon_{mech}=-\varepsilon_{th}\) in oop TMF cycles. Since the TMF cycle started at a minimum temperature of 250 \({}^{\circ}\)C, the strain ratio of mechanical strain was \(R_{\varepsilon,\ mech}=-\infty\). The differently pronounced thermal expansion of P91 and Crofer(r)22 H, which is mainly caused by substantially different Cr content, leads to deviations in \(\varepsilon_{th}\), and thus, in \(\varepsilon_{mech}\). While P91 was loaded with \(\varepsilon_{mech}=0.54\%\), Crofer(r)22 H was charged with \(\varepsilon_{mech}=0.509\%\). Consequently, one may expect a higher stress range for P91, but in TMF tests, a higher stress level was observed for Crofer(r)22 H (see Figure 3), caused by pronounced Laves phase strengthening of Crofer(r)22 H (see Reference [5]). Moreover, both materials show a decrease in stress range during TMF-loading, which is more pronounced for Crofer22 H. The results of TMF-experiments are discussed in detail in Reference [5]. Despite slightly higher maximum stress, Crofer(r)22 H exhibits in Reference [5] higher TMF-lifetime in relation to P91 (see Figure 3), which might be explained by differently pronounced damage tolerance. To analyze the changes in cyclic hardening potential, which correlates with damage tolerance [17; 18], during TMF-loading, instrumented cyclic indentation tests (CITs) were conducted at the initial state (IS), as well as after defined numbers of TMF-cycles \(N_{TMF}\). Therefore, the CITs were performed in the volume of the gauge length of the respective specimen with a maximum indentation force of \(F_{max}=1000\) mN and in total ten indentation cycles with a frequency of \(f=1/12\) Hz. Because this requires a destructive preparation of the specimens' gauge length, different specimens were loaded for various \(N_{TMF}\). For CITs, a Fischerscope H100C (Helmut Fischer GmbH, Germany), which enables continuous detection of the indentation depth \(h\) and indentation force \(F\), was used.