Comparative study of dynamic recrystallization behavior, microstructural characteristics, and mechanical properties of high-speed-extruded AZ31 and BA56 magnesium alloys

Abstract

This study compares the microstructural evolution, dynamic recrystallization (DRX) behavior, tensile properties, and age-hardenability between the newly developed high-speed-extrudable BA56 alloy and those of the widely recognized AZ31 alloy in industry. Unlike the AZ31 alloy, which retains partially unrecrystallized grains, the high-speed-extruded BA56 alloy demonstrates a coarser but entirely recrystallized and more homogeneous microstructure. The fine-grained structure and abundant Mg₃Bi₂ particles in the BA56 extrusion billet significantly enhance its DRX behavior, thus enabling rapid and complete recrystallization during extrusion. The BA56 alloy contains band-like fragmented Mg₃Bi₂ particles and numerous fine Mg₃Bi₂ particles distributed throughout the material, in contrast to the sparse Al₈Mn₅ particles in the AZ31 alloy. These features contribute to superior mechanical properties of the BA56 alloy, which achieves tensile yield and ultimate tensile strengths of 205 and 292 MPa, respectively, compared to 196 and 270 MPa for the AZ31 alloy. The superior strength of the BA56 alloy, even with its coarser grain size, can be explained by its elevated Hall-Petch constant and the strengthening contribution from the fine Mg₃Bi₂ particles. Additionally, the BA56 alloy demonstrates significant age-hardenability, achieving a 22% enhancement in hardness following T5 aging, attributed to the precipitation of nanoscale Mg₃Bi₂ and Mg₁₇Al₁₂ phases. By contrast, the AZ31 alloy shows minimal hardening due to the absence of precipitate formation during aging. These findings suggest that the BA56 alloy is a promising candidate for the production of extruded Mg components requiring a combination of high productivity, superior mechanical performance, and wide-ranging process adaptability.