| As a lightweight representative of metal matrix composites,aluminium matrix composites have become one of the most common and important metal matrix composites by virtue of their high specific strength and stiffness,good high temperature performance,more fatigue and wear resistance,good damping properties and low coefficient of thermal expansion.Carbon nanotubes offer new ideas for achieving high performance aluminium matrix composites due to their excellent comprehensive properties.However,dispersing carbon nanotubes uniformly in the aluminium matrix and forming a good interfacial bond with the aluminium matrix is the key to obtain high performance aluminium matrix composites,which is also the problem that limits their application.To solve the above problems,this paper proposes to modify the surface of carbon nanotubes with nickel nanoparticles(CNTs@Ni)from the surface structure design in order to give full play to the strengthening effect of CNTs in aluminium matrix composites.In view of the dispersion of CNTs,high-energy ultrasound-assisted casting was used to prepare CNTs@Ni-reinforced 2024(2024-CNTs@Ni)aluminium matrix composites.The influence of the microstructure of the composites on their mechanical and electrochemical properties was investigated by changing the process parameters,and the corresponding strengthening mechanisms were discussed.The effects of high-energy ultrasound and carbon nanotubes on the nucleation of the aluminium melt are summarised by numerical simulation techniques and combined with solidification theory.To further improve the properties of 2024-CNTs@Ni aluminium matrix composites,the prepared composites were deformed by equal channel angular pressing(ECAP),and their microstructure,mechanical properties and corrosion performance were discussed and analysed by numerical simulations and experimental validation.The main elements are as follows.(1)CNTs@Ni powders were prepared by the"palladium"reduction method and the oil bath dynamic reduction method,and the distribution of the prepared Ni nanoparticles was compared between the two methods.The wetting behaviour of the CNTs/Al system and the CNTs@Ni/Al system were compared at high temperatures using the seat drop method.The contact angles at 1133 K for CNTs/Al system and CNTs@Ni/Al system were 139.6°and 81.9°,respectively,which indicating that the introduction of Ni nanoparticles on the CNTs surface was sufficient to significantly improve the wettability with the Al substrate,generate a more spontaneous chemical reaction of Al Ni,release more heat,reduce the surface tension of molten 2024Al surface tension and surface free energy at the solid-liquid interface.At the same time,the generated Al Ni will break the boundary structure of molten 2024Al and reduce its surface tension.(2)The 2024-CNTs@Ni aluminium matrix composites were prepared with the assistance of high-energy ultrasound,and the influence of the microstructure on the mechanical and electrochemical properties of the composites was investigated by changing the process parameters.The results showed that the high-energy ultrasonic-assisted casting with the process parameters of ultrasonic frequency of 20 k Hz,ultrasonic power of 1000 W and ultrasonic time of 15 min could effectively improve the dispersion effect of CNTs@Ni.When the content of CNTs@Ni is 1.5 wt.%,the refinement of grains and second phase was most obvious and the addition of excessive CNTs@Ni would produce agglomeration and damage to the composite the organization of the composites.(3)The 2024-CNTs@Ni aluminium matrix composites exhibited excellent mechanical and electrochemical properties.The mechanical properties of the 2024-CNTs@Ni aluminium matrix composites were the best,with Young’s modulus,yield strength,ultimate tensile strength,elongation and hardness 9.63%,36.81%,28.99%,16.37%and 10.08%higher than those of 2024Al,respectively.The enhancement of the mechanical properties of 2024-CNTs@Ni aluminium matrix composites was a synergistic coupling of grain refinement strengthening,thermal mismatch strengthening,Orowan strengthening and load-bearing strengthening.Among them,the load transfer strengthening mechanism played a dominant role in the strengthening of the mechanical properties of 2024-1.5 wt.%CNTs@Ni composites.Meanwhile,the2024-1.5 wt.%CNTs@Ni composite had the best electrochemical properties with the highest corrosion potential of-603.792 mv and the lowest corrosion current density of102.332μA/cm~2.because the formation of thicker protective films was accelerated by the large number of dislocations,the introduction of Ni made fewer oxygen vacancies available for erosion by Cl~-ions,and in addition,the microstructure refinement allowed the generation of a"large anode-small cathode"phenomenon,inhibiting the continued corrosion of the matrix Al as the anodic phase.(4)Numerical simulations based on the Population Balance Model were carried out to discuss the influence law of different ultrasonic process parameters and carbon nanotubes on the nucleation of aluminium melt.The numerical simulation results showed that the number of nuclei in the Al melt increased with increasing ultrasonic power,and the number of nuclei in the Al melt also increased with increasing the content of carbon nanotubes.The application of ultrasound and the addition of carbon nanotubes affected the subcooling degree of the melt,thus reducing the nucleation work of the Al melt and increasing the number of nuclei,which was beneficial to the refinement of the composite grains.However,in combination with the experiments,the thermal effect of too high power can lead to an increase in the number of large-sized nuclei,as well as poor dispersion of high content of carbon nanotubes,which in turn affected the microstructure refinement of the composites.(5)During the ECAP,the stresses are mainly concentrated in the area close to the inner corner.The microstructure of the 2024-1.5 wt.%CNTs@Ni aluminium matrix composite was observed and it was found that the closer the inner corner was to the inner corner the finer the grain size was and the more dispersed the second phase and CNTs@Ni was after ECAP.The hardness of the as-cast 2024-1.5 wt.%CNTs@Ni aluminium matrix composite was not uniformly distributed,with an average hardness of approximately 94.55 HV.After ECAP,the average hardness was approximately136.54 HV,an increase of 44.41%compared to the as-cast state.The yield strength,tensile strength and elongation were increased to 283.49 MPa,399.18 MPa and 11.69%respectively,which were approximately 44.99%,45.97%and 344.49%higher than those of the as-cast state.In addition,the corrosion performance and electrochemical properties of the 2024-1.5 wt.%CNTs@Ni aluminium matrix composite were better than those of the as-cast state after ECAP.The corrosion potential was-563.643 mv,an increase of 6.65%compared to the as-cast state,the corrosion current density was88.319μA/cm~2,a decrease of 12.33%compared to the as-cast state,and the charge transfer resistance increased to 145.26Ω-cm~2.Simulation of the corrosion behaviour of the composites in atmospheric marine and other environments using COMSOL software showed that the composites after ECAP taked longer to immerse in the electrolyte to the same depth and the average anode current density was lower than that of the cast composites,and that ECAP process improved the applicability of the composites in atmospheric marine and other environments.In summary,this paper adopts an innovative idea of introducing Ni nanoparticles at the CNTs/Al interface to achieve a controlled preparation of 2024-CNTs@Ni aluminium matrix composites,effectively improving the dispersion and interfacial bonding strength of CNTs in the aluminium matrix and elucidating the strengthening mechanism of the CNTs@Ni reinforced phase.At the same time,a combination of high-energy ultrasonic-assisted casting and ECAP processes was used to achieve uniform dispersion of the CNTs@Ni reinforcing phase in the aluminium matrix and fully realise its strengthening effect,which provides an experimental basis and theoretical reference for the preparation and application of lightweight and high-strength structural and functional integrated aluminium matrix composites. |
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