In-Situ Synthesis,Deformation Behavior,Strengthening And Toughening Mechanisms Of Graphene/Aluminum-matrix Composites

To meet the increasingly urgent demands of frontier applications such as high-end equipment manufacturing and automotive light-weighting,the development of aluminum matrix composites（AMCs）with light-weight and high-strength has become a focus in the research of metallic materials.Due to its excellent intrinsic properties such as low density,large specific surface area,and high strength/modulus to density ratio,graphene is considered as an ideal reinforcement for AMCs.However,achieving its uniform dispersion and strong interfacial bonding remains challenging to synergistically improve the strength and toughness of composites.Therefore,the development of innovative preparation techniques and new strengthening/toughening mechanisms is of great importance for obtaining graphene reinforced AMCs with high performances.In this thesis,graphene nanosheets（GNS）were synthesized on the surface of aluminum（Al）matrix powders by thermal decomposing the solid carbon source of glucose(C₆H₁₂O₆)with copper（Cu）catalyst,then the GNS reinforced AMCs were fabricated by powder metallurgy method.The issues including the in-situ synthesis methods of graphene,the mechanism of graphene inducing grain structure evolution and its effect on the mechanical properties,and the influence of plastic deformation on the graphene morphology and strengthening efficiency were investigated.The main research contents and results are as follows:（1）A novel method was invented for the in-situ preparation of GNS/Al-Cu composites.Using Al powders,Cu powders and C₆H₁₂O₆ as raw materials,the GNS/Al-Cu composite powders were synthesized by stirring hot-drying and in-situ calcination,then the GNS/Al-Cu bulk composites with layered microstructure were fabricated via hot pressing and multi-pass hot rolling.The in-situ synthesis mechanism of GNS,as well as the microstructure and mechanical properties of the composites,were investigated.The results show that the in-situ synthesized GNS is achieved by a dehydration-melting-decomposition-graphitization process of C₆H₁₂O₆from room temperature to 600℃with Cu catalyst.The multi-pass hot rolling promotes the formation of layered microstructure and drives the dissolution of Cu element and the precipitation of the Al₂Cu phase,which significantly improves the dislocation density and mechanical properties of composites.（2）A new strategy for the preparation of GNS wrapping/loading Cu nanoparticles reinforced Al matrix（GNS@Cu/Al）composites was proposed.A mixed aqueous solution composed of Al powders,Cu（NO₃）₂·3H₂O and C₆H₁₂O₆ was freeze-dried and in-situ calcinated to synthesize the GNS@Cu/Al composite powders,then the bulk composites were fabricated by spark plasma sintering（SPS）.The effect of preparation parameters on the carbon content,microstructure,relative density,and mechanical properties of the composites was investigated.The results show that the structural integrity of GNS@Cu hybrid layers is maintained by the SPS condition of 400℃/400MPa,and the reaction of Cu nanoparticles that are loaded on the GNS surfaces with the Al grains is promoted by the SPS condition of 500℃/100 MPa,the latter of which results in the formation of a large amount of Al₂Cu particles,thus increasing the microhardness and elastic modulus of the composites.（3）The graphene induced bimodal Al grain structure was obtained and the formation and strengthening/toughening mechanisms of this structure were clarified.The GNS@Cu/Al composites were fabricated through short-time high-energy ball milling of in-situ synthesized composite powders,combining the cold compaction,sintering,hot extrusion and solid solution treatment.The microstructure,mechanical properties,and strengthening/toughening mechanisms of composites were studied.The results show that a bimodal grain structure consisting of ultrafine and coarse grains is formed in the composites,and the GNS@Cu nanoplatelets are distributed at the grain boundaries of ultrafine grains.The formation of bimodal grain structure is caused by the induction mechanisms of GNS@Cu nanoplatelets,which involves promoting dynamic recrystallization of powder-shell grains to form ultrafine grains during ball milling and inhibiting their growth during hot consolidation processes.This grain structure improves the strain hardening ability of composites and the back stress of coarse grain regions,which makes the composites achieve excellent strength and toughness.（4）The deformation behaviors and strengthening mechanisms of GNS/Al composites under thermal plastic deformation processes were revealed.The effects of hot extrusion deformation and two kinds of hybrid deformations,including hot extrusion plus few-pass（3 passes）hot rolling and hot extrusion plus multi-pass（15 passes）hot rolling,on the microstructure and mechanical properties of composites were systematically investigated.The results show that the GNS-rich zones exist in the extruded composites,and these zones are eliminated to finely disperse GNS by the subsequent hot rolling,where the effect of few-pass hot rolling is much more significant than that of multi-pass one.The fine distribution of GNS promotes the homogenization of the recrystallized Al grains around it,hence enabling an extent of strain hardening ability of composites.The high shear stress of rolling deformation causes the GNS layers to exfoliate and form boundary-type defects,which promotes the interdiffusion of Al and C atoms at the interface to generate the Al-GNS interfacial transition zone.This zone raises the load transfer capability and the strengthening efficiency of GNS by enhancing the interfacial shear strength.The results of this thesis provide theoretical and experimental guidance for the preparation of high-performance graphene reinforced AMCs by in-situ synthesis and for understanding the relationship between the microstructure and macroscopic properties of the composites.