Research On Solidification Of Cu-25%Ag Alloys Under A High Magnetic Field And Their Heavily Deformed Strengthening Mechanism

Cu-Ag composites have been used in both DC and pulsed high-field magnets because of their high strength and high conductivity. It’s desirable to improve further the combination properties of Cu-Ag alloys to meet the increasing requirements from higher strength pulsed magnets. Cu-25%Ag alloys with good combination properties were selected to explore the influence of a12-T high magnetic field on their solidification processing, which is financially supported by National High Technology Research and Development projects (863Project)(No.2007AA03Z519), Wuhan National High Magnetic Field Center (Grant No. WHMFCKF2011007and the National Natural Science Foundation of China (Grant No.51004038). The mechanism of high magnetic field on the solidification processing for Cu-25%Ag alloys was investigated by quantitative analysis of as-cast microstructures. The solidified Cu-25%Ag alloys were heavily deformed by cold drawing to form the composite wires with different deformation rates. The relations between the deformed microstructure, properties and as-cast microstructure were determined, which will indicate the influence mechanism of high magnetic field on solidification processing of Cu-Ag alloys, and increase the combination properties.In the case of continuous cooling solidification (1050℃for40minutes then furnace cooling), the quantitative analysis of Cu-25%Ag as-cast microstructure under a high magnetic field indicates that, the influence of the high magnetic field on the morphology and distribution of Cu dendrites on transverse sections is not apparent, but as for Cu dendrites on longitudinal sections, the length is decreased and the dendrites spacing is increased, which is caused by the damped convection under the high magnetic field at the scale of Cu dendrites (about80μm). And the eutectic spacing on transverse sections is increased, because a new motion along the radial is induced at the scale of eutectic lamella (about1μm) with the high magnetic field which is beneficial to the atomic long-range diffusion on transverse sections during the growth of eutectic. As a comparison, the eutectic spacing on longitudinal sections is decreased, because the atomic diffusion was not only along the radial on longitudinal sections and the diffusion coefficient is decreased during the short-range diffusion. In the case of twostep cooling solidification (1050℃for40minutes, and cooling to790℃holding for30minutes, then furnace cooling), the quantitative analysis of Cu-25%Ag as-cast microstructure under a high magnetic field indicates that, the Cu dendrites were coarsened during the eutectic temperature holding times and the morphology change caused by high magnetic field is covered. In addition, the eutectic spacing is increased after applying high magnetic field which is caused by the decreased nucleation rate with the new motion.In the case of both solidification condition, the results of EDS analysis show that the weight percent of Ag in Cu dendrites is increased with the high magnetic field, which is attributed to the increased solute concentration in front of the interface and the unchanged equilibrium partition coefficient.The Cu-25%Ag alloys in both continuous and twostep cooling solidification were cold drawn to wires, and the influence of different deformation rate on Cu-25%Ag alloys with and without a high magnetic field was investigated.1) The quantitative analysis of microstructure indicates that, the microstructure on longitudinal sections is changed from network organization into fibrous tissue, and the spacing of eutectic fibers is decreased during the deformation. The microstructure on transverse sections is still the network structure, but the size is decreased. By the contrast of microstructure, it is found that the as-cast microstructure has the heredity to the drawn microstructure.2) The results of mechanical property indicate that, the ultimate tensile strength of the alloy is increased and the plasticity is dropped with increasing deformation rate. In the case of continuous cooling solidification, the strength is increased with the applied magnetic field which is attributed to the enhanced precipitation hardening and eutectic hardening. In the twostep cooling solidification cases, the strength is decreased with applied magnetic field which is attributed to the reduced eutectic hardening.3) The results of electrical conductivity indicate that, the conductivity is slightly decreased with the applied magnetic field, which is caused by the increased precipitation scatter because of the increased Ag content in Cu dendrites.The research from the master’s thesis shows that the high magnetic field could effectively improve the microstructure of Cu-25%Ag alloys and enhance the mechanical property, which provides a novel method to fabricate the Cu-Ag alloys with high performances.