The effects of Sb on the lattice and microstructure characteristics of hypo-eutectic Sn-Bi alloys

Sn-Bi alloys emerged as promising candidates for low temperature solder materials due to their low cost and low liquidus temperature. However, the Sn-Bi solders tend to have low ductility at high strain rates. The addition of Sb is found to improve the ductility of Sn-Bi alloys and they have recently received strong commercial interest. This study employs scanning electron microscopy (SEM), electron probe microanalysis (EPMA), in-situ heating synchrotron powder X-ray diffraction (PXRD) and cooling curve analysis of solidification events to investigate the effects of Sb on the microstructure, crystal structure and solidification behaviour of hypoeutectic Sn-37wt%Bi-xSb alloys, where x is up to 3 wt%. Calculations of phase diagrams (CALPHAD) and density functional theory (DFT) simulations are used to predict the phases and to calculate the energy of formation. It was found that Sb does not necessarily refine the microstructure of Sn-37wt%Bi. Between room temperature and 120 °C, the lattice parameters of both Sn and Bi are dependant on temperature and Sb concentration. However, the effect of temperature on the relationship between Sb concentration and the lattice parameters differs, being more complex for the Sn phase. A concentration of 0.5 wt% Sb was found to result in the lowest energy of formation during the dissolution/precipitation of Bi.