Improvement in tensile ductility and aging response of high-speed-extruded Mg–5Bi–3Al alloy via trace Zn addition

This study examines the effects of trace Zn addition on the microstructure, tensile properties, and precipitation behavior of a high-speed-extruded Mg–5Bi–3Al (BA53, wt.%) alloy. While this alloy exhibits excellent extrudability and high strength, its low ductility and prolonged aging time to achieve peak hardness limit its commercial viability. To address these limitations, 0.5 wt.% Zn was added to the BA53 alloy to enhance its ductility and reduce its peak-aging time without compromising its strength. Subsequently, BA53 and Mg–5Bi–3Al–0.5Zn (BAZ530) alloys were extruded at 400 °C with a die-exit speed of 40 m/min. Microstructural analysis revealed that trace Zn addition had minimal impact on grain size or Mg 3 Bi 2 particle distribution but reduced the maximum texture intensity of the high-speed-extruded alloy. The BAZ530 alloy demonstrated a 38% increase in tensile elongation compared to the BA53 alloy, with no reduction in tensile yield strength, primarily due to the activation of non-basal slip systems. In addition to improving ductility, Zn addition significantly reduced the peak-aging time at 180 °C from 128 h for the BA53 alloy to 16 h for the BAZ530 alloy, an eightfold decrease, while achieving comparable peak hardness values. This acceleration is attributed to the substitution of Bi atoms by Zn atoms in the Mg 3 Bi 2 phase, which lowered the formation enthalpy and increased nucleation rates. Overall, our findings demonstrate that trace Zn addition effectively enhances tensile elongation and accelerates precipitation in high-alloyed Mg–Bi–Al alloys.