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Acta Metall Sin  2016, Vol. 52 Issue (2): 143-150    DOI: 10.11900/0412.1961.2015.00279
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Xiaowei ZUO,Rui GUO,Bailing AN,Lin ZHANG,Engang WANG()
Key Laboratory of Electromagnetic Processing of Materials (Ministry of Education), Northeastern University, Shenyang 110819, China
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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.

Key words:  Cu-Ag alloy      magnetic field      microstructure      hardness      electrical resistivity     
Received:  26 May 2015     
Fund: Supported by National Natural Science Foundation of China (Nos.51474066 and 51004038), National High Technology Research and Development Program of China (No2007AA03Z519), Special Research Fund for Doctoral Disciplines Program of Chinese Higher Education (No.2012004211008) and Programme of Introducing Talents of Discipline to Universities of China (NoB07015)

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Fig.1  Schematic of directional solidification apparatus with a transverse static magnetic field (1: thermal insulation, 2: liquid molten metal, 3: solid sample, 4: quartz crucible, 5: cooling water, 6: Ga-In-Sn liquid metal, 7: radiant plate, 8: transverse static magnet, 9: induction heating coils, 10: water input, 11: water output)
Fig.2  Macro-morphologies of directionally solidified Cu-6%Ag alloys (a1~c1), OM images of proeutectic Cu (a2~c2), SEM images of eutectic (a3~c3) and TEM images (a4~c4) under external magnetic field with intensities of 0 T (a1~a4), 0.80 T (b1~b4) and 1.12 T (c1~c4) (Inset in Fig.a4 indicates the SAED pattern; inset in Fig.b4 indicates the rod-like TEM morphology of Ag precipitation; B—external magnetic field, λ1—primary arm spacing of dendrite, λ2—secondary arm spacing of dendrite, L1, L2—test line lengths,. N—number of feature)
Fig.3  Average dendrite arm spacing of primary and secondary dendrites (a), microhardness and electrical resistivity (b) of directionally solidified Cu-6%Ag alloy with different magnetic field intensities
Fig.4  Schematic of solidification process of directionally solidified Cu-6%Ag alloy without and with a transverse magnetic field (L—liquid, S—solid, TL—liquidus temperature, TE—eutectic temperature, c0—initial alloy concentration, k0—equilibrium distribution coefficient)
Magnetic field intensity
0 16.61 4.02 0.03 20.66 18.6
0.80 16.61 7.67 0.03 24.31 20.87
1.12 16.61 8.45 0.03 25.09 21.9
Table 1  Calculated and measured electrical resistivities in Cu-6%Ag alloy
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