Effect of isothermal aging on mechanical properties and strain rate sensitivity of the eutectic Sn-58Bi solder alloy

Mechanical properties and strain rate sensitivity of the eutectic Sn-58Bi solder alloy, before and after isothermal aging at 100 °C for 200 h, were investigated. Results show that the grain coarsening occurs in the Sn-58Bi solder alloy with prolonging the isothermal aging time, and the lamellar microstructure of the Sn-58Bi alloy is changed, in which the Bi-rich phase exhibits network-like morphology. The tensile strength of the thermally aged alloy increases slightly and the percentage elongation at fracture decreases at high strain rates. On the contrary, the tensile strength decreases and the elongation increases at low strain rates. Mechanical properties and fracture mode of both the as-cast and thermally aged Sn-58Bi alloy specimens exhibit a strong dependence on the strain rate under tensile loading. At low strain rates, both types of the alloys exhibit good ductility, but their tensile strength decreases greatly, and the values of the SRS index (m) are much higher than that at high strain rates. Fractographic analysis reveals that the fracture mode of the Sn-58Bi alloy specimens gradually changes from typical ductile fracture to brittle fracture with the decrease of the SRS index, in which high strain rates are used. It is demonstrated that the content of Bi element in the Sn-rich phase increases gradually during isothermal aging process, introducing the increased lattice distortions to the Sn-rich phase, which may in turn affect the deformability of the Sn-58Bi solder alloy at different strain rates.