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Microstructural, Mechanical And Electrical Characteristics Of Cu-Ag Filamentary Microcomposites

Posted on:2006-07-01Degree:DoctorType:Dissertation
Country:ChinaCandidate:L ZhangFull Text:PDF
GTID:1101360182973076Subject:Materials Processing Engineering
Abstract/Summary:PDF Full Text Request
Cu-Ag microcomposites strengthened from the nanostructured filamentary phases have become the most important candidates for conductor materials used in high-field pulsed magnets and other application fields because of their outstanding strength and conductivity. Heavy cold deformation has been employed to obtain the nanosized filamentary microstructure. During the process forming the filamentary distribution from micron to nanometer dimensions, the evolution of microstructure in Cu-Ag microcomposites will strongly influence the mechanical and electrical properties.The Cu-Ag filamentary microcomposites with different Ag content were prepared by heavy cold drawing and intermediate heat treatments. The characteristics and evolution of filamentary microstructure during heavy cold deformation were investigated. The mechanical and electrical properties of the microcomposites dependent on the evolution of microstructure were measured. The mechanisms responsible for the strengthening and electrical conductive behavior and the dimensional effects of nanosized structure were discussed and relative analysis models were built up. Moreover, the microalloying effects on the microstructure and properties were investigated by adding Cr and rare earth elements into the alloys. The influence of final heat treatments after heavy cold drawing on the microstructure, mechanical and electrical properties was also investigated.The microcomposites usually contain Cu-rich matrix and lamellar eutectic product. With the increase of Ag content, the morphology of eutectic products in Cu-Ag alloys changes from discontinuous particles or island-like colonies to continuous net-like structure surrounding the pro-eutectic Cu dendrites. During cold drawing, a filamentary bundle structure dispersed in Cu matrix is formed from the lamellar eutectic structure. A lot of Ag precipitates produced in intermediate heat treatments can also evolve into thin filaments during drawing.Subgrain boundaries observed in the eutectic bundle of filamentary grains evolve gradually into low-angle or even high-angle grain boundaries with the increase of strain level, which can cause the separation of filamentary phases or subgrains. Heavy strain deformation results in tiny bend and rotation of the grains or local distortion of the grain boundaries on the cross section perpendicular to the drawing direction.The dislocation cell structure was observed in the Cu matrix grains. The cell walls transform into subgrain boundaries with the increase of strain level or decrease of grain size. At high strain range, the dislocation density greatly reduces with the decrease of filamentary diameter and the twin structure exists in the filamentary grains. The fibrous Ag precipitates usually locate on the cell walls or subgrain boundaries and adjust the strain difference between the precipitates and Cu matrix by interfacial misfit dislocations. There is obvious structural change at the interface between the filament phase and Cu matrix at heavy strain conditions. The structural change induces grain gliding along the grain boundaries and produces disturbed or distorted interfaces and microcrystalline transition layers.The Cu-Ag alloys maintain effective strain strengthening and high electrical conductivity due to the fibrous bundle structure. At heavier strain conditions, the nanosized filamentary structure further enhances the strength level of the alloys but obviously reduces the conductivity. The abundant fibrous Ag precipitates can also improve the strength and reduce the conductivity in low strain range but have insignificant effects on the properties in high strain range.In the range of the drawing strain lower than a certain value or the fibrous bundle spacing greater than a certain scale, the ultimate tensile strength of the filamentary composites dependent on the spacing of eutectic fibrous bundles accords with the Hall-Petch relationship. The mechanism of pile-up of dislocation can be suggested to be responsible for the strengthening benefit. As the strain level is over a certain degree or the spacing of eutectic fibrous bundles less than the certain scale, the strain strengthening becomes weak and the strength dependent on the spacing of eutecticfibrous bundles deviates from the Hall-Petch relationship. This can be attributed to the strain mechanism transformation from dislocation movement in grains to atomic glide at grain boundaries and phase interfaces because dislocation density greatly decreases under the heavy strain conditions. The strengthening benefit is mainly produced from the athermal obstacles at the boundaries and interfaces.The resistivity increase of the Cu-Ag alloys with drawing strain is mainly attributed to the enhanced scattering of conduction electrons at the abundant internal Cu/Ag interfaces. A size-effect conduction model based on interfacial scattering and the quantitative relationship between the resistivity and draw ratio has been drawn and can be utilized to describe the effects of the filamentary structure on the electrical properties. The additional scattering from the fibrous Ag precipitates obviously increases the resistivity in a certain strain range. The decrease of probability of electronic elastic scattering induced by increasing Ag content or phase interface results in a further significant reduction of conductivity with increasing strain.Adding l%Cr into the Cu-6%Ag induces more dispersive filamentary bundles transformed from eutectic colonies and fewer Ag fibers transformed from precipitates. Solution strengthening of Cr solute results in the strength improvement. The reduction in dislocation density in small scale of the fibrous structure invalidates solution-strengthening contribution of Cr solute to a certain degree and results in the loss of continuous strain hardening at high strain levels. Additional solute scattering of Cr atoms dissolved in the Cu matrix significantly reduces the electrical conductivity in the alloys. Cr addition also decreases the interval of the interface between the filamentary bundle and Cu matrix and the probability of electronic elastic scattering, which results in more obvious reduction in conductivity with increasing strain level. Adding minor rare earth elements produces more eutectic and finer microstructure in the as-cast alloys. As a result of the microstructural modification, the hardness level of the microcomposites is enhanced and the electrical conductivity improved in the range of high deformation. Suitable microalloying of the rare earth elements in the Cu-Ag alloys contained with Cr addition further enhances the tensile strength with an improvement ofthe electrical conductivity to a certain degree.For the microcomposites annealed at 200 °C, the morphology of filamentary structure formed during cold drawing shows an insignificant change and the ultimate tensile strength slightly decreases, while hardness and conductivity slightly increase. For the microcomposites annealed at temperatures higher than 400 °C, the strength and hardness decrease and the conductivity increases significantly. Moreover, the equiaxed grains recrystallized from filamentary structure and aligned along the wire axis can be observed and the fibrous morphology disappears in the micro structure annealed at high temperature.
Keywords/Search Tags:Cu-Ag alloy, cold drawing, heat treatment, filamentary microcomposite, microstructure, mechanical property, electrical property, alloying, rare earth element
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