*3.2. Microstructure in the Mid-Radius Area*

In contrast to the inhomogeneous microstructure observed in the central part of the processed sample, the grain structure in the mid-radius area was rather uniform. The average grain size of both D15 and D3 were significantly smaller than those in the central area (Table 1) and grain morphology was equiaxed (Figure 3b).

The SEM-BSE images revealed that the volume fraction of the second-phase particles was slightly smaller than that of the central area. Furthermore, no precipitates in the grains after HPTE were seen in TEM bright field (BF) images (Figure 6a).

**Figure 6.** Microstructure of the HPTE Cu in the mid-radius area of the longitudinal section; all the images are at the same scale. (**a**) TEM BF image; (**b**) ACOM TEM OIM map in which LABs (3◦ < Θ < 15◦ are highlighted with white lines) and HABs (Θ > 15◦) are highlighted with black lines; (**c**) GND distribution map from ACOM TEM (all GBs are marked with white lines; and (**d**) EBSD OIM GBs network (LABs marked with red lines, HABs marked with blue lines).

The BF TEM image shown in Figure 6a also illustrates the grain substructure of copper after HPTE in the mid-radius area. The majority of triple junctions had contact angles close to 120◦ (Figure 6a,b). Some grains contained a higher dislocation density compared to that of the adjacent grains, manifested by the diffraction contrast in the image. In general, the TEM observations were consistent with the EBSD maps with respect to the grain size values (Table 2).


**Table 2.** Vickers hardness (HV), yield strength (YS), ultimate strength (UTS) and elongation to failure (δ) of CP Cu after HPTE and after some other SPD processing methods.

Grain size histograms for transverse and longitudinal sections demonstrated similar lognormal distributions. The maxima in both distributions corresponded to 0.8–0.9 μm in the transverse and to 0.7–0.8 μm in the longitudinal sections (Figure 4). However, in the transverse section the number of very small grains, of sizes less than 0.3 μm, was larger than that present in the longitudinal section (Figure 4). The grain size distribution was broader in the transverse section, with grains in range 3 μm < D15 < 6 μm, whereas no grains larger than 3 μm were observed in the longitudinal section (Figure 4). From this, we concluded that the grain structure in copper after HPTE in the mid-radius area was quite similar both in transverse and longitudinal sections, which was not the case for the sample´s central area.

From GB misorientation distributions, it can be observed that the volume fraction of HABs in longitudinal section was higher than that in the transverse one (49% and 33%, respectively, Table 1). This difference was similar to the difference found in the central area and was most likely due to the same grain refinement mechanism.

The differences in grain size and grain size distribution provide evidence that in the mid-radius area the HPTE creates conditions for efficient grain refinement, resulting in smaller grain size than that in the central area of the sample (Table 1).

Examination of the CP copper microstructure after HPTE in the mid-radius area by means of ACOM TEM provided additional microstructural details. Grain substructure was examined in detail using the BF images like that shown in Figure 6a. A typical orientation map is shown in Figure 6b, whereas a GND spatial distribution is shown in Figure 6c. It should be noted here that twins were observed inside the ultrafine grains, which is the reason for the characteristic peak at 60◦ present in the boundary misorientation histogram (Figure 6b). The volume fraction of the twin-type boundaries was 14% and this value was greater by 5.5% compared to that obtained by the EBSD method (Table 1). Most probably, this difference could be the result of the different step sizes used in the two methods as well as by higher spatial resolution in ACOM TEM.

The average thickness of the twins was about 40 nm (Figure 6b), and the average twin size, estimated using equal circles method, was about 110 nm, i.e., smaller than the measured grain size, D3 = 250 nm. The nano-size twins and grains less than 50 nm could not be resolved in SEM, using a 50 nm step size. Figure 6a,b shows that a high number of very small subgrains, bounded by LABs, was present in the HPTE-processed sample. In general, the majority of grains, bounded by HABs (and marked with blue lines in Figure 6d) contained one or more LABs.

We can see in the ACOM TEM maps (Figure 6b,c) that many twin-type GBs were formed during the HPTE deformation within the fine grains, in this way dividing them into smaller grains. The GND density differed significantly in neighboring (sub)grains (Figure 6c), which means that both LABs and twin-types GBs provided effective barriers for the dislocation slip and they should be taken into account in the analysis of possible strengthening factors, in particular Hall–Petch strengthening. Since the volume fraction of twin boundaries increases from the sample´s center to the edge (Table 1), we can conclude that twinning was more active at higher strains. From the nano-twins morphology, it is hypothesized that twinning occurred in the newly formed small grains.
