Age-hardening response of Mg-0.3Â at.%Ca alloys with different Zn contents

We have investigated the age-hardening responses and corresponding microstructures of Mg-0.3Ca-xZn (x = 0.0, 0.1, 0.3, 0.6, 1.0, 1.6 at.%) alloys by hardness test, transmission electron microscopy (TEM), and high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM). Zn additions up to x = 0.6 lead to enhanced age-hardening responses with the highest peak hardness of HV = 69 for x = 0.6. Further addition of Zn degraded the age-hardening responses. HAADF-STEM images revealed that the finely dispersed monolayer G.P. zones with internal ordered structure are the major contributor to the age-hardening. Excess addition of Zn resulted in the formation of Ca2Mg6Zn3 precipitates suppressing the formation of the ordered G.P. zones.