# Microstructure and Properties of AA6061/SiCp Composites Sintered under Ultra High-Pressure
## 3 Results and Discussion
### Effect of the Volume Fractions of N-SiC on Microstructure and Interface
For AA6061/5 vol.% N-SiCp, the two main peaks of the Al4C3 phase are detected at 26 of 31\({}^{\circ}\)-32\({}^{\circ}\) while the peak is not detected for the sample with 1 or 3 vol.% N-SiCp. Combined with the SEM micrograph of AA6061/3 vol.% N-SiCp, a short rod-like phase appears at a certain position. The larger volume fraction of N-SiCp content is, the more N-SiCp around the Al particles, and the larger the interface surface exists between N-SiCp and Al particles. Moreover, the probability of reaction will increase, resulting in more product. Figure 4: SEM micrographs of AA6061/N-SiC composites sintered at 550\({}^{\circ}\)C with 3 (**a**), 5 (**b**), 10 (**c**) and 15 (**d**) vol.% N-SiCp. Figure 5: EDS-SEM analysis: (**a**) The analysis of point 1 of AA6061/10 vol.% N-SiC, (**b**) The analysis of line scanning 2 of AA6061/15 vol.% N-SiC sintered at 550 \({}^{\circ}\)C.  
Figure 6: XRD diffraction pattern of AA6061/N-SiCp sintered at 4.5 GPa and 550 \({}^{\circ}\)C with different content of N-SiCp particles. Figure 7a shows the TEM micrographs of AA6061/1 vol.% N-SiCp. Figure 7b shows the high-resolution microstructure of the interface in Figure 7a. The dark gray ellipsoidal particles in Figure 7a are N-SiC particles with a size of 50 nm, and the white edge is the Al matrix. The interface of the AA6061/1 vol.% N-SiCp is clean and combined tightly and continuously. Figure 7c shows an inverse Fourier transform diagram of the high-resolution map of the C region in Figure 8b. There are many dislocations near the N-SiCp interface. There usually exist thermal misfit dislocations at the interface due to the difference in the thermal expansion coefficient during sintering and cooling. [PERSON] and [PERSON] [21] derived the formula of the dislocation density increment for the ideal cubic shape reinforcement particle composite:  
\[\Delta\rho=12\Delta\alpha\Delta TV_{P}/BD \tag{2}\]  
where \(\Delta\alpha\) and \(\Delta T\) are the difference in thermal expansion coefficient between the reinforcement and the matrix and the temperature difference between the sintering temperature and room temperature, respectively. \(B\) means Berner vector, which could be determined by the Berner loop of the dislocation, the Berner vector of the dislocation loop, or half-dislocation loop, which is relatively complicated [22]. \(D\) and \(V_{p}\) are the average sizes of the reinforcement particles and the volume fraction of the reinforcement, respectively. Figure 7: (**a**) TEM micrograph of AA6061/1 vol.% N-SiCp sintered at 550 \({}^{\circ}\)C and 4.5 GPa, (**b**) IFFT image of zone D in Figure 7c, (**c**) HRTEM image of zone E in Figure 7a. Equation (2) illustrates that the dislocation density is related to the thermal expansion coefficient of the reinforcement material and the matrix material, the sintering temperature, the size and volume fraction of the reinforcement particles. The smaller the reinforcement is, the larger dislocation densities that can be obtained. Furthermore, increased dislocation density can also improve the strength of the material. Figure 8: OM micrograph of AA6061/5 vol.% N-SiC composites sintered at 450 (**a**), 550 (**b**), 600 (**c**), 650 (**d**) \({}^{\circ}\)C.