Effect of Y addition on microstructure, tensile properties, and corrosion behavior of high-speed-extruded Mg–5Bi–3Al alloy

This study investigates the effect of Y addition on the microstructure, tensile properties, and corrosion behavior of high-speed-extruded Mg–5Bi–3Al (BA53) alloy. Y-free BA53 and Y-containing BAW53 x alloys ( x = 0.2, 0.5, and 1.0 wt.%) are extruded at a die-exit speed of 67 m/min, and the role of Y is examined with emphasis on second-phase evolution and its influence on mechanical and corrosion behavior. Y addition progressively changes the second-phase characteristics from fine intragranular Mg 3 Bi 2 particles to coarse BiY particles concentrated mainly along grain boundaries, while distinct grain refinement appears only at the highest Y content. The tensile yield strength and ultimate tensile strength decrease slightly with increasing Y content, whereas tensile elongation remains comparable and increases slightly at 0.2 wt.% Y. The strength reduction is mainly associated with the reduced strengthening contribution of fine Mg 3 Bi 2 particles. Longitudinal-section observations near the fracture surface show that coarse BiY particles do not act as dominant crack-initiation sites; instead, fracture remains governed mainly by twin-related cracking, which accounts for the limited change in tensile ductility despite the marked change in second-phase characteristics. The corrosion behavior shows a strongly non-monotonic dependence on Y content. A minor addition of 0.2 wt.% Y markedly improves corrosion resistance, whereas larger additions lead to severe deterioration. The Volta potential difference between BiY and the α -Mg matrix is substantially larger than that between Mg 3 Bi 2 and the α -Mg matrix, indicating a stronger local galvanic driving force. Early-stage and cross-sectional corrosion observations further show that BA53 is more susceptible to widespread Mg 3 Bi 2 -assisted corrosion initiation, whereas high-Y alloys increasingly develop BiY-assisted localized corrosion, corrosion-front propagation along grain-boundary regions, and corrosion-film instability. These results demonstrate that a minor addition of 0.2 wt.% Y provides a favorable combination of high-speed extrudability, strength–ductility balance, and enhanced corrosion resistance in the present BA53-based alloy system.