Microstructure And Mechanical Properties Of Laminated SWCNT/6061Al Nanocomposites With Different Matrix Flake Thickness

Aluminum matrix composites reinforced by carbon nanotubes(MWCNT/SWCNT)exhibit excellent mechanical properties such as high strength,high modulus and low density,with broad application prospects in aerospace,national defense and energy industry.The "brick-masonry" laminated composite structure inspired by natural shell pearl layer improves the plasticity and toughness of aluminum matrix composites,and achieves excellent matching between strength and plasticity.However,there have been few studies related to the microstructure and mechanical properties of biomimetic laminated composites with different matrix flake thickness.In this paper,biomimetic laminated SWCNT/6061 Al composites were fabricated by flake powder metallurgy technique,and the research focused on the coupling relationship between microstructure and properties of composites with different flake thickness of 6061 Al matrix.Besides,the corresponding strengthening mechanism was explored.The main research results are listed as follows:(1)The SEM and TEM characterization results showed that the prepared SWCNT/6061 Al composites illustrated ordered "brick-masonry" laminated composite configurations at micro and nano scales.As SWCNT content increased,the elastic modulus,tensile strength and yield strength of the composites got improved as well,while the elongation at break decreases gradually.Under the content of 1.5 vol.%,SWCNTs were mostly uniformly dispersed on the surface of 6061 Al flakes.The elastic modulus was 79.5 GPa.Tensile strength of 414 MPa was obtained,which was 28.6% higher than that of pure 6061 Al matrix after 6 hours of ball milling,and the plasticity was also well maintained.(2)The characterization on the flaking behavior of 6061 Al matrix showed that with the increase of milling time,the lamellar diameter of the matrix increased first and then decreased,and the lamellar structural integrity would be damaged if the milling time was extended too long.The thickness of 6061 Al lamellae was observed 1 ?m,650 nm,350 nm and 325 nm at 2 h,4 h,6 h and 8 h of ball milling,respectively.The diameter-thickness ratio reached the maximum at 6 h.The preferred orientation of grains in 6061 Al matrix also changed correspondingly,and the average grain size decreased.With the decrease of flake thickness,the strength and hardness of pure 6061 Al matrix material increased,while the elongation at break and density decreased gradually.(3)With the decrease of matrix thickness,the specific surface area and absorption space increased.The uniformity of SWCNT dispersion on 6061 Al surface increased continuously and achieved highest level under 6 h ball milling.Further prolongation of ball milling time led to the re-entanglement of SWCNT across the microporous defects of broken surface.The decrease of matrix thickness accelerated the interfacial reaction and increased the lattice defects of SWCNT.The average grain size of the composites diminished with the decrease of the matrix flake thickness,which were measured around 870 nm,755 nm,667 nm and 642 nm at 2 h,4 h,6 h and 8 h of ball milling,respectively.Uniaxial tensile tests showed that with the decrease of matrix thickness,the yield strength of the composites increased first and then decreased,the tensile strength increased continuously,the elongation decreased monotonously and the elastic modulus increased slightly as a whole.The tensile strength of composites reached 414 MPa after milling for 6 h,22.8% and 13.7% higher than that after milling for 2 h and 4 h,respectively,while maintaining a high elongation of 9%.The excellent matching of strength and plasticity was then achieved.(4)The strengthening effect of the composites mainly came from the stress loading of SWCNT and the grain refinement strengthening of 6061 Al matrix.Through shear lag model,the theoretical contributions of stress loading of SWCNT to strengthening effect of composites under ball milling for 2 h,4 h,6 h and 8 h were calculated 45.8 MPa,49 MPa,54.4 MPa and 54.8 MPa,respectively.Based on the Hall-Petch relationship,the contributions of grain refinement strengthening at milling time of 4 h,6 h and 8 h were calculated 8.1 MPa,17 MPa and 19.9 MPa,respectively,with the sample under 2 h milling as reference basis.ABAQUS was used to build microstructural finite element models of the composites with different flake thickness.The numerical simulation results of the tensile stress-strain relationship agreed well with the experimental values,which strongly verified the key role of the matrix flake thickness effect.The results of flake thickness effect in this study can be extended to other composite systems with laminated configurations,providing guidance and basis for the microstructural design of biomimetic laminated composites.