Preliminary Study On Hierarchical Structure Of Graphene Reinforced AZ91 Magnesium Matrix Composites

Magnesium matrix composites have light-weight and high-strength properties,and are currently a hot spot in the research of“lightweight”materials.Graphene is considered to be one of the most ideal reinforcements for magnesium matrix compositesdue to its excellent mechanical properties.However,the graphene-enhanced magnesium matrix composites material has the bottleneck that the strong plasticity cannot be improved synergistically,and the graphene has problems such as poor dispersion and weak bonding with the magnesium matrix interface.In this paper,a ball mill powder metallurgy method was used to react graphene oxide with metal lanthanum powder to prepare an in-situ lanthanum oxide coated graphene(La₂O₃coated Graphenes,La₂O₃@GNS)reinforced AZ91 composite material,which was achieved through structural design and interface control.The comprehensive mechanical properties of composite materials are improved.The specific research work is as follows:(1)When the prefabricated powder is ball milled for 6 hours,it changes from a uniform flake shape to a ratio of thickness and fineness,which provides favorable conditions for the formation of a hierarchical structure.At this time,graphene oxide has fewer defects and a more complete structure,which provides reaction conditions for the formation of in-situ La₂O₃@GNS.La₂O₃@GNS(0?1.8 wt.%)reinforced AZ91 composites were prepared by sintering and extrusion.(2)The grain size of the sintered structure increases with the content of La₂O₃@GNS,and the grains of the magnesium matrix are refined.And the morphology of the organization has also changed from a square shape to a slender strip shape,and the hierarchical structure is becoming more and more obvious.The microhardness gradually increases with the increase of La₂O₃@GNS content.When the content reaches 1.5 wt.%,the microhardness reaches 68.2 HV.At the same temperature,La reacts with-COOH,-OH and C-O-C and other functional groups with less heat of chemical reaction than Mg.La is easier to chemically react with GO,and finally La₂O₃is formed.(3)During the extrusion process,the crystal grains are subjected to the extrusion force along the extrusion direction and the shear force perpendicular to the extrusion direction,causing the structure to gradually change from block-shaped coarse crystals to slender strip-shaped fine crystals along the growth in the extrusion direction.The finally formed extruded magnesium matrix composites material has mainly slender and elongated grains,and the grain size is smaller than that of the sintered state,the density of the structure is improved,the defects are significantly reduced,and the hierarchical structure is more obvious.With the increase of in-situ La₂O₃@GNS content,the mechanical properties of the extruded magnesium matrix compositesincrease significantly.When the content is 1.5 wt.%,the hardness reaches93.3 HV,and the elongation,yield strength and tensile strength are higher than those of the original AZ91 alloy.Increased by 149.3%,68.3%and 50.2%respectively,the toughness increased,and the fracture mode changed from brittle fracture to ductile fracture.With the increase of in-situ La₂O₃@GNS content,the friction performance of magnesium matrix compositesis improved,the friction coefficient decreases from0.50 to 0.32,and the wear characteristics change from adhesive wear to abrasive wear.A yield strength prediction model is established:the yield strength increment is the superposition of the strengthening mechanisms of the magnesium matrix composites.The hierarchical structure hinders the crack propagation during the deformation process.The strain gradient caused by the deformation of the coarse and fine grain structure forms dislocation plugging at the coarse and fine grain interface,forming back stress strengthening,and finally achieving the strong plasticity of the magnesium matrix composites material.The synergy to improve.In this paper,an in-situ La₂O₃@GNS reinforced AZ91 composite was prepared by ball milling powder metallurgy process.Through the interface control and structural design of the magnesium matrix composites material,the synergistic improvement of strong plasticity was realized,and it provided a way for the preparation of high-performance magnesium matrix composites materials.