Origin of the age-hardening and age-softening response in Mg-Li-Zn based alloys

The origin of the age-hardening and age-softening response in the Mg-10Li(-Zn)(-Er) alloys is studied. The results show that the age-hardening of the Mg-10Li-5Zn (LZ105) alloy quickly after quenching is due to the precipitation of the semi-coherent (Mg, Li)3Zn phase. The Zn atom occupation of the precipitate is confirmed. Beside precipitate coarsening, Mg atoms in the (Mg, Li)3Zn precipitate are gradually replaced by Li/Zn atoms, primarily accounting for the origin of age-softening. The substitution of Li for Mg is dominant at low aging temperature, while the substitutions of both Li and Zn for Mg are prominent at high aging temperature. The reduced modulus strengthening leads to the strength reduction of the LZ105 alloy during low-temperature aging. High-temperature aging brings about significant coarsening of the precipitate and the strength decreases dramatically. Er addition retards the evolution of the (Mg, Li)3Zn precipitate, resulting in the deceleration of age-softening observed in the LZ105 alloy modified by 0.5 wt% Er addition during aging at 50 °C. The considerable Mg-Zn-Er phases along with trace Er atoms dissolved in the matrix lower the available Zn concentration for (Mg, Li)3Zn precipitation and retard the diffusion of Li and Zn atoms during aging. A novel aging mechanism based on the (Mg, Li)3Zn precipitate evolution in Mg-Li-Zn-based alloys is proposed in this study and provides a theoretical basis for deeply understanding the aging behaviors of Mg-Li-Al/Zn alloys.