Microstructure And Mechanical Properties Of CNTs@SiCp Reinforced Magnesium Composite

Carbon nanotubes（CNTs）were synthesized on silicon carbide（Si Cp）surface and urchin-like reinforcement CNTs@Si Cp was prepared via chemical vapor deposition（CVD）and was introduced into Mg-6Zn alloy by semisolid stirring assisted with ultrasonic vibration method.After extrusion,Mg composite with laminated structure was obtained.SEM,TEM,TG,Raman spectroscopy,were adopted to study the morphology,the yield and the graphitization degree of the synthesized CNTs.SEM,TEM,EBSD and neutron diffraction were applied in the analysis of microstructure and mechanical properties of Mg-6Zn and composites.The strengthening and toughening mechanism of composites were clarified.The urchin-like reinforcement CNTs@Si Cp was constructed via CVD process and the synthesis process of CNTs on Si Cp surface was systematically studied.The Ni nanoparticles were attached to Si Cp by impregnation method and proceeding calcination and reduction process and the precursor Ni@Si Cp was obtained.The morphology,particle size and distribution of Ni nanoparticles on Si Cp surface were influenced by the components and contents of the catalyst.By adding La（NO₃）₃·6H₂O and Al（NO₃）₃·9H₂O into Ni（NO₃）₂·6H₂O（Mass ratio:La₂O₃:Al₂O₃:Ni=1:1:98）,the size of Ni nanoparticles were significantly refined.After calcination（450^oC for 1 h）and reduction（450 ^oC for 2 h）,the Si Cp was coated by spherical Ni nanoparticles,the nanoparticle size is between 10 to 20 nm with 0.3%mass fraction of Ni.Ni nanoparticles were formed on the Si Cp surface,acting as the catalyst in the synthesis process of CNTs.CNTs@Si Cp were prepared by CVD method and acetylene（C₂H₂）was used as the carbon source.Four synthesis parameters i.e.,Ni catalyst size,synthesis temperature,deposition time and C₂H₂ gas flow rate were taken into consideration.to investigate their effects on the morphology,yield and graphitization degree on the as-fabricated CNTs.By controlling these synthesis parameters,homogeneous dispersed CNTs with high graphitization degree and ideal morphology were synthesized on Si Cp surfaces.The CNTs synthesized at 700 ^oC for 60 min with 50 sccm gas flow rate has high purity without any extra amorphous carbon with diameter between 10 to 20 nm.The synthesized CNTs show typical straight-multiwalled and hollow structure with a smooth surface and maintained their layer structure with an average interlayer spacing of 0.34 nm.The CNTs grew up with the catalyst particle rooted on its base,which is known as“base-growth model.The urchin-like CNTs@Si Cp reinforcement lays a foundation for subsequent preparation of Mg-6Zn composite.The Mg-6Zn composite reinforced by urchin-like reinforcement CNTs@Si Cp was fabricated via semisolid stirring assisted with ultrasonic vibration method.In the as-cast composite,10 vol.%Si C particles were uniformly segregated in the matrix,which were like necklaces along the grain boundaries.0.95vol.%CNTs were mainly located around Si Cp in the as-cast composite.The CNT-（Mg-6Zn）interface layer was formed in the CNTs@Si Cp/Mg-6Zn composite around Si Cp.The CNT-（Mg-6Zn）interface layer could release the thermal residual stresses in the composite in two ways:firstly,the gap of the coeffcient of thermal expansion（CTE）between the Si Cp and the Mg matrix is reduced;secondly,the thermal diffusivity of the composite is declined,leading to lower cooling rate of the as-cast composite in which a portion of residual stresses are released.The Type II thermal residual stresses of composite were investigated by neutron diffraction.The residual stress of Mg in CNTs@Si Cp/Mg-6Zn（35 MPa）is less than half of that in Si Cp/Mg-6Zn composite（74 MPa）.Compared with mono-Si Cp reinforced Mg-6Zn composite,the Mg-6Zn composite with urchin-like reinforcement enhanced the elastic modulus,yield strength,ultimate tensile strength and elongation by 9.6%,14%,20%and 18.2%respectively.The presence of CNT enhances the interfacial adhesion between Si Cp and the matrix,delaying the interfacial cracking and hindering the process of crack propagation.The CNT-（Mg-6Zn）interface layer also led to the increase of the size of the dislocation hardening zone and enhanced the dislocation strengthening effect of Si Cp.Mg-6Zn composite with laminated structure was constructed by hot extrusion method.After hot extrusion,The CNT layers were peeled off from Si Cp surfaces by extrusion,the peeled CNT layers changed to CNT strips,which aligned along the extrusion direction in the Mg matrix.the layered distribution of CNTs introduced laminated structural in the matrix,which was consisted of fine-grains zone and ultra-fine grains zone.After extrusion,the typical basal plane texture was formed,and the basal planes（0002）tended to be parallel to extrusion direction.The reinforcements in the composite promoted the dynamic recrystallization of the matrix alloy and induced the formation of twins,which result in weakened basal plane texture.The reinforcements induced twinning.Promoted the weakening of the fiber texture of Mg.Compared to the as-extruded Mg-6Zn alloy,the YS and UTS in as-extruded CNTs@Si Cp/Mg-6Zn composite were improved by 41.6%and20.7%,respectively.Combining the tensile test results and the calculation results,the dominant strengthening mechanism is the load transfer effect of CNTs.In addition,the Si Cp and CNTs play a synergistic enhancement effect and stress back strengthening introduced by laminated structural play important role on the enhancement of mechanical properties.In the CNTs@Si Cp/Mg-6Zn composite with urchin-like structure,the CNT-（Mg-6Zn）interface layer was formed between the Si Cp and Mg matrix,which could relieve the interface stress concentration.The additional CNTs enhances the bonding strength between Si Cp and Mg matrix.and impede microcrack propagation in the matrix under applied load by pulled-out and bridging mechanism.In the CNTs@Si Cp/Mg-6Zn composite with laminated structure,the direct toughening mechanisms of the composite is the crack bridging and pulled-out CNTs.The weakened fiber texture,formation of tension twinning and the promoted initiation of nan-basal slip induced by reinforcements were the indirect toughening mechanisms.In addition,the toughness of the composite was enhanced by the coordination of the fine-grain zone and the ultra-fine-grain zone in the composite with laminated structure.