Role of Zn on the rapid age-hardening in Mg-Ca-Zn alloys

Rapid age-hardening is critical for developing bake-hardenability in heat-treatable wrought magnesium alloys. This study investigates the role of Zn on the rapid age-hardening in Mg-0.3Ca-0.6Zn (at.%) alloy using positron annihilation lifetime spectroscopy (PALS), scanning transmission electron microscopy (STEM), and atom probe tomography (APT). Although trace Zn addition into the Mg-0.3Ca alloy accelerates the age-hardening response, PALS analysis indicates that it cannot be explained by excess quenched-in vacancies, which play a critical role in accelerating the kinetics in aluminum alloys. Instead, smaller trapping sites, i.e., open spaces, were detected in both samples. 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. Therefore, the rapid age-hardening in the Mg-0.3Ca-0.6Zn alloy is attributed to the formation of a large number of Ca-Zn co-clusters facilitated by open spaces.