| Carbon nanotubes(CNTs)reinforced aluminum(Al)matrix composites(AMCs),as structural-functional integration composites with lightweight and high strength,show broad application prospects due to the combination of excellent performance of Al matrix and CNTs.However,the development of CNTs reinforced AMCs is restricted by some problems,i.e.agglomeration of CNTs,poor wettability and interface bonding between CNTs and Al matrix,and the uncontrollability of CNTs/Al interfacial reaction.In view of the above bottlenecks in CNTs-Al composites,a new idea of introducing nano SiC layer with specific structure into CNTs/Al interface was proposed to improve the interfacial wettability and interface bonding,and to regulate the interfacial reaction,so as to give full play to the strengthening effect of CNTs in Al matrix according to the design of the interface structure of AMCs.Based on the preparation of nano SiC coated CNTs reinforced AMCs by variable speed ball milling-powder metallurgy-hot extrusion process,the effects of the content,structure and preparation parameters on the microstructure,interface,mechanical performance and electrical properties of AMCs were systematically studied.On this basis,the relationship between the characteristics of reinforcements and the strengthening effect of AMCs was revealed.And then the strengthening and toughening mechanisms of nano CNTs-SiC composite phase reinforced AMCs were elucidated.It provides a new research idea and method to balance matching of strength and plasticity/toughness of CNTs reinforced AMCs.The results show as follows:Nano CNTs-SiC composite powders with different structures were prepared by adjusting the ratio of CNTs and proper amount of nano Si powder and process parameters.The uniform mixing of CNTs and nano silicon particles was realized by using synchronous ultrasonic stirring dispersion.It is determined that the initial reaction temperature of in-situ synthesis of nano SiC layer on CNTs surface is about 1030℃.The nano silicon powder can be completely reacted with the outerwalls of CNTs to form a continuous SiC layer when the heat treatment temperature reaches above 1200℃,while the inner walls of CNTs still keep its original structure and composition.In the process of in-situ reaction,Si atoms preferentially nucleate on the surface defects and attached active groups of CNTs by diffusion and evaporation condensation.This reaction process mainly depends on the interstitial diffusion of C atoms and the vacancy migration of Si atoms.The preferential SiC layer can hinder the longitudinal diffusion of the atoms,meanwhile,SiC layer tends to stack along the<111>direction of the close packed surface to maintain the minimum energy requirement.As a result,SiC tends to grow along the axis which parallel to CNTs and finally form a continuous transition layer.It is found that the introduction of SiC layer reduces the surface energy of CNTs and decreases the specific gravity difference between CNTs and matrix,which improves the dispersion uniformity of the reinforcements in matrix.It also hinders the interfacial reaction between CNTs and Al matrix and prevents the formation of Al4C3 hard brittle phase,improving the structural stability of AMCs manufactured by variable speed ball milling-powder metallurgy-hot extrusion process.The dispersed CNTs-SiC composite phase plays an effective pinning role in grain boundary migration in sintering process,which prevents the growth of Al matrix grains.The introduction of SiC transition layer transforms the interfacial structure between CNTs/Al4C3/Al into that of CNTs/SiC/Al,which enhances the interface bonding strength between Al matrix and reinforcements under the premise of ensuring the structural stability of composite interface.The results also show that the strength of CNTs/SiC-Al composites increases while the ductility remains good(>19%)with the rise of the content of CNTs-SiC reinforcements.However,the mechanical performance of CNTs-Al references decreases continuously with the increase of the content of CNTs.The coating uniformity is improved with the increase of SiC proportion in CNTs-SiC composite phase.The adhesive work of CNTs/SiC/Al interface increases from 200 mJ/m2 of CNTs/Al to 1300 mJ/m2,and its interfacial shear strength is also significantly strengthened compared with CNTs/Al.On the basis of the wettability and interfacial bonding strength of reinforcements/matrix are significantly enhanced,the load transfer efficiency through the interface can be effectively improved,so that the bearing capacity of CNTs in Al matrix can be brought into full play.Therefore,the mechanical properties of CNTs/SiC-Al composites show a better trend with the increase of SiC layer thickness.The tensile strength of 2.0(1CNTs-1SiC)-Al reaches 212 MPa,which is 66.9%,43.2%and 35.0%higher than that of pure Al,1.0CNTs-Al and 1.0SiC-Al composites,respectively.Meanwhile,its elongation and conductivity still maintain at 18.8%and 48%IACS.The enhancement of the strengthening efficiency of AMCs is mainly originated from the improved wettability and interface bonding between CNTs/Al.The information such as load and phonon can be transferred more effectively under the better bonding effect of CNTs/SiC/Al composite interface.Therefore,the contribution from CNTs bearing strengthening to the strength of AMCs is significantly increased,while maintaining the good plastic/toughness and electrical properties of Al matrix.In addition,the synergistic effect of dislocation strengthening and Orowan strengthening result from thermal mismatch between reinforcements and Al matrix,and grain refinement strengthening caused by nano CNTs-SiC particles play important roles in the improvement in mechanical performance of AMCs.In conclusion,based on the innovative idea of introducing nano SiC layer into CNTs/Al interface,the controllable preparation of CNTs-SiC composite phase reinforced AMCs is realized,which effectively improves the common bottleneck problems of CNTs strengthening effect in Al matrix.And the matching strengthening/toughening mechanisms in CNTs-SiC reinforced AMCs are also illustrated.It provides experimental basis and theoretical reference for the development and application of lightweight and high-strength AMCs. |
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