Microstructure and creep characteristics of Zn–3Cu–xAl ultra high-temperature lead-free solders

This study investigates the microstructure and impression creep behavior of the ultra high-temperature Zn–3Cu–4Al (ZCA34), Zn–3Cu–5Al (ZCA35), and Zn–3Cu–6Al (ZCA36) solder alloys under constant punch stress in the range of 70–800 MPa and at temperatures in the range of 345–495 K. The results showed that for all loads and temperatures, increasing Al content of the alloys results in higher creep rates, and thus lower creep resistances. This is attributed to the partial spheroidisation of the lamellar eutectic structure, and the four phase transformation α+ε→T′+ηα+ε→T′+η. The former may ease grain boundary sliding at high temperatures and the latter results in a lesser amount of the more creep resistant ε-phase. The stress exponents of 5.0–7.9 and activation energy values of 52.5–100.3 kJ mol−1, are indicative of a dislocation climb controlled creep mechanism. The observed increasing trend of creep activation energy with temperature means that two parallel mechanisms of lattice-diffusion-controlled and pipe-diffusion-controlled dislocation climb are competing. Dislocation climb controlled by pipe diffusion is the rate controlling mechanism at low temperatures, whereas dislocation climb controlled by lattice diffusion is the dominant creep mechanism at high temperatures.

► Increasing Al content of Zn–3Cu–xAl deteriorated creep resistance. ► Lower creep resistance was due to the four phase transformation α+ε→T′+ηα+ε→T′+η. ► At low temperatures creep mechanism was pipe-diffusion-controlled dislocation climb. ► The high temperature creep mechanism was dislocation climb controlled by lattice diffusion.