Unusual texture evolution in extruded AZ31 Mg alloy plates with bimodal grain structures

AZ31 Mg alloy plates with bimodal grain structures were fabricated via conventional extrusion under varying temperatures and speeds to investigate the mechanisms governing dynamic recrystallization (DRX) and texture evolution. Although all samples exhibited similar DRXed grain sizes (5.0–6.5 µm) and fractions (76%–80%), they developed distinct c-axis orientations and mechanical properties. The P1 sample (350 °C, 0.1 mm/min) exhibited the lowest yield strength (~192 MPa) but the highest elongation (~18.2%), whereas the P3 sample (400 °C, 0.6 mm/min) showed the highest yield strength (~241 MPa) and the lowest elongation (~14.2%). The P2 sample (400 °C, 0.1 mm/min) demonstrated intermediate behavior (~226 MPa, ~17.7%). These variations were primarily attributed to differences in c-axis orientations, particularly their alignment with respect to the normal direction (ND) and their slight deviation from the extrusion direction (ED). Microstructural analysis revealed that distinct DRX mechanisms were activated under different extrusion conditions. P1 predominantly exhibited twinning-induced dynamic recrystallization (TDRX) and continuous dynamic recrystallization (CDRX), whereas P3 primarily showed CDRX and discontinuous dynamic recrystallization (DDRX). These DRX mechanisms, in combination with the activated slip systems governed by the evolving local stress state, collectively contributed to orientation rotation and texture development. During the early stage of extrusion, tensile strain along the ED promoted basal <a> slip, rotating the c-axes toward the ND. As deformation progressed, compressive strain along the ND became dominant. In P1, basal <a> slip remained active, aligning the c-axes along the ND and forming a smaller angle with the ED. In contrast, P3 exhibited predominant pyramidal <c + a> slip, resulting in a pronounced deviation of the c-axes from the ND and a slightly larger angle relative to the ED. The P2 sample exhibited a transitional texture state between those of P1 and P3.