## 3 Results and Discussion
### Effect of Tungsten Addition on Laves Phase Precipitation  
The effect of tungsten content on the precipitation behavior during thermomechanically induced precipitation was investigated with the alloys Fe17 Cr2.6W and Fe17 Cr3.6W. An optimized thermomechanical process route with a deformation temperature of 800 \({}^{\circ}\)C, a deformation grade of 0.5, and isothermal tempering at the deformation temperature after a forging process of 60 s was chosen. Representative SEM-BSE micrographs of Laves phase evolution are shown in Figure 6. The SEM micrographs show the microstructure states of model alloys achieved with the mentioned constant forging parameters and depict the perpendicular view of the longitudinal cross-section of the deformed central areas of the TTS samples. The new developed process induces a very fine and homogeneous dispersed precipitation structure of the Laves phase. The created rectangular precipitates appear in a linear arrangement form. Particle-free zones along the grain boundaries reach an average width of 630 nm for Fe17 Cr2.6W and 825 nm for Fe17 Cr3.6W, respectively. Higher tungsten content in the Fe17 Cr3.6W model alloy obviously results in the precipitation of slightly larger particles. Due to the higher number of tungsten atoms available in the matrix, the precipitates' growth is promoted. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Model Alloy** & **C** & **Si** & **Mn** & **Cr** & **Nb** & **W** \\ \hline Fe17 Cr2.6W & 0.0021 & 0.25 & 0.19 & 17.1 & 0.99 & 2.6 \\ Fe17 Cr3.6W & 0.0043 & 0.25 & 0.21 & 17.1 & 1.01 & 3.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition in wt. % of the manufactured model alloys Fe17 Cr2.6W and Fe17 Cr3.6W. Figure 5: Results of thermodynamic equilibrium calculations of Laves phase amount as a function of tungsten content at 650 \({}^{\circ}\)C (ThermoCalc®, database TCFES). The high-tungsten model alloy Fe17 Cr3.6W with a tungsten content of 3.6 wt. % exhibits an approximately 5% higher strengthening effect in comparison to the low-tungsten model alloy Fe17 Cr2.6W with 2.6 wt. % of tungsten (see Figure 7). In other words, increasing the tungsten content results in the precipitation of bigger Laves phase particles (Figure 7a) in a more significant phase fraction (Figure 7b) and enables the reaching of a higher ultimate tensile strength and yield strength (Figure 7c). Due to the higher availability of the dissolved tungsten in the ferritic matrix, the diffusion-conditioned precipitation process is accelerated. Therefore, a faster nucleation and growth of the Laves phase particles are expected. In general, alloying tungsten leads to a more effective solid solution strengthening and supports the Laves phase precipitation. Additionally, tungsten is the main formation element of the stable strengthening HT Laves phase; it promotes the precipitation process and stabilizes the precipitated particles. Corresponding to the results, a 1 wt. % increase in tungsten content results in the precipitation of slightly bigger (approximately 10 nm) particles with a 25% higher phase fraction. The favorable combination of solid solution and precipitation strengthening of the high tungsten model alloy Fe17 Cr3.6W enables the reaching of a higher ultimate tensile stress (UTS) of 712 MPa in comparison to the model alloy Fe17 Cr2.6W, which results in an UTS of only 683 MPa. The higher strengthening effect is also evident through the increasing of the yield strength value from 412 MPa in the Fe17 Cr2.6W alloy to 434 MPa in the Fe17 Cr3.6W alloy.