## 3 Results and Discussion  
In this study, the SCFs were added in 2024 Al matrix with volume fraction ranging from 2 to 8%. The SEM morphology for the SCFs/2024 Al are presented in Figure 1. As shown in Figure 1, SCFs are dispersed uniformly in the 2024 Al matrix with no obvious clustering being observed. It suggests that the clustering of SCFs could be avoided by reducing the aspect ratio. It is also clear that no pore or cracking is found between SCF and matrix. The density of 2024 Al against different volume fractions of SCFs/2024 Al is displayed in Figure 2a. As shown in Figure 2a, the addition of SCFs results in a lower density of SCFs/2024 Al than that of 2024 Al matrix (2.763 g/cm\({}^{-3}\)). In the meantime, with the volume fraction of SCFs increasing from 2 to 8 vol. %, the density decreases from 2.746 to 2.692 g/cm\({}^{-3}\). The theoretical density of the composite is determined by  
\[\rho_{theoretical}=\rho_{C}V_{C}+\rho_{M}V_{M} \tag{4}\]  
where \(\rho_{C}\) (1.80 g/cm\({}^{-3}\)) and \(\rho_{M}\) (2.78 g/cm\({}^{-3}\)) are the theoretical density of carbon fiber and 2024 Al matrix, respectively. \(V_{C}\) and \(V_{M}\) are the volume fraction of carbon fiber and 2024 Al matrix, respectively. Therefore, the theoretical density for the composites with 2, 4, 6 and 8 vol. % is 2.760, 2.741, 2.721 and 2.702 g/cm\({}^{-3}\), respectively. The relative density \(\rho_{relative}\) is determined by  
\[\rho_{relative}=\rho_{measured}/\rho_{theoretical} \tag{5}\]where \(\rho_{measured}\) is the measured density. The relative density of SCFs/2024 Al and 2024 Al matrix is shown in Figure 2b. All composites with different fractions of SCFs achieve above 99% in relative density, indicating that low porosity is achieved through powder metallurgy. Figure 3a presents the XRD patterns for 8 vol. % SCFs/2024 Al composite and 2024 Al. According to the XRD results, major aluminum peaks are detected at 38.4\({}^{\circ}\) (1 1 1), 44.8\({}^{\circ}\) (2 0 0), 65.1\({}^{\circ}\) (2 2 0) and 78.3\({}^{\circ}\) (3 1 1) (PDF#65-2869) for 2024 Al. In contrast, the four aluminum peaks detected in the XRD pattern for 8 vol. % SCFs/2024 Al composite all shift \(\sim\)0.2\({}^{\circ}\) to a higher value, indicating aluminum grains in the SCFs/2024 Al composite are subjected to tensile strain. Owing to the sensitivity of XRD, no peaks representing typical precipitates (such as Al\({}_{2}\)Cu) or interfacial product (such as Al\({}_{4}\)C\({}_{3}\)) are observed in the XRD patterns for both 2024 Al and SCFs/2024 Al composites. It is worth noting that no carbon peak representing carbon fiber is detected in the XRD pattern for SCFs/2024 Al composite with the highest SCFs content (8 vol. %). [PERSON] [27] reported that PAN-based carbon fibers are largely amorphous, as evidenced by the relative lack of preferential orientation in the graphite basal planes. Figure 3b shows the average crystallite size obtained based on XRD patterns and Scherrer's equation [40]. 2024 Al has the largest average crystallite size with 81 nm. The addition of SCFs results in a decrease in crystallite size of aluminum matrix, and the crystallite size decreases with the increasing SCFs content. Figure 1: SEM images for the 2024 Al matrix composites with (**a**) 2, (**b**) 4, (**c**) 6 and (**d**) 8 vol. % short carbon fibers (SCFs). Figure 2: The (**a**) measured density and (**b**) relative density for the composites with different volume fraction of SCFs. Figure 4 illustrates the morphology of the interface between carbon fiber and matrix in 4 vol. % SCFs/2024 Al composite. As shown in Figure 4a, different from the XRD results, the interface is rich in the rod-like interfacial products (highlighted by white arrows), which have an average size of 28 nm in width. The interfacial products nucleate discontinuously at the fiber surface and grow into the matrix in various directions. Meanwhile, Figure 4b shows that large amounts of dislocations are accumulated in the matrix, especially in the vicinity of carbon fiber.