Fabrication And Synergistic-strengthening Mechanism Of CNTs-SiC_p Dual-nano Reinforcements In Aluminum Matrix Composites

Carbon nanotubes (CNTs) reinforced aluminum matrix composites (AMCs) are widely concerned due to the excellent performance of integrated CNTs and Al. However, uniform dispersion of CNTs and the control of the interfacial reaction between CNTs and Al are the bottlenecks of its development. In order to solve the above problems, a new method of low energy wet ball milling and CNTs-SiCp double nano-phase reinforced AMCs were proposed in this paper. CNTs/Al and CNTs-SiCp/Al powder mixtrue were prepared by planetary ball milling(PBM), spark plasma sintering (SPS) technology was used to consolidate powder mixture, and finally AMCs were fabricated by hot extrusion (HE). The effects of sintering temperature,CNTs and SiCp content, dual-nano reinforcements on the microstructure and properties of AMCs were clarified through thermodynamic process, microstructure observation, interfacial analysis and mechanical properties test.The result shows that the relative densities of AMCs are more than 99% after sintering at 630 ℃ and hot extrusion. CNTs can be uniformly and individually dispersed in Al matrix when its content is not more than 0.5 vol.%, and the actual strengthening effect is in agreement with the theoretical value; otherwise, it will produce the phenomenon of local aggregation,which leads to defects such as holes and microcracks in AMCs, it may reduce the strengthening effect of CNTs and finally decrease the tensile strength, ductility and conductivity of AMCs.Microstructure observation shows that CNTs can effectively refine the grain size of Al matrix,and the grain refinement effect is more obvious with the increase of CNTs content; The analysis of the interface shows that the interfacial reaction between CNTs and Al starts from the outer wall of CNTs and begins to form Al4C3 when the temperature rises to 630 ℃; The result of tensile test shows that the mechanical properties of AMCs are improved with the increase of sintering temperature. The appropriate preparation mothod was determined through the research on CNTs-Al system, specifically in the current conditions. It is easy to agglomerate when the content of CNTs exceed to 0.5vol%, which leads to a decrease in mechanical properties and thermal conductivity.In order to improve the technical bottlenecks of CNTs/Al such as the decrease of toughness and electrical conductivity on the basis of the study on CNTs-Al system, nano SiCp was introduced into CNTs/Al as second reinforcement. CNTs-SiCp reinforced AMCs were prepared by "ultrasonic dispersion and PBM-SPS-HE" route. The result shows that the addition of SiCp is beneficial to promote the dispersion of CNTs and improve the wettability between CNTs and Al matrix. As one kind of reinforcement, SiCp can be used to inhibit the growth of grains and hinder the movement of dislocations, and it can play the role of grain refinement and dispersion strengthening. The SiC particles distributed around CNTs can inhibit or delay the pulling out and peeling of CNTs to further enhance the strengthening effect of CNTs by pinning effect, and assist CNTs to strengthen matrix material. So that the enhancement effect with dual-nano reinforcements is better than that of single enhancement phase and more effective to refine the grain in AMCs. The tensile strength of 0.5CNTs-0.5SiCp/Al composites reaches 247 MPa, which is increase by about 94% compare with pure Al (σ b=127 MPa), and compare to 1.0CNTs/Al ( σb=217 MPa) and 1.0SiCp/Al ( σ b=158 MPa) also has a lifting of 14% and 56%.There is a nano-scale reaction transition layer between CNTs and A1 matrix through the analysis of interface, it greatly improve the interfacial bonding strength. The load can be effectively transferred from A1 matrix to reinforcements, thereby significantly improving the strength of AMCs. At the same time, the existences of SiCp can also inhibit the excessive interfacial reaction, it makes the mechanical properties of AMCs increase while still maintaining a similar elongation (20%) and conductivity (50%IACS) of pure Al. The main strengthening mechanisms of CNTs-SiCp reinforced AMCs include load transfer, grain refining and dispersion strengthening, it is the result of synergistic effect between the two kinds of nano reinforcements.