Elevated-temperature shear strength and hardness of Zn–3Cu–xAl ultra-high-temperature lead-free solders

This study examines the shear strength and hardness of the ultra-high-temperature Zn—3 wt% Cu—4 wt% Al (ZCA34), Zn—3 wt% Cu—5 wt% Al (ZCA35), and Zn—3 wt% Cu—6 wt% Al (ZCA36) solder alloys in the temperature range of 25–225 °C. Evaluation of the data revealed that the ZCA34 alloy, with the lowest aluminum content, had the highest shear yield stress (SYS), ultimate shear strength (USS) and hardness (Hv) at elevated temperatures. This behavior was in contrast to that of the more concentrated ZCA36 alloy which exhibits the best mechanical properties at room temperature. Higher Al contents of the alloys resulted in the evolution of a refined spheroidized structure, and encouraged the four phase transformation α + ε → T ′ + η . The refined structure may give rise to grain boundary sliding at high temperatures, and the four phase transformation results in a lesser amount of the harder ε-phase in the microstructure, thus causing softening. The shear strength of these high temperature solder alloys is found to be much higher than those of the Zn–Sn and Pb–Sn high-temperature solders.