Fabrication And Strengthening-Toughening Mechanisms Of Graphene Reinforced Aluminum Matrix Composites

Graphene,the ideal reinforcement for metal matrix composites,has been widely used in aluminum matrix composites due to its excellent mechanical,electrical and thermal properties.The main factors that affect the performance improvement and application of graphene/Al composites including the interface bonding between graphene and aluminum matrix,the dispersion of graphene in aluminum matrix,and the synthesis of graphene.There is still a long distance between the actual performance and theoretical value of graphene/Al composites,and most of the as obtained graphene/Al composites exhibited improved strength but poor ductility.Therefore,the key to develop graphene reinforced aluminum matrix composites is seeking for new fabrication methods which could overcome the above problems.In this paper,metal nano-particles modified graphene nanoplates(MPGNPs)was synthesized by the combination of spray-drying and chemical deposition technique on a large scale firstly,then graphene nanoplates(GNPs)reinforced aluminum or Al-Cu matrix composites were fabricated by powder metallurgy.The relationship between preparation methods,microstructure and mechanical properties was studied systematically,the strengthening and toughening mechanism as well as the fracture mechanism of GNPs reinforced aluminum matrix composites were discussed,which provided theoretical basis and realization way for the preparation of high-performance graphene reinforced aluminum matrix composites.Nickel nanoparticles modified GNPs(Ni-GNPs)and copper nanoparticles modified GNPs(Cu-GNPs)were synthesized by the combination of spray-drying and chemical deposition technique.Then Ni-GNPs/Al and Cu-GNPs/Al composites were prepared by powder metallurgy,and the effects of metals modified on the surface of GNPs on the interface and mechanical properties of GNPs/Al composites were investigated.The conclusion is as follows:those nanoparticles modified on the surface of GNPs could improve the dispersibility of GNPs,inhibit or delay the reaction between GNPs and aluminum matrix;nickel nanoparticles would react with aluminum and form Al₃Ni nanoparticles/layers near the interface in the Ni-GNPs/Al composite,which is beneficial for the interfacial bonding between GNPs and aluminum and accumulation of dislocations at the interface,leading to the increased ductility of Ni-GNPs/Al composites;in Cu-GNPs/Al composite,copper would diffuse into the matrix and Al-Cu solid solution formed near the interface,which has little effect on the interfacial bonding,however,Cu-GNPs/Al composite shows higher strength due to the solution strengthening of Cu element.A novel milling strategy-segmented ball milling was used for the preparation of Cu-GNPs/Al composites powders,and Cu-GNPs/Al composites were fabricated by cold forming,sintering and hot extrusion process subsequently.The conclusion is as follows:Cu-GNPs located on the surface of the at the early stage of ball milling,thus the dispersion of Cu-GNPs is positive correlation with the surface area of the ball milled aluminum powders,and high milling speed is beneficial for improving the surface area of aluminum powders as well as the dispersion of Cu-GNPs;low temperature ball milling could accelerate ball milling process and make composite powders cold-welded,which is also beneficial for the dispersion of Cu-GNPs;using segmented ball milling strategy,the maximum dispersion content of Cu-GNPs could reach 3.5 wt.%,and the corresponding Cu-GNPs/Al composite has a yield strength of 418 MPa,a tensile strength of 480 MPa and an elongation of 9.4%,which is 250%,240%and 48%of that of pure aluminum,respectively.The impacts of Cu-GNPs on the microstructure and mechanical properties of the composites were investigated and the strengthening and toughening mechanisms were discussed.The conclusion is as follows:Cu-GNPs could accelerate ball milling process,make composite powders cold-welded,refine the powder size and grain size;Cu-GNPs could also pin the migration of grain boundaries and impede grain rotation,block dislocation movement and pile-up dislocations effectively;the main strengthening mechanisms of Cu-GNPs/Al composites including the thermal mismatch strengthening caused by the difference in coefficient of thermal expansion between GNPs and aluminum matrix,Orowan strengthening caused by the blocking effect of GNPs on dislocations,and the load transfer strengthening caused by the load bearing of GNPs,grain refinement and the solid solution strengthening play a role;the improvement of work hardenability in the composites caused by the accumulation of dislocations near GNPs/Al interface is the main toughening mechanism.GNPs/Al-Cu composites were fabricated by the segmented ball milling strategy,the microstructures and mechanical properties of GNPs/Al-Cu composites under different heat-treatment conditions were investigated.The conclusion is as follows:the impacts of Cu-GNPs on Al-Cu alloys is similar to that on pure aluminum;GNPs could accelerate precipitation in Al-Cu alloys,shorten the time to reach the peak hardness;in GNPs/Al-Cu composites,GNPs and precipitation influence each other,leading to the decreased strength improvement after peak aged;with general consideration of all respects,solution treated GNPs/Al-Cu composite with GNPs content of 2.1 wt.%got the best performance,with yield strength of 482 MPa,tensile strength of 615 MPa and elongation of 9%,which is 180%,143%and 54%of that of solution treated Al-Cu alloy.