# Production and Characterization of Cu/CNT Nanocomposites
## 3 Results and Discussion
### Strengthening Mechanisms
The load transfer mechanism was observed with the increase in the mechanical properties of the nanocomposites but confirmed by the characterization of the fracture surface of the nanocomposites by SEM (shown in Figure 8). Elongated and fractured CNTs were detectable on this surface. This led to the conclusion that one of the factors responsible for the difference in hardness and tensile strength between the copper matrix and the nanocomposite is the load transfer mechanism. The increase in tensile strength observed for the nanocomposites can be attributed to a uniform CNT distribution and effective load transfer between the matrix and CNTs due to a strong matrix/CNT interface. [PERSON] et al. [18] observed similar results in Cu/CNT nanocomposites produced by powder metallurgy and cold rolling. The fracture surfaces shown in the CNTs prove the good interface bonding between the strengthening material and the matrix that enhances the load transfer mechanism. However, other mechanisms may play a role in reinforcing Cu/CNT nanocomposites. In this regard, in addition to the load transfer mechanism, Orowan, dislocation, and grain boundary or texture strengthening mechanisms were considered in this work. The effect of CNTs in grain refinement due to grain growth inhibition during nanocomposite sintering was investigated using EBSD maps. Figure 9 shows the unique color grain maps and grain size distributions of the as-sintered Cu and Cu/CNT samples. These results \begin{table}
\begin{tabular}{c c c c} \hline
**Sample** & **Ultimate Tensile Strength (MPa)** & **Elongation (\%)** & **Relative Density (\%)** \\ \hline Cu matrix & \(92\pm 2\) & \(13\pm 3\) & 97.6 \\ Cu/CNT nanocomposites & \(154\pm 4\) & \(6\pm 2\) & 95.8 \\ \hline \end{tabular}
\end{table} 
Table 1: Tensile strength, elongation, and relative density of the Cu matrix and Cu/CNT nanocomposite as-sintered samples. Figure 7: Stress–strain curves of the Cu matrix and Cu/CNT nanocomposite as-sintered samples. show that the samples have a similar average grain size, indicating that adding CNTs does not significantly influence the grain size. The pole figures (PFs), inverse pole figures (IPFs), and IPF maps shown in Figure 10 allowed the evaluation of the crystallographic orientation of the grains. Neither sample shows a preferential orientation but a strong crystallographic orientation in the ND and TD directions. However, some slight differences in crystallographic orientation are detected for the nanocomposites. Unlike what was observed for Al and Ni matrices, where the crystallographic orientation of the matrix changes significantly, CNTs do not considerably affect the orientation of the Cu matrix [20; 24]. In this context, it is evident that the CNTs do not influence the grain size or texture of the Cu matrix during the sintering process. Another mechanism that can contribute to strengthening the Cu matrix is the dislocation or Orowan strengthening that occurs in the presence of reinforcement material. Figure 11 shows the image quality (IQ) maps with the high- and low-angle boundaries (HAGB and LAGB, respectively) and the estimated geometrically necessary dislocation (GND) density maps for the Cu and Cu/CNT samples. Based on these results, it is clear that the addition of CNTs does not significantly increase the copper matrix's dislocation density (1.8 \(\times\) 10\({}^{14}\) m\({}^{-2}\) and 1.9 \(\times\) 10\({}^{14}\) m\({}^{-2}\) are the estimated dislocation densities for  
Figure 8: SEM image of the Cu/CNT fracture surface of the nanocomposites produced by ultrasonication, where the arrows point to elongated and fractured CNTs. Figure 9: Unique color grain maps of (**a**) Cu matrix and (**b**) Cu/CNT nanocomposites, and (**c**) grain size distribution of Cu matrix and Cu/CNT nanocomposite sintered samples. Cu and Cu/CNT samples, respectively). There is only an increase in the dislocation density near the CNT clusters, as seen at higher magnification. However, it is possible to observe dislocations of cells related to CNTs embedded inside the grains.