Deformation and Fracture of Pb-Sn-Eutectic Under Tensile and Fatigue Loading

Room temperature, displacement-controlled tensile test and displacement-controlled tension/compression fatigue tests were performed on as-cast specimens of eutectic Pb-Sn solder with lamellar microstructure. A range of strain rates were used to identify how failure varies with strain rate. Under tensile loading at high strain rate (6.7 × 10⁻²/s) failure was observed after about 20 percent strain. There was evidence of shear localization, and failure was transgranular with a dimpled fracture surface. The low strain rate (6.7 × 10⁻⁶/s) tensile specimen failed by cavitation. This cavitation coincided with sliding at boundaries between colonies of the eutectic (grain boundaries); there was little evidence of sliding in the high strain rate specimen. Two behaviors were observed in the fatigue tests. At low frequency and high strain range (2 × 10⁻³/s, 4 percent, N_f ≈ 300−1000) numerous intergranular cracks initiated at the surface, eventually linking up and propagating along grain boundaries to the interior. What appeared to be fatigue striations were found on the crack face. At high frequency and low strains (>0.1/s, <1 percent) transgranular failure dominated.