| Al matrix composites（AMCs）,are widely used in aerospace,electronics and automotive industries owing to their low density,excellent ductility,strong corrosion resistance and outstanding thermal conductivity.However,the traditional AMCs are difficult to meet the mechanical property requirements of high strength and high toughness in structural materials due to the limitations of the size and performance of the reinforcement.Compared with traditional reinforcements,graphene is considered to be an ideal reinforcement for Metal Matrix Composites（MMCs）on account of its unique two-dimensional structure and ultra-high mechanical properties.However,the weak interfacial bonding between Al matrix and graphene made it difficult for graphene to fulfill its optimal effect in AMC.Herein,the interface structure between Al and graphene was designed and manipulated by preparing novel graphene hybrid reinforcements to reinforce AMCs thus achieving excellent comprehensive mechanical properties.In this thesis,focusing on the design of new interface structures,graphene hybrid reinforcements decorated with metal nanoparticles were prepared by freeze-drying technology combined with in-situ synthesis method.Thus,different interface structures were constructed in AMCs by powder metallurgy process.The effects of interfacial structure on the strength,ductility,strain hardening ability and fracture behaviors of composites were systematically studied,which could provide theoretical basis and realization approach for the preparation of AMCs with comprehensive mechanical properties.The main research contents of this thesis are as follows:1)First,using self-assembled Na Cl particles as template,glucose as carbon source and Ni（NO3）2·6H2O as catalyst precursor,the reinforcement of graphene nanosheets（GNS）decorated with Ni nanoparticle（NiNPs@GNS）was prepared by the method of high-temperature calcination.Based on the principle of Cu-catalyzed graphene growth and the strengthening effect on the Al matrix,Cu（NO3）2·3H2O was selected as the catalyst precursor to further synthesize the reinforcement of GNS decorated with Cu nanoparticles（CuNPs@GNS）.To take this one step further,carbon nanotubes（CNTs）were added to the precursor to form a novel hybrid reinforcement,in which GNS decorated with CuNPs and CNTs（Cu@GNS-CNTs）simultaneously.The results indicate that the NiNPs and CuNPs can catalyze the decomposition of carbon source effectively and promote its final transformation into high-quality multilayer graphene due to the combined space-confined effect of the NaCl template and the catalytic effect of metal salts.2)The NiNPs@GNS,as reinforcement for AMCs,was uniformly mixed with Al powders by intermittent ball milling and further consolidated by SPS sintering.The results revealed that the hybrid reinforcement facilitates the construction of an interfacial interlocking structure and accounted for the distinctive strengthening and toughening mechanisms of the composites.It demonstrated that the covalent bonding interface of Ni-GNS with a specific orientation relationship was constructed caused by the template effect of Na Cl and the catalytic effect of NiNPs on the growth of GNS during the in-situ synthesis,thereby largely improving the stability of the reinforcement structure.Hence,the hybrid particles of Ni-Al3Ni were in-situ generated at the Al-GNS interface through diffusion reaction,which appeared to form a coherent interface with the Al matrix and maintained a covalent bond with the graphene.In this way,the load transfer efficiency of the GNS were significantly enhanced thanks to the interfacial interlocking structure of Al-Al3Ni-Ni-GNS.Meanwhile,this interface structure was verified to suppress the nucleation and propagation of cracks,endowing the composites with an excellent strengthening efficiency and elongation.3)For CuNPs@GNS reinforced AMCs,the outstanding strain hardening ability of composites due to the interface regulation and the corresponding strengthening mechanism was revealed.Al-CuNPs@GNS composites were prepared by combining the technologies of segmented ball milling,cold pressing,sintering and hot extrusion.The results revealed that CuNPs@GNS were uniformly dispersed at the grain boundary of the composites,in which CuNPs were still uniformly decorated on the surface of GNS so that the Al2CuNPs were generated at the Al-GNS interface during the molding process.As a result,the interface bonding strength of Al-GNS was improved and the tensile strength of the composites was increased about twice as that of pure Al.The special orientation relationship and semicoherent interface between Al2CuNPs and Al matrix could promote the release and storage of dislocations from the primary slip system to the secondary slip system during the deformation process,resulting in the plastic relaxation dislocations near the interface.Thus,the dislocation storage capacity near the interface and thus the deformability in the local areas were enhanced,benefiting the elevated uniform elongation of composite.4)Aiming for the configuration design of graphene,Cu@GNS-CNTs was selected as the reinforcement and gained significant improvement of mechanical properties.The underlying strengthening and toughening mechanisms were thoroughly revealed.Cu@GNS-CNTs was added into the matrix by shift speed ball milling and then the composites were prepared by cold pressing,sintering and hot extrusion.The results demonstrated that CuNPs on GNS surface could not only protect the structure of GNS from damage,but also strengthen the interfacial bonding between base plane of GNS and Al matrix.The covalent bonding between CNTs and GNS on the surface of GNS could improve the load-carrying ability and also enhance out-of-plane strength of GNS.It also indicated that CNTs at the edge of GNS could react with Al matrix to form Al4C3nanorods,which further strengthened the interfacial bonding between the edge of GNS and Al matrix.Based on the unique configuration design of graphene and stable interface structure,the strengthening efficiency of Cu@GNS-CNTs was superior to the single components.From another aspect,the composites reinforced by Cu@GNS-CNTs has better dislocation storage capacity so that a good combination of strength and ductility of the composite was achieved.To sum up,the regulation and optimization for the strength and ductility of the composites were realized by the comprehensive effect of the well-designed interface structure in graphene/Al composites.The research results can provide theoretical and experimental basis for the design of interface structure of graphene reinforced MMCs,which can supply a realization approach for the preparation of high-performance MMCs. |
