Recrystallization behavior and microstructure evolution of Mg–5Bi–3Al alloy during very high-speed extrusion

In our previous study, we extruded Mg–5Bi–3Al (BA53, wt.%) alloy at a very high speed of 70 m/min, and the high-speed-extruded alloy exhibited an unusual fine grain structure. Since dynamic recrystallization (DRX) behavior determines the microstructure and corresponding mechanical properties of the extruded alloy, understanding its origin is crucial for further improvement and optimization of alloys. Herein, the DRX behavior during high-speed extrusion of the recently developed BA53 alloy was investigated in detail by analyzing the microstructure and texture of the extrusion butt along the extrusion path obtained by water quenching the remaining part of the billet immediately after extrusion. During the initial stages of extrusion with a high extrusion speed of 70 m/min and a high temperature of 400 °C, tension twinning predominantly occurred in grains with a high Schmid factor (SF), switching the c-axis of the grains toward the transverse direction of the butt and causing the formation of a ring basal texture. In the intermediate stage, the microstructural changes were dominated by continuous and discontinuous DRX, resulting in the formation of fine recrystallized grains and orientation distribution originated from the parent grain. DRX was completed in the late stage wherein unrecrystallized grains with low SF for basal slip contributed to the late-stage recrystallization, strengthening the ring basal texture. Additionally, DRXed grains with favorable orientation for basal slip underwent lattice rotation, contributing to the formation of the obvious ring basal texture. Finally, preferential grain growth occurred after the DRX, leading to an increase in texture intensity and grain size of the extruded alloy. Therefore, the fine grain structure and ring basal texture obtained through high-speed extrusion were primarily attributed to the activated discontinuous DRX facilitated by a high strain rate and temperature.