## 3 Results
### Material Properties  
The representative tensile stress-strain curve of the prepared composites and 2024 Al matrix is shown in Figure 8. Correspondingly, the yield strength (YS), ultimate tensile strength (UTS), and microhardness obtained from the test are provided in Table 2. It can be seen that the mechanical properties of the composites were significantly improved after adding an appropriate amount of GNPs. The average UTS and YS exhibited were approximately 170.6 MPa and 257.4 MPa, which represent an increase of 12.2% and 18% compared to the matrix value, respectively. In addition, the composite fabricated shows better mechanical properties than the original 0.5 wt.% GNPs/2024 Al composite when the reinforcement is replaced with modified GNPs. The YS and UTS of the nanocomposite reinforced with CeO\({}_{2}\)-GNPs were 184.1 MPa and 271.8 MPa, increasing by 21.1% and 24.7%, as compared to the matrix. The carbon-based reinforcement with coated particles  
Figure 7: Mapping analysis of the 0.5 wt.% GNPs/2024 Al composite fabricated. provides a pinning effect on the dislocation during the deformation process, leading to further improvement in the mechanical properties of the composites [25]. Figure 8(a)-d shows the typical fracture morphology and EDX results of composites reinforced with GNPs and CeO\({}_{2}\)-GNPs. The fracture morphology of the 0.5 wt.% GNPs/2024 Al composite is mainly composed of cleavage fracture, torn edges, and lots of platforms, as shown in Figure 8(a). It means that the fracture of the composite is a mainly brittle fracture. When the reinforcement has been replaced by CeO\({}_{2}\)-GNPs, the number of brittle platforms areas shows a decreasing trend, as can be seen in Figure 8(d). Furthermore, it can be clearly observed from the microstructure comparison in Figure 8(b),d that there are more obvious gaps between the original GNP\({}_{\text{S}}\) and the matrix. Moreover, the decorated GNPs have a better bonding with the matrix compared with the original GNPs. It means the interface bonding between GNPs and the matrix has been improved due to the surface decoration effect of the rare earth oxide CeO\({}_{2}\). \begin{table}
\begin{tabular}{c c c c} \hline \hline
** Composites** & **YS (MPa)** & **UTS (MPa)** & **Microhardness (Hv)** \\ \hline
2024 Al & 152 & 218 & 101.2 \\
2024 Al composite/0.5 wt.\% GNPs & 170.6 & 257.4 & 111 \\
2024 Al composite/0.5 wt.\% & 184.1 & 271.8 & 128 \\ CeO\({}_{2}\)-GNPs & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Corresponding mechanical properties of composites prepared. Figure 9: _Cont._  
Figure 8: Tensile strain-stress curves (**a**) and microhardness (**b**) of the composites fabricated and 2024 Al alloy matrix. Coefficient of thermal expansion (CTE) mismatch strengthening, load transfer, and grain refinement mechanisms are considered to be the three main strengthening mechanisms for graphene-reinforced metal matrix composites. Coefficient of thermal expansion (CTE) mismatch strengthening refers to the enhancement of the mechanical properties due to the huge difference in the coefficient of thermal expansion (CTE) between GNPs and Al alloys promoting the generation of dislocations at the interfaces. The TEM image of dislocations is formed in the CeO\({}_{2}\)-GNPs/2024 Al composite, as shown in Figure 10.  
Load transfer mechanism, in addition, is considered to be the critical mechanism with the greatest contribution to matrix strengthening among them. It mainly refers to the load transfer from the matrix to GNPs during the deformation of the composites. Furthermore, it can also be seen from Figure 6 that the addition of GNPs promoted grain refinement, which is beneficial to the improvement of the mechanical properties of the composites. Correspondingly, all three of them can be used to calculate the yield strength increase numerically through their calculation models.