The effect of micro-alloying addition of Cr on age hardening of an Mg–Zn alloy

A trace amount of Cr is soluble in the magnesium lattice in the presence of Zn. The presence of Cr does not notably affect the constituent particles, but it significantly improves the age hardening response of Mg–Zn alloy by increasing the number density of the precipitates. As a result of Cr addition, the hardness increment produced by artificial ageing at 160 °C is almost doubled compared to binary Mg–2.8 at.% Zn alloy. In particular, Cr noticeably accelerates the kinetics of precipitation and the near-peak hardness can be reached after only 4 h of ageing. Cr also moderately accelerates the natural ageing. The highest level of hardening in Mg–Zn–Cr alloy is produced by ageing at intermediate temperatures (e.g. 70 °C). Cr represents a suitable novel micro-alloying addition for the development of alloys with improved age hardening response that is particularly effective in enhancing the nucleation of precipitates.