Abstract
Braze joints have been formed between Ag/W electrical contact tips and Sn-coated Cu straps using a Cu–Ag–P-based filler metal by short-cycle direct conductive heating from the Cu strap side. The microstructures of the braze joints have been investigated using a combination of scanning electron microscopy, energy-dispersive X-ray spectroscopy, electron backscatter diffraction, focused ion beam milling, and transmission electron microscopy. The braze joints exhibit regions with two distinct types of microstructures. Type I microstructures contain three micro-constituents: FCC (Ag), FCC (Cu), and a ternary eutectic [(Ag) + (Cu) + Cu3P]. The difference between the proportions of these phases and those in the baseline filler metal can be accounted for on the basis of P loss due to self-fluxing during the brazing process. Type II microstructures contain mostly (Ag) and (Cu) with a small amount of Cu3P but no ternary eutectic mixture. Other differences from the Type I regions include a lower P content, incomplete dispersion of the Sn layer, and a refined grain size in the majority (Cu) phase. These effects are accounted for on the basis of local differences in interfacial pressure due to the geometry of the Ag/W contact surface.
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Notes
Following Takemoto et al. [8] we designate the Cu- and Ag-rich face-centered-cubic (FCC) solid solutions in this system as (Cu) and (Ag), respectively.
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Acknowledgements
This work was supported by a research grant from GE Industrial Solutions under a GE-UConn partnership agreement, and by the award of a GE Graduate Fellowship to Haibo Yu. The authors would like to thank Jonathan Potter and Jason Harmon from GE for helpful discussions, and Dr. Richard McLaughlin of Oxford Instruments for assistance with the EBSD experiments.
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Yu, H., Sun, Y., Alpay, S.P. et al. Microstructure effects in braze joints formed between Ag/W electrical contacts and Sn-coated Cu using Cu–Ag–P filler metal. J Mater Sci 50, 324–333 (2015). https://doi.org/10.1007/s10853-014-8591-7
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DOI: https://doi.org/10.1007/s10853-014-8591-7