## 3 Results and Discussion
### Mechanical Properties of G/6061 Al Composite  
The effect of the sintering temperature and the content of graphite on hardness is shown in Figure 7a,b. During hardness testing, it was found that the hardness of HP-sintered 5 vol.% G/6061 Al was HBS 39.3--too soft to be measured by an HRA tester, while the UHP-sintered G/6061 Al was much  
Figure 5: HRTEM micrographs of (**a**) hexagon or (**b**) bar-type and (**c**) nano-Al\({}_{4}\)C\({}_{3}\), and (**d**) selected area electron diffraction of (**c**). Figure 6: XRD patterns of the sintered with G content of (**a**) 5 vol.% and (**b**) 10 vol.%, at different temperatures under 5.2 GPa. harder. Figure 7a,b presents that the hardness of the UHP-sintered G/6061 Al increases with the increase of graphite and with raising the sintering temperature. Figure 7a also illustrates that the hardness of the 10 vol.% is lower than that of the 5 vol.% at sintering temperature lower than 650 \({}^{\circ}\)C, while it is the opposite at sintering temperature of 650 \({}^{\circ}\)C or higher, similar to the comparison of relative density in Figure 3a. Based on the foresaid microstructure analysis, it is obviously caused by the formation of Al4C3 and relative density. Table 1 lists out the tensile strengths of the G/6061 Al composites in this investigation and other reported. Table 1 shows that the tensile strength of the UHP-sintered 6061 Al with 0 vol.% graphite is 338 MPa, equivalent to the tensile strength of the 6061 Al after T6 heat treatment; the tensile strength of the HP-sintered 6061 Al is only 179 MPa, much lower than the UHP-sintered 6061 Al. With the addition of 5 vol.% G, the tensile strengths of the UHP-sintered and HP-sintered G/6061 Al decreased. However, the tensile strength of the UHP-sintered 5 vol.% G/6061 Al is 183 MPa, slightly higher than the HP-sintered 6061 Al, and about two times of the HP-sintered 5 vol.% G/6061 Al.It is evident that the strength of the HP-sintered is much lower than that of the UHP-sintered, but is comparable to that of the reported [2,5,9]. This can also demonstrate that the ultra-high pressure has an important effect on promotion of strength of the G/6061 Al. An investigation of fracture surface shows big dimples in the fracture surface of the 0 vol.% G/6061 Al sintered at 5.2 GPa, which generally represents ductile fracture, as shown in Figure 8a. However,the dimpleis alike to grain boundary, meaning that the interface boundary between the Al particles is still the weak region, which can act as the crack source and the crack propagation path. It is deemed that 6061 Al particles were extruded together with small relative slippage under high isostatic pressure and thesurface impurities of oxidation were retained in part of the interface. Accordingly, the bonding strength at interface was lower than that at the interior, thus the interfaceacted as the crack source and the propagation path. Though graphite could promote the slither of 6061 Al powders during the UHP sintering, which was propitious to destroy the oxide layer and improved the connection \begin{table}
\begin{tabular}{c c c} \hline \hline
**Processing Parameters** & **Yield Strength Rp\({}_{02}\) (MPa)** & **Tensile Strength Rm (MPa)** \\ \hline
5.2 GPa, 650 \({}^{\circ}\)C, 0 vol.% G & 293 & 338 \\
5.2 GPa, 650 \({}^{\circ}\)C, 5 vol.%G & 142 & 183 \\
35 MPa, 520 \({}^{\circ}\)C, 0 vol.% G & 81 & 179 \\
35 MPa, 520 \({}^{\circ}\)C, 5 vol.% G & 74 & 77 \\
6101 Al, casting \(\sim\) T6 heat treatment [2] & - & 164 \\
0 wt.% G/6016 Al, sintering + hotstruding [9] & 75 & 106 \\
5 wt.% G/6061 Al, sintering + hot extruding [9] & 67 (Measured from the curve) & 97 (Measured from the curve) \\
2 wt.% Graphene [5] & 48 & 94 \\
2 wt.% Graphite [5] & 44 & 75.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Tensile strengths of the composites in this investigation and other reported literature. Figure 7: Effect of (**a**) sintering temperature and (**b**) graphite content on the hardness of the G/6061 Al. between particles, graphite could also be the source of the crack.