## 3 Results  
### Material Characterization  
Figure 3a shows the TEM image of the CeO\({}_{2}\) surface-decorated GNPs. It can be seen from the image that the surface of GNPs changed considerably after the surface decorating process. From the enlarged image shown in Figure 3b, it can be seen that even after the ultrasonic pretreatment for the TEM test, these generated particles are still tightly adhered to and distributed on the surface of the GNPs. This implies the existence of strong interface bonding [20]. The corresponding EDX spectrum, as shown in Figure 3d, indicates that the particles mainly consisted of Ce and O. Close examination of the rare earth oxides on the surface of the GNPs (Figure 3b) shows a d-spacing with an observed fringe separation of 0.31-0.32 nm. Consistent with the interlayer separations of the (111) [21] crystal plane of CeO\({}_{2}\) (Figure 3c). Therefore, it could be confirmed that CeO\({}_{2}\) particles have been successfully generated and decorated on the surface of GNPs. To assess the damage degree of the surface-decorated and ball milling process to the GNPs. The comparison of the Raman spectra of original GNPs and Al-CeO\({}_{2}\)-GNPs mixture after ball milling is shown in Figure 4. The better the structural integrity of GNPs, the  
Figure 3: TEM photographs of CeO\({}_{2}\)-GNPs: (**a**) TEM and (**b**) HRTEM images of CeO\({}_{2}\)-GNPs; (**c**) SAED patterns; (**d**) corresponding EDX result of the nanoparticles. smaller the ID/IG value in general. In this study, it is mainly to show the change of its value after processing. The results show that the apparent peaks are located at ~1350 cm\({}^{-1}\) and ~1580 cm\({}^{-1}\)[22], corresponding to the D-band and G band of GNPs, respectively. The calculated values (ID/IG) were 0.66 and 0.82, which means that even after the ball milling process, these characteristic values only slightly increased. Namely, compared with the original GNPs, the ball milling process only caused slight damage to the surface of GNPs. The structural integrity of the GNPs in the mixed powder was not damaged. The optical micrograph and grain size distribution of 2024 matrix alloy, 0.5 wt.% SNPs/2024 Al and CeO2-GNPs/2024 Al composites are shown in Figure 5. The average values of the grains are also labeled in Figure 5d-f. The comparison of Figure 5a-c reveals that the grains of matrix alloy was refined after adding an appropriate amount of GNPs. This phenomenon is usually caused by the GNPs distributed in the grain boundaries hindering the growth of the grains [23]. On the other hand, it can be observed through Figure 5b,c that the grain size of the composite prepared by adding the reinforcement CeO2-GNPs was smaller than that of adding raw GNPs. As can be seen from Figure 5e,f, the average grain size of the 0.5 wt.% GNPs/2024 Al composite was 26.4 \(\upmu\)m, while that size of the 0.5 wt.% CeO2-GNPs/2024 Al composite was further reduced to 22.9 \(\upmu\)m. In addition, there were fewer micropores revealed on the surface of the 0.5 wt.% CeO2-GNPs/2024 Al composite compared with the others. Generally, the micropores structure is considered to be the agglomerations of GNPs at the grain boundaries. An appropriate amount of GNPs has a grain refinement effect on the matrix alloy existing in the grain boundary, as mentioned above. However, a large amount of agglomerated GNPs impedes the continuity of the metal material, resulting in the deterioration of the mechanical properties of the composites [24]. These observations could be attributed to the decorating process that optimizes the wettability of GNPs with 2024 alloy and, therefore, improves the dispersibility of the GNPs in the matrix. The improvement of the wettability between the GNPs and the matrix promotes the combination of GNPs and alloy, which leads to a reduction in structural defects during the process. Figure 6 shows the TEM and HRTEM analysis of CeO2-GNPs/2024 Al composites prepared with this current process.