# Thermomechanically Induced Precipitation in High-Performance Ferritic (HiperFer) Stainless Steels
## 3 Results and Discussion  
### Mechanical Testing  
Figure 3 presents the tensile properties of the different TM-processed 17 Cr5 model alloys in comparison to the 17 Cr2 base alloy in the MSP state [8; 9]. Because of a lack of strengthening precipitates, the ultimate tensile strength (UTS) of the recrystallized (RX: 1100-1125 \({}^{\circ}\)C/25 min [8]) 17 Cr5 alloy was about 470 MPa, with an offset yield strength (Y\({}_{0.2}\)) of approximately 370 MPa. After standard MSP treatments (MSP1: RX + 530 \({}^{\circ}\)C/2-5 h/WQ + 600-650 \({}^{\circ}\)C/0.5-5 h/WQ; MSP2: RX + 540 \({}^{\circ}\)C/5-10 h/WQ + 600-650 \({}^{\circ}\)C/5-10 h/WQ), the tensile strength was almost doubled with sufficient rupture elongation (Figure 3). \begin{table}
\begin{tabular}{c c c} \hline \hline \(\mathbf{T_{\varphi}}\) (\({}^{\circ}\)C) & \(\varphi\) (–) & \(\mathbf{t_{H}}\) (s) \\ \hline
**800, 950** & 0, 0.2, 0.5 & 0, 60, 80, 300 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: TMP parameters. The 17 Cr5_2 TMP material (parameters: T: 800 \({}^{\circ}\)C, \(\varphi\): 0.5, t\({}_{\rm H}\): 60 s, cf. Figure 3) reached a UTS of approx. 710 MPa with a 0.2% offset yield strength (\(Y\)S\({}_{0.2}\)) of 495 MPa. Despite the limitations of miniature specimen geometry and comparatively large grain size (800 to 1500 \(\mu\)m), a promising rupture elongation of about 10% was achieved. Prolonged holding (80 s) after deformation did not yield improved UTS, but did yield an increase in rupture elongation. Considering specimen geometry, i.e., gauge cross-section (2 mm \(\times\) 1 mm), in relation to grain size, the measured rupture elongations were quite conservative. For uniaxial testing specimens with adjusted grain size (200-300 \(\mu\)m), higher elongation values can be expected [32]. Nevertheless, economic, integrated thermomechanical processing can provide tensile properties close to those achieved via standard multistep processing. The microstructures of the TMP and MSP materials were observed via HR-FESEM (cf. Figure 4). Typical microstructures from MSP 17 Cr5 steel (schematic in Figure 4a) are displayed in Figure 4b,c. Microstructures from TMP steel (800 \({}^{\circ}\)C/0.5/60 s) are depicted in Figure 4d,e. Figure 4: _Cont._  
Figure 3: Ambient-temperature stress–strain curves of MSP and TMP 17 Cr5 alloys in comparison to the base 17 Cr2 alloy in the MSP state. Precipitation of a multitude of small Laves-phase particles was obviously induced by both MSP and TMP. The particle coverage of high-angle grain boundaries appeared more complete in MSP specimens, because of pronounced diffusion-controlled precipitate growth [10; 33; 34]. The enrichment of key elements for Laves-phase nucleation at grain boundaries decreased the concentration of these in the surrounding matrix and led to delayed nucleation and growth of the Laves-phase particles. In turn, this caused the formation of characteristic particle-free zones (PFZs) along grain boundaries [28; 29; 30] and a tendency towards larger particles towards the grain interiors. As a consequence, the intragranular precipitates in the MSP material had a bimodal size distribution and were typically larger in the centers (cf. Figure 4b; Figure 5a, MSP IGc) than in the exterior (near PFZs) areas of grains (cf. Figure 4c; Figure 5a, MSP IGe). Intragranular particles, which are located at the exteriors of grains close to the PFZs (IGe; cf. Figure 4c, Figure 5a: MSP IGe) and the PFZ width itself were smaller (appr. 35-40 nm) in the case of the MSP material. With a plateau from 20 to 100 nm in the size distribution, the TMP material, in contrast, did not display such pronounced interrelations of grain boundaries, PFZs, and particle size (cf. Figure 4d,e; Figure 5a, TMP800C/0.5/60s IGc vs. TMP800C/0.5/60s IGe). Figure 4: Schematic drawing of precipitate microstructure (**a**).