Obtaining ultra-fine and uniform two-phase structures: Evolution of morphology and phase redistribution in a dual-phase Mg–Li alloy during submerged friction stir processing

Abstract

This study analyzed the microstructure and mechanical properties of a dual-phase Mg–Li alloy produced through submerged friction stir processing. The study focused on the recrystallization and grain refinement of distinct α-Mg and β-Li phases. Two methods were identified for refining the grains. The first method involved dynamic recrystallization in large deformed grains, resulting in recrystallized grains that had minimal impact on the two-phase distribution. The second method utilized a phase transition for the growth of new α-Mg and β-Li grains occurred through heterogeneous nucleation in the other phase. The relationship between the α-Mg and β-Li phases after the phase transition was observed to have an orientation of Mg[11–20]//Li[100], Mg(0–110)//Li(011). Maelstrom-affected zones were carefully observed near the advancing side, where severe recrystallization, phase transition, and second phase refinement occurred in this region. Submerged friction stir processing prevented grain growth due to a high density of phase boundaries and rapid heat dissipation. As a result, micro-nano grains were observed in the maelstrom-affected zone. The ductility of the interface was enhanced by the maelstrom-affected zone. Therefore, the maelstrom flow can improve mechanical performance through grain refinement and redistribution for dual-phase Mg–Li alloys during submerged friction stir processing.