Research On Strengthening And Toughening Design And Fracture Behaviors Of Multilayer Graphene/6063Al Composite

Simultaneously enhanced strength and ductility of metal is a long-standing research goal that has been eager to achieve in material science.However,the factors that increase the strength of metals often lead to decrease in ductility,exhibiting a trade-off relation between strength and ductility.Metal matrix composites is a widely used method to increase the strength and stiffness of material at the same time by artificially introducing rigid particles into metal matrix.However,due to the large difference in elastic modulus between particles and matrix,the inconsistent deformation results in stress concentration,inducing early nucleation of crack,and the ductility is largly lower than that of matrix.In order to solve the problem of restriction between strength and ductility,this paper takes the graphene/6063Al composite as research object.The yield strengthening,graphene orientation and distribution,and interfacial structure are considered for strengthening and toughening design.A three-step powder metallurgy process including flake ball milling,horizontal sintering and vertical extrusion are utilized successfully to prepare high strength-toughness multilayer graphene/6063Al composites.The microstructures evolution during preparation process are systematically studied,and the strengthening mechanism and enhanced toughness of graphene are clarified.Furthermore,in-situ tensile test is used to reveal the deformation process and fracture behavior of composites.The main results are as follows:High strength-toughness multilayer graphene/6063Al composits is designed and prepared.After theoretical design,the high strength-toughness graphene/aluminum composites should satisfy the following conditions:interface is clean and well-bonded,and high content,low damage graphene is uniformly dispersed and aligned on grain boundaries and inside grain.This paper proposes a new strategy to improve the distribution and interface bonding of graphene in aluminum to meet the above conditions of high strength and toughness.The preparation process includes a three-step powder metallurgy process,which consists of flake ball milling,horizontal sintering and vertical extrusion.The flake ball milling process synchronously realizes the long-term(?13h)flake deformation of aluminum powder and the uniform dispersion of graphene with low damage.Under the action of the ball milling shear force,graphene is peeled off and layer number of graphene is reduced.Then,the flake powders are stacked horizontally to form layered preform by spark plasma sintering,and the vertical hot extrusion is performed.The large plastic deformation of nearly 90°causes the graphene to be further exfoliated.The original agglomerated graphene nanosheets are transformed into uniformly dispersed multilayer graphene with a thickness of about 5?10nm,and uniformly dispersed on grain boundary and inside grain.The multilayer graphene is aligned in the extrusion direction.The angle between the intragranular graphene plane and extrusion direction is about15?34°.The orientation relationship is as follows:(111)Al//(0001)GNSs&[2-1-1]Al//[-12-10]GNSs.The crushing effect of mechanical ball milling and the adjustment of alloying elements remove the surface oxide film of aluminum powder.The interface is mainly composed of C-Al.The Mg element reacts with Al₂O₃ to form rod-shaped Mg Al₂O₄.The harmful interfacacial reaction product Al₄C₃ is avoided by low-temperature rapid sintering.The microstructure and mechanical properties of the extruded composite material are studied.As the graphene content increases from 0wt.%to 1.0wt%,the grain size decreases from 912nm to 595nm,and the intensity of<111>fiber texture increases from 5.99 to 12.59.With increasing content of graphene,the yield strength and tensile strength of composites increase monotonously with high ductility.The 0.5wt.%graphene/aluminum composite exhibits the best balance between strength and ductility,and the yield strength and tensile strength increase about 22%and 15%than that of aluminum alloy prepared by the same process,which are about 280MPa and 341MPa,respectively.At the same time,the total elongation is 16.3%,which is slightly higher than that of the aluminum alloy(14.6%),combined with obvious necking.The strengthening mechanism of the composite is calculated and analyzed in two stages of yield strength and strain hardening.The strengthening mechanism of yield strength is mainly composed of grain refinement strengthening,dislocation strengthening,load transfer strengthening and Orowan strengthening of intragranular graphene.When the content increases to 1.0wt.%,Orowan strengthening accounts for up to 52%.The results of strain hardening curve fitting show that forest dislocation hardening is the main mechanism,and the influences of intergranular graphene and intragranular graphene on dislocation accumulation are different.On one hand,intergranular graphene hinders the movement of dislocations and causes pile-up of dislocation.On the other hand,it reduces the grain boundary diffusion coefficient and reduces the absorption and annihilation of edge dislocations by grain boundary.Intragranular graphene increases the dislocation density by proliferation of Orowan dislocation rings,but it will increase the annihilation of screw dislocation.The deformation process and fracture behavior of composites are revealed.The in-situ tensile results show that graphene coordinates the deformation of matrix by rotation,straightening,and interlayer slidding,which reduces the interface stress concentration and nucleation of crack,making the plastic deformation continues to the onset of necking.During the fracture process,the densely distributed graphene disperses and blocks the distribution and coalescence of cracks in matrix,bridging cracks in matrix.and then interlayer sliding and gradually tears.The coordinated deformation behavior and step-by-step delamination fracture of graphene provide permanent deformation and bridging effects,which are the main reasons for the excellent ductility of composite.The torn few-layer graphene are arranged densely along the tear ridge of dimples and extended more than 100nm along the force direction,forming a special"cicada wing-like"fracture.The research results of this paper provide intuitive experimental evidence for understanding the strengthening and toughening of graphene in metal matrix,and provide significant guidance for the design and preparation of metal matrix composites with high strength and toughness.