High-speed extrusion of dilute Mg-Zn-Ca-Mn alloys and its effect on microstructure, texture and mechanical properties

Three dilute Mg-Zn-Ca-Mn alloys were successfully extruded at 24 m/min and the alloy with lowest Zn content (0.21 wt%) can even be extruded at 60 m/min without any surface defects, which was ascribed to the thermally stable Mg2Ca phase and high solidus temperature (∼620 °C). The alloys extruded at die-exit speed ≥6 m/min showed a fully dynamically recrystallized (DRXed) microstructure and weak rare earth (RE) texture at the position between [21̅1̅4] and [21̅1̅2] parallel to the extrusion direction. Besides, fine Mg2Ca and α-Mn particles dynamically precipitated during extrusion, acting as effective pinning obstacles against the DRXed grain growth via Zener drag effect. Due to the deformation temperature rise with increasing extrusion speeds, the grain size increased gradually, which can be understood from the relationship between DRXed grain size and Zener-Hollomon parameter. The RE texture contributed to high uniform elongation of ∼23%, but the increased grain size (>30 µm) deteriorated post-uniform elongation due to the prevalence of {101̅1} contraction and {101̅1}-{101̅2} double twins during post-uniform deformation.