## 3 Results and Discussion
### Microstructure Characterization after TMT
#### 3.2.2 Transmission Electron Microscopy  
Since the grain structure after 3.0 + 550 (5) and 3.0 + 600 (5) was too fine for the EBSD system that we used, TEM analysis of the structure was performed. Figure 6 presents the TEM bright- and dark-field images, along with the corresponding SAED patterns. The obtained TEM images exhibit an ultrafine-grained submicrocrystalline structure of the \(\beta\)-phase. An exception is the SAED pattern of the 18-15 alloy, where reflections of the \(\alpha\)-phase are present after 3.0 + 550 (5) (Figure 6b), which confirms the results of the XRD analysis (Figure 2). The TEM images show that after annealing at 550 and 600 \({}^{\circ}\)C for 5 min, the low-angle misorientations are still preserved with a predominance of high-angle boundaries. The high-angle boundaries (Figure 6) are confirmed by the high-angle azimuthal misorientations in the selected area electron diffraction (SAED) patterns and the observation of individual grains in the dark-field (DF) images. However, some internal low-angle boundaries can also be seen in the dark-field images of all alloys. These subboundaries divide grains surrounded by high-angle boundaries into subgrains (Figure 6a-f). Thus, after 3.0 + 550 (30) and 3.0 + 600 (30) TMT, a mixed submicrograined and submicrosubgrained structure of the \(\beta\)-phase with a grain/subgrain size in the 100 to 1000 nm range is formed. Figure 6: TEM images of 22-6 (**a,d**), 18-15 (**b,e**), and 41-12 (**c,f**) alloys after 3.0 + 550 (5) (**a–c**) and 3.0 + 600 (5) (**d–f**). Based on the bright-field images, grain/subgrain histograms (i.e., without separation into grain and subgrain size distributions) were built (see Figure 6) and the average grain size \(\overline{D}\), was calculated and compared with \(\overline{D}\), (calculated for other TMT routes in Figure 7). The histograms of all alloys exhibit a log-normal distribution with a single peak, except for one 22-6 alloy after 3.0 + 600 (5), which does not follow a log-normal distribution. This deviation can be attributed to non-uniform recrystallization due to non-uniform initial deformation. It can be observed in Figure 4 that 0.3 + 600 (30) TMT leads to the formation of a plate structure of the \(\beta\)-phase where the calculation of grain size \(\overline{D}\), is not possible. Increasing the PDA temperature to 800 \({}^{\circ}\)C leads to a recrystallized structure in all alloys. In 22-6, however, an equiaxed grain structure forms with an average size of 31.8 \(\pm\) 3.6 \(\upmu\)m, while in 18-15 and 41-12, an elongated structure forms with average grain sizes \(\overline{D}\), of \(\sim\)50.1 \(\pm\) 5.4 \(\times\) 16.5 \(\pm\) 1.8 \(\upmu\)m and 88.0 \(\pm\) 8.1 \(\times\) 36.9 \(\pm\) 3.1 \(\upmu\)m, respectively (Figures 4 and 7). The decrease in grain size \(\overline{D}\), in all alloys after PDA at 800 \({}^{\circ}\)C, compared to the initial state, is associated with the occurrence of static recrystallization [24]. Intermediate 1.5 + 600 (30) creates a predominantly fine-grained structure with an average grain size of 4.5 \(\pm\) 0.6 \(\times\) 2.1 \(\pm\) 0.3 \(\upmu\)m for 22-6, 4.5 \(\pm\) 0.6 \(\times\) 3.5 \(\pm\) 0.5 \(\upmu\)m for 18-15, and 30.2 \(\pm\) 4.4 \(\times\) 16.3 \(\pm\) 2.1 \(\upmu\)m for 41-12. It consists of a mixture of recrystallized equiaxed grains and non-recrystallized grains (containing a polygonized dislocation substructure due to static polygonization) (Figure 4). After 1.5 + 800 (30) TMT, a fully statically recrystallizedfine-grained structure in 22-6 and 18-15 and a coarse-grained structure in 41-12 are formed. The average equiaxed grain sizes are 21.4 \(\pm\) 1.9 \(\upmu\)m for 22-6 and 75.1 \(\pm\) 8.2 \(\times\) 68.36 \(\pm\) 7.7 \(\upmu\)m for 41-12, and the average elongated grains sizes are 21.5 \(\pm\) 1.8 \(\times\) 8.1 \(\pm\) 0.7 \(\upmu\)m for 18-15.