*3.1. Microstructural Characterization*

A typical feature of HPT-deformed materials is an increasing hardness and decreasing microstructural size with an increasing distance from the sample's center. As soon as a microstructural saturation is reached, the hardness does not increase further [32]. For a valid comparison of samples of varying Co-content (Figure 2), Vickers hardness values were taken from the saturation region of constant hardness. Measurements within these regions show a clear trend of increasing hardness with increasing Co-content. Figure 2 was drawn using samples from this work and data from samples presented in reference [40]. The trend breaks down for pure Co of highest purity, whose hardness was found to be 368 HV. In the latter case, neither impurities (as would be the case for the less pure Co-powder) nor another element obstruct grain boundary motion; the grains are larger and the material becomes softer.

**Figure 2.** Hardness HV as a function of Cu-Co-content, showing a linear trend for room temperature deformed samples. The highlighted measurements were made on samples Cu55Co45 (large HPT, elevated temperature) and Cu49Co51 (small HPT, 200 ◦C) Hardness values from samples presented in reference [40] are included. The linear fit between 10 wt % and 70 wt % Co, representing a simple rule of mixture, is a guide for the eye.

SE micrographs already give a first indication of the enhanced, mutual solubility of Cu- and Co-phases upon HPT. Figure 3 shows micrographs taken in tangential direction from the saturation region for binary CuCo composites. They were made at a radius of 3 mm (εv.M. ~ 2200) with the exception of r = 10 mm (εv.M. ~ 1300) for the large sample (Figure 3c).

At room temperature and for 200 ◦C deformed samples, the microstructure appears to be homogeneous on a scale of ~1 μm, while the microstructure for the samples deformed at elevated temperature (Figure 3c) and 300 ◦C (Figure 3f) appears to be granular. EDX measurements of the sample presented in Figure 3c) using low energy electrons (5 kV, smaller excitation volume) show changes in the chemical composition at the same length scale (Figure 4). The small sample Cu49Co51 (200 ◦C deformation temperature) does not show this segregation of Co. Therefore, the microstructure of the large sample resembles more like the one of the Co-rich Cu22Co78, which experienced a deformation temperature of 300 ◦C.

**Figure 3.** SEM micrographs of HPT-deformed samples, taken at r = 3 mm (with the exception of (**c**)) in tangential direction. (**a**) Cu81Co19; (**b**) Cu64Co36; (**c**) Cu55Co45 (r = 10 mm, large HPT-tool, deformed at elevated temperature); (**d**) Cu52Co48; (**e**) Cu49Co51 deformed at 200 ◦C; (**f**) Cu22Co78 deformed at 300 ◦C.

For the determination of the actual chemical composition and elemental distribution of the deformed samples and to detect possible deviations from the nominal composition of the powder mixture, several EDX spectra were recorded at large radii, and their mean values are presented for the chemical composition in Table 1. To demonstrate the good co-deformation and the increasing intermixing with high applied strains, 60 EDX spectra were recorded right at the center and at radii of 1, 2, and 3 mm. As an example, the sample Cu85Fe15 was used. In Figure 5a, it can be seen that still some Fe-particles (dark particles) can be found for small radii (r = 0 mm, r = 1 mm) and the distribution of results of EDX-analysis (Figure 5d) is widespread (blue lines), due to the existence of Cu-rich and Fe-rich regions. For r = 2 mm and r = 3 mm, the distribution become narrower, demonstrating improved intermixing of Cu and Fe on the scale of the EDX-analyzed volume. This is also confirmed by micrographs in Figure 5b,c.

**Figure 4.** SEM micrograph of the large HPT sample Cu55Co45, taken at r = 10 mm, with EDX maps of the same region showing the distribution of Cu (**center**, blue) and Co (**right**, green).

**Figure 5.** SEM micrographs of the sample containing Cu85Fe15 at (**a**) r ~ 0 mm, (**b**) r = 1 mm, (**c**) r = 2 mm, from left to right (No. rotations = 100). The progressing dissolution of Fe into the Cu matrix can be seen. (**d**) Distribution of EDX-results for the same sample and for different radii. 60 spectra were recorded at each position. As a guide for the eye, a normal distribution was fitted to the data.

