# Microstructure and Properties of AA6061/SiCp Composites Sintered under Ultra High-Pressure
## 1 Introduction  
Al matrix composites reinforced by micron-sized particles can significantly improve the properties of materials, such as the hardness, wear-resistance, etc. [1; 2; 3; 4]. However, the large size particles in composite materials may contribute to the appearance of holes around the reinforcement during the preparing process and will break or crack during service, reducing the mechanical properties of the composite. Thus N-SiCp is increasingly used as the reinforcement for composites. Commercial N-SiCp is mostly with irregular shapes, and hence, cracks occur at the sharp corners of the SiCp during the sintering process or service due to stress concentration. However, the N-SiCp prepared by high-energy ball milling is approximately spherical, which can significantly avoid the stress concentration and reduce the generation or propagation of cracks, resulting in the enhancement of the properties of composites. Although N-SiCp can improve the strength of the composite without reducing its plasticity in the related research [2; 3; 4] on Al matrix composites reinforced by N-SiCp, there are still some problems in the research of composites reinforced by nanoparticles due to the large specific surface area, easy agglomeration, and the low volume fraction of N-SiCp. Besides, the reinforcement particles are inclined to agglomerate and are not easy to sinter for the composite with the large volume fraction [5; 6; 7; 8]. [PERSON] [8] pointed out that the smaller SiCp represented the larger interface area between SiC and matrix in composite materials is, indicating an increased yield strength and tensile strength. [PERSON] [9; 10] pointed out that N-SiCp could make the yield strength and tensile strength of composites reach up to 102.5 and 175.0 mPa, respectively, leading to an augment of 50% and 30% than that of matrix alloy. [PERSON] [11] pointed out that the grain size of the Al matrix composites reinforced by single N-SiCp couldbe significantly refined, showing an improved hardness and wear resistance. [PERSON] (2018) prepared Al matrix composites reinforced by micron SiCp and N-SiCp through the stirring casting method. It was discovered that the nanoparticles can significantly improve the tensile strength and yield strength of the composites. [PERSON] et al. (2018) also studied the hot-pressed sintered Al matrix composites reinforced by N-SiCp. The results showed that the hardness and compressive strength of composites reinforced by N-SiCp were higher than that reinforced by micro-SiC at the same volume fraction. Besides, the grain refinement effect of N-SiCp is significantly better than that of micron SiC. In summary, researchers have gradually studied the effect of particle size on Al matrix composites from micron to single nanometer ([PERSON] et al., 2018; [PERSON] et al., 2018; [PERSON] et al., 2018). During the stirring casting process, matrix and reinforcement were mixed through mechanical stirring or magnetic stirring, followed by casting, which makes the SiC more easily agglomerated because of the large specific surface energy of the N-SiCp and the poor wettability between the reinforcement and matrix ([PERSON] et al., 2018). Although powder metallurgy combined with ball mill dispersion can reduce the agglomeration phenomenon, the sample with a low density and poor bonding is still prone to generating cracks. There are some certain disadvantages for the preparation methods with the low pressure in previous studies about Al/micro-SiC composites--for example, low density, low strength, shrinkage, and porosity due to the insufficient pressure ([PERSON] et al., 2018; [PERSON] et al., 2018). Therefore, an effective preparation method is particularly important and in urgent need. UHPS, which is characterized by ultra-high-pressure during sintering, has received considerable attention over the past few years because it could solve the problem of the defects produced during conventional powder metallurgy techniques ([PERSON] et al., 2018).