Study On Microstructure And Mechanical Properties Of Hybrid Reinforced CNTs-Mg₂Si_p/AZ91D Composites

Carbon nanotubes(CNTs)is ideal reinforcements for magnesium-based composites due to their excellent mechanical properties.However,the CNTs in the metal matrix has serious problems of agglomeration and poor CNTs/matrix interface bonding.This causes the performance improvement of CNTs/Mg-based composites to be much lower than the excellent performance of CNTs themselves.This obviously limits the popularization and application of CNTs/Mg-based composite materials.So,the key is to improve the dispersibility and interfacial bonding strength of CNTs and Mg2Si in magnesium-based composites.Mg2Si nanoparticles(Mg2Sip)have high strength,elastic modulus and good wear resistance,and are widely used as reinforcement phases in metal matrix composites.One-dimensional CNTs and zero-dimensional nanoparticles multi-scale hybrid reinforcement has the significant advantage of improving the material properties.In this paper,CNTs,nano silicon particles(Sip)and AZ91D alloy powders are mixed and pressed into a block.The bulk undergoes semi-solid partial remelting,thixoforming and other processes.Finally,AZ91D magnesium-based composites reinforced with CNTs and in-situ Mg2Sip hybrid reinforcement and magnesium-based composites reinforced with single Mg2Sip or CNTs are prepared.This paper explores the surface modification process of CNTs,the micro-morphology and micostructure of acid oxidized CNTs(o-CNTs),and its dispersion in CNTs/Mg and CNTs-Sip/Mg composite powders.The thixoformed microstructure of magnesium-based composites with different composition ratios of CNTs-Mg2Sip hybrid reinforcement and different contents of Mg2Sip reinforcement were studied.The distribution position of CNTs and in-situ Mg2Sip in Mg,the interface reaction of CNTs/Mg and Mg2Sip/Mg,the interface structure,the mechanical properties and fracture behavior of composite materials were analyzed.At the same time,the strengthening mechanism of composites were also quantitatively analyzed.The following conclusions could be drawn from the experiments:The o-CNTs obtained by oxidation treatment with mixed strong acids are rich in oxygen-containing functional groups such as carboxyl groups and hydroxyl groups,and there are a few pits and defects in the local area of the outer wall of CNTs.The acid oxidation process starts from the defect of the CNTs wall itself,so that vacancy defects are generated there.o-CNTs can be uniformly dispersed in a mixed solution of absolute ethanol-deionized water.The o-CNTs suspension and the Sip suspension were mixed by stirring,and then ball milled with magnesium powders to obtain composite powders with uniform distribution of CNTs-Sip on the surface of the magnesium powders.CNTs-Sip/AZ91D composites,CNTs/AZ91D composites and Mg2Sip/AZ91D composites were fabricated by semi-solid powder thixoforming technology.The microstructure and mechanical properties of the composites were analyzed.In-suit Mg2Sip and o-CNTs can be evenly distributed among the ?-Mg primary particles of 0.3CNTs-1.2Mg2Sip/AZ91D composites.The tensile strength(UTS),yield strength(YS),and elongation of CNTs-Mg2Sip/AZ91D hybrid reinforced composites are 271 MPa,213 MPa,and 6.3%,which are 41.1%and 54.3 higher than those of the AZ91D alloy respectively but the elongation decreased by 1.7%.Compared with 1.5Mg2Sip/AZ91D composites and 1.5CNTs/AZ91D composites,its UTS increased by 27.8%and 37.6%,respectively.This shows the advantages of Mg2Sip-CNTs hybrid reinforcement to improve the properties of composites.The improvement of the mechanical properties of CNTs-Mg2Sip/AZ91D composites is mainly related to improve the dispersity of Mg2Sip-CNTs hybrid reinforcement and high synergistic strengthening efficiency.Mg2Sip/AZ91D composites with different Mg2Sip volume fractions were fabricated by semi-solid powder thixoforming technology and the effects of Mg2Sip contents on the microstructure and mechanical properties of the composites were studied.The results indicated that the tensile strength and elongation of the composites increased gradually with the increase of Mg2Sip contents.However,when the Mg2Sip contents exceeds 3%,the increase in mechanical properties is significantly reduced.The 4.5vol%Mg2Sip/AZ91D composite has the best mechanical properties,the UTS,YS and the elongation are 288 MPa,205 MP and 6.9%,respectively.Compared with AZ91D alloy,the UTS and YS were respectively increased by 50%and 48.6%,but the elongation was decreased 9.2%.In particular,the mechanical properties of 0.3CNTs-1.2Mg2Sip/AZ91D composites were only reduced by 5.9%and 8.7%compared to the UTS and elongation of 4.5 Mg2Sip/AZ91D composites,respectively,on the contrary the YS of hybrid reinforced composites increased by 3.9%.So,there is a limit to improve the mechanical properties of composites simply by increasing the content of the reinforcement.However,by adjusting the co ntent ratio of CNTs-Mg2Sip hybrid reinforcement,the synergistic strengthening efficiency of multi-scale nano hybrid reinforcement can be more significantly enhanced.