研究了横向磁场对定向凝固Cu-6%Ag合金组织、硬度和电阻率的影响. 结果表明, 施加磁场增加了先共晶Cu枝晶的一次枝晶间距和体积分数, 先共晶Cu中捕捉了更多的过饱和Ag, 最终升高了合金的硬度和电阻率. 从磁场抑制熔体对流作用的角度建立了一次枝晶间距与施加磁场强度、液相扩散系数等的关系, 并结合溶质分配系数的影响解释了Ag过饱和度增加的原因. 利用TEM和EDS分析结果, 结合建立的合金电阻率模型, 定量分析了合金电阻率的改变和固溶Ag原子对固溶体Cu-6%Ag合金导电性的主导作用.
Cu-Ag material with excellent combination of high strength and high conductivity is an important conductor for both direct current resistive and pulsed high-field magnets. The strength and electrical conductivity of Cu-Ag microcomposite are closely related to the microstructure of proeutectic Cu because of its high volume fraction. The morphology of proeutectic Cu, Ag precipitation and concentration of Ag in Cu can be controlled by application of external field and the addition of the third elements. In this work, the microstructural evolution, concentration contributions, the resulting microhardness and electrical resistivity of Cu-6%Ag alloy, which was directionally solidified under a transverse magnetic field were studied. The effect of the magnetic field on the microstructure was analyzed by OM, SEM, TEM and EDS. The results demonstrate that in macro scales, the growth direction of columnar grains is gradually deflected along the axial and heating flow directions with increasing magnetic field intensity. In micro scales, the increasing magnetic field increases both the primary dendrite arm spacing and volume fraction of proeutectic Cu, and traps more supersaturated Ag in proeutectic Cu. No obvious effect on the secondary dendrite arm spacing of proeutectic Cu is observed. In nano scales, SAED pattern in TEM indicates a small quantity of fine nanostructured Ag precipitations in proeutectic Cu. A relationship among the primary dendrite arm spacing, external magnetic field intensity and the initial diffusion coefficient in liquid was established from the viewpoint of suppressed convection by the magnetic field. The increased supersaturated Ag in proeutectic Cu is thought to be caused by the influence of magnetic field on the solute redistribution coefficient. The changes of microstructure induced by magnetic field result in the increases of the microhardness and electrical resistivity in Cu-6%Ag alloy. A model was proposed to clarify the changes of electrical resistivity in terms of the resistivity of Cu matrix, the impurity-scattering resistivity from dissolved Ag in Cu and the scattering resistivity from vacancy, where the interface-scattering resistivity from precipitation of Ag is assumed to be ruled out. The result shows that the impurity-scattering resistivity from dissolved Ag in Cu, which is increased by the application of external magnetic field, plays an important role in determining the overall resistivity of the alloy.
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