Solidification of ternary eutectic and near eutectic alloys in the Ag – Cu – Sn system

Abstract

The techniques for quenching samples during freezing, slow unidirectional solidification and differential scanning calorimetry were used to evaluate the freezing characteristics of eutectic and near eutectic ternary Sn rich Ag – Cu – Sn alloys. The ternary eutectic temperature was verified as 217·2±0·4°C and composition as 3·5 wt-%Ag, 0·9 wt-%Cu, 96·5 wt-%Sn. The ternary eutectic was observed to be of the faceted (Ag₃Sn) faceted (Cu₆Sn₅) nonfaceted (Sn) type. The Ag₃Sn phase forms a broken lamellar microstructure while the Cu₆Sn₅ phase forms a fibrous microstructure in which the fibres become ‘hollow’ hexagonal prisms (filled by Sn) at very low growth rates. The volume fraction occupied by the two compound phases was measured as 5·5±0·5 vol.-% and calculated as 5·63 vol.-%. It was observed that the compound phases are poor nucleation catalysts for Sn. The presence of faceting phases results in the formation of a skewed coupled zone, causing formation of primary Sn dendrites on freezing of a melt even of eutectic composition. When fractured, both compound phases showed brittle behaviour.