Study On The Preparation And Mechanical Properties Of Multi-scale In-situ Mg₂Si And CNTs Hybrid Reinforced AZ91D Composite

Magnesium alloy is the lightest metal structural material currently,but its low strength,poor toughness,and poor creep resistance still have many problems in practical applications.The use of ceramic particles,fibers,carbon nanotubes(CNTs)and other reinforcing phases is one of the effective ways to overcome the defects of mechanical properties of magnesium and magnesium alloys.CNTs have ultra-high strength and elastic modulus,and can be used as an ideal reinforcement material.But its high surface energy and large aspect ratio make it easy to agglomerate.Magnesium silicide particles(Mg2Si)are an ideal reinforcing phase for magnesium-based composite materials due to their high hardness,low density and low coefficient of thermal expansion.In this paper,AZ91D magnesium alloy is used as the matrix material,and CNTs,multi-scale in-situ micron Mg2Si(Mg2Sim)and nano-Mg2Si(Mg2Sin)are used as the reinforcing phases.The spark plasma sintering technology(SPS)is used to prepare multi-scale in-situ Mg2Si including Mg2Sin and Mg2Sim and CNTs hybrid reinforced CNTs-Mg2Si/AZ91D composites.The influence of surface modification of CNTs and micrometer Si particles(Sim)and powder mixing process on their structure and morphology,as well as the influence of dispersion degree in CNTS-Si/AZ91D composite powder were investigated.The total content(1.5 vol.%)of CNTs and in-situ Mg2Sim was unchanged,while the content ratio(0.3/1.2,0.75/0.75)was different.The microstructure of CNTs-Mg2Si/AZ91D composites,the distribution of hybrid reinforcing phase,the interfacial reaction between Mg matrix,CNTs and Mg2Si,the evolution mechanism of interfacial structure and its influence mechanism on the microhardness,compressive mechanical properties and fracture behavior of the composites were investigated.In addition,the influence of different CNTs content on the microstructure and mechanical properties of CNTs-Mg2Si/AZ91D composites was studied by keeping Mg2Sim content(1.2 vol.%),and the strengthening mechanism of the composites was explored.The main research conclusions are as follows:(1)The surface of CNTs treated by acid oxidation can carry a variety of oxygen-containing functional groups such as-COOH,and their agglomeration,impurity content and structural damage are effectively reduced.The surface modified CNTs were bonded with Sim with hydroxyl group(-OH)on the surface by hydrogen bond,and the uniform pre-dispersed CNTs/SIM composite powder on the surface of Sim micron powder was obtained.The CNTs/Sim composite powder was added into the pre-milling mixed silicon nano-particles(Sin)and AZ91D powder,and the CNTS-Si/AZ91D powder evenly distributed on the AZ91D powder and Sim surface was obtained by short-time ball milling.It is proved that the surface modification,short-time ball milling process and the Sim carrier can effectively realize the uniform dispersion of nano-powders.(2)CNTS-Mg2Si/AZ91D composites were prepared by SPS with the total content of CNTs and in-situ Mg2Sim unchanged(1.5 vol.%),but the content ratio of CNTs-Mg2Si/AZ91D was different.The microstructure is composed of spherical ?-Mg particles,Mg17Al12 eutectic phase and multi-scale in-situ Mg2Si and CNTs hybrid reinforcers distributed between a-Mg particles.The CNTs/Mg interface is a hybrid interface structure composed of MgO and Mg-O-C atomic diffusion layers.When the ratio of CNTs/Mg2Sim is 0.3/1.2,The microhardness,compressive ultimate strength(UCS),compressive yield strength(YCS)and compressive elongation of 0.3vol.%CNTs-2.7vol.%Mg2Si/AZ91D composites are 92.7 HV,408.2 MPa,238.3 MPa and 24.5%,respectively.Compared with the Mg matrix alloy,they are increased by 15.6%,53.2%,50.4%,2.9%,respectively,and compared with the 0.75vol.%CNTs-2.25vol.%Mg2Si/AZ91D composite are increased by 5.0%,20.2%,29.1%,8.4%,respectively.This is mainly attributed to the uniform dispersion of the multi-scale hybrid reinforcing material in the matrix and its good interface bonding with the matrix material.Based on the analysis of the strengthening mechanism of the composites,it is found that heat mismatch strengthening contributes the most to the improvement of the properties of the composites,followed by Orowan strengthening and load transfer strengthening.(3)The effects of CNTs content(0.1 vol.%,0.3 vol.%,0.5 vol.%)on the microstructure and properties of CNTs-Mg2Si/AZ91D composites were studied when Mg2Sim content(1.2 vol.%)remained unchanged.With the increase of CNTs content,the mechanical properties of CNTs-Mg2Si/AZ91D composites were firstly improved and then decreased.The mechanical properties of 0.3vol.%CNTs-2.7vol.%Mg2Si/AZ91D composites are the best,and their UCS and YCS are 408.2 MPa and 238.3 MPa,respectively,and their elongation is 24.5%.Compared with 0.1vol.%CNTS-2.7vol.%Mg2Si/AZ91D composites,the UCS and YS are increased by 10.5%and 11.4%,respectively,and the elongation decreased by 1.2%.However,when the content of CNTs reaches 0.5 vol.%,the properties of the composites are reduced.The reason is that the agglomeration of CNTs counteracts the strengthening effect of hybrid reinforcers on the composites,leading to the rapid fracture failure of the composites.Therefore,by introducing nano Mg2Sin,CNTs,micron Mg2Sim and hybrid reinforcement surface functional group modification and mixing ratio change,the mechanical properties of composites can be regulated and optimized.