Unexpected effect of pre-aging prior to extrusion on microstructure and mechanical properties of extruded Mg–8Sn–6Zn–0.1Mn alloy

Abstract

Pre-aging prior to extrusion (AE), aiming to achieve high strength without sacrificing ductility, has shown considerable potential in Mg-Sn alloys containing low content of Zn element. For Mg–8Sn–6Zn–0.1Mn alloy containing high contents of Sn and Zn elements, however, an unexpected decrease in mechanical properties after AE treatment is observed and comprehensively investigated in this paper. We demonstrate that after pre-aging treatment, the size and volume fraction of bulk Mg2Sn particles are not decreased as expected, but some fine particles are segregated around the bulk Mg2Sn phases due to high contents of Sn and Zn in this alloy. The bulk phases are prone to facilitate the occurrence of particle-stimulated nucleation and continuous dynamic recrystallization mechanisms during extrusion. Since pre-aging treatments produce a high number of fine particles, these particles exert a pinning or drag effect on grain boundaries, inhibiting the nucleation of dynamic recrystallization, which reduce the volume fraction of dynamic recrystallized grains (DRXed). Unexpectedly, solution-extruded (SE) alloy has finer and higher volume fraction of DRXed grains than AE alloy. Additionally, the fine particles produced by dynamic precipitation in SE alloy are densely distributed and smaller than those in AE alloy. The coarse particles in AE alloy serve as the crack initiation sites under external force, and reduce the ductility of the alloy. Consequently, SE alloy with excellent strength-ductility synergy can be attributed to a combination of refinement of grain structure and fine densely distributed particles.