Effects of Al addition on the microstructure, mechanical properties, and age-hardening behavior of high-speed-extruded Mg–Bi alloys

This study investigated the effects of Al addition on the microstructures, tensile properties, and age-hardening behaviors of high-speed-extruded Mg–5Bi alloys. To this end, Mg–5Bi–xAl (x = 0, 2, 4, and 6 wt%) alloys—designated as B5, BA52, BA54, and BA56—were extruded at a die exit speed of 40 m/min. Increasing the Al content from 0 to 6 wt% significantly reduced the average grain size of the extruded alloys from 46 to 25 μm, primarily owing to enhanced grain-boundary pinning caused by the increased formation of Mg₃Bi₂ particles. The increase in Al content from 0 to 6 wt% also enhanced the tensile yield strength from 172 to 205 MPa and ultimate tensile strength from 213 to 292 MPa. These improvements were attributed to grain refinement, Al solute strengthening, and the higher density of Mg₃Bi₂ particles. Tensile elongation also showed a marked improvement, increasing from 2.4 % for the base B5 alloy to 9.2 %, 14.2 %, and 11.9 % for the BA52, BA54, and BA56 alloys, respectively. After peak-aging treatment at 180 °C, the hardness values of the extruded B5, BA52, and BA54 alloys increased by 4.3–6.4 Hv owing to the formation of Mg₃Bi₂ precipitates. Conversely, the BA56 alloy exhibited a greater hardness increase of 13.3 Hv, which was attributed to the combined formation of Mg₁₇Al₁₂ and Mg₃Bi₂ precipitates. These findings demonstrate that Al addition enhances the grain refinement, strength and ductility, and age-hardening capability of high-speed-extruded Mg–5Bi alloys, broadening their potential applications across various industries.