Improved age-hardening behavior of Mg–Sn–Mn alloy by addition of Ag and Zn

The effect of 0.5 at% Ag and its combination with 0.4 at% Zn on the precipitation hardening behavior of Mg–1.5Sn–0.5Mn (at%) alloy have been investigated using hardness measurements and transmission electron microscopy (TEM). The results show that sole addition of Ag can remarkably improve the peak hardness from 58HV to 77HV after ageing at 200 °C, which is attributed to the increased number density, the size and so the volume fraction of the Mg2Sn lath precipitates. A combination addition of Ag and Zn further improves the peak-hardness to 83HV and the time to peak hardness has been reduced from 120 h to 100 h. The joint addition of Ag and Zn both increases the number density and refines the Mg2Sn precipitates. Zn also promotes the Mg2Sn precipitates lying on the non-basal plane of matrix, as well as the blocky Mg54Ag17 phase which has an orientation relationship with α-Mg matrix as (0 0 1)ɛ′//(0 0 0 1)α, [2 0 0]ɛ′//[−1 −1 2 0]α, [0 −2 0]ɛ′//[1 −1 0 0]α. In the peak-aged Ag modified alloys, Ag atoms are segregated in the Mg2Sn precipitates. Nucleation, growth and coarsening of the Mg2Sn precipitates simultaneously occur as the ageing proceeds. The growth of Mg2Sn precipitates exhibits a significant anisotropic behavior. Addition of Ag can promote the precipitation process during the underage period and retarding the growth of Mg2Sn during the overage period, leading to a more thermal stable microstructure of the Ag and Ag + Zn modified alloys, from which only a small hardness decrease after 700 h ageing is detected.