Effect of icosahedral phase on the thermal stability and ageing response of a duplex structured Mg–Li alloy

•I-phase formation improves the thermal stability of the β-Li phase.•Suppression of Li diffusion by I-phase causes age-hardening response of the α-Mg matrix.•Age-softening response of the β-Li phase is due to the Li diffusion.

The formation of I-phase (Mg3Zn6Y, icosahedral structure) can suppress the lithium diffusion from β-Li (Mg solid solution in Li) to α-Mg (Li solid solution in Mg) and then remarkably improve the thermal stability of β-Li. Although the lithium diffusion is suppressed by I-phase pockets, quick precipitation and growth of MgLiZn particles can lead to the age-softening response of β-Li. However, a normal age-hardening response of α-Mg can be maintained. Thus, the macro hardness of the Mg–6%Li–6%Zn–1.2%Y alloy depends on the competition between the age-softening of β-Li and age-hardening of α-Mg.