Directional Solidification and Liquidus Projection of the Sn-Co-Cu System

This study investigates the Sn-Co-Cu ternary system, which is of interest to the electronics industry. Ternary Sn-Co-Cu alloys were prepared, their as-solidified microstructures were examined, and their primary solidification phases were determined. The primary solidification phases observed were Cu, Co, Co₃Sn₂, CoSn, CoSn₂, Cu₆Sn₅, Co₃Sn₂, γ, and β phases. Although there are ternary compounds reported in this ternary system, no ternary compound was found as the primary solidification phase. The directional solidification technique was applied when difficulties were encountered using the conventional quenching method to distinguish the primary solidification phases, such as Cu₆Sn₅, Cu₃Sn, and γ phases. Of all the primary solidification phases, the Co₃Sn₂ and Co phases have the largest compositional regimes in which alloys display them as the primary solidification phases. There are four class II reactions and four class III reactions. The reactions with the highest and lowest reaction temperatures are both class III reactions, and are L + CoSn₂ + Cu₆Sn₅  =  CoSn₃ at 621.5 K (348.3 °C) and L + Co₃Sn₂ + CoSn = Cu₆Sn₅ at 1157.8 K (884.6 °C), respectively.