## 3 Results and Discussion
### Thermomechanically Triggered Precipitation in Crofer(r)22 H  
As outlined in Section 3.1, Crofer(r)22 H strengthens by about 25% over the first 35 cycles (Figure 3a,b), where it constitutes a peak in stress range and consecutively softens slightly by appr. 5% until the end of the initial phase. The underlying microstructural mechanisms are complex.  
Figure 5: Evolution of martensite lath aspect ratio in P91 with an increasing number of TMF cycles.  
Figure 6: Electron backscatter diffraction (EBSD) image (band contrast image, grain boundaries added) of the as-received state (**a**) of and grain size evolution (**b**) in ferritic Crofer®22 H.  
First, the generation of dislocations by cyclic plasticization within the first 10 cycles (cf. Figure 7a or Figure 8a: Almost no precipitates detectable yet) leads to strain hardening (observable from widening of the hysteresis loop and an increase in stress range, cf. Figure 3d), which by itself would be non-permanent. Second, the induced dislocations accelerate nucleation of small, intergranular Laves phase particles, which become clearly visible in between 10 and 20 cycles (Figure 7a,b). This further increases the stress response (Figure 3d). Third, the thermomechanically induced precipitates [28; 29; 30; 31; 32] pin the dislocations and through this make (at least part of the) dislocation-induced strain hardening quasi permanent. With the applied heating/cooling rate of 10 Ks\({}^{-1}\) the first 20 cycles accumulate up to a dwell time of just 200 s (3, 33 min.) at temperatures higher than 600 \({}^{\circ}\)C, which is necessary to induce strengthening of Laves phase precipitation in Crofer(r)22 H in a reasonable time (i.e., hours or less) [28]. In isothermal, isochoric (without cyclic plasticization) precipitation annealing at 650 \({}^{\circ}\)C, about 30 min is needed to obtain a comparable number of Laves phase precipitates [39]. Deformation, in this case induced during TMF cycling, thus has a strong accelerating effect on precipitation kinetics. At 35 cycles, the peak stress range (Figure 3a,b) and the maximum area of the hysteresis loop, which correlates to the maximum deformation energy induced within a single cycle, are reached (Figure 3d). In the following, stress response (Figure 3a,b) and area of the hysteresis loop (Figure 3d) start decreasing slowly, while the increase in the number of particles saturates just towards 250 cycles (Figure 7c: 35 cycles, i.e., \(\sim\)6 min. @ T \(\geq\) 600 \({}^{\circ}\)C vs. Figure 7d: 250 cycles, i.e., \(\sim\)42 min. @ T \(\geq\) 600 \({}^{\circ}\)C, cf. Figure 8a).  
Exhaustion of Laves phase-forming elements from the alloy matrix and consequently a deceleration of precipitation kinetics are the cause of this phenomenon. Dislocations may be generated in greater number or more rapidly than they can be populated by newly nucleating particles. Recovery then establishes a dynamic balance between generation and annihilation of dislocations in the stable curve region, but on a higher stable stress range level than without precipitation. Enhanced Laves phase volume fraction, which is obtainable by increased levels of the main forming elements W and Nb [23; 31; 32] and adjusted precipitation kinetics, accessible by adapted Si content [40; 41; 42], do facilitate establishment of the dynamic balance on an even higher level. Consequently, the drop in the stress response after the peak value could be avoided, the hardening range extended and thus an even higher level of fatigue resistance could be achieved [33]. Aging by particle growth becomes (Figure 7e vs. f, g: 2000 vs. 3000, 4342 cycles, i.e., \(\sim\)333 vs. 500, 721 min. @ T \(\geq\) 600 \({}^{\circ}\)C) observable from the decreasing total number of particles at almost constant total particle area beyond 2000 cycles (Figure 8b).