| The pulsed high magnetic field experimental device is an important experimental device focusing on physics.materials,biomedicine and other major scienti fic fields.In the development process of pulsed high magnetic field,the magnetic winding material is the focus of research.The accumulative drawing and bonding steps（ADB）as a SPD（Severe Plastic Deformation）technology developed by Northwest Institute of Nonferrous Metals research,has obtained highperformance copper-based composite wires with nanostructures,and has been successfully applied in the pulsed high magnetic fields.With the continuous deepening of material research,the results indicate that this nanofibers reinforced copper-matrix composite wires has excellent strength and conductivity,which will promotes the wide application of materials in aerospace,military fields.However,under extreme conditions such as space,weapons and equipment,material structure and performance need to have high temperature stability,so it is urgent to carry out thermal stability research of materials.In this paper,a new technology of coiling+bundle drawing is used to prepare high-strength and high-conductivity Cu-Nb composites,and obtained the semi-coherent interface of Cu/Nb,in which the Nb core wire exists in the form of nano-grain along the radial direction of the material,and formed a multi-scale pure copper layer from nano-scale to micro-scale around the Nb core wire.Under the influence of temperature and time,this special structure changes greatly,and the interface structure,core wire morphology,grain boundary configuration,and of the material all show different changes,this affects the thermal deformation behavior and electron scattering characteristics of the material,and brings about changes in the mechanical and electrical properties of the material.Therefore,it has great significance to study the rheological stress characteristics of materials under thermal conditions,the evolution of interface and the thermodynamic dynarriics of materials.It is major project that studied the thermal deformation behavior of the materials at different temperatures and strain rate conditions,obtained the effect mechanism of temperature on the microstructure of the materials,clarified the rheological stress charareteristics of the materials,and constructed the constitutive model of Cu-Nb composites.Furthermore,we also studied the evolution law of the interface structure of the material under thermal conditions,understand the mechanism of spheroidization and columnarization of the Nb core wire with the increase of temperature,and revealed the corresponding relationship between the microstructure and properties of the material under high temperature conditions,providing experimental support for the engineering application of the material.The main conclusions are as followed:1.The strengthening mechanism and conductive mechanism of the winding Cu-Nb composite are clarified.The nano-Cu matrix and Nb layer of prepared composites wire are composed of multi-layer（layer thickness 73 nm）and single-layer elongated grain（layer thickness 22 nm）,respectively.When the annealing temperature increases to 800℃,the thickness of nanoCu layer and Nb layer increases to 311 nm and 112 nm.At high temperature,the grains migrate at large angle grain boundaries,the dislocation density and intragranular defects of the material are eliminated,and a high proportion of E3 lattice grain boundaries are formed,which is conducive to the stable deformation of the material.At the same time,driven by grain boundary surface energy,curvature potential and chemical potential energy,the internal Nb core wire is coarsened and spheroidized to form cylindrical and bamboo-like structure.The Cu-Nb composites formed<111>Cu//<110>Nb annealing texture in annealing process.With the decrease of the interface density,the hardness and tensile strength of the wire decreased significantly,and the conductivity increased.2.The thermal deformation law and microstructure evolution mechanism of Cu-Nb composites were studied.The dynamic recovery and recrystallization behavior of multiscale Cu matrix and Nb grains occurred in stages during the hot deformation process.The material undergoes rapid dynamic recrystallization at low critical strain（σc=0.701σp）,and the dynamic recrystallization behavior develops and continues to increase with the degree of deformation.At low strain rate,the material has ductile fracture,uneven grain size and good hot working performance.At low temperature and high strain rate,the nano-Cu and nano-Nb core wire is debonded and pulled out,which has good reinforcement and toughening effect,the parameter of high temperature and low strain rate are beneficial to the dynamic recrystallization and softening of the material and improve the hot working performance of the material.The coincidence of the simulated data of the hyperbolic Arrhenius constitutive equation and the experimental data is about 93%,which has a good prediction for the stress-strain curve of hot drawing,and can optimize and guide the hot working process of high-performance Cu-Nb composites.3.Based on the DMM（Dynamic Material Model）model,the good hot working region of the material was analyzed,and the corresponding relationship between the hot working parameters and the microstructure of the high performance composite wire was studied by EBSD.The Cu-Nb composite is prone to crack during thermal deformation at low temperature,and the thermally deformed material will form adiabatic shear band at high temperature and high strain rate,resulting in rheological instability.When the hot working parameters are in the safe hot working area with temperature of 710℃-745℃ and strain rate of 0.0032 s-1-0.033 s-1,the material has good hot workability.At high temperature and low strain rate,the material undergoes significant dynamic recrystallization and forms equiaxed crystal structure,which leads to the destruction of Nb fiber strength carrier.With the increase of strain rate and the decrease of temperature in the safe area of hot working,the integrity of Cu and Nb fiber in the material is improved,and Cu-Nb composite wire with high-performance is obtained. |
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