R. Yoshimura 1, H. Esaka 1 and K. Shinozuka 1
1 Department of Materials Science and Engineering, National Defense Academy
In order to investigate the effect of the addition of a small amount of Zn to a Sn-based eutectic alloy, the thermal histories during solidification were measured and the solidified structures of the Sn-2Ag-xZn alloys were characterized. The Zn content, x, was varied from 0 to 0.6 mass%. Five grams of the alloy were melted and cooled at a constant cooling rate of 0.05 K/s. The temperature of the alloy was measured using a fine, K-type thermocouple and recorded using a data recorder. The undercooling for the nucleation of primary(β-Sn), ΔTexp, was defined as the temperature difference between the minimum temperature before nucleation and the maximum after recalescence. It was found that ΔTexp decreased with increasing x. When x was between 0 and 0.15 mass%, the slope of this decrease was large. On the other hand, when x exceeded 0.15 mass%, the slope became small. The solidified structures of these alloys consisted of the primary(β-Sn)phase and the eutectic structure, which were typical for hypoeutectic alloys. The volume fraction of primary(β-Sn), f β-Sn, was larger than the equilibrium value and decreased slightly with increasing x. This indicated that the undercooling for the nucleation of the primary(β-Sn)phase is linked to the volume of the primary phase; however, this relation is weak.
lead-free solder, β-tin, volume fraction, lever rule, undercooling, equilibrium
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