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1. Twinning characteristics in rolled AZ31B magnesium alloy under three stress states

   https://doi.org/https://doi.org/10.1016/j.matchar.2021.111050  

   Carneiro, L. ; Culbertson, D. ; Zhu, X. ; Yu, Q. ; Jiang, Y. ( May 2021 , Materials characterization)

   null (Ed.)

   Stress-strain responses and twinning characteristics are studied for a rolled AZ31B magnesium alloy under three different stress states: tension along the normal direction (NDT), compression along the rolled direction (RDC), and torsion about the normal direction (NDTOR) using companion specimens interrupted at incremental strain levels. Tension twinning is extensively induced in twinning-favorable NDT and RDC. All the six variants of tension twin are activated under NDT, whereas a maximum of four variants is activated under RDC. Under NDTOR, both tension twins and compression twins are activated at relatively large strains and twinning occurs in a small fraction of favored grains rather than in the majority of grains. Secondary and tertiary twins are observed in the favorably-orientated grains at high strain levels. Deformation under each stress state shows three stages of strain hardening rate: fast decrease (Stage I), sequential increase (Stage II), and progressive decrease (Stage III). The increase in the hardening rate, which is more significant under NDT and RDC as compared to NDTOR, is attributed to the hardening effect of twin boundaries and twinning texture-induced slip activities. The hardening effect of twin boundaries include the dynamic Hall-Petch hardening induced by the multiplication of twin boundaries (TBs) and twin-twin boundaries (TTBs) as well as the hardening effect associated with the energetically unfavorable TTB formation. When the applied plastic strain is larger than 0.05 under NDT and RDC, the tension twin volume fraction is higher than 50%. The twinning-induced texture leads to the activation of non-basal slips mainly in the twinned volume, i.e. prismatic slips under NDT and pyramidal slips under RDC. The low work hardening under NDTOR is due to the prevailing basal slips with reduced twinning activities under NDTOR. 

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2. Effects of initial {10-12} twins on cyclic deformation and fatigue of magnesium alloy at low strain amplitudes

   https://doi.org/https://doi.org/10.1016/j.matchar.2019.01.017  

   Wang, F. ; Lui, M. ; Sun, J. ; Feng, M. ; Jin, L. ; Dong, J. ; Jiang, Y. ( March 2019 , Materials characterization)

   An extruded AZ31B (Mg-3Al-1Zn-0.5Mn) magnesium alloy with a twin volume fraction of 60% was subjected to fully reversed strain-controlled tension-compression along the extrusion direction at strain amplitudes ranging from 0.23% to 0.45%. Dislocation