On the Sn-Bi-Ag ternary phase diagram

The selection and evaluation of Pb-free solders depend on the information that is best determined through a knowledge of ternary and higher order phase diagrams. Thermodynamic models of the phase diagram of the Sn-Bi-Ag ternary system are reported in this paper. A phase diagram of the Sn-Bi-Ag ternary system was constructed by theoretically extrapolating the experimental and calculated phase diagram data from the Ag-Sn, Ag-Bi, and Sn-Bi binary systems. Microstructure of the as-cast alloy with 96.2 at% Sn and 3.8 at% Ag showed the presence of primary (Sn)-dendrites. Thermodynamic functions for the various phases obtained by fitting measured data for the three constituent binary systems were extrapolated to the ternary system using the method of Muggianu. Modeling results were compared to the preliminary experimental data for the ternary system and were applied in the calculation of the solidification path. An agreement between the predicted and measured ternary eutectic temperatures was found to be a good criterion to judge the accuracy of the extrapolation. The difference between the predicted and measured eutectic temperatures was 1.2 deg C, which was fairly small.