An enhanced age hardening response in Mg–Sn based alloys containing Zn

Additions of Zn and Zn + Na to an age hardenable Mg–1.3 Sn (at.%) alloy have been examined using hardness measurements and transmission electron microscopy. Zn additions resulted in a substantial increase (300%) in the hardening increment after aging at 200 °C but the time to peak hardness was relatively unaffected. The additions were found to have little effect on the number density of precipitates formed and no effect on the identity of precipitates but significant changes to particle morphology were observed. Combined additions of Zn and Na (an element previously illustrated to be an effective microalloying element for the Mg–Sn system) showed features typical of separate additions of Na and Zn: a large increase in the observed hardening increment, an acceleration of the time to peak hardness and a change in particle morphology. Furthermore, the combined Zn + Na additions resulted in synergistic effects on precipitate stability during overaging and the time to peak hardness. The stability of Mg 2Sn precipitates formed in the Mg–Sn–Zn–Na alloy was much greater than those usually observed in the Mg–Sn–Na system and the time to peak hardness was much reduced from ∼58 h in the Mg–Sn–Na alloy to 7 h in the Mg–Sn–Zn–Na alloy.