# 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  
Figure 3 shows the microstructures of the composites with 1, 3, 10, 15 vol.% N-SiCp sintered at 550 \({}^{\circ}\)C under the compacting pressure of 4.5 GP. As shown in Figure 2(a),b, Al particles without the agglomerations are uniformly coated with a small layer of the N-SiCp. There are uniform grains  
Figure 1: SEM morphology of AA6061 powder. Figure 2: SEM micrographs of N-SiC powder (**a**) and AA6061/N-SiC powder mixture (**b**). with a small difference and smooth grain boundaries. The particles change from a hexagon shape in Figure 3a-d to more circular shapes in Figure 3e at 4.5 GPa. Increasing the volume fraction of the N-SiCp, agglomeration appears at the boundary of the matrix in Figure 3c,d and the roundness of the Al particles increases gradually from an irregular polygon to a circular shape, as shown in Figure 3e. During the process of UHPS, the Al particle is pressed and deformed with each other. As the volume fraction of the N-SiCp increases, the N-SiCp particles surrounded by the Al particles begin to withstand the certain sintering pressure, resulting in the Al particles retaining a good roundness [20]. Besides, as the content of the N-SiCp continues to rise, the particle becomes uneven and increasingly small. During the sintering process, the pores between the particles will diffuse and migrate to the boundary of the matrix, and the small pores merge into the large pores, which make the pores disappear, and the volume of the Al particles becomes smaller. Figure 4 presents a high-magnification microstructure of AA6061/N-SiCp sintered at 4.5 GPa and 550 \({}^{\circ}\)C with different contents of N-SiCp particles. As shown in Figure 4a for AA6061/3 vol.% N-SiCp, N-SiCp lined at the grain boundary of Al particles. What is more, the discontinuous short rod phase (off-white color) appears at the boundaries of Al particles in Figure 4c. The EDS point of Area 1 in Figure 4c is analyzed for the phase, as shown in Figure 5a. The elemental content of this point was obtained from the EDS point analysis. Based on the elemental content, the bulk composition could be judged roughly in the first step. The main elements are Al and O, and the C element is  
Figure 3: OM micrograph of AA6061/N-SiCp composites sintered at 550\({}^{\circ}\)C with 1 (**a**), 3 (**b**), 5 (**c**), 10 (**d**) and 15 (**e**) vol.% N-SiCp. relatively low, which was obtained from Figure 5a. Depending on the element content, it can be judged that the bulk phase is Al4C3 or Al2O3. The line scan analysis of Area 2 in Figure 4d is shown in Figure 5b. The C element and the O element is relatively stable throughout the whole scanning distance. Al content begins to reduce at the intersection of SiC and Al when scanning distance is about 6 \(\upmu\)m. In comparison, Si content presents an increasing trend. The lowest Al content could be found at approximately 7.5 \(\upmu\)m distance and the infiltration zone lasts for 1 \(\upmu\)m. Similarly, an elemental interpretation zone of about 0.8 \(\upmu\)m appears on the other side at 13 \(\upmu\)m. The interdiffusion of the elements promotes the reaction between the elements in the interface region, causing the formation of compounds. However, the bonding strength increases when the amount of Al4C3 is small. When the amount of Al4C3 reaches a critical value, the interface will be damaged. The influence of the specific product content on the properties of the composite material has not been specifically reported before. To further determine the composition of the phase, X-ray diffraction was used to examine the constituents of the AA6061/N-SiCp composites with different volume fractions of N-SiCp, as shown in Figure 6. With the increase in SiCp content, the peak intensity of SiC gradually increases at 32\({}^{\circ}\).