Effect of rare earth element additions on the impression creep of Sn-9Zn solder alloy

摘要

Creep behavior of the Sn-9Zn-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 eutectic Sn-9Zn alloy. The tests were carried out under constant punching stress in the range 40-135 MPa and at temperatures in the range 298-420 K. Results showed that for all loads and temperatures, Sn-9Zn-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-RE second phase precipitates which act as the main strengthening agent in the RE-containing Sn-Zn alloys. RE contents higher than 0.25 wt.%, resulted in a lower creep resistance due to a reduction in the volume fraction of Zn-rich phase caused by the formation of Sn-Zn-RE intermetallics. The average stress exponents of 6.8, 6.9, 7.1, 6.8 and activation energies of 42.6, 40.6, 43.0 and 44.9 kJ mol~(-1) were obtained for Sn-9Zn, Sn-9Zn-0.1RE, Sn-9Zn-0.25RE, and Sn-9Zn-0.5RE, respectively. These activation energies were close to 46 kJ mol~(-1) for dislocation climb, assisted by vacancy diffusion through dislocation cores in the Sn. This, together with the stress exponents of about seven suggests that the operative creep mechanism is dislocation climb controlled by dislocation pipe diffusion.