Fabrication,Microstructure And Properties Of Magnesium Matrix Composites Reinforced By MgO Particles

Magnesium alloy has many advantages such as high specific strength and stiffness,strong electromagnetic shielding performance,strong damping ability and so on.However,low absolute strength,poor plasticity and poor high temperature properties restrict the further application of magnesium alloys in industry.In this paper,MgO/AZ31 composites with specific spatial distribution and a good combination of strength and toughness were prepared by in-situ reaction method and ex-situ method respectively by combining mechanical ball milling and hot extrusion process.The effect of MgO content on the microstructure and properties of the composites under different preparation methods was studied.The effect of hot extrusion temperature on the microstructure and properties of the composites.The formation process of MgO specific spatial distribution and the strengthening and toughening mechanism of MgO specific spatial distribution composites were analyzed and discussed.The results show that the MgO particles in the MgO/AZ31 composites prepared by ball milling and hot extrusion are honeycombed in the matrix.The MgO is distributed in banded parallel to the extrusion direction.In the vertical extrusion direction,MgO is distributed in network along the particle boundary.The grain size in MgO-rich area is much finer than the MgO-lean area,which could up to submicron,.The average grain size of in-situ MgO/AZ31 composites is refined with the increase of MgO particle content after hot extrusion at 360℃.The average grain size of matrix alloy is 3.1μm and decreased to 2μm when MgO content reaches 6 wt%.Under the different temperature of hot extrusion,the study of the microstructure of0.75MgO/AZ31 composite showed that the average grain size of the composite increases with extrusion temperature rises.However,the grain size in MgO-rich area has not changed a lot while the grain size in MgO-lean area increased fast.The average grain size of the composite is 1.6μm and 3.6μm at the extrusion temperature of 320℃and 400℃respectively.In the in-situ MgO/AZ31 composites after hot extrusion at 360℃,the yield strength and tensile strength of the composites increased with the increase of MgO content,while the elongation gradually decreased with the increase of MgO content.When MgO content is 0.75wt%,the yield strength and tensile strength of the composite are 245 MPa and 343 MPa,respectively,and the elongation reaches 18.5%,which is5.6%,3%and 5.1%higher than that of the matrix,respectively.The hardness of the composite increases linearly with the increase of MgO content.The hardness of the matrix is 78.2HV,and the hardness of the sample reaches 96.8HV when the content of MgO is 6%.The mechanical properties of the in-situ 0.75MgO/AZ31 composites under different hot extrusion temperatures show that the yield strength and tensile strength of the composites decrease with the increase of extrusion temperature,and the elongation first increases and then decreases.The yield strength and tensile strength of the specimen extruded at 360℃are 245 MPa and 343 MPa,respectively and the elongation reaches 18.5%.The grain size of ex-situ MgO/AZ31 composites is similar to that of in-situ MgO/AZ31.The grain size decreased gradually with the increase of MgO content,and the grain size decreased to about 2μm when MgO content is 4 wt%.Compared with the composites prepared by in-situ method,the spacing between the MgO-rich area in the extruded MgO/AZ31 composites parallel to the extrusion direction is more uniform.The yield strength and tensile strength of ex-situ MgO/AZ31 composites increased with the increase of MgO content.Compared with in-situ MgO/AZ31 composites with similar strength,the composites exhibited better plasticity.When MgO content is 1wt%,the yield strength and tensile strength are 246 MPa and 344 MPa,respectively,and the elongation reaches 21.5%,which is increased by 6%,3.3%and 22.2%compared with matrix alloy,respectively.The hardness of the composite increases linearly with the increase of MgO content.The hardness of the matrix is 78.2HV,and the hardness of the sample reaches 89.8HV when MgO content is 4 wt%.TEM analysis showed that in-situ MgO/AZ31 composites and ex-situ MgO/AZ31composites,the interface of MgO and Mg Matrix is clean and well bonded and there are dislocations near the MgO particles.EBSD analysis showed that the MgO-rich area showed higher residual strain,higher KAM value and lower Schmitt factor,while the MgO poor area showed lower residual strain value and KAM value and higher Schmitt factor.Besides,the basal texture of（0001）plane in MgO-rich area is weaker than that of MgO-lean area.Theoretical calculation showed that the main strengthening mechanisms of MgO/AZ31 composites areΔσ_Hall-Petch、Δσ_CTE andΔσ_Load.The effect of Orowan strengthening on the YS of the composite is almost negligible compared to other mechanisms.