Microstructure And Properties Of Magnesium-coated Graphene/AZ91 Magnesium Matrix Composites Under Hot-pressing

AZ91 magnesium alloy has the advantages of high specific strength,high specific rigidity,good shock absorption performance,and low density.With the rapid development of my country's aerospace and weaponry fields,a single magnesium alloy has limited mechanical strength,poor friction properties,and can not meet the requirements.Therefore,it is imminent to prepare high-performance reinforced magnesium matrix composites.Carbon atom sp~2-hybridized two-dimensional monolayer graphene(GNP)has excellent Young's modulus,yield strength,and tensile strength.Adding it to magnesium alloys can effectively improve the comprehensive properties of composites.In this study,a novel organic chemical reduction method was used to prepare Mg nanoparticles in situ coated with graphene,which improved the wettability between GNPs and Mg substrates and promoted the tight interfacial bonding with the substrates.Then,the uniform dispersion of magnesium-coated graphene(GNPs)in AZ91 alloy powder was achieved by ultrasonic stirring and dispersion method,and the x GNPs/AZ91 magnesium matrix composite was successfully prepared by vacuum hot-pressing sintering technology.The effects of hot-pressing process and different addition amounts of graphene on the microstructure evolution,grain size,precipitation phase distribution and phase structure of the composite were systematically studied.In addition,the effects of different heat treatment mechanisms on the microstructure,phase composition,chemical composition,phase orientation relationship and interfacial bonding of the composites were studied.By analyzing the microhardness,friction and wear,and compressive properties of GNPs/AZ91 composites,the reinforcement mechanism and friction and wear mechanism of the composites were explored.The main contents and conclusions are as follows:(1)The magnesium-coated graphene composite structure was successfully prepared by the organic chemical reduction method.Magnesium nanoparticles with a size of 30-50 nm were heterogeneously nucleated on the graphene surface,and were uniformly and densely coated on the graphene surface.And the close interfacial bonding between Nano-Mg and graphene is maintained,thereby improving the interfacial wettability with the alloy matrix and ensuring the uniform mixing of GNPs in the magnesium alloy matrix.The magnesium-coated graphene reaction system is a spontaneous process with increased disorder,and increasing the temperature is beneficial to the reaction.(2)With the increase of GNPs content,the average grain size gradually decreased.The?-Mg₁₇Al₁₂ precipitates with different morphologies(long rod,fusiform,and granular)in the composite organization can heterogeneously nucleate on GNPs,resulting in significant grain refinement.At the same time,a finer second precipitate phase is precipitated in the matrix.The GNPs/AZ91 composites formed at a pressure of 60 MPa and sintered at 450°C for 2 h under vacuum and with a graphene content of 2.5 wt%had the densest structure,with a relative density of 99.50%.Compared with the AZ91 alloy,the average grain size of the 2.5wt%-GNPs/AZ91 composite decreased from 40.78?m to 25.39?m,a decrease of 37.7%.Nanoscale contact interfaces and diffusion bonding interfaces were formed between GNPs,the precipitation phase and the magnesium matrix,which significantly improved the interface bonding strength between GNPs and the metal matrix,and played a role in effective load transfer.(3)The microstructure of 2.5wt%-GNPs/AZ91 composites changed significantly after different heat treatments.With the prolongation of the solution treatment time of T4 heat treatment,the?phase in the AZ91 magnesium alloy gradually decomposes and dissolves into the?phase,forming a single-phase supersaturated solid solution,and the hardness decreases.After T5 aging treatment,the GNPs in the composite were mainly distributed near the grain boundaries,and a large amount of discontinuous?second phase precipitated in the grain boundaries and grains.With the prolongation of aging time,the hardness increases obviously,while the compressive strength decreases.The T6 state solution aging heat treatment promotes the diffusion of GNPs into the crystal and the matrix,which was conducive to the continuous precipitation of a large number of fine?second phases in the crystal.The interfacial bonding strength was significantly enhanced,showing a good strengthening effect and high mechanical stability.(4)After the optimal heat treatment system(T6 state solution aging:415°C×8 h+200°C×12 h),the largest and smallest?phase precipitates in the composite structure,showing good strengthening effect and high mechanical stability.The texture strength of the composite decreases significantly,the proportion of high-angle grain boundaries increases significantly,and recrystallization occurs.The area fraction of tissue increased from 24.49%to 75.72%.The average microhardness of the composite reaches 137.1 HV_0.1,and the compressive strength reaches 313 MPa,which is 40.3%and 61.3%higher than that of unheated GNPs/AZ91,and the fracture shows cleavage damage caused by shearing.(5)In the microstructure of the GNPs/AZ91 composites after heat treatment at T6 state,the second phase?-Mg₁₇Al₁₂ precipitated finely dispersed around the GNPs.At the same time,the GNP surface coating reacted in situ on the molten Mg matrix to form Mg O nanoparticles,which promoted interfacial wetting and played a good bridging role between the GNP and the matrix.The nanoscale contact and tightly bonded interface greatly enhance the interfacial bonding strength with the matrix and significantly improve the wear resistance of the composite.Compared to AZ91 alloy,the wear rate and wear depth were reduced by 57.13%and 41.78%,and the wear mechanism changes from severe delamination to a mixture of adhesive,oxidative,and abrasive wear.The remarkable increase in wear resistance of the composites is attributed to the synergistic resistance of GNPs with high strength and high heat capacity mainly at grain boundaries through the formation of a lubricating layer between the sliding surfaces,and the in situ generated hard Mg O nanoparticles to resist deformation.(6)The strengthening mechanisms of GNPs/AZ91 composites include grain refinement,load transfer,thermal mismatch strengthening,second phase strengthening,and Orowan strengthening.