The Swift effect, namely the axial response accompanying torsion, is strongly affected by twinning mechanisms in magnesium alloys, yet detwinning occurs during torsion, and its quantitative connection to the axial response remains insufficiently clarified. In this work, an extruded AZ31 Mg alloy bar is studied under a pre-compression-free-end torsion loading path to tailor various initial {10–12} extension twin fractions. A continuous transition of the Swift effect from axial contraction to axial elongation is observed with increasing pre-strain, indicating the evolutionary change in the dominant twinning-related contribution to the axial response. Moreover, the elastic visco-plastic self-consistent model with twinning and detwinning scheme, together with torsion-specific finite-element approach (TFE-EVPSC-TDT) reproduces the first-order shear response and captures the overall evolution trend of the second-order axial strain. A novel criterion based on the modified global Schmid factor (GSF) for twinning and detwinning under torsion is proposed: the nucleation and growth of {10–12} extension twin occur in grains or twin structures with a positive GSF, while detwinning is favored in prefabricated twins with a negative GSF, and the orientations of pre-twins may promote either re-twinning or detwinning depending on the orientations of the parent grains. In addition, an analytical “twinning-only” upper-bound model is established to quantify the axial contribution of extension twinning under torsion. The analysis indicates that the maximum twinning-related axial contribution reaches ~6.66%, and the remaining deviation of the measured axial strain can be attributed to the additional slip-assisted axial extension that becomes increasingly important as shear straining. The findings in the present work provide a new and significant understanding of the twinning and detwinning mechanism in the Swift effect of Mg alloys.

Grain refinement and precipitation are conventionally employed to enhance the mechanical properties of magnesium alloys. However, there remains a challenge in obtaining a fine grain structure together with a high-density precipitates, particularly in rare-earth containing magnesium alloys. In this study, a strong and ductile Mg-RE (WE43) alloy featuring a fine twin structure and dense nano-precipitates was fabricated via a processing combining multi-directional compression with multi-intermediate aging. The mechanical characterization demonstrated that the fabricated WE43 alloy exhibits an exceptional work-hardening capacity and enhanced ultimate tensile strength, albeit with some compromise in yield strength. Microstructural investigations reveal that the multi-directional compression promotes extensive grain refinement through the formation of nanostructured deformation twins, while the multi-intermediate aging inhibits twin expansion via solutes and precipitates pinning along twin boundaries. Further transmission electron microscopy analysis revealed the formation of high-density nano-precipitates within the matrix. The fine twins and dense precipitation structure strongly promote dislocation multiplication and accumulation, by interaction among dislocations, twin boundaries and nano-precipitates, leading to the significantly improved work-hardening capability and ultimate strength. The current study presents a new approach for the fabrication of rare-earth containing magnesium alloys with high ductility and ultimate strength.

Based on the magnetic-fluid-thermal multiphysics transient coupling numerical simulation results of the magnesium alloy direct-chill (DC) casting, the effects of conventional vibration electromagnetic field (VMF), differential phase vibration electromagnetic field (DP-VMF), conventional low-frequency electromagnetic field (LFMF), and differential phase low-frequency electromagnetic field (DP-LFMF) on melt flow were systematically investigated from the perspective of impulse. Based on thermal behavior and crystal growth theory, the relationships between the velocity field, temperature field, and the morphology of the solidification structure were discussed, and the effect and mechanism of different electromagnetic fields in improving the solidification structure were revealed. Simultaneously, the effects of different electromagnetic fields on AZ31B and AZ80 alloys were investigated. The DC casting experiment verified the theoretical results. Results show that applying low-frequency electromagnetic fields (LFMF and DP-LFMF) can effectively inhibit the formation of columnar grain, but the effect of microstructure refinement is weak; the impact of vibration electromagnetic fields (VMF and DP-VMF) is precisely the opposite. The structure refinement effect of DP-VMF and the inhibition effect of DP-LFMF on columnar grains are better than those of their conventional electromagnetic fields. In the presence of DP-VMF, the average grain size of the center, 1/2 radius, and the edge of the ingot decrease by about 42%, 49%, and 77%, respectively, compared with no electromagnetic field.

Hall-Petch slope (k) is an important material parameter, while there is a great challenge to accurately predict the k value of magnesium alloys due to a high dependence of k on the material parameters, deformation history and testing conditions. The present study demonstrates that machine learning could provide opportunities to overcome this challenge. Two machine learning models, artificial neural network (ANN) and random forest (RF), were built and validated using 138 data. The results showed that increasing the training data set would enhance the prediction efficiency of both models. Comparing to the RF model, the ANN model showed higher accuracy. The correlations between individual attribute and k values were also discussed.