Precipitation Behavior in Low-alloyed Mg–Ca–Zn Alloys

Abstract

Low-alloyed Mg–Ca–Zn alloys are promising as a base system of heat-treatable wrought alloys because of their rapid age-hardenability. The trace addition of Zn plays a critical role in accelerating the age-hardening response and following precipitation behavior of Mg–Ca binary alloys. This study reports the role of Zn on the rapid age-hardening and precipitation sequence in a Mg–0.3Ca–0.6Zn (at.%) alloy during isothermal aging at 200 °C using positron annihilation lifetime spectroscopy (PALS), scanning transmission electron microscopy (STEM), and atom probe tomography (APT). PALS analysis indicates the absence of excess quenched-in vacancies in the as-quenched condition. Instead, the smaller trapping sites, i.e., open spaces, can facilitate the formation of Ca–Zn co-clusters even in the as-quenched condition. APT analysis reveals that the number density of Ca–Zn co-clusters in the Mg–0.3Ca–0.6Zn alloy increases in the early stage of aging, while that of Ca clusters tends to decrease in the Mg–0.3Ca alloy. These results indicate that the rapid age-hardening is attributed to the formation of a large number of Ca–Zn co-clusters. Microstructure analysis using aberration-corrected STEM provides further insights into the precipitation process of the Mg–0.3Ca–0.6Zn alloy. The atomic structures and stability of precipitates are identified by first-principles calculations. A precise precipitation sequence is established as: S.S.S.S → G.P. zones → η″ → η′ → η′ pairs and stacks/η1 → η.KeywordsMagnesium alloysPrecipitationAge hardeningHAADF-STEMAtom probe tomography