Phase Equilibria of the Ternary Sn-Zn-Co System at 250°C and 500°C

Abstract

The isothermal sections of the ternary Sn-Zn-Co system (<60 at.% Co) at 250°C and 500°C have been experimentally determined. A series of Sn-Zn-Co alloys of various compositions were prepared and annealed at the respective temperatures to reach phase equilibrium. The equilibrium phases in these alloys were examined metallographically and characterized by electron probe microanalysis and x-ray diffraction. In this system, the ternary solubilities of all the binary Sn-Co and Co-Zn intermetallic compounds (IMCs) are very limited. For the phase equilibria at 250°C, two ternary IMCs, T₁ and T₂, were found, whose compositions were Sn-25 at.%Zn-25 at.%Co and Sn-15 at.%Zn-41 at.%Co, respectively. For the phase equilibria at 500°C, in addition to the T₂ phase, another ternary IMC, namely T₃ (Sn-18 at.%Zn-37 at.%Co), was also found. Moreover, the phase stability of the T₁ and T₃ phases was investigated at temperatures of 260°C to 400°C in detail. The equilibrium phase was the T₁ phase below 300°C, and changed to the T₃ phase at 400°C. The crystal structures of these three ternary IMCs were also studied. The T₁ phase has a cubic structure (Pm3m), and the T₂ and T₃ phases are orthorhombic in space group Cmcm and Pnma, respectively.