Study On The Fracture Toughening Mechanisms In Graphene-Al Nanolaminated Composites

Compounding is an effective way to improve the mechanical properties of metal materials.Adding specific or excellent second phase into the metal matrix can achieve Metal matrix composites?MMCs?which can improve overall performance by compensating for some disadvantages of the metal matrix.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.In recent years,nano-reinforced phase has attracted much attention due to its unique physical,mechanical and other performance advantages,especially represented by carbon nanotubes and graphene which have become a trend in the field of composite materials.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 order to further reveal the toughening mechanism of graphene reinforced aluminum matrix nanocomposites,we extended our study by carrying out in-situ bending test on cantilevers fabricated from the RGO-Al nanolaminated composites of different laminated orientations.By controlling the laminated orientations of RGO-Al nanolaminated composites,this paper quantitatively evaluates the toughening effect of graphene under different orientations.Meanwhile,the in-situ testing technology ensures the observation of whole fracture process of cantilever beams during bending test.The results show that the microstructure has a significant hindrance to the crack propagation when the crack propagation direction is perpendicular to the laminated structure,which causes obvious crack deflection during the crack propagation.When the direction of crack propagation is parallel to the laminated structure,the resistance of the crack is reduced during the crack propagation,and the crack tends to rapidly propagate between the layers,thereby further delaying the material.In the meantime,the macro-three-point bending experiment was conducted to investigate the toughening mechanism of RGO-Al nanolaminated composites under different graphene contents.As we know,Fracture toughness(K_IC)are commonly used to characterize the hindrance of crack expansion.It was found experimentally that increasing the content of graphene in the composite can gradually convert the graphene in the matrix from a discontinuous reinforcement to a continuous reinforcement.At the same time,as the content of graphene increases,the fracture toughness value increases continuously,which indicates that the increase of graphene content has an impediment to the crack growth.In addition,SEM observation of the crack propagation path on the surface of the specimen shows that the crack propagation path becomes tortuous when graphene content increases,further TEM test analysis shows that the fracture process of composite materials not only include translamellar fracture,also obvious interlamaller fracture.By studying the detailed 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.