Influence of Ca addition on microstructural characteristics and mechanical properties of Mg–5Bi–3Al alloy extruded at extremely high speed

In this study, the effects of Ca addition on the microstructural characteristics and mechanical properties of a Mg–Bi–Al alloy extruded at high speed were investigated by analyzing extruded Mg–5Bi–3Al (BA53) and Mg–5Bi–3Al–0.5Ca (BAX530) alloys fabricated at a speed of 70 m/min. The homogenized BA53 billet contained fine and coarse Mg₃Bi₂ particles, whereas the homogenized BAX530 billet contained only coarse Mg₂CaBi₂ particles. Both alloys exhibited fully recrystallized microstructures and similar grain sizes upon extrusion at the high speed. Ca-induced texture weakening did not occur in the extruded BA53 alloy because the added Ca was completely consumed during the formation of Mg₂CaBi₂ particles. Abundant fine Mg₃Bi₂ particles were uniformly distributed throughout the extruded BA53 alloy, whereas the extruded BAX530 alloy contained coarse Mg₂CaBi₂ particles. The addition of Ca to the BA53 alloy decreased the tensile strength by ∼10%, primarily due to weakened particle hardening. Under tension, an increased number of {10–11} contraction and {10–11}-{10–12} double twins were formed and a larger number of twin variants were activated in the extruded BAX530 alloy than in its Ca-free counterpart; consequently, Ca addition enhanced the tensile elongation of the extruded alloy by ∼21% via suppressing the local strain concentration. Thus, the addition of a small amount of Ca can enhance the tensile ductility of high-speed-extruded Mg–Bi-based alloys with high Bi contents without altering the alloy texture.