As typical examples taken from different ranges of the CuCo system, the following three specimens were subjected to synchrotron diffraction phase analysis: Cu81Co19, Cu55Co45, Cu22Co78. The results, logarithmic intensity as a function of scattering vector q are presented in Figure 6 for different radii. To compare with, the peaks of fcc-Cu (lattice constant d = 3.615 Å [49]), fcc-Co (d = 3.554 Å [50]) and of hcp-Co, data from reference [51], are included. For the low Co-containing material, small peaks of hcp-Co can be found for r = 1 mm; however, these peaks vanish for larger radii. The Cu-peak slightly deviates from the position of pure Cu, due to supersaturating the crystal with Co. This effect is stronger for the higher-Co containing material (Figure 6b); however, traces of hcp-Co can be found for all radii. No hcp-Co can be found in the large sample Cu55Co45 (Figure 6c,d). The plateau-like shape of the peaks – especially at high q – is explained most likely due to the occurrence of two fcc-phases for Co and Cu.

**Figure 6.** Synchrotron data for three CuCo specimens (**a**) Cu81Co19 deformed at room temperature, (**b**) Cu22Co78 deformed at 300 ◦C, (**c**) Cu55Co45 deformed at elevated temperature and lower applied strain rate. Note the larger radius of the specimen, (**d**) like (**c**) but showing a different regime of scattering vector q.

Further microstructural investigation, using TKD, was made for Cu55Co45 to obtained deeper insights into the as-deformed microstructure. As an example for TKD results, Figure 7a is shown. Although the hcp-Co phase was searched for, there are no clear indications of hcp-Co grains of detectable size. Distinction between grains rich in fcc-Co and rich in fcc-Cu cannot be made since the difference in lattice parameter is too small. Identified grains are in the 100 nm regime and a texture analysis, which is not shown here, yielded the shear texture typical of fcc metals [52]. In total, four scans were made, and their aggregated area weighted grain size distribution *f*(*d*) was fitted by a lognormal distribution.

$$f(d) = \frac{1}{\sqrt{2\pi}\sigma d} e^{-\frac{\left(\ln\left(d\right) - \mu\right)^2}{2\sigma^2}}\tag{3}$$

With the mean μ and the standard deviation σ, the median value of the grain size *e*<sup>μ</sup> is found to be 79 nm. Stückler et al. [40] found similar results for other CuCo materials, which were processed the same way: A decreasing grain size of 100 nm, 78 nm, and 77 nm for increasing Co-content of Cu-Co28 wt %, Cu-Co49 wt %, and Cu-Co67 wt %, respectively.

**Figure 7.** (**a**) Inverse pole figure map of Cu55Co45, taken at a radius of ~10 mm in an axial direction. As an overlay in the lower left part, the Cu-phase map (fcc–phase map, respectively) of the identical area is shown. After the clean-up, all identified grains are found to be fcc. No hcp-particles can be detected. (**b**) Aggregated grain size distribution from four TKD scans. In total, 1648 grains were taken into account.

Regarding spatial resolution, TKD-EBSD is better than conventional EBSD in many cases. Ge et al. [17] made a related analysis on Co-particle sizes in electrodeposited Cu84Co16, using TEM, before and after annealing for 30 min at 695 K and found mean values of 10 nm and 12 nm respectively, being even below the resolution limit of TKD. Thus, a high angle annular dark field (HAADF) image was recorded in scanning TEM, Figure 8. HAADF is sensitive to the atomic number, while Co-enriched (dark) and Cu-enriched (bright) regions can be identified within the matrix.

**Figure 8.** High angle annular dark field (HAADF) image of Cu55Co45, using the same specimen, which was used for deriving the results shown in Figure 7. The Co-particles (dark regions) with a size of several tens of nanometers become visible.
