Breaking the strength-ductility trade-off of magnesium alloys via the twinning-DRX symbiosis cycle under cross-critical temperature ARPT processing

The strength-ductility trade-off in magnesium alloys, in which an increase in strength is accompanied by a decrease in ductility, has long restricted their widespread application. Based on the self-proposed alternating ring-groove pressing and torsion (ARPT) process, experiments were conducted at 250 °C and 300 °C, which straddle the critical temperature for dynamic recrystallization (DRX) of AZ31 magnesium alloy. The mechanism of the “twinning-recrystallization symbiosis cycle” and its influence on the strength-ductility balance were systematically revealed. Challenging the conventional view that twinning and recrystallization are competing mechanisms, this study proposes and validates a new synergistic evolution mechanism—the “twinning-recrystallization symbiosis cycle”. At 250 °C, twins act as sites for storing deformation energy and as nucleation sites for DRX. In turn, DRX grains further promote secondary twinning under stress, forming a positive-feedback loop of “twinning → stored energy accumulation → DRX nucleation → secondary twinning”, thereby promoting continuous grain refinement and synergistic improvement of strength and ductility. At 300 °C, twinning is no longer observed, and continuous DRX becomes the dominant mechanism, resulting in a fine equiaxed grain structure with an average grain size of approximately 3.4 µm and a high-angle grain boundary fraction of 74.4%. Mechanical tests showed that, compared with the original sheet, the tensile strength of the 300 °C-ARPTed sample increased by 58 MPa and the elongation increased by 6.24 percentage points, successfully breaking through the strength-ductility trade-off in magnesium alloys. This study provides a new theoretical basis and process strategy for the severe plastic deformation of high-performance magnesium alloy sheets.