## 3 Results and Discussion
It indicates that in SCF is able to enhance the hardness and modulus of the nearby matrix, and the enhancement weakens with the increasing distance from the 2024 Al matrix to the fiber. Figure 9a-c shows the engineering stress versus strain curves for 2024 Al and 2-8 vol. % SCFs/2024 Al composites at room temperature, 423 K and 523 K respectively, while Figure 9d-f and Table 1 summarize the typical tensile properties for all samples, including yield strength (YS), ultimate tensile strength (UTS) and fracture strain (\(\varepsilon_{f}\)). The mechanical strengths for 2024 Al and SCFs/2024 Al composites display a sound dependence on temperature and SCFs content. The increasing temperature gives rise to the deterioration in mechanical strengths for 2024 Al and SCFs/2024 Al composites. The YS and UTS of the composites first increase and then decrease with an increasing volume fraction of SCFs. Figure 7: The variation tendency of Vickers hardness with the volume fraction of SCFs, and the inset shows the indentation with high hardness. At all temperatures, the samples for 4 vol. % SCFs/2024 Al composites possess the highest YS and UTS. In comparison with 2024 Al, the YSs for 4 vol. % SCFs/2024 Al composites are 19.6%, 45.6%, and 36.7% higher than those of 2024 Al at room temperature, 423 K and 523 K, while the UTSs increase 4.6%, 13.7%, and 16.2%, respectively. It is worth noting that the addition of SCFs results in the significant decrease in elongation for SCFs/2024 Al composites. From the engineering stress-strain curves for 2024 Al and SCFs/2024 Al composites (Figure 9a), unlike the apparent plastic deformation for 2024 Al, SCFs/2024 Al composites are found fractured immediately after yielding at room temperature. Meanwhile, with the temperature increasing, the engineering stress-strain curves for SCFs/2024 Al (Figure 9b,c) exhibit a relatively obvious plastic deformation after yielding. It indicates that temperature plays a critical role in affecting the fracture behavior of SCFs/2024 Al composites. The fracture morphologies of 4 vol. % SCFs/2024 Al composites at different temperatures are displayed in \[\rho_{theoretical}=\rho_{C}V_{C}+\rho_{M}V_{M} \tag{4}\]0 For the fracture surface of 4 vol. % SCFs/2024 Al composite at room temperature, the existence of dimples is observed on the 2024 Al matrix, indicating the ductile failure for the 2024 Al matrix. The primary reason causing the dramatic decrease in fracture strain is the addition of SCFs. It is worth noting that the debonding between SCF and aluminum matrix is hard to observe, and most of the SCFs are subjected to shear failure (as shown in Figure 10a,b). At 423 K, the amount of sheared SCFs decreases. Meanwhile, the debonding between SCFs and matrix is existent, and some SCFs are observed to be pulled out from the matrix (Figure 10c,d). When the temperature increases to 523 K, the predominant fracture morphologies for the composite debond and pull-out SCFs (Figure 10e,f). Figure 11 presents the TEM micrographs for the 4 vol. % SCFs/2024 Al composites after the tensile test at 523 K. As shown in Figure 11a, after the tensile test at 523 K, the composite has a larger density of Al\({}_{4}\)C\({}_{3}\) at the interface. Meanwhile, the interfacial product has an average size of 70 nm in width, which is larger than the size of Al\({}_{4}\)C\({}_{3}\) at the interface in the composite at room temperature (as shown in Figure 4a). The plate-shaped \(\theta^{\prime}\) (Al\({}_{2}\)Cu) precipitates along \(<\)101\(>\) direction of \(\alpha\)-Al matrix also have a larger average size of \(\sim\)150 nm (Figure 11b), indicating the coarsening of precipitates at high temperature (523 K). Figure 11c shows that the dislocation accumulation takes place in the Al matrix near the interface between carbon fiber and 2024 Al. The STEM and EDS mapping results reveal that large amounts of Al\({}_{2}\)Cu segregation form in the area around Al\({}_{4}\)C\({}_{3}\), and the segregation coarsens a lot compared to that in SCFs/2024 Al composites at room temperature (as displayed in Figure 6).