Microstructure And Mechanical Properties Of Graphene Reinforced Magnesium Matrix Composites Via Vapor（CO₂）-Liquid（Mg） Reaction

In view of the urgent demand for ultra-light metal matrix composites in aerospace,defense and military industry,the research on high-performance magnesium（Mg）matrix composites is of great engineering significance.Graphene（Gr）is considered as an ideal reinforcement for composites because of its excellent physical and chemical properties.However,the poor dispersion and weak interface bonding of graphene are most difficulties that limit the development of magnesium matrix composites（MMCs）.In this paper,a novel in-situ liquid metallurgical process is developed for the fabrication of bulk graphene reinforced MMCs.This preparation strategy directly grows Gr in Mg melt and simultaneously forms Mg O modified structure.The effects of Mg melt temperature and CO₂ on the quality and growth rate of graphene were systematically studied,and the related growth mechanism was discussed.The microstructure of the as-fabricated composites was studied by SEM,TEM,EPMA and EBSD,and the effect of in-situ graphene on the microstructure evolution was clarified.The mechanical properties and strengthening mechanism of the composites were studied by the investigation of microstructure.The results show that the vapor-liquid reaction system between CO₂ and Mg melt can realize the green,controllable and uniform synthesis of graphene.In addition,a production process of extracting graphene powder directly from Mg melt by using the mixed molten salt of Na Cl and KCl was proposed.The increase of temperature promote graphene to have higher crystallinity,but it would also produce porous structure on the surface of graphene by the etching effect of CO₂.The bubble growth in the melt is simulated through the similarity test.The size difference of CO₂ bubbles formed under different gas flow rates is small,but the increase of gas flow rate accelerates the growth of bubbles and the time of escaping from the surface.Small flow rate make the graphene possesses higher growth speed and lower defect density.Mechanical stirring increase the contact area between CO₂ bubbles and Mg melt,which improve the conversion efficiency of CO₂ significantly.The growth speed of graphene reach 180 mg/min after mechanical stirring at a fixed flow rate of 900 m L/min and at a fixed temperature of 680℃.The uniform dispersion and distribution of graphene in Mg melt can be achieved to the following factors.On the one hand,Mg O effectively reduces the Van der Waals force between graphene sheets,so that graphene can achieve self-stabilized dispersion in Mg melt.On the other hand,a high energy barrier preventing graphene from being pushed to grain boundaries during solidification owing to a reasonable wettability between Mg O coating on the graphene and molten Mg.During the hot extrusion,the dislocations accumulated around graphene in the composite form low angle grain boundaries through rearrangement gradually.After the formation of low angle grain boundaries,the orientation difference is gradually increased by continuing to absorb the dislocations,resulting in high angle grain boundaries.Meantime,the migration of high angle grain boundaries is restrained due to the pinning of graphene to the grain boundaries.Accordingly,The recrystallization process of high angle grain boundary is completed by the continuous rotation of grain boundary.Therefore,the distribution of graphene in the composite gradually changes from the inside of the grain to the grain boundary.The composites were prepared by in-situ vapor-liquid reaction combined with hot extrusion process,which realized the synergistic improvement of strength and modulus.The elastic modulus,yield strength and tensile strength of the composites were about 21%,50%and 29%higher than those of Mg-6Zn matrix.The as-extruded composites could also maintain high plastic deformation ability,thus the composites achieve excellent comprehensive mechanical properties.On the one hand,Mg O nanoparticles are distributed and tightly bonded on the surfaces of graphene by the carboxyl bonding,facilitating the stress in the interface area more uniform and avoiding the interface cracking caused by large stress.This special interfacial structure obtained via the in-situ vapor-liquid reaction can give full play to the inherent properties of graphene and improve the load transfer efficiency.On the other hand,the composite obtained fine grain size during extrusion.Because graphene distributed at the grain boundary hinders the dislocation movement,fine grain strengthening also plays an important role in improving yield strength.