Phase Equilibria of Sn-Co-Cu Ternary System

Sn-Co-Cu ternary alloys are promising lead-free solders, and isothermal sections of Sn-Co-Cu phase equilibria are fundamentally important for the alloys' development and applications. Sn-Co-Cu ternary alloys were prepared and equilibrated at 523 K, 1073 K, and 1273 K (250 A degrees C, 800 A degrees C, and 1000 A degrees C), and the equilibrium phases were experimentally determined. In addition to the terminal solid solutions and binary intermetallic compounds, a new ternary compound, Sn3Co2Cu8, was found. The solubilities of Cu in the alpha-CoSn3 and CoSn2 phases at 523 K (250 A degrees C) are 4.2 and 1.6 at. pct, respectively, while the Cu solubility in the alpha-Co3Sn2 phase is as high as 20.0 at. pct. The Cu solubility increases with temperature and is around 30.0 at. pct in the beta-Co3Sn2 at 1073 K (800 A degrees C). The Co solubility in the eta-Cu6Sn5 phase is also significant and is 15.5 at. pct at 523 K (250 A degrees C).