Study On The Strengthening And Toughening Mechanisms In Graphene-al Nanolaminated Composite

An effective way to improve the mechanical properties of metallic materials is to form metal matrix composites(MMCs).In traditional MMCs,a uniform distribution of the reinforcements in the metal matrix is often desired,as microstructrual homogeneity may prevent severe stress concentration and the ensuing premature failure of the composites that are frequently caused by reinforcement agglomeration.However,these MMCs characterized by a uniform spatial distribution of constituent phases and microstructures are unfavorable to fully take advantage of the synergistic and coupling effects,generally making a compromise between strength/stiffness and plasticity/toughness,which greatly hinders their broad engineering applications.Instead of fabricating composites with a homogeneous microstructure,by using a modified powder metallurgy fabrication route,our research group utilized graphene(in the form of reduced graphene oxide,RGO)with high strength and high modulus as the reinforcement and developed bulk RGO-Al composites with a bioinspired nanolaminated structure.On this basis,in this study,we extended our study by carrying out uniaxial compression tests on micro-pillars fabricated from the RGO-Al nanolaminated composites of different laminate orientations and RGO concentrations,with the aim of pinpointing their detailed strengthening,deformation and toughening mechanisms.In particular,the control of the angle that the laminates formed relative to the uniaxial loading axis enabled quantitative evaluation of the load-bearing capacity of RGO in the composites as well as the cohesive property of the RGO/Al interface.It was found that the variation in the load-bearing capacity of RGO in different laminate orientations relative to the loading direction could make a significant influence on the strengths of RGO-Al nanolaminated composites.The strengthening effect and efficiency of RGO could be enhanced by orienting the RGO layers parallel with the loading direction.In addition,raising the RGO concentration of the composite could make discontinuous RGO layers turn to be more continuous,which consequently raised the strength of the RGO-Al composite.The stress-strain response of all the micro-pillars was populated with discrete bursts,and the stress increments of the bursts scaled with the RGO concentration,regardless of the laminate orientation relative to the loading direction.This observation was interpreted by the dislocation annihilation at the RGO/Al interface,and the dislocation sink strength scaled with the total area of RGO/Al interfaces which could be increased by raising the RGO concentration of the composite.Furthermore,by orienting the RGO/Al interface parallel with the maximum resolved shear stress orientation,the shear strength of RGO/Al interface was directly measured to be higher than 133 MPa.Such a robust RGO/Al interface was proved to effectively divert the cracks towards the orientation parallel with the laminates,extending the crack propagation path,which consequently raised the toughness of the composite.By studying the detailed strengthening,deformation and toughening mechanisms in RGO-Al nanolaminated composites,this work proves that the intrinsic mechanical properties of the reinforcement can be fully exerted in the composite with a bioinspired nanolaminated microstructure and a robust interface between the reinforcement and the metallic matrix,which can efficiently make increase in both strength and toughness of the composite.This work underscores the importance of structural design and control in the strengthening and toughening of MMCs,and the methodology developed may be applied to other composites with microstructural heterogeneity to probe their specific mechanical behaviors and structure-property correlations,leading to improved design and tailoring of architectured MMCs.