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 β1' phase and plate-like β2' 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 β1' precipitates and Mg2Sn precipitates.