Effect of Zn additions on age‐hardening response and precipitate microstructure of Mg–Pb alloys

Abstract The age‐hardening response and related precipitate microstructure in Mg–Pb alloys with different Zn contents have been examined systematically. The age‐hardening response significantly improves with increasing Zn content. Specifically, the addition of 3 wt% Zn results in a twofold increase in hardness increment, from 9.7 to 21.3 HV, during ageing at 160 °C. The precipitation kinetics is also facilitated, leading to a shorter time to reach peak hardness from 3000 to 200 h. The enhanced age‐hardening response is attributed to the threefold increase in the number density of γ‐Mg 2 Pb precipitates and the uniform contribution of rod‐like Zn‐rich precipitates in the Mg–Pb–Zn alloy. In the Zn‐free alloy, γ precipitates typically have a basal lath‐like morphology, whereas in the Mg–Pb–Zn alloy, prismatic and pyramidal γ lath precipitates are also frequently observed, leading to the formation of two types of novel orientation relationships. Zn solute atom segregation is detected at the γ precipitate/matrix interface, which slows the coarsening rate, refines precipitate size, and hence increases the number density of γ precipitates by a factor of three in the Mg–Pb–Zn alloy. Additionally, each γ precipitate always contacts with a rod‐like precipitate in the peak‐aged samples. Microstructure characterization performed at the early stage of ageing suggests that rod‐like Zn‐rich precipitates act as abundant heterogeneous nucleation sites for γ‐Mg 2 Pb precipitates, which should be another reason for the refinement of γ precipitate in the Zn‐containing alloys. These results provide new insights into the development of novel precipitation‐hardenable Mg alloys via micro‐alloying elements.