Research On Texture Evolution, Mechanical And Electrical Properties Of Cu-Nb Composite Wires

Cu-Nb composite wires fabricated by accumulative drawing and bundling (ADB),and tranditional drawing process have been studied by means of electron channelcontract (ECC), electron back-scattered diffraction (EBSD), transmission electronmicroscope (TEM) and X-ray diffraction (XRD). The microstructure, mechanical andelectrical properties of the wires at different drawing strains and annealed temperatureshave been characterized systematacially. Interface density (Sv) and microstructureparameter at+d were introduced to estimate the effect of interface on the properties. TheHall-Petch relationship has been modified to clarify the strengthening of the Cu-Nbcomposite wires. The roles of size effect, interface effect and magnetic fields on theproperties were discussed.Based on these studies, the main conclusions can be drawn as following:①The curling of Nb filaments after large strain deformation is considered to berelative to not only the body centered cubic crystallographic structure of Nb but also theCu-Nb interface.②The deformation textures of Cu matrix can be characterized as multiple fibertextures parallel to drawing direction (DD): the major <111>, and minor <100> and<112>. Nb filaments develop a stable and strong <110> fiber texture, parallel to DD.The Cu and Nb follow (111)Cu//(110)Nbrelationship. The intensity of <111>Cutexture isimhomogenous from the center to the surface with the strongest in the center and theweakest near the surface. The density of both <111>Cuand <110>Nbfiber texturesincreases with increasing strains, but the density of <111>Cuis weakened due to thedynamic recrystallization at larger strains.③The interface density is considered to be critical to the mechanical andelectrical properties of Cu-Nb composite wires. At low strains, the interface density islow with hexgonal Nb filaments in the cross section, inducing low microhardness andelectrical resistance. At high strains, the interface density increases rapidly due to thecurling of Nb filaments, resulting in high microhardness and electrical resistance.④The Cu-Nb composite wires with extra Cu in the center show a goodcombination of strength and electrical properties. This could be a new way to obtainhigh strength and high conductivity Cu-Nb composite wires.⑤There are some limitations of Hall-Petch relationship for Cu-Nb composite wires due to the interface effect and size effect. A new model has been developed toestimate the interface density in the wires. Modified Hall-Petch relationship wasdeveloped, which suggests three parts for the strengthening mechanism: interfacestrenghening, solid solution hardening and work hardening of the Cu-Nbmacrostructure.⑥At room temperature, the magnetic field doesn’t show obvious impact on theresistance (magnetoresistance) of the Cu-Nb composite wires. While at77K, themagnetoresistance inceases with increasing magnetic fields. When the Cu matrixreached nanoscale, a drop in resistance as increasing magnetic fileds can be observed.⑦Microhardness and TEM results of annealed Cu-Nb composite wires indicatethat when the microstructure size is reduced to nanoscale, the recystallization of nanoCu matrix is delayed due to the size and interface effects, which suggests a high thermalstability in Cu-Nb composite wires. After annealing at high temperatures, recovery andrecrystallization occur in Cu matrix. Spheroidization of Nb filaments are observedduring higher temperature leading to the formation of cylinders and coarsening.Annealing process can be devided into three stages:1) At low temperatures,recrystallization in nano Cu matrix is delayed due to size and interface effects resultingin small reductions of microhardness and resistivity;2) At medium temperatures,recovery and recrystallization occur in nanoscale Cu matirx, and macroscale Cu grainsgrow inducing sharp decrease of microhardness nad resistivity;3) At high temperatures,revrystallization and growing occur in both Cu matrix and Nb filaments resulting infurther decrease of hardness and resistivity.⑧Comparisons of microstructure, mechanical and electrical properties are madebetween the ADB and in situ Cu-Nb composite wires. The results suggest a morehomogenous microstructure, and a better combination of strength and electricalconductivity in ADB Cu-Nb composite wires, which is contributed to size effect andinterface effect.