Cryogenic pretreatment and multi-directional compression mediated multi-variant twinning and strength-ductility synergy in LAZ931 alloy

To clarify the influence of cryogenic pretreatment on the subsequent deformation behavior and mechanical properties, a dual-phase Mg-9Li-3Al-1Zn alloy was subjected to multi-directional compression (MDC) through two processing routes. The non-pretreated group was compressed directly, whereas the cryogenically pretreated group was immersed in liquid nitrogen at −196 °C for 30 min, returned to room temperature (RT), and then compressed under otherwise identical conditions. Both groups received 1, 6, and 9 MDC passes at an engineering strain of 8% per pass and an initial strain rate of 5 × 10 −4 s −1 , The phase constitution remained α-Mg, β-Li, AlLi, and MgZn₂ after processing. The { 10 1 ̅ 2 } extension twin was the principal twinning mode, with twin area fractions of 6.52%, 4.09%, and 6.45% in the non-pretreated group and 1.71%, 15.08%, and 1.42% in the cryogenically pretreated group after 1, 6, and 9 passes, respectively. Only a minor contribution from { 10 1 ̅ 1 } contraction twinning was detected. The nonmonotonic evolution of twin area fraction is consistent with competition among twin nucleation, growth, and partial detwinning under successive changes in the compression direction. R9 was dominated by parent grains containing a single extension-twin variant, whereas all six crystallographically equivalent variants made measurable contributions in C6. The average twin-lamella width decreased from approximately 0.83 μm in R9 to 0.23 μm in C6. Moreover, C6 exhibited a lower but more spatially uniform geometrically necessary dislocation density ( ρ GND ) than R9, together with a representative C14 MgZn₂/α-Mg interface exhibiting the basal-plane alignment ( 0001 ) MgZn 2 ‖ ( 0001 ) α-Mg . C6 achieved an ultimate tensile strength （UTS） of 181.01 MPa, a yield strength (YS) of 102.60 MPa, an elongation to fracture (EL) of 24.82%, and a hardness of 79.96 HV. By comparison, R9 attained higher strengths but a substantially lower elongation of 15.72%. The favorable strength-ductility balance of C6 is associated with closely spaced twin boundaries, broader twin-variant participation, and spatially distributed dislocation storage, which increase the resistance to dislocation motion while limiting severe strain localization.