Grain refinement and mechanical enhancement in high-speed-extruded Mg–5Bi–3Al Alloy via Si microalloying

This study investigates the influence of minor Si addition on the microstructural evolution and mechanical properties of a high-speed-extruded Mg–5Bi–3Al (BA53, wt.%) alloy processed at a die-exit speed of 67 m/min. A comparative evaluation is performed between the base BA53 and its Si-added counterpart, Mg–5Bi–3Al–0.5Si (BAS530). The addition of 0.5 wt% Si leads to the formation of coarse Chinese-script-shaped Mg₂Si intermetallics during solidification. These brittle particles subsequently fragment and align along the extrusion direction, forming continuous particle bands that impose a strong pinning effect on migrating grain boundaries, resulting in a 38 % reduction in average grain size. Both extrudates exhibit fully recrystallized microstructures and strong basal textures, with Si addition having a minimal impact on crystallographic orientation. BAS530 exhibits improved tensile and compressive properties, with a modest rise in tensile yield strength (+7 MPa) and a more notable increase in compressive yield strength (+26 MPa), resulting in reduced tension–compression yield asymmetry compared to BA53. These enhancements are primarily attributed to grain refinement, although particle strengthening decreases due to the reduced number density of submicron Mg₃Bi₂ particles. Additionally, the finer grains in BAS530 increase the twinning activation stress and hinder twin propagation under compression, leading to a lower twin area fraction than in BA53. Overall, the minor addition of Si provides an effective approach to improving both tensile and compressive strength and to reducing tension–compression anisotropy in high-speed-extruded Mg–Bi–Al alloys without compromising ductility.