## 3 Results and Discussion
These dislocations are mainly formed due to the large thermal expansion mismatch between carbon fiber and matrix during the cooling period. Figure 4c displays the magnified morphology of the interfacial product embedding in the interface between carbon fiber and matrix. Figure 4d shows the high resolution transmission electron microscopy (HRTEM) image of the selected area in Figure 4c. The HRTEM image shows that lattice fringes with a spacing of 0.83 nm lie parallel to the crystal axis. It is reported that the Al\({}_{4}\)C\({}_{3}\) parallel to the c-axis is three fringe spacings, or 2.49 nm [41]. Furthermore, the inserted fast Fourier transformation (FFT) pattern confirms the compounds as Al\({}_{4}\)C\({}_{3}\), which is in hexagonal structure with lattice parameters of a = 0.33 nm and c = 2.49 nm [41]. Figure 4: (**a**) Bright-field transmission electron microscopy (TEM) morphologies of the fiber/matrix interface in the SCFs/2024 Al composites, (**b**) enlarged view of the dislocation accumulation in the matrix, (**c**) enlarged view of the Al\({}_{4}\)C\({}_{3}\) embedding at the interface and (**d**) corresponding HRTEM image of the selected area in Figure 4c. Figure 3: (**a**) XRD patterns for 2024 Al and 8 vol. % SCFs/2024 Al composites, (**b**) the relationship between crystallite size of the aluminum matrix and SCFs content. Figure 5 presents the TEM micrograph of \(\theta^{\prime}\) (Al\({}_{2}\)Cu) precipitates in the aluminum matrix. The plate-shaped \(\theta^{\prime}\) precipitates are observed along \(<\)101\(>\) direction of the \(\alpha\)-Al matrix, and its size is about 90 nm in length. Figure 6a shows the interface morphology between Al\({}_{4}\)C\({}_{3}\) and \(\alpha\)-Al matrix. It can be seen that white precipitates are formed at the interface between Al\({}_{4}\)C\({}_{3}\) and \(\alpha\)-Al matrix. From the EDS mapping results (Figure 6b-d), these precipitates are rich in aluminum and copper, indicating the segregation of \(\theta^{\prime}\) (Al\({}_{2}\)Cu) surrounding Al\({}_{4}\)C\({}_{3}\). Figure 6: (**a**) High angle annular dark field (HADDF) STEM image (Z-contrast) for the interfacial microstructure of the SCFs/2024 Al composite, (**b–d**) EDS mapping analysis for Figure 6a. The Vickers hardness results of 2024 Al and SCFs/2024 Al are presented in Figure 7. 2024 Al has the lowest hardness (127.8 HV). The Vickers hardness increases with the increasing SCFs content. The samples for 8 vol. % SCFs/2024 Al composite possess the highest Vickers hardness (160.6 HV), which is 25.6% higher than that of 2024 Al. It is worth noting that a wider error range is obtained for the composites with higher volume fractions of SCFs. The inset in Figure 7 displays the indentation for the measured point with the highest hardness (170.1 HV). It can be seen that the indentation covers several SCFs. It is obvious that the indenter is more likely to press on the zone around SCFs with the increasing volume fraction of SCFs, and thus, results in the fluctuation in Vickers hardness for the composites with high volume fractions of SCFs. In order to further study the influence of SCFs on the hardness of the 2024 Al matrix, nanoindentation was performed on a single SCF and the matrix around it. The indentation load-displacement curves for the SCF and matrix are shown in Figure 8a, in which both the indentation tests have a fixed maximum displacement of 200 nm. The peak load of SCF (2.31 mN) is much higher than that of the 2024 Al matrix (1.65 mN), indicating that the SCF possesses a higher hardness than the nearby matrix. Figure 8b displays the hardness (H) and modulus (E) variation as indentation moving from a single SCF to the aluminum matrix. The SCF has the highest hardness at 4.58 GPa on average and lowest modulus at 28.32 GPa on average. For the aluminum matrix near the single SCF, the indentation in the vicinity of carbon fiber is provided with highest modulus (93.53 GPa) and hardness (2.51 GPa). As the indentation moves far away from the carbon fiber, the modulus and hardness for the matrix both decrease.