Fabrication And Properties Of Aluminum Matrix Composites Based On In-situ Al-CuO Reaction

Al-CuO system is one of the research focuses in the development of aluminum matrix composites（AMCs）with promising mechanical properties due to the promising reinforcement-precipitate synergistic strengthening.However,AMCs prepared by in-situ Al-CuO reaction can hardly possess high strength and fracture toughness due to the coarsening of Al₂O₃ and the grain boundary（GB）segregation of second phase induced by the interfacial energy mismatch between Al₂O₃ and Al-melting.Therefore,it is of great significance to develop a novel in-situ reaction process based on Al-CuO system and optimize the reinforcement configuration for obtaining AMCs with outstanding strength and toughness.In the present work,AMCs were successfully fabricated by in-situ solid-state reaction of Al-CuO system using Al powders and CuO powders as raw materials via shift-speed ball milling（SSBM）combined with hot-pressing（HP）.The reaction mechanism,the microstructure and composition of Al₂O₃-Al interface and the dependence of nano-sized Al₂O₃ distribution on mechanical properties were systematically studied.This work aims to reveal the effect of non-uniform distribution of nano-scale Al₂O₃ on strength,toughness and fracture mechanism of AMCs,and provide a basic theoretical framework and instruction for the preparation of AMCs with excellent mechanical properties based on thermite reaction.The main research contents and results are as follows:（1）AMCs were successfully prepared by HP based on in-situ solid-state reaction of Al and 5 wt.%CuO powders.The effect of SSBM on thermite reaction and the solid-state reaction mechanism were discussed.The results show that SSBM promotes the uniform dispersion of Al-CuO powders and reduces the activation energy of chemical reaction.Meanwhile,the intense thermal effect originated from thermite reaction promotes the interdiffusion behavior of O and Cu atoms at the Al-CuO interface,and the diffusion-assisted nucleation mechanism is account for the in-situ generation and intragranular distribution of Al₂O₃.As a result,theδ^*-Al₂O₃ particles with an average size of 200 nm,as well asγ-Al₂O₃ whiskers with an average length of 150 nm and thickness of 20 nm,is produced.Meanwhile,the reduced Cu atoms react with the matrix to form Cu Al₂ phase.（2）The effect of structure and composition of in-situ Al₂O₃-Al interface on strength and plasticity and the fracture behavior of AMCs was revealed.It shows that the in-situ Al₂O₃-Al interface is identified as semi-coherent interface.Affected by the atomic diffusion behavior,the"Al₂O₃-intermixing region-Al"interfacial structure is formed for enhancing the interfacial bonding strength and plastic stability.The segregated Cu atoms and formed Cu Al₂ at the Al₂O₃-Al interface inhibit the nucleation of interfacial dislocations and interfacial slippage.The result also shows that the crack passivation,deflection and bridging constitute the extrinsic toughening mechanism of Al-CuO composite,while the weak bonding force of ex-Al₂O₃-Al interface causes interfacial debonding.（3）The evolution of microstructure and properties of Al-CuO composite during the heat treatment was discussed,thereby optimizing the heat treatment process routing.The results show that the high dislocation density of the composite caused by coefficient of thermal expansion（CTE）mismatch between the Al₂O₃ and Al matrix during solution treatment promotes the precipitate nucleation in the grain interior.After solution treatment at 793 K for 6 h followed by aging at 393 K for 18 h,the Al-CuO composite reaches the peak-aging,which is shortened 4 h compared with the matrix.The reduction in the size and the decreased number density of intragranular precipitates are the main factors for the simultaneous improvement of the strength and plasticity of composite after aging treatment.The tensile strength of the peak-aged composite is 480MPa,and the fracture elongation is 16.8%,which is 96 MPa and 3.6%higher than that of the as-extruded composite,respectively.（4）The work hardening behavior of the Al-CuO composite integrating isotropic hardening and kinematic hardening models was investigated.The loading-unloading test and the characterization of dislocation evolution during deformation show that the in-situ generated Al₂O₃ contributes significantly to the kinematic hardening of the composite.On basis of the visualization of strain evolution during in-situ SEM tensile process and the analysis of fracture morphologies,it is confirmed that the distribution feature of intragranular reinforcement is the main factor for the composite to achieve strain delocalization.（5）The effect of Al₂O₃ on the strengthening and toughening behavior of the composite was studied.Based on Orowan mechanism,an equivalent distribution model of Al₂O₃ is established,and it is proposed that the nano-sized intragranular reinforcement contributes to the high yield strength.By comparing the distribution characteristics of reinforcements,it is suggested that the alternately distributed heterogeneous structure of"Al₂O₃-rich zone"-"Al₂O₃-poor zone"is responsible for the high toughness of Al-CuO composites.This work contributes to the instruction for the preparation of Al-CuO composite via in-situ solid-state reaction,and has important theoretical significance for understanding the strengthening and toughening behavior of the composite reinforced by nano-sized intragranular reinforcement.