# Thermomechanically Induced Precipitation in High-Performance Ferritic (HiperFer) Stainless Steels
## 3 Results and Discussion
### Mechanical Testing
SEM micrographs of 17 Cr5 steel: Precipitate morphology in the grain interiors (IGc) after MSP2 (**b**) and TMP800C/0.5/60s (**d**) and intragranular particle morphology in exterior areas (IGe) close to PFZs in MSP2 (**c**) and TMP800C/0.5/60s (**e**) materials. In terms of deformation, TMP induces additional dislocations and thus may alter the energy levels of the grain interiors and the grain boundaries near areas, which in turn may result in a more homogenous particle size distribution. Furthermore, the comparably high temperature of TMP accelerates the growth of precipitates. Nevertheless, TMP yielded an average particle size comparable to that of MSP (Figure 5b, ECD). While the intragranular particles were homogenously distributed in the MSP material, TMP produced a preferentially linear alignment of Laves-phase particles (Figure 4, bottom). The higher particle density of the MSP material (Figure 5a) provided a higher tensile strength. Nevertheless, even with just one fifth of the particle density (Figure 5a) and about half the particle area fraction (Figure 5b), the TMP-treated alloy, most probably because of the increased dislocation density from TMP deformation in comparison to the MSP material, yielded a UTS that was just 10% lower at ambient temperature (Figure 3). Because of the comparable tensile strength values of the material TMP-treated at T = 800 \({}^{\circ}\)C, \(\varphi=0.5\), and prolonged holding time of tH = 80 s (cf. Figure 3) and the MSP1 material (cf. Figure 3), the microstructures of these two variants were compared in detail by TEM analysis (Figure 6). FIB lamellae were cut perpendicular to the linearly aligned precipitates (cf. \"plane of view\" indicated by arrows in the left column of Figure 6) in the case of the TMP variant. At the same tensile and offset yield strength values, the TMP800C/0.5/80s material presented much larger precipitates than the MSP1 material did. Shear bands with clusters of dislocations and particles precipitated along them were observed in the TMP800C/0.5/80s lamellae. A higher dislocation density at shear bands favors the formation of Laves-phase particles and led to the observed linear alignment of precipitates. Both treatments mainly provided largely disc-shaped particles (Figure 6, left column). Volumetric expansion into all three dimensions was observed, with sizes ranging from 40 to 400 nm in diameter and 20 to about 80 nm in thickness in the TMP800C/0.5/80s variant. In contrast to this, both needle-shaped and comparably thin, disc-shaped particles were observed in the MSP1 lamellae, which may have been caused by observation of thin disc-shaped particles from different relative orientations. In former studies, Laves-phase particles mainly presented volumetric expansion into two dimensions, leading to disc shapes of typically less than 20 nm in thickness and 20 to 200 nm in diameter [4; 8; 18; 28; 29; 30; 31]. The TMP800C/0.5/80s material provided tensile properties in the range of the MSP1 material, which obviously presented more, finer, and more homogenously distributed particles. As expected, the thermomechanically processed TMP800C/0.5/80s material not only provided dispersion, but also combined dislocation strengthening, which obviously compensated the drawback in dispersion hardening. Figure 5: Quantitative image analysis results of (**a**) particle size distribution and (**b**) averaged particle area fractions against averaged ECDs (equivalent circle diameter) of intragranular precipitates.