## 3 Results and Discussion
### Novel Materials for Improved Structural Safety
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Material** & \(\mathbf{\sigma_{h}}\) & \(\mathbf{N_{h}}\) & \(\mathbf{N_{d}}\) & \(\mathbf{N_{f}}\) & **Damage Range** \\ \hline Crofer22 H & \(\approx\)205 MPa (38\%) & 30 & 2400 & 2930 & 18.1\% \\ HiperFer 17 Cr2 & \(\approx\)236 MPa (42\%) & 70 & 974 & 1389 & 29.9\% \\
316L & \(\approx\)47 MPa (5.4\%) & 40 & 1143 & 1326 & 13.8\% \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Strengthening during thermomechanical fatigue (TMF) loading (\(\mathbf{\sigma_{h}}\)), hardening cycles (\(\mathbf{N_{h}}\)), damage initiation (\(\mathbf{N_{d}}\)), technical TMF lifetime (\(\mathbf{N_{f}}\)), and damage ranges (i.e., relation of \(\mathbf{N_{f}}\) and \(\mathbf{N_{d}}\); experimental parameters: 50–650 \({}^{\circ}\)C, \(\epsilon_{\text{mech}}=-\epsilon_{\text{th}}\), dT/dt = 10 \(\text{Ks}^{-1}\), \(\mathbf{t_{h}}=0\) at \(\mathbf{T_{\text{min}}}\) and \(\mathbf{T_{\text{max}}}\)). Figure 19: Cyclic crack propagation curves of additively manufactured 316L (AM316L–ZY), conventionally manufactured 316L, and Crofer22 H (after precipitation annealing: 0.5 to 10 h from 600–650 \({}^{\circ}\)C) at ambient temperature. In comparison to additively/conventionally manufactured austenitic 316L (cf. curves \"AM316L-ZY, 2 nd\"/\"316L, 2 nd\" in Figure 19), it takes almost twice/1.5 times the stress intensity to initiate crack propagation in conventionally processed Crofer22 H. Over the whole stress intensity range, the crack propagation rate was lower than in conventional 316L. Crack propagation in the second, additively manufactured 316L specimen (AM316L-ZY) was slightly lower in comparison to Crofer22 H, but the specimen already hit the test termination criterion (a/W = 0.7) when Crofer22 H just reached the stable region of the cyclic crack propagation curve. However, HiperFer-like steel can only exploit its maximum potential at temperatures around 600 \({}^{\circ}\)C or above. As outlined in the previous section (cyclic), plastic deformation in fatigue loading in this temperature range results in \"reactive precipitation hardening\" [32], which not only leads to higher strength, but to active crack obstruction [17]. On the basis of this intrinsic mechanism, HiperFer 17 Cr2 (conventionally manufactured variant) can reach up to an order of magnitude of higher residual fatigue lifetime than austenitic 316L (Figure 20). Crack propagation in HiperFer 17 Cr2 remained at an almost constant velocity over a comparatively wide range of the cyclic crack propagation curve, despite increasing stress intensity \(\Delta\)K.  
In order to investigate the potential of the alloying system for AM, we developed a first model alloy (HiperFerAM (Additive Manufacturing) for the additive manufacturing processes. The alloy composition was 22 Cr, 2 W, 1.5 Nb, and 1 Si (wt.-%). The increased Nb and Si contents in comparison to HiperFer 17 Cr2 in combination with higher cooling rates in the AM process resulted in a highly supersaturated solid solution in the as-processed state, which in turn led to rapid precipitation kinetics, caused by high precipitation pressure in the direction of equilibrium. This resulted in a large amount of fine Laves phase particles (Figure 21), even after the comparatively short high temperature periods of in situ heat treatment. Figure 22 displays a typical microstructure of LMD-processed (by Fraunhofer ILT, Aachen) HiperFerAM powder. In addition, the grain boundaries were occupied by Laves phase particles (Figure 21) in the as-manufactured state, stabilizing the grain structure without embrittling the material. As outlined in [16; 17], this is an important issue to enable profit from dynamic strengthening. Pores contained in additively manufactured materials (cf. Figure 21) acted as notches and led to accumulation of plastic strain in the surrounding material during cyclic loading.