*3.1. Microstructure in the Central Area*

The OIM analysis was performed for the central, mid-radius, and edge areas of the samples, previously processed by HPTE in v1w1 regime and the corresponding OIM maps are shown in Figure 3. Grains displaying different crystallographic orientations are indicated by different colors, according to the standard stereographic triangle in the pole figure. Corresponding histograms of the grain

size distribution and a GB misorientation distribution from the OIM maps are shown in Figure 3. Microstructure parameters, i.e., grain size at respective boundary misorientation threshold (D3 and D15), average misorientation angle (Θav), the HABs volume fraction (VHAB), twin boundaries volume fraction (Vtwin), second-phase particles' size (dp) and their volume fraction (*f*), are given in Table 1.

**Figure 3.** OIM maps from (**left**) transverse and (**right**) longitude sections of the sample areas at (**a**) the center; (**b**) the mid-radius; and (**c**) the edge, after HPTE.

The D15 grain size distribution in the transverse section of central area was bimodal with two maxima, at 16.7 μm for coarse grains and at 1.2 μm for fine grains (Figure 4a). These two fractions had a specific area ratio of 1:3. Grain size distribution in the longitude section was of a lognormal type. For this section the average grain size of 1.4 μm matched the histogram maximum (Figure 4b).

**Figure 4.** D15 grain size distribution by the grains specific area (S/S*i*) after HPTE in the (**a**) transverse and (**b**) longitude sample sections.

According to the EBSD data, the GB misorientation histograms were bimodal in both sample sections, with one maximum being close to 2◦ in the low angle range, and a second one at 47◦–48◦ in the high angle range (Figure 5).

**Figure 5.** GB misorientation distributions after HPTE in the (**a**) transverse and (**b**) longitudinal sample sections.

In the central part, the HABs volume fraction in the longitudinal cross-section was larger than that of the transverse cross-section (41% vs. 25%, see Table 1 and Figure 5a,b). This difference in the volume fraction can be explained by the grain morphology, i.e., the coarse grains were elongated in the transverse section, containing numerous LABs (Figure 3a). The grain size that was determined at the 3◦ (D3) boundary misorientation threshold was 0.8 μm in both cross-sections, whereas D15 in the longitudinal section was four times smaller than that in the transverse section (refer to Table 1). Due to this difference in the D15 value, the volume fraction of HABs in the longitudinal section was larger than that in the transverse section.
