Study On The In-Situ Synthesis Mechanism And Strengthening Behavior Of Multi-Scale SIC Particles In Al-Si-C Alloys

In this work, based on the liquid-solid heterogeneous reaction principle, multi-scale β-SiCp were in-situ synthesized in Al-Si melts using the carbon plasmids method, and then the SiCp reinforced Al matrix composites were prepared. Firstly, the reaction conditions of in-situ synthesis SiCp were studied, and the gradual reaction mechanism of in-situ synthesis multi-scale SiCp was put forward though the thermodynamics and kinetics analysis. Secondly, the hereditary effect of carbon plasmids on the morphological evolution of SiCp in Al melts was studied, and the parallel layered growth and oriented absorption stacking mechanism for in-situ synthesized SiCp were put forward. The effect of the Al doping on the crystal structure and electron density distribution by first-principles method based on density functional theory was studied, and the correlation between chemical bonding and hardness of doping SiCp was established. Finally, the synergy strengthening mechanism and elevated skeleton strengthening mechanism of the in-situ synthesis multi-scale SiCp reinforced Al matrix composites were put forward based on the analysis of the mechanical performance. In this paper, the main research results can be describes as follows:(1) The in-situ synthesized mechanism of multi-scale in Al meltsThe effect of the Si content, carbon content, reaction temperature and holding time on the synthesis of SiCp in the Al melts were systematically studied, and the critical reaction conditions of in-situ synthesis SiCp were studied. The synthesis mechanism of in-situ synthesis multi-scale SiCp was put forward:the gradual reaction mechanism based on the Al4C3 as the transitional phase, the micro or submicro scale SiCp were obtained due to the hereditary influence of the Al4C3; and the direct reaction mechanism based on the reaction between dissolved Si and C in the Al melts, the nano scale SiCp was formed.The effect of Cu addition on the in-situ synthesis SiCp were studied, the results show the solidus and liquidus temperature of the alloys decrease with Cu addition, which leads to the synthesis reaction temperature reduced from 750℃ to 700℃, and the size of SiCp decrease. It can improve the mechanical properties of the composites.In addition, the reaction mechanism of Al-Si-SiO2-C system was also studied, then and the in-situ synthesis of SiCp and Al2O3p hybrid reinforced Al matrix composites.(2) Growth mechanism and chemical bonding of doping SiCp in Al meltsMorphological evolution of SiCp in Al-Si-C system were studied, the morphology of SiCp transforms from the hexagonal flake to the truncated pyramid and finally change to the irregular polyhedral with the increase of holding time. Crystal structure analysis indicated the growth rate of three crystal faces is as follows:V(110)>V(100)> V(111), and the equilibrium crystal morphology is the hexagonal flakes with lowest surface energy. Si and C atoms preferentially stacking along <110> and<100> direction and the growth rate of them were faster, the growth rate of<111> direction was slowest. Thus, the growth mechanism of in-situ synthesized SiCp in the Al melts was the parallel layered growth and oriented absorption stacking mechanism.Brinell hardness of the SiCp reinforced Al matrix composites increase from 88.8HBW to 109.8HBW and the wear weight loss of them reduced from 0.371 mg/min to 0.272 mg/min with the decrease of the Al content. The chemacial structure of doped SiCp with different Al doping and was calculated by first-principles theory, it is found that the atomic arrangement of (011) change as the increase of the Al doping content, it lead to the decrease of the covalent bond proportion in SiCp, electron density around carbon atoms and the hardness of doped SiCp. Thus, the correlation between chemical bonding and hardness of doped SiCp was established.The hereditary effect of the size and variety of carbon plasmids on the morphology of SiCp, the results show that the decrease in the size of carbon plasmids can accelerate the reaction rate and the morphology of the SiCp is closer to polyhedron shape.(3) The strengthening mechanism of in-situ synthesis of SiCp reinforced Al-Si alloyThe multi-scale SiCp reinforced Al matrix composites were fabricated using the carbon plasmids method, it’s Rockwell hardness, wear resistance and thermal expansion coefficient were studied, and then the synergy strengthening mechanism were put forward. The Brinell hardness and elevated temperature tensile strength of the SiCp/ZL111 composite were improved by using the hot isostatic pressing technology.The new preparation technology for SiCp reinforced Al-Si composites by using the alloyed carbon plasmid was put porward. The elevated temperature tensile strength of SiCp reinforced Al-Si alloys with the heat treatment and the hot extrusion conditions were studied. It is found that the elevated temperature tensile strength of the composites with the heat treatment condition was about 130MPa at 350℃ and the elevated skeleton strengthening mechanism were studied. Since the network skeleton structure forming between SiCp and the intermetallic compounds in the composites were destroyed, the elevated tensile strength of SiCp reinforced Al-Si alloys decrease under the hot extrusion.