Microstructure and Mechanical Properties of Graphene Nanoplatelets Reinforced Magnesium Matrix Composites

Author:Xiang Shu Lin

Supervisor:wu kun


Degree Year:2018





The chemical predispersion process of graphene nanoplatelets(GNPs)was explored first,which provided the basis for the preparation of magnesium matrix composites.Liquid metallurgy methods were used to obtain the homogeneously and heterogeneously distributed GNPs reinforced magnesium matrix composites,respectively.The influence of GNPs on the microstructure and mechanical properties of the composites were studied systematically.Accordingly,the mechanism of strengthening and toughening of GNPs reinforced magnesium matrix composites was revealed.Moreover,the effect of GNPs on the precipitate characteristics in the heterogeneous bimodal structural Mg–6Zn(wt.%)matrix composites was investigated.The raw GNPs were agglomerated in large clusters.Pre–dispersion of the GNPs and preventing their re–agglomeration are prerequisites for the subsequent preparation of composites.The chemical predispersion processes of GNPs was explored.Ultrasonic assisted aqueous dispersion of GNPs was first performed using sodium dodecyl sulfate(SDS)as the surfactant,and the high concentration of GNPs dispersion was obtained by continuous SDS addition.The SDS molecules existed as monomers in the suspension during continuous surfactant addition,which reduced and prevented the formation of local micelles of SDS.Small SDS monomers were more effective for the dispersion of GNPs in water.Subsequently,the polyvinyl alcohol(PVA)coated Mg chips(Mg@PVA)were used as the carriers for the dispersed GNPs.By utilizing the good foaming ability of SDS,the dispersed GNPs were transferred to and bonded to the surface of Mg@PVA via the liquid membrane of bubbles.In addition,the formation of bubbles also effectively prevented the reagglomeration of GNPs caused by the pile–up of Mg@PVA.The Mg@PVA–GNPs composite chips obtained by the bubbles assembly method dispersed and carried GNPs effectively when the mass ratio of Mg@PVA/GNPs suspension was 1/10.Mg@PVA–GNPs composite chips were ideal"intermediate composites"for the subsequent fabrication of magnesium matrix composites.GNPs reinforced Mg matrix composites with a homogeneous distribution were fabricated by decomposing melt deposition.With the increasing content of GNPs,the strength and elongation of composites have been improved simultaneously.The improvement of yield strength(YS),ultimate tensile strength(UTS)and fracture elongation(εf)of 26 MPa,39 MPa and 7%were realised in the 1.6 vol.%GNPs/Mg composites.According to the microstructure analysis of Mg matrix composites,the content of tensile twins in Mg matrix gradually increased with the increase of GNPs content.On the other hand,the intensity of the basal fiber texture of the composite decreased.GNPs promoted the weakening of the fiber texture of the composites mainly due to the large number of GNPs induced?101?2?tension twins.The parent matrix in the twinning system retained the parts of the grains that far from the fiber texture.According to the shear–lag model and finite element method(FEM),load transfer mechanism of GNPs is the main strengthening effect for the composites.The direct toughening mechanism of the composites is the crack bridging,tearing and pull–out of GNPs.In addition,the weakened fibre texture and the promoted initiation of non–basal slip induced by GNPs are the indirect toughening mechanisms.GNPs reinforced Mg–6Zn matrix composites with a heterogeneous distribution were fabricated by modified ultrasonic vibration.The strength of the composites has been greatly enhanced,however,the elongation decreased but it still maintained the acceptable deformation.The improvement of elastic modulus(E),YS and UTS of 9 GPa,169 MPa and 92 MPa were realised in the 1.6 vol.%GNPs/Mg–6Zn composites.The layered distribution of GNPs induced the bimodal structural in the matrix,which consisted of ultra–fine grains(UFG)region and coarse equiaxed grains(CEG)region.The strength of the composites was enhanced largely by building up local high–strength UFG in the composites.The proportion of the UFG region increased with the rising GNPs content in the composite.The load transfer effect has surpassed the grain refinement effect gradually and become the main strengthening mechanism as the content of GNPs is increasing,because of the huge specific surface area provided by the two–dimensional GNPs.Under applied flow stress,the UFG region helped maintain the high strength of the composites.Meanwhile,the CEG regions produced pronounced strain hardening to sustain the useful deformation to large strains and hindered the propagation of the initiation crack in the UFG region.Heterogeneously distributed GNPs reinforced Mg–6Zn matrix composites displayed the inhomogeneous age–hardening response.The aging process of the composites has been accelerated,especially in the GNPs–rich region.The main strengthening phase in the composites was determined as??([0001]Mg),and the orientation relationship between the??phase and the matrix is determined.According to the experimental and FEM analysis,nucleation sites and high–velocity diffusion channels were produced by dense tangles of dislocations at the GNPs/matrix during quenching.Furthermore,the Zn solute atoms tended to segregate at the interface of the GNPs/matrix due to the lattice distortion around.Both the processes accelerated the precipitation of??phase.The age–hardened composites combined the load transfer effect of GNPs and the Orowan strengthening effect of??rods when the external load was applied along the extrusion direction.As a result,the GNPs and??rods have built a hybrid strengthening network in the three–dimensional space,which further enhanced the strength of the composites.