Effects of Cu and In Trace Elements on Microstructure and Thermal and Mechanical Properties of Sn-Zn Eutectic Alloy

The effects of addition of small amounts of copper (Cu) and indium (In) on the microstructure and thermal and mechanical properties of Sn-Zn eutectic alloy have been investigated. Cu and In were added in varying amounts to Sn-14.9 at.%Zn alloy by replacing an equal amount of Sn. Addition of Cu changed the eutectic composition, leading to the appearance of primary phases followed by the formation of intermetallic compounds (IMC) CuZn5 and Cu5Zn8. However, addition of indium (In) did not lead to formation of any new IMCs. With addition of Cu or In to the binary eutectic, the eutectic microstructure coarsened (the eutectic spacing increased). Addition of 1.919 at.% indium decreased the Sn-Zn eutectic temperature from 198.8°C to 192.0°C. On the other hand, addition of Cu did not affect the eutectic temperature as much as In addition did. The mushy (or pasty) range (the temperature range between the eutectic temperature and the melting temperature of the primary phase) increased with addition of Cu or In. Addition of a small amount of copper increased the yield strength of the binary eutectic alloy. The maximum hardness and yield strength of 24 ± 2 HV and 60 ± 3 MPa, respectively, were found with addition of 0.841 at.% Cu; this improvement in strength can be attributed to formation of hard IMCs. In contrast, addition of a small amount of In initially decreased the strength, but the strength increased to 50 MPa at 1.919 at.% In. This can be attributed to the effect of solid-solution strengthening due to the In solute in soft β-Sn.