Effects of Pb/Sn additions on the age-hardening behaviour of Mg–4Zn alloys

The effects of Pb/Sn additions on the age-hardening responses and corresponding microstructures of a Mg–4Zn alloy (wt%) were investigated by hardness test and transmission electron microscopy (TEM). Sn additions significantly refine the precipitates and lead to enhanced age-hardening response of Mg–4Zn alloys, while the addition of Pb slows down the ageing kinetics. The yield stress of peak-aged Mg–4Zn significantly improved from 87 MPa to 129 MPa through the addition of Sn, but slightly decreased to 84 MPa with the addition of Pb. TEM showed that Mg–4Zn with Pb-added alloys consist of two main precipitates, rod/lath-like β 1 ʹ phase and plate-like β 2 ʹ phase. The precipitates in the peak-aged Mg–4Zn–1Pb alloy have much lower number density and larger size than that of the peak-aged Mg-4Zn alloy. In contrast, Sn-containing alloys show much finer precipitates, and high melting temperature Mg2Sn precipitates formed when the Sn content increased to 3 wt%. Cliff–Lorimer plots of Mg–4Zn–1Pb and Mg–4Zn–3Sn alloys indicate that no Pb atoms preferentially exist in the precipitates, while Zn and Sn atoms are incorporated in the rod/lath-like β 1 ʹ precipitates and Mg2Sn precipitates.