# Microstructure and Properties of AA6061/SiCp Composites Sintered under Ultra High-Pressure
## 2 Materials and Experiments  
AA6061 alloy powders with an average size of 40\(\sim\)53 \(\upmu\)m prepared by JW-25 vacuum metal atomization furnace (Chenhua Science Technology Corp., Ltd., Shanghai, China) were used as matrix material. The measured density and the purity are 2.65 g/cm\({}^{3}\) and 99.8%, respectively, and have the color of silver gray, as shown in Figure 1. The N-SiCp, with an average size of 50 nm, the density of 3.227 g/cm\({}^{3}\), gray-green color, ellipsoidal shape, purity of 99.9% and the specific surface area of 60 m\({}^{2}\)/g, is depicted in Figure 2a. The N-SiCp were used as reinforcement and provided by Yaotian New Material Technology Co. Ltd. in Shanghai, China. The chemical compositions of the AA6061 are listed in Table 1. N-SiCp weighed in 1, 3, 5, 10, and 15 vol.% were premixed with AA6061 alloy powder in a mortar and then milled into a high-speed vibrating ball mill (NanDa Instrument Plant, Nanjing, China). The mixing time and speed is 2 h, 300 r/min, respectively. The mixed powders are shown in Figure 2b. To make N-SiCp distribute partly on the surface of the Al particles, the Al particles and the N-SiCp were mixed in a mortar according to the patent CN201210103182.6. The surface energy of the nanoparticles is high, and the coating layer improves the wettability of the material, hence it reduces the agglomeration of the SiC particles.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Si** & **Cu** & **Mg** & **Zn** & **Mn** & **Ti** & **Gr** & **Fe** & **Al** \\ \hline
0.393 & 0.246 & 0.96 & 0.001 & 0.086 & 0.015 & 0.05 & 0.1 & balance \\ \hline \hline \end{tabular}
\end{table}
Table 1: Chemical component of AA6061 (wt%).  
The 6 MK-3 WY-60 six-face pressing machine was used as the sintering equipment. The powder is pre-compressed and then filled into the pyrophyllite block for sintering at 4.5 GPa, which is selected according to the experimental data, for 40 min and at 450, 550, 600, and 650 \({}^{\circ}\)C, respectively. The pressure was produced by a pressure cylinder, which is connected to a hinge. It uses oil pressure in the pipeline to generate high pressure, which is adjusted by the cross-sectional area. The pressure could be calculated according to Equation (1). P1, P, S1, S, stands for the pressure generated by a pressure cylinder, the pressure applied to the sample, the cross-sectional area of the hammer, the area contacted between the sample, and the hammer, respectively. An optical microscope (OM, Olympus GX-51, Tokyo, Japan) and a field emission scanning electron microscope (SEM, Merlin Compact, Carl Zeiss NTS GmbH, Jena, Germany) were used for microstructure characterization of the polished specimens. An X-ray diffractometer (XRD, SmartLab, Rigaku Corporation, Tokyo, Japan) was used for phase analysis using CuK\(\alpha\) radiation in the step scan mode and with the scanning speed of 10\({}^{\circ}\)/min. The interface microstructure was characterized using a high-resolution transmission electron microscope (TEM, JEM-2100, JEOL Ltd., Tokyo, Japan). The density was measured by the Archimedes method using the density tester (ED-300C, TecFront Electronics Corp., Shanghai, China). The hardness was measured by a Rockwell hardness tester (HBE-300, Wobot Measuring Instruments Co., Ltd, Shanghai, China), under the load of 50 Kgf for 10 s. The wear tests were conducted by a wear tester (M-2000, Hongda Testing Instrument Co. Ltd., Jinan, China). Every test was repeated three times to check the reproducibility and the reliability of the results.  
\[\text{P}=\text{P}_{1}\text{S}_{1}\text{/S} \tag{1}\]