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.

Elevated temperatures lead to a reduction in the strength of magnesium-based alloys. At elevated temperatures (200–300 °C), even rare-earth reinforced Mg alloys experience notable strength deterioration. In this paper, the additions of different composition (0.4 and 0.8 wt.%) of Ca or Al in Mg-7Y-3Zn-0.4Mn alloy is added to increase the service temperature. The strength of Mg-7Y-3Zn-0.4Mn is effectively improved by Al element and higher after more Al is added, particularly at 300 °C. Specifically, the ultimate tensile strength (UTS) of Mg-7Y-3Zn-0.4Mn alloy increases from 181 MPa to 213 MPa by adding 0.8 wt.% Al. Remarkably, the UTS declines merely by 36 MPa (from 249 to 213 MPa) between 200 °C and 300 °C. And at both RT and elevated temperature (300 °C), Al alloying effectively improves the EL of Mg-7Y-3Zn-0.4Mn alloy, with improvements from 5.4% to 14.4% and 8.2% to 23.1%, respectively. Al element has dramatically increased the thermal stability and more significant effect at higher temperature. After prolonged annealing at 475 °C for 48 h, the Mg-7Y-3Zn-0.4Mn-0.8Al alloy shows only a limited increase in grain size from 16.9 to 23 µm. Even after annealing at 525 °C for 48 h, the Mg-7Y-3Zn-0.4Mn-0.8Al alloy maintains a grain size of approximately 41 µm. However, the growth of grain in basic alloy and 0.4 wt.% Ca alloy exceeds 150 µm. For microstructure, Ca addition primarily promotes LPSO phase formation and subsequent coarsening. Alloying with Al leads to three distinct microstructural changes: the LPSO phase transitions from interconnected networks to discrete blocks, Al₂Y particles precipitate homogenously, and second phases distribute more uniformly. And the texture is also weakened by Al element. Essential mechanisms of Ca/Al effects on mechanical properties, thermal stability, and microstructure in Mg-7Y-3Zn-0.4Mn alloys are investigated.

Atomistic simulations were adopted to study the solute segregation effect on dislocation transmutation across the {1012} twin boundaries in magnesium. For pure magnesium, the dislocation-twin reaction resulted in the formation of sessile dislocations accompanied by the fast migration of the twin boundary, and no 〈c + a〉 dislocation occurred. With Al segregation, instead, two basal dislocations transmuted into one prismatic 〈c + a〉 dislocation in the twin. Twin migration was significantly impeded, and the resultant twin disconnections stayed localized and had a higher step character than in pure Mg. To reveal the mechanism of the effect of solute segregation, the Peierls barriers of twin disconnections were calculated, and the dynamic evolutions of twin disconnection dipoles were simulated. The results suggested that Al segregation softened the Peierls barrier of twin disconnections but imposed a high pinning force on twin disconnections, thus attenuating their mobility. Moreover, given the same Al segregation, the twin disconnection dipole with a higher step showed greater stability, which explained the presence of localized twin disconnections with a higher step in the cases with Al segregation than in pure magnesium. The solute segregation induced low mobility of twin disconnections contributed to the occurrence of 〈c + a〉 dislocations.

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.