Natural ageing responses of duplex structured Mg-Li based alloys

Natural ageing responses of duplex structured Mg-6% Li and Mg-6% Li-6% Zn-1.2% Y alloys have been investigated. Microstructural analyses revealed that the precipitation and coarsening process of alpha-Mg particles could occur in beta-Li phases of both two alloys during ageing process. Since a certain amount of Mg atoms in beta-Li phases were consumed for the precipitation of abundant tiny MgLiZn particles, the size of alpha-Mg precipitates in Mg-6% Li-6% Zn-1.2% Y alloy was relatively smaller than that in Mg-6% Li alloy. Micro hardness measurements demonstrated that with the ageing time increasing, the alpha-Mg phases in Mg-6% Li alloy could have a constant hardness value of 41 HV, but the contained beta-Li phases exhibited a slight age-softening response. Compared with the Mg-6% Li alloy, the age-softening response of beta-Li phases in Mg-6% Li-6% Zn-1.2% Y alloy was much more profound. Meanwhile, a normal age-hardening response of alpha-Mg phases was maintained. Tensile results indicated that obvious ageing-softening phenomenon in terms of macro tensile strength occurred in both two alloys. Failure analysis demonstrated that for the Mg-6% Li alloy, cracks were preferentially initiated at alpha-Mg/beta-Li interfaces. For the Mg-6% Li-6% Zn-1.2% Y alloy, cracks occurred at both alpha-Mg/beta-Li interfaces and slip bands in alpha-Mg and beta-Li phases.