Research On Structure And Properties Of Directionally Solidified Cu-6%Ag Alloy Under A Magnetic Field

High strength and high conductivity Cu-Ag alloys are widely used as winding coil conductors in high-field pulsed and DC resistant magnets. The properties of these materials are closely related to the control of solidification and precipitation behaviors. In this study, a static magnetic field was superimposed to the directional solidification of Cu-6wt.%Ag alloys. The influences of the external magnetic field on structure and properties of as-cast, as-rolled and as-heat-treated samples were investigated. It is the purpose to clarify the relation among the external magnetic field, solidification structure, solute composite and as-rolled microstructure evolution, electrical and mechanical properties of the samples. The results could provide fundamental and experimental data for the technology development of Cu-Ag alloy by application of electromagnetic field.Investigations on the influence of an external static magnetic field on solidification structure of directionally solidified (DS) Cu-6%Ag alloys indicated that with increasing magnetic field,1) the growth direction of macro columnar grains were gradually deflected along the axial and heating flow directions; 2) the dendrites within each macro grains grew along the crystallographic growth direction, and the deflection angle of micro-scaled dendrite was consistent with that of macro-scaled grains; 3) both the primary dendrite arm spacing and the length of dendrite were increased; 4) the solubility of Ag in Cu matrix was increased, but the volume fraction of eutectic colonies was decreased; no Ag precipitates in Cu matrix were detected by morphology observation and EDS analysis of TEM. A schematic model of solidification process was proposed based on our experimental results.Differential scanning calorimetric (DSC) results on as-cast Cu-6%Ag alloys indicated that the precipitation behavior of Ag atoms was dominant between 300℃ and 360℃, however the dissolution behavior was predominant as the temperature was over the temperature of 420℃. The activation energy of Ag precipitation from Cu matrix was approximately estimated to be 129.3KJ/mol. The results from both the hardness and conductivity of as-cast Cu-6%Ag alloys as a function of aging temperatures show that the annealing at the temperature of 300℃ could present better conductivity property and annealing at 450℃ could get maximum hardness.The tests on the hardness and resistivity of as-cast Cu-6%Ag alloys solidified under different magnetic field, and the modeling results of resistivity demonstrated that the imposition of magnetic field enhanced the solubility of Ag in Cu matrix. It increased the resistivity and deteriorated the conductivity properties of alloys. However, the solute hardening improved the hardness of alloys. Microstructure evolution of as-rolled samples of Cu-6% Ag alloys with increasing reduction showed that, after the magnetic field was introduced,1) the volume fraction of off-eutectics was a little lower; 2) with lower total reduction, slower deformation microstructure decreased the hardness; 3) with further increasing reduction, the Ag precipitates induced by rolling enhanced the hardness, which made the samples with magnetic field have higher hardness. The ageing at the temperature of 300℃ promoted the Ag atoms to precipitate from Cu matrix. It optimized the combination of strength and conductivity of alloys.