Nanoscale quantification of slip activity and pre-twin-assisted strength-ductility synergy in AZ31 Mg alloy at cryogenic temperatures

Current research on the cryogenic mechanical behavior of Mg alloys predominantly focuses on macroscopic strengthening effects, while the quantitative plastic deformation mechanisms at the micro-scale remain insufficiently understood. This lack of mechanistic insight poses a major challenge for designing microstructures that can overcome the inherent trade-off to achieve a favorable strength-ductility synergy. Here, high-resolution digital image correlation (HRDIC) combined with relative deformation ratio (RDR) analysis is employed to quantitatively resolve the slip activity in an AZ31 Mg alloy at -180 °C. Furthermore, a structural tailoring strategy via pre-twinning is specifically introduced to alter the deformation pathway. The results show that in the as-extruded alloy, basal slip is strongly suppressed at cryogenic temperatures, contributing only 35.5% to the overall plastic deformation, whereas non-basal slip dominates (64.5%), with first-order pyramidal 〈c+a〉 slip being the most active mode. By introducing a controlled population of {10 ̅ 1 ⁢ 2 } tensile twins through 1.6% pre-compression along the extrusion direction, a simultaneous enhancement in tensile strength (336 MPa) and elongation (17.4%) is achieved compared with the as-extruded counterpart under cryogenic loading. Quantitative analysis reveals that pre-twinning further increases the fraction of non-basal slip to 77.8%, among which second-order pyramidal 〈c+a〉 slip contributes 36.7%. More importantly, this work demonstrates that pre-existing {10 ̅ 1 ⁢ 2 } tensile twins do not merely act as static barriers or grain-refinement units, but actively reconfigure the cryogenic deformation pathway through detwinning-mediated promotion of pyramidal 〈c+a〉 slip. This slip-mode-resolved evidence provides a mechanistic basis for achieving strength-ductility synergy in rare-earth-free Mg alloys under cryogenic loading.