Thermodynamic study of phase equilibria in the Sn-Ag-Bi-Zn quaternary system

The phase equilibria in the Sn-Ag-Bi-Zn quaternary system have been studied experimentally and using thermodynamic calculations. The determined values of the thermodynamic parameters of the Sn-Ag-Zn and Sn-Ag-Bi systems were applied in the calculation of the phase diagrams. Thermodynamic evaluation of the Sn-Bi-Zn and Ag-Bi-Zn systems was performed by considering a two-phase separation of the liquid phase. The phase boundaries in some vertical sections of the quaternary system were determined using differential scanning calorimetry to confirm the calculated results. The calculation of the phase diagrams when 10%Bi was added to Sn-Ag-Zn alloy shows that the regions of the primary crystals in the quaternary system did not show a large discrepancy from the Sn-Ag-Zn ternary system. The eutectic temperature decreased to about 203.7°C on addition of Bi to Sn-Ag-Zn alloy. The rate of change was estimated to be 1°C per 1% Bi added. The solidification structure was investigated using scanning electron microscopy and energy dispersive X-ray microanalysis. The microstructure was composed of an Sn-Bi-based eutectic, Ag₅Zn ₈, Ag₃Sn, and coarsened Bi of about 50 μm. Based on these results, a non-equilibrium solidification process using the Scheil model was simulated and compared with the observed structures. Our calculations reasonably explain Bi-enrichment in the final solidification zone.