Effects of Sn addition on the microstructure and mechanical properties of extruded Mg–Bi binary alloy

We investigated the effects of Sn addition on the microstructural characteristics and mechanical properties of an extruded Mg–Bi binary alloy by comparing Mg–5Bi (B5) and Mg–5Bi–4Sn (BT54). Both the extruded alloys exhibit a partially recrystallized grain structure with a strong extrusion fiber texture and numerous Mg3Bi2 precipitates. However, the addition of Sn significantly decreases the average grain size of the extruded alloy from 123.9 to 75.2 µm. The Sn solute atoms inhibit the activity of dislocation slip, which reduces the internal strain energy accumulated in the dynamically recrystallized (DRXed) grains during extrusion. Consequently, this reduced strain energy leads to the decrease in the DRXed grain size owing to weakened grain growth during natural air-cooling. The extruded BT54 alloy exhibits higher tensile strength and ductility than the extruded B5 alloy. The improvement in the strength by the Sn addition is attributed to the combined effects of grain refinement, Sn solute atoms, and increased dislocation density. The formation of {10–11} and {10–11}-{10–12} twins during tension is suppressed by the grain refinement, thereby improving the tensile elongation considerably.