Yoshiko Takamatsu, Hisao Esaka and Kei Shinozuka
Department of Materials Science and Engineering, National Defense Academy of Japan, Yokosuka 239-8686
In this study, the solidification process has been investigated to clarify the formation mechanism of Sn-Cu6Sn5 binary eutectic structure after growth of primary β-Sn in undercooled Sn-1.0Ag-0.5Cu alloy by using thermal analysis, interruption tests and energy dispersive X-ray spectroscopy. First Cu-enriched zone was formed around β-Sn after growth of primary β-Sn, then the Cu-enriched zone changed into a round shape as the temperature decreased. Eventually some spherical Cu-enriched droplets, which were 10 μm in diameter, were separated from the remained liquid. The concentration of Cu and Ag in the remained liquid was higher than the expected value from equilibrium phase diagram since the considerably large volume fraction of β-Sn nucleated in this study. Moreover, Cu was piled-up around β-Sn with the growth of β-Sn.
lead-free solder, tin-silver-copper melt, copper-enriched zone, undercooling, nucleation, solidification structure
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