## 3 Results and Discussion
\[\rho_{theoretical}=\rho_{C}V_{C}+\rho_{M}V_{M} \tag{4}\]1 Engineering stress-strain curves and evolution curves for the tensile properties of 2024 Al and SCFs/2024 Al composites at (**a**) and (**d**) room temperature, (**b**) and (**e**) 423 K and (**c**) and (**f**) 523 K.  
\[\rho_{theoretical}=\rho_{C}V_{C}+\rho_{M}V_{M} \tag{4}\]2 (**a-c**) TEM micrographs, (**d**) HADDF STEM micrograph and (**e**,**f**) corresponding EDS mapping results for the 4 vol. % SCFs/2024 Al composite after tensile test at 523 K.  
\[\rho_{theoretical}=\rho_{C}V_{C}+\rho_{M}V_{M} \tag{4}\]3 The SEM images for the fracture surface of 4 vol. % SCFs/2024 Al composites at (**a**,**b**) room temperature, (**c**,**d**) 423 K and (**e**,**f**) 523 K.  
From the results of Vickers hardness tests (Figure 7), the addition of SCFs gives rise to the significant improvement in Vickers hardness of SCFs/2024 Al composites, and the Vickers hardness increases with the increasing volume fraction of SCFs. Corresponding to the results of nanoindentation (Fig 8a), carbon fiber, as a hard phase, processes much higher hardness than 2024 Al. From the view of the rule of mixture, the increasing volume fraction of SCFs is able to cause higher hardness of SCFs/2024 Al composites. On the other hand, the addition of SCFs results in the decrease in the crystallite size of the aluminum matrix (Figure 3b), which indicates that SCFs could efficiently impede the coarsening of aluminum grains during the thermal process. According to Hall-Petch relationship, grain refinement creates more grain boundaries, and the high free energy at grain boundaries is able to resist the dislocation motion [42]. Meanwhile, dislocation accumulation is observed in the aluminum matrix near the interface (Figure 4b), resulting in the strengthening of aluminum matrix and leading to a higher hardness, which is in agreement with the results of nanoindentation (Figure 8b). The results of tensile tests show that SCFs give rise to the improvement in yield strength of the SCFs/2024 Al composites compared to that of 2024 Al. At all three temperatures, the yield strength of the composites first increases and then decreases with the SCFs content increasing. Due to the thermal expansion mismatch between carbon fiber and aluminum matrix, residual stress is generated at the interface. Meanwhile, residual stress is able to cause the dislocation accumulation in the matrix near the interface (as shown in Figure 4b). It is important to note that the dislocation accumulation observed at the interface plays a key role in improving the yield strength of SCFs/2024 Al. In addition, the residual stress can be calculated in accordance with the equation below [43]:  
\[\rho_{theoretical}=\rho_{C}V_{C}+\rho_{M}V_{M} \tag{4}\]4  
where \(\sigma_{CF}\) is the stress carbon fiber suffered in radial direction, \(\alpha_{CF}\) and \(\alpha_{m}\) is the thermal expansion coefficient of carbon fiber in radial direction (\(7.2\times 10^{-6}\) K\({}^{-1}\)[35]) and aluminum matrix (\(23.6\times 10^{-6}\) K\({}^{-1}\)[36]), \(\Delta T\) is the difference between sintering temperature and room temperature, and \(E_{CF}\) is the modulus of carbon fiber. Due to the residual stress, the carbon fiber in composites are subjected to compressive stress, as \(\alpha_{CF}\) is much smaller than \(\alpha_{m}\). Reversely, the aluminum matrix is subjected to tensile stress, which is in agreement with the shifted aluminum peaks observed in the XRD patterns for SCFs/2024 Al composites (Figure 3a). The inner tensile stress is adhesive to increasing the yield strength for SCFs/2024 Al. With the SCFs content increasing, the inner tensile stress becomes the predominant disadvantage and causes the deduction in strength. At high temperature (423 K and 523 K in present work), 2024 Al and SCFs/2024 Al composites all sustain an obvious decrease in tensile strength, because of the coarsening of Al\({}_{2}\)Cu phase (as shown in Figure 10b).