# Microstructure and Properties of AA6061/SiCp Composites Sintered under Ultra High-Pressure
## 3 Results and Discussion
### Research on Mechanical Property  
Some of the literature [27; 28; 29; 30; 31; 32] studied the tensile strength of Al/N-SiCp composites with the different content of SiCp. In this paper, AA6061/N-SiCp composites with 3, 5, and 10 vol.% were prepared at 550 \({}^{\circ}\)C. It showed the largest tensile strength of the composite, 314.7 MPa for AA6061/3 vol.% N-SiCp without further treatment. Compared with the AA6061/N-SiCp prepared by powder metallurgy at 600 \({}^{\circ}\)C, the tensile strength increased by about 88.2%. The fracture morphology is shown in Figure 13. The surface of the port has a stone-like fracture morphology, which is due to the dispersion of N-SiCp at the grain boundary. The dispersion of N-SiCp forms a large number of fine dimples distributed along the grain boundary. The N-SiCp location is inclined to cause stress concentration, and thus the crack is easy to form at the grain boundary and expands along the grain boundary, resulting in brittle grain boundary fracture. Therefore, the fracture mechanism is an intergranular fracture.  
Predecessors [28; 29; 30; 31; 32; 33; 34; 35] studied the different Al/SiCp composites with different particle sizes and sintered them at different temperatures. The tensile properties of the Al/SiC matrix composites make a difference at various temperatures.  
In this study, AA6061/5 vol.% N-SiCp composites were sintered at 450, 550, and 600 \({}^{\circ}\)C, respectively. The highest tensile strength of the composite was obtained at 550 \({}^{\circ}\)C, which was about 227 MPa. The fracture morphologies of sample sintered at different temperatures are shown in Figure 14. Figure 14 shows the fracture morphology of AA6061/5 vol.% N-SiCp composites. The fracture mode is similar to that of intergranular fracture. The irregularity in this figure is attributed to the AA6061 particles, which are pressed against each other under UHPS, and are changed from spherical particles to polyhedral shapes. The material fracture first occurs at the sintered neck between the particles. The micro-oxidation of the impurities on Al particles causes the performance of the sintered neck to reduce, corresponding to the internal grain boundary of the material. Thereby, it causes fracture from the neck and leads to intergranular fracture.  
Figure 12: The curve of the hardness of AA6061/N-SiCp composite at 4.5 GPa: Relationship between hardness and volume fraction: 0, 1, 3, 5, 10, 15 vol.% (**a**), Relationship between hardness and sintering temperature: 450, 550, 600, 650 \({}^{\circ}\)C (**b**) for AA6061/5 vol.% N-SiCp sample.  
Figure 14b shows the fracture morphology of AA6061/5 vol.% N-SiCp composites prepared at 4.5 GPa and 550 \({}^{\circ}\)C. The fracture mechanism is a quasi-cleavage fracture, and there is a flat \"cleavage-like\" region in the figure. The small plane and micropores appear. The sample of Figure 14b is similar to the sample in Figure 14a. Increasing the temperature is more conducive to the elimination of voids and defects so that the sintered neck with excellent performance is formed between the AA6061 particles. The fracture morphology in Figure 14b has a great difference from that in Figure 14a. During the tensile process, the AA6061 matrix is plastically deformed, but N-SiCp distributed among them are small. The uncoordinated deformation causes micro-voids to occur in the small range, which nucleate, grow, and finally connect. Therefore, breaking, dimples, and tearing edges can be inspected. Figure 14c shows the fracture morphology of AA6061/5 vol.% N-SiCp composites sintered at 600 \({}^{\circ}\)C. It can be discovered that there are many smaller dimples on the fracture surface. This is mainly due to the agglomeration of N-SiC in the localized area and the SiC will be pulled out during the stretching process.