## 3 Results and Discussion  
### Alloy Design  
The alloy concept of fully ferritic stainless steels strengthened by the Laves phase focuses on high creep strength and steam oxidation resistance. The design is based on experience from the development of the commercial-grade Crofer(r) 22 H [17; 18] and on the previous development of ferritic 17 wt. % Cr steels at IEK-2 [19; 20; 26]. The chemical composition of the experimental melting was developed by systematically varying the concentration of the mean alloying, Laves phase-forming elements. Thermo-Calc(r) calculations (utilizing the TCFE8 database) were performed for the reduced chromium content concept of 17 wt. % to accomplish the stability reduction of the embrittling \(\sigma\)-phase. The aim of the design was to maximize the volume of the Laves phase at the operating temperature (650 \({}^{\circ}\)C). The presence of carbides in ferritic steels limits the creep strength, due to their size and tendency to coarsen further. As a result, the formation of additional finer precipitate is suppressed [15; 19]. The intermetallic phase particles exhibit less tendency toward coarsening compared to the carbides; therefore, they significantly improve the creep strength [27]. The objective of optimization is to minimize losses of the Laves-phase-forming alloying elements tungsten and niobium due to the undesired formation of carbide, nitride, and carbonitride phases. In order to avoid this formation, the contents of carbon, nitrogen, and titanium are kept low [18; 19]. Alloying silicon during production for the deoxidation of the melt positively influences the precipitation kinetics of the Laves phase, accelerating its formation by decreasing the solubility of tungsten in the matrix [27; 28]. Additionally, the silicon itself is incorporated into the Laves phase precipitates [29]. According to the thermodynamic calculations, the combined addition of the alloying elements W, Nb and Si leads to the ferritic structure shown in Figure 3. This thermodynamic calculation was performed with the nominal chemical composition. The phase diagram predicts that the reduced chromium content implies a suppressed \(\sigma\)-phase stability under 620 \({}^{\circ}\)C as required. Two different Laves phases are present, denoted as the low-temperature Laves phase (LT Laves phase) and the high-temperature Laves phase (HT Laves phase) depending on their temperature range of stability. Tungsten and niobium show low solubility in ferrite; therefore, they extend the stable ferrite area and improve the stability of the precipitates. Due to their large atomic radii, these alloying elements contribute significantly to strengthening the solid solution. Tungsten, as the main Laves phase-forming element in this steel class, serves in general as the stimulator and stabilizer of the precipitates. The phase diagram shows the stability ranges of the respective phases as a function of the temperature and tungsten content. An increasing tungsten content slightly increases the stability of the HT Laves phase and the \(\sigma\)-phase. Maximum stability of the LT Laves phase is reached at 737 \({}^{\circ}\)C with a tungsten content of 2.3 wt. %. The target operating temperature of 650 \({}^{\circ}\)C is indicated by the red line and defines the relevant interval of 1.0 to 3.7 wt. % tungsten, in which the two Laves phases occur together. Based on the calculation shown in Figure 4, the Laves phases exhibit the same crystal structure (hexagonal C14). However, they are lightly differentiated in chemical composition. The LT Laves phase (Fe,Cr,Si)\({}_{2}\)(Nb,W) exhibits higher niobium content, while the HT Laves phase (Fe,Cr,Si)\({}_{2}\)(W,Nb) contains more tungsten. Corresponding to the chemical composition of both Laves phases and based on the higher niobium diffusivity in the ferritic matrix, a metastable nature is expected of the LT Laves phase. The metastable nature implies specific mechanisms of the formation and change of chemical composition of the LT Laves phase into the final stable HT Laves phase [24; 30].