Impression creep of the rare-earth doped Sn–2%Bi lead-free solder alloy

Creep behavior of the Sn–2Bi–RE alloys containing 0.1, 0.25 and 0.5 wt% rare earth (RE) elements was studied by impression testing and compared to that of the Sn–2Bi alloy. The tests were carried out under constant punching stress in the range 70–190 MPa and at temperatures in the range 298–370 K. Results showed that for all loads and temperatures, Sn–2Bi–0.25RE had the lowest creep rate, and thus the highest creep resistance among all materials tested. This was attributed to the formation of Sn–Bi, Sn–RE, and Sn–Bi–RE intermetallic particles which act as both strengthening agent and grain refiner in the RE-containing Sn–2Bi alloy. RE contents higher than 0.25 wt%, resulted in a lower creep resistance due to the formation of the same intermetallics but with much higher Bi content. This consumes the Bi content of the matrix and reduces the corresponding solid solution hardening, resulting in a lower creep resistance of the material. The stress exponents in the range 8–10.5, 8.4–11.5, 8.8–12.3, 8.4–11.6 and average activation energies of 64.5, 65.1, 67.4 and 68.0 kJ mol⁻¹ were obtained for Sn–2Bi, Sn–2Bi–0.1RE, Sn–2Bi–0.25RE, and Sn–2Bi–0.5RE, respectively. Although these activation energies are close to the activation energy of lattice self diffusion for β-Sn, the relatively high stress exponents of about 8–12 suggests that creep mechanisms associated with dislocation movement such as dislocation creep are prevailing.