In magnesium alloys, detwinning typically refers to the phenomenon in which the 101 2 twins revert to their original orientation before twinning occurs. This study revealed the correlation between detwinning and macroscopic strain-localized banding in AZ31 (Mg-3Al-1Zn-0.3Mn, wt pct) Mg alloy and explored the origin of this correlation. Using digital image correlation (DIC) and in situ X-ray diffraction analysis, a collective detwinning phenomenon occurring alongside macroscopic strain-localized bands was observed. These strain-localized bands formed and propagated repetitively in the gauge part of the specimen during tensile deformation. The heterogeneous deformation was prominent in the yield plateau region of the stress–strain curve, where intensive detwinning was observed. Quantitative assessment of the local strain performed using correlative electron backscatter diffraction–DIC analysis revealed that the detwinning shear significantly contributes to macroscopic strain localization; however, detwinning alone cannot account for the observed strain localization. This study demonstrated that detwinning occurs collectively, inducing the formation and propagation of macroscopic strain-localized bands during tensile deformation.

In Mg alloys, basal (a ) dislocation slip is the preferential slip system that is activated at room temperature, while non-basal slips are typically difficult to activate owing to their high critical resolved shear stress. Until now, minimal focus has been directed towards the influence of loading direction on slip behavior in pre-twinned AZ31 (Mg-3Al-1Zn-0.3Mn, wt%) Mg alloys. This study employed transmission electron microscopy to demonstrate that non-basal (a ) slips, specifically prismatic and pyramidal I slips, are activated under deformation conditions where de-twinning is difficult in a pre-twinned AZ31 Mg alloy. When the tensile loading direction is parallel to the precompression direction, de-twinning and basal (a ) slip are the primary deformation modes. Conversely, when the tensile loading direction is perpendicular to the precompression direction, where de-twinning is challenging to activate, both basal (a ) and non-basal (a ) slips, such as prismatic and pyramidal I slips, emerge as the primary deformation modes. These results indicate that the pre-twinned AZ31 Mg alloy cannot deform solely through basal (a ) slips, and the activation of either de-twinning or non-basal (a ) slips is necessary to satisfy the von Mises criterion. Our findings in this study demonstrate the impact of non-basal slip activity on macroscopic yield stress and overall deformation, hence enhancing the understanding of magnesium alloy deformation mechanisms.

In this study, we present a distinctive phenomenon of strain localization observed in pre-strained AZ31 alloy during tensile deformation. The strain localization behavior involves the repetitive formation, propagation, and subsequent annihilation of localized deformation bands within the gage section of the specimen. Although this strain localization behavior bears resemblance to the Portevin–Le Chatelier (PLC) effect commonly observed in steels and Al-Mg alloys, it is driven by a fundamentally different mechanism. Through detailed microstructural analysis, we reveal that this unique strain localization is closely associated with the process of de-twinning. Our findings contribute to a deeper understanding of the deformation behavior in magnesium alloys and offer new insights for the strain localization in metals.

The extrudability, microstructural characteristics, and tensile properties of the Mg-5Bi-3Al (BA53) alloy are investigated herein by comparing them with those of a commercial Mg-8Al-0.5 Zn (AZ80) alloy. When AZ80 is extruded at 400 degrees C, severe hot cracking occurs at exit speeds of 4.5 m/min or more. In contrast, BA53 is successfully extruded without any surface cracking at 400 degrees C and at high exit speeds of 21-40 m/min. When extruded at 3 m/min (AZ80-3) and 40 m/min (BA53-40), both AZ80 and BA53 exhibited completely recrystallized microstructures with a < 10-10 > basal texture. However, BA53-40 has a coarser grain structure owing to grain growth promoted by the high temperature in the deformation zone. AZ80-3 contains a continuous network of Mg17Al12 particles along the grain boundaries, which form via static precipitation during natural air-cooling after the material exits the extrusion die. BA53-40 contains coarse Mg3Bi2 particles aligned parallel to the extrusion direction along with numerous uniformly distributed fine Mg3Bi2 particles. AZ80-3 has higher tensile strength than BA53-40 because the relatively finer grains and larger number of solute atoms in AZ80-3 result in stronger grain-boundary and solid-solution hardening effects, respectively. Although BA53 is extruded at a high temperature and extrusion speed of 400 degrees C and 40 m/min, respectively, the extruded material has a high tensile yield strength of 188 MPa. This can be primarily attributed to the large particle hardening effect resulting from the numerous fine Mg3Bi2 particles. (c) 2021 Chongqing University. Publishing services provided by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer review under responsibility of Chongqing University

Herein, the evolution of long-period stacking ordered (LPSO) phases in the as-cast Mg-6Gd-1Zn-0.6Zr (wt.%) alloy are investigated via transmission electron microscopy (TEM) and atom probe tomography (APT). The TEM results reveal that two types of LPSO phase (a bulky interdendritic phase and a plate-like matrix LPSO phase) are formed in the as-cast sample. Most of the LPSO phases are confirmed to be of the 14H type, with a smaller proportion being of the 18R LPSO. Further, the APT results reveal that the composition of the interdendritic LPSO phase is closer to that of the ideal 14H phase compared to the matrix LPSO phase, and both the interdendritic and matrix LPSO phases exhibit a Gd/Zn ratio of 2.5, thereby indicating a deficient Zn content compared to the ideal 14H phase (i.e., 1.3). In addition, the influence of the LPSO phases on the deformation behavior is investigated at different compressive plastic strains using electron backscatter diffraction (EBSD) analysis to reveal twinning and slip behavior during deformation. The results indicate that the LPSO phase induces additional work hardening in the late stage of deformation via the suppression of {101¯1} compressive twinning and the activation of non-basal slip systems.

We investigated the effects of Sn addition on the microstructural characteristics and mechanical properties of an extruded Mg–Bi binary alloy by comparing Mg–5Bi (B5) and Mg–5Bi–4Sn (BT54). Both the extruded alloys exhibit a partially recrystallized grain structure with a strong extrusion fiber texture and numerous Mg3Bi2 precipitates. However, the addition of Sn significantly decreases the average grain size of the extruded alloy from 123.9 to 75.2 µm. The Sn solute atoms inhibit the activity of dislocation slip, which reduces the internal strain energy accumulated in the dynamically recrystallized (DRXed) grains during extrusion. Consequently, this reduced strain energy leads to the decrease in the DRXed grain size owing to weakened grain growth during natural air-cooling. The extruded BT54 alloy exhibits higher tensile strength and ductility than the extruded B5 alloy. The improvement in the strength by the Sn addition is attributed to the combined effects of grain refinement, Sn solute atoms, and increased dislocation density. The formation of {10–11} and {10–11}-{10–12} twins during tension is suppressed by the grain refinement, thereby improving the tensile elongation considerably.

In this study, the effects of the combined addition of Ca and Gd on the microstructure and mechanical properties of rolled Mg alloys are investigated by the addition of 0.5 wt% Ca and 0.5 wt% Gd to a commercial AZ31 alloy. The combined addition of Ca and Gd leads to the formation of undissolved second phases such as Al2Ca and Al2Gd, which promote dynamic recrystallization during rolling via the particle-stimulated nucleation phenomenon. As a result, the rolled AZ31–0.5Ca–0.5Gd (wt%) (AZXG3100) alloy shows a finer recrystallized grain structure than the rolled AZ31 alloy; the average grain size of the former (11.9 µm) is considerably smaller than that of the latter (22.4 µm). The combined addition of Ca and Gd improves the tensile yield strength of the rolled material from 131 to 144 MPa, which is attributed mainly to the combined effects of Hall–Petch hardening enhanced by grain refinement and dispersion hardening induced by the undissolved particles. The tensile elongation of the rolled material also increases from 14.6 to 18.3% upon the combined addition of Ca and Gd, because the activation of twinning during tension is less pronounced in the rolled AZXG3100 alloy owing to its smaller grain size. These results demonstrate that the combined addition of Ca and Gd simultaneously improves the tensile strength and ductility of the rolled AZ31 alloy. Combined addition of Ca and Gd to rolled AZ31 alloy.

Low material cost and high extrudability for ensuring price competitiveness with Al alloys, as well as excellent mechanical properties, are essential for expanding the application range of Mg extrudates. Bi is a promising alloying element for developing extruded Mg alloys that satisfy such requirements. Bi is inexpensive, exhibits a high solubility limit, and forms a thermally stable Mg 3 Bi 2 phase, which improves the commercial viability and enables the high-speed extrusion of Mg–Bi alloys. In this study, the effects of Bi addition on the dynamic recrystallization（DRX） and dynamic precipitation behaviors during hot extrusion of a pure Mg and the resultant microstructure and mechanical properties of the extruded materials were investigated. The addition of 6 wt% and 9 wt% Bi to a pure Mg yielded numerous fine Mg 3 Bi 2 precipitates during the early stage of hot extrusion. Consequently, the area fraction of dynamic recrystallized（DRXed） grains decreased because of DRX-behavior suppression by the Zener pinning effect.However, the DRXed grain size was substantially reduced through the grain-boundary pinning effect.The size and number of undissolved Mg 3 Bi 2 particles in the homogenized billets increased when the Bi content was increased, which resulted in increased DRX fractions owing to the enhanced levels of particle stimulated nucleation. Bi addition yielded considerable strength improvement of the extruded pure Mg. However, the extruded Mg–Bi binary materials were less ductile than the extruded pure Mg material. This lower ductility resulted from the cracking at twins formed in the coarse un DRXed grains of the Mg-6Bi material and the cracking at large undissolved Mg 3 Bi 2 particles in the Mg-9Bi material.

This study demonstrates that the addition of Al to an Mg-5Bi alloy effectively promotes dynamic recrystallization during hot extrusion and significantly improves the mechanical properties of the extruded alloy. When the Al content increases from 0 wt% to 9 wt%, the area fraction of recrystallized grains increases from 69.3% to 100%, which can be attributed to the greater number of recrystallization nucleation sites due to grain refinement of the billet, enhanced particle-stimulated nucleation due to the greater abundance of undissolved particles, and the facilitation of dislocation accumulation due to the lower stacking fault energy. The addition of Al also simultaneously improves the microstructural homogeneity, tensile and compressive strength, ductility, and yield symmetry of the extruded alloys. The tensile yield strength x elongation value of the extruded material dramatically increases from 412 to 3489 MPa.% with the addition of 6 wt% Al to the Mg-5Bi alloy. The variation in dynamic recrystallization and dynamic precipitation behaviors with changes in Al content and the association between the microstructural characteristics and mechanical properties of the extruded alloys are discussed in detail. (C) 2019 Elsevier B.V. All rights reserved.

A novel high-alloyed Mg–5Bi–3Al (BA53, wt%) alloy with extraordinary extrudability and high strength is developed. It is successfully extruded at a die-exit speed of 67 m/min without any hot cracking, which is over 10 times the maximum extrusion speeds of commercial high-alloyed Mg alloys, e.g., AZ80 and ZK60. The BA53 alloy extruded at 67 m/min shows high tensile yield strengths in the as-extruded and peak-aged states (188 and 214 MPa, respectively). These excellent extrudability and high strength are mainly attributed to the formation of thermally stable Mg3Bi2 phase. Undissolved coarse and fine Mg3Bi2 particles effectively suppress grain growth during and after extrusion through grain-boundary pinning, leading to the formation of a relatively fine grain structure. Fine rod-type Mg3Bi2 precipitates formed on the prismatic plane along the <0001>Mg direction during air-cooling after existing the die lead to precipitation hardening. The formation of numerous nanosized Mg3Bi2 precipitates along the <01¯10>Mg direction during subsequent aging improves the material strength without ductility loss. The developed BA53 alloy, which comprises inexpensive alloying elements Bi and Al, can be extensively used for manufacturing extruded Mg products because of its high cost-competitiveness, processing efficiency, and mechanical properties.

A high-strength AZ91 alloy is produced via hot extrusion using flakes fabricated through the rapidly solidified flaky powder metallurgy. The AZ91 alloy flakes have an extremely fine dendritic structure without any second-phase particles owing to the fast cooling rate during solidification; these microstructural features considerably promote dynamic recrystallization and precipitation behaviors during extrusion process. As a result, the AZ91 alloy extruded using the flakes exhibits an almost fully recrystallized microstructure with a very small average grain size of 1.2 µm owing to an increase in the number of nucleation sites for recrystallization, and it shows a high microstructural homogeneity owing to the numerous Mg17Al12 precipitates uniformly distributed throughout the material. This extruded AZ91 alloy has a tensile yield strength of 345 MPa, ultimate tensile strength of 417 MPa, and total elongation of 5.6%. These superior tensile strengths are mainly attributed to the combined effects of precipitation hardening caused by abundant fine precipitates and grain boundary hardening caused by fine recrystallized grains.