## 3 Results and Discussion
### Microstructure, Relative Density and Phase Analysis of the G/6061 Al Composite  
The microstructure of the UHP-sintered G/6061 Al consists of Al matrix (light region) and graphite (dark region), with graphite flakes uniformly distributing in the Al matrix, which signifies the effect of pre-coating on scattering the graphite flakes, shown in Figure 2. It is observed in the optical images that no significant difference has been caused by variation of the sintering temperature, seen from Figure 2b,d. However, some agglomerations appear when the graphite content increased to 15 vol.%, as shown in Figure 2c. Due to the limited magnification, no pores are found in the optical microcopies, shown in Figure 2. However, the relative density is measured to be only 91.35%, implying the existence  
Figure 1: Scanning electron microscopy (SEM) micrographs of (**a**) nano-Al powders, (**b**) graphite flakes, (**c**) graphite flakes coated with nano-aluminum (NAGF) powders and (**d**) the mixed composite powders with graphite content of 10 vol.%. of pores in the G/6061 Al HP-sintered at 35 MPa and 520 \({}^{\circ}\)C; meanwhile, the relative density of the G/6061 Al with 10 vol.% graphite, sintered at 5.2 GPa and 450 \({}^{\circ}\)C, is 98.5% and is the lowest among the UHP-sintered, seen in Figure 3a,b. It is also illustrated in Figure 3a that the relative density increases when raising the temperature from 450 \({}^{\circ}\)C to 650 \({}^{\circ}\)C, but slightly decreases as the sintering temperature is raised to 750 \({}^{\circ}\)C at 5.2 GPa. When the temperature is not higher than 550 \({}^{\circ}\)C, the relative density of the composite bearing 5 vol.% graphite is higher than that bearing 10 vol.% graphite, while an inverse result is presented when the  
Figure 3: Effect of (**a**) sintering temperature and (**b**) graphite content on the relative density of the G/6061 Al. Figure 2: Optical images of the sintered G/6061 Al with (**a**) 5 vol.%, (**b**) 10 vol.% and (**c**) 15 vol.% graphite at 5.2 GPa and 650 \({}^{\circ}\)C and (**d**) that with 10 vol.% at 5.2 GPa and 550 \({}^{\circ}\)C, or (**e**) at 35 MPa and 520 \({}^{\circ}\)C.  
sintering temperature is higher than 550 \({}^{\circ}\)C, shown in Figure 3a. In addition, it is also observed that adding more graphite increases the relative density when the G/6061 Al is sintered at 5.2 GPa and 650 \({}^{\circ}\)C, as shown in Figure 3b. It is reported that the relative density of G/Al decreases with the increase of graphite [9; 12; 13; 14], different from the above result, which is considered to probably be caused by the reaction of Al and graphite during the UHP sintering at a high temperature. More precise observations were carried out by SEM that demonstrate some pores existing in the HP-sintered, marked by a white arrow in Figure 4a, but pores are still scarcely observed in the UHP-sintered G/6061 Al, shown in Figure 4b-d. Furthermore, the shape of graphite in the UHP-sintered G/6061 Al is changed to be quite different from its original. During the UHP sintering, relative movement between graphite and 6061 Al matrix was constricted by the hydrostatic pressure, correspondingly the graphite appearance depended on the shape of the gap between 6061 Al particles. Under the ultra-high pressure, the 6061 Al particle was also deformed and could not keep its original spherical shape, leading to flat boundaries at the parts contacting with graphite, similar to that of stable grain boundaries. At the sites contacting with other 6061 Al particles, the original surface disappeared and the two Al particles combined with each other, shown in Figure 4b-d. Concurrently, the gaps between the 6061 Al particles were substantially reducedand the pores at the interface of the original graphite flake and the 6061 Al particle could also be eliminated by the ultra-high pressure, leading to a high relative density. For instance, Figure 3a shows that relative densities of 5 vol.% and 10 vol.% graphite G/6061 Al are 99.19% and 98.9% individually, sintered at 550 \({}^{\circ}\)C and 5.2 GPa.