Phase diagrams and microstructures related to the miscibility gap of the Ag-Cu-Se system

The Ag-Cu-Se ternary system is widely used in thermoelectric materials. The liquidus projection and the 300 °C isothermal section were determined based on the experimental results of this study and the phase diagrams of their constituent binary systems. For the liquidus projection, there are eight primary solidification phases. These phases include (Ag), (Cu), Ag₂Se, Cu₂Se, CuSe, CuSe₂, (Se) and a ternary compound, AgCuSe. Additionally, based on the thermal analysis results, there are nine invariant reactions, including four Class I reactions, four Class II reactions and one Class III reaction. Interestingly, two wide compositional range of liquid miscibility gaps are observed in liquidus projection. Various kinds of interesting microstructures were observed in alloys solidified through miscibility gaps, which resulted from compositional differences, density differences, and surface tension. For the 300 ^oC isothermal section, the sample alloys were quenched and equilibrated at 300 ^oC for six months. There are seven three-phase regions in the Ag-Cu-Se system, including Cu₂Se + Ag + Cu, AgCuSe + Cu₂Se + CuSe, AgCuSe + CuSe + Ag₂Se, CuSe + Ag₂Se + CuSe₂, Ag₂Se + CuSe₂ + Liquid, AgCuSe + Ag₂Se + Ag, and AgCuSe + Cu₂Se + Ag. Furthermore, Ag, Cu and Ag + Cu mixed whiskers were observed along the Cu-Cu₂Se-Ag₂Se-Ag section, which overlaps the miscibility gap.