Relation between the Solidification Condition and Volume Fraction of Rod-Like Eutectic Cu₆Sn₅ Phase in the Eutectic Structure in Sn-Cu Alloys

Abstract

The Sn-Cu alloy is a typical rod-like eutectic system. The eutectic Cu₆Sn₅ phase is a minor phase exhibiting a rod-like structure. The volume fraction of this eutectic Cu₆Sn₅ (f_E-Cu6Sn5) is characterized as a function of copper content and growth velocity using an image analyzer. The volume fraction variation of the eutectic Cu₆Sn₅ relative to copper content is classified into three regions depending on the solidified structure. In the primary β-Sn + eutectic structure and primary Cu₆Sn₅ + eutectic structure regions, the f_E-Cu6Sn5 increased with increasing growth velocity, independent of the copper content. These were qualitatively explained by the undercooling for the growth of both phases. On the other hand, in the eutectic structure region, the f_E-Cu6Sn5 increased with copper content, independent of the growth velocity. The mechanism for the volume fraction change of the eutectic Cu₆Sn₅ under this growth condition was discussed. In order to exhibit the eutectic structure even for the off-eutectic composition, the volume of eutectic Cu₆Sn₅ was primarily altered by their changed rod diameters.