A DSC analysis of thermodynamic properties and solidification characteristics for binary Cu–Sn alloys

Abstract

The liquidus temperatures and enthalpies of fusion for Cu–Sn alloys are systematically measured across the whole composition range by differential scanning calorimetry (DSC). The liquidus slope vs. Sn content is derived on the basis of the measured results. The measured enthalpy of fusion is related to the Sn content by polynomial functions, which exhibit one maximum value at 55wt.% Sn and two minimum values around 28.9wt.% Sn and 90wt.% Sn, respectively. The undercoolability of those liquid alloys solidifying with primary α (Cu) solid solution phase is stronger and can be further enhanced by increasing the cooling rate. However, other alloys with the preferential nucleation of intermetallic compounds display smaller undercoolings and are not influenced by cooling rate. Microstructural observations reveal that peritectic reactions can rarely be completed. With the increase in undercooling, the primary α (Cu) dendrites are refined in the peritectic Cu–22wt.% Sn alloy. For the hyperperitectic Cu–70wt.% Sn alloy, typical peritectic cells are formed in which the peritectic η(Cu6Sn5) phase has wrapped the primary ε(Cu3Sn) phase. The DSC curves of metatectic-type Cu–Sn alloys indicate that the metatectic transformation γ→ε+L upon cooling is an exothermic event, and a large undercooling of 70K is required to initiate this transformation in metatectic Cu–42.5wt.% Sn alloy. The metatectic microstructures are characterized by (ε+η) composite structures. The η phase is mainly distributed at the grain boundaries of the coarse ε phase, but are also dispersed as small particles inside ε grains. The volume fraction of the η phase increases with the Sn content.