## 2 Materials and Methods  
T700 polyacrylamide(PAN)-based carbon fiber was provided by Toray Co. Ltd., Tokyo. The density of the PAN-based carbon fiber was 1.80 g/cm\({}^{-3}\), and the average diameter was 7 \(\upmu\)m. The PAN-based carbon fiber was immersed in acetone for 4 h to remove sizing agent, and then desized into SCF by ball milling. The final aspect ratio of SCF was less than 42.8. The as received 2024 Al powder (Chaowei Nanotechnology Co. Ltd., Shanghai, China) was \(\sim\)20 \(\upmu\)m in D50 particle size and 2.78 g/cm\({}^{-3}\) in density. The composition of the matrix was: 0.125 wt % Fe, 0.087 wt % Si, 3.952 wt % Cu, 0.450 wt % Mn, 1.283 wt % Mg, 0.137 wt % Zn and the balance aluminum.  
SCFs were uniformly dispersed in 2024 Al powder through mechanical mixing using ethyl alcohol as mixing agent. The blended powders were then placed in a graphite mold with \(\Phi\)60 mm diameter and vacuum dried at 343 K for 2 h to remove ethyl alcohol. SCFs reinforced 2024 Al matrix composites were fabricated by heating in a vacuum furnace at 853 K for 40 min, during which a 30 MPa pressure was applied on the powders. The SCFs/2024 Al were then T6-treated (solution treated at 768 K for 60 min, water quenched and then aged at 463 K for 12 h). Besides, the comparative sample 2024 Al was prepared by powder metallurgy (PM) using the same method.  
The microstructures were characterized by a scanning electron microscopy (SEM.; S-4800, Tokyo, Japan) and a transmission electron microscopy (TEM, Titan G2 60-300, Hillsboro, USA) equippedwith scanning transmission electron microscopy (STEM) and energy dispersive spectrometer (EDS). The phase constituents of samples were identified by X-ray diffraction (XRD, D/Max 2500 PC, Tokyo, Japan) using Cu K\(\alpha\) radiation in step mode from 20\({}^{\circ}\) to 80\({}^{\circ}\). The density of SCFs/2024 Al was measured through Archimedes' principle [36]. The specimens for the tensile test were prepared in accordance with ASTM Standard E-8/E8M-09 with a 10 mm gauge length. Tensile tests were performed at a strain rate of \(8.3\times 10^{-4}\) s\({}^{-1}\) on INSTRON-1121 material testing machine at room temperature 423 and 523 K. The Vickers hardness was measured via a microhardness tester (1600-5122 VD Microment 5104, Chicago, USA) under an applied load of 100 g for 15 s. At least seven measurements were performed to ensuring the accuracy of results. Further, nanoindentation measurement was done using an MTS XP nanoindentation with a Berkovich type pyramidal diamond indenter. The indent with 2 \(\upmu\)m spacing between neighboring indents and 200 nm depths were used. The load-displacement curve of the indentation was obtained by recording the applied forces on the indenter and the corresponding displacements. The contact hardness H was determined by  
\[\mathrm{H}=\frac{P_{max}}{A_{c}} \tag{1}\]  
where \(P_{max}\) and \(A_{C}\) was the peak load and the real contact area, respectively.  
The elastic modulus \(E\) was calculated using the following equation:  
\[\frac{1}{E_{r}}=\frac{1-v^{2}}{E}+\frac{1-v_{i}^{2}}{E_{i}} \tag{2}\]  
where \(E_{i}\) was the modulus of the indenter, \(v\) and \(v_{i}\) were the Poisson's ratio for the specimen and the indenter, respectively. The values \(E_{i}\) = 1141 GPa, \(v_{i}\) = 0.07 [37] for the indenter, \(v\) = 0.33 for the aluminum matrix and \(v\) = 0.30 for the carbon fiber [38] were used in all computations. The effective elastic modulus, \(E_{r}\), which accounts for elastic displacements in both the specimen and the indenter, was evaluated from  
\[E_{r}=\frac{1}{\beta}\frac{\sqrt{\pi}}{2}\frac{S}{\sqrt{A_{c}}} \tag{3}\]  
where \(\beta\) was a correction factor, which depends on the indenter geometry. For Berkovich indenter, \(\beta\) was a constant with a value of 1.034 [39].