Crystallographic orientation and variant selection of trimodal microstructure in dual-phase Mg-Li alloy after solid solution treatment

The orientation relationship (OR) between precipitated needle-shaped α'-Mg and matrix β-Li in a dual-phase Mg–Li alloy has been investigated. Most α'-Mg within the interior of β-Li matrix (α_M'-Mg) follow the Burgers OR: $\left\{0001\right\}\parallel \left\{110\right\},〈11\overline{2}0〉\parallel 〈111〉$, while α'-Mg at phase boundaries (α_PB'-Mg) deviate from the Burgers OR, forming specific angles with the original α-Mg in the common $〈11\overline{2}0〉$ direction. When the OR between the original α-Mg and β-Li is closer to the Burgers OR, more α_PB′-Mg with the same orientation as the original α-Mg precipitate, which generates a strong variant selection for α_M'-Mg that follows the Burgers OR. These α_M'-Mg preferentially precipitate when they share common $〈11\overline{2}0〉$ direction with α'-Mg that deviate from the Burgers OR to minimize overall strain. With increasing solid solution time, the increased α_M'-Mg that deviate from the Burgers OR reduces deformation compatibility across phase boundaries, leading to diminished ductility.