# Microstructure and Properties of AA6061/SiCp Composites Sintered under Ultra High-Pressure
## 3 Results and Discussion
### Effect of the Sintering Temperature on Microstructure and Interface  
The samples in this work were fabricated by UHPS method. In this case, the sintering temperature is different from that under normal pressure conditions. Under UHPS conditions, the melting point increase with the pressure rising, according to the Simon-Glatzel Equation (3) [23]  
\[T_{m}=T_{\rm m0}\left(\rm P/(A+1)\right)^{B} \tag{3}\]  
where P is pressure in GPa and \(T_{\rm m0}\) is melting temperature in Kelvin at 100 kPa. Therefore, it plays a great role in melting. [PERSON] [24] prepared AA6061/30 vol.% N-SiCp composite by hot pressing sintering in vacuum at 540, 560, 580, 600, 650 \({}^{\circ}\)C, and 70 mPa, respectively. At 540 \({}^{\circ}\)C, SiC particles agglomerated in the microstructure of composite material, resulting in more obvious voids between each other. As the temperature increases, the voids between the SiC particles became fewer, and there were no large voids in the matrix. At 600 \({}^{\circ}\)C, the SiC particles were filled with the Al matrix, and no reinforcement voids appeared, suggesting that the interface bonding is good. [PERSON] et al. [25] prepared an Al/SiC powder mixture by high energy ball milling and sintered it by spark plasma sintering at 540 \({}^{\circ}\)C and 40 mPa. From the perspective of the microstructure, the overall distribution of SiC particles is uniform in the Al/SiC composite. For the same content of SiC particles, the agglomerations of nano-scale Al/SiC composites are more common and serious than those of micro-scale Al/SiC composites. For the same size of SiC particles, the Al/SiC composites show more agglomeration as the SiC content increases. Figure 8 presents the microstructure of AA6061/5 vol.% N-SiCp composites sintered at different temperatures at 4.5 GPa. It seems that, as the temperature increases, the composite has a slight agglomeration of N-SiCp. The boundary between the Al particles and the N-SiCp layer is smooth at 450 \({}^{\circ}\)C as indicated in Figure 8a. However, from the enlarged figure in Figure 8b-d, as the temperature rises, some glitch from the interface to the Al particles, indicating that the particles extend from the boundary to the Al particles on both sides, which makes the boundary rough. Additionally, an increasing number of small size particles could be inspected. Because of the low temperature, there are a large number of connected pores between the Al particles or between the N-SiCp, and between the Al particle and N-SiCp. These small pores could diffuse on the sample surface. Due to the continuous reduction and disappearance, the micropores begin to increase, and the similar small pores can be merged by the micropores. This will contribute to the reduction in the total pores and the volume fraction of the composite, as well as the increases in the density. As the temperature further increases, the pore surface becomes smooth and begins to round, but the pores do not reduce or disappear. At this time, under the action of the pressure, the pores move toward the surface of the Al particles, and the atoms in the Al particles move in the opposite direction. The pores are filled and grown into smaller Al particles, illustrating the increment of small-sized particles in the metallographic photograph. Figure 9 shows the high-magnification microstructure of AA6061/5 vol.% N-SiCp composites at different sintering temperatures and 4.5 GPa. Judging from the microstructure at 450 \({}^{\circ}\)C in Figure 9a, the boundary between the particles is relatively smooth. The granules begin to appear at 550 \({}^{\circ}\)C, as shown in Figure 9b. The increasing temperature could provide more energy required for the reaction, so the particles tend to grow, as shown in Figure 9d. To determine the phase at the boundary, the XRD analysis was performed on the sample. As shown in Figure 10, the diffraction angle 20 was partially magnified from 30\({}^{\circ}\) to 32.5\({}^{\circ}\).