Study On Preparation And Properties Of Multimodal Microstructural Cu-Y₂O₃ Composites

Particle-reinforced copper matrix composites exhibit important engineering applications in the fields of integrated circuit lead frames,high-voltage substation equipment switches,automotive spot welding electrodes due to their high strength,high conductivity,excellent thermal stability,wear resistance and arc erosionresistance.However,the current processes of preparing particle-reinforced copper matrix composites are often accompanied by a sharp decline in the room-temperature processing performance while achieving a significant increase in the strength of the material.This phenomenon seriously restricts their application in the field of precision electronic equipment.As one of the effective strategies to alleviate the strength-ductlity trade-off problems of metals,the construction of multimodal grain structure has been widely concerned in academia and has become a research hotspot.Therefore,we used brittle copper oxides and nano-yttria as raw materials.A novel process to prepare Cu-Y₂O₃ composites which can achieve finely dispersed reinforcments and multimodal grain structure of matrix were developed by combining mechanical ball milling,atmosphere reduction and spark plasma sintering.With the help of scanning electron microscope,high-resolution transmission electron microscope,electron backscatter diffraction and other analysis techniques,the evolution of Y₂O₃ content on the reinforcment distribution,grain structure and mechanical properties under this process were systematically studied.The key steps of the preparation process that affect the microstructure and performance of the materials were proposed.The strengthening mechanism and microscopic deformation behavior of the Cu-Y₂O₃ composites were revealed,which provide a good theoretical and experimental ground for the further development and research of the strength and ductlity matching particle reinforced copper matrix composites.In this paper,iterative optimization of the preparation process of Cu-Y₂O₃ composite nano-powders was carried out.One-step ball milling and two-step reduction process（ball milling,CO mild reduction and then H₂ deep reduction）can produce pure and no obvious sintered copper nano-powders,but the distribution of reinforcement after spark plasma sintering is a quasi-continuous network.Two-step ball milling and two-step reduction process（ball milling,CO mild reduction,ball milling and H₂ deep reduction）can synthesise high volume fraction finely dispersed nano Y₂O₃ particles in the matrix by external physical addition method.At the same time,the microstructure of Cu-Y₂O₃ composite after spark plasma sintering exhibits the characteristics of multimodal grain structure with micro grains embedded in the nano grains.The study shows that Y₂O₃ content has a significant effect on the dispersion of reinforcement and the size and distribution of grains in Cu-Y₂O₃ composites.When the content of Y₂O₃ is 4 vol.%,though the matrix has a multimodal microstructure,the agglomeration of Y₂O₃ nanoparticles can be oberserved in the final bulk.This is because of the sintering and coarsening of copper nanoparticles during the reduction process in CO atmosphere.The hardness and yield strength of Cu-4vol.%Y₂O₃ composite are only 99.7HV and 274.1 MPa,respectively.When the content of Y₂O₃ is 10 vol.%,the Y₂O₃ nanoparticles are finely dispersed in the matrix,and the multimodal microstructure is basically disappeared.Although the hardness and yield strength of Cu-10vol.%Y₂O₃ composite are 198.7HV and613.3 MPa respectively,it loses the strain hardening ability.When Y₂O₃ content is 7 vol.%,Y₂O₃ nanoparticles are uniformly dispersed in the matrix and the multimodal microstructure of the matrix is obvious.The composite exhibits not only high microhardness and yield strength which can reach to 182.3HV and 437.9 MPa respectively,but also good true compressive strain（0.463）,and the conductivity can still maintain 62.9%IACS.Based on the theoretical strengthening models of particle reinforced metal matrix composite,the main mechanisms of yield strength improvement of Cu-Y₂O₃ with multimodal microstructure developed in this paper are grain boundary strengthening mechanism and thermal mismatch strengthening mechanism.The experimental datum of yield strength can be fitted well with the theoretiacal values carried out by root mean square additivity rule.Microscopic deformation behavior result shows that the hard domains composed of matrix fine grains and nano-Y₂O₃ receive most of the strain energy,and the soft domains composed of coarse grains bear the main plastic deformation.It also indicates that strain partitioning caused by the strain gradient between coarse-grain and fine-grain domains plays an important role in coordination of heterogeneous plastic deformation between hard domains and soft domains.The results of compressive fracture morphologies show that the fracture mechanisms of Cu-Y₂O₃composites change from ductile fracture to mixed fracture and finally to intergranular fracture with the increase of Y₂O₃ content.In view of the difficulty of controlling the morphology of Cu-Y₂O₃ composite powder in the process of CO reduction,the reduction kinetics of the preparation of Cu-Y₂O₃ composite powder by CO reduction under different Y₂O₃ contents were studied.The results show that the preparation of Cu-x vol.%Y₂O₃（x=0,4,7,8）composite powders by CO reduction at 120 ^oC is in three stages:the early stage of slow reaction,the middle stage of fast reaction and the late stage of slow reaction.The Y₂O₃ content has no effect on the three-stage kinetic control mechanisms,but has two effects on the three-stage reaction rate.On the one hand,the addition of Y₂O₃nanoparticles is beneficial to the refinement of CuO particles during the ball milling process,thereby promoting the reaction rate at each stage and shortening the redox reaction time.On the other hand,the addition of Y₂O₃ will block the gas flowability inside the agglomerated particles,which is not conducive to the progress of the gas-solid reaction and prolongs the redox reaction time.These two effects compete with each other.When the content of Y₂O₃ is low,the first effect is dominant and the overall reduction time is shortened.If the reduction time is too long,it will cause the sintering and coarsening of the fully reacted nano-copper powder.When the Y₂O₃content is high,the second effect dominates and the overall reduction time is extended.If the reduction time is too short,it will cause insufficient reaction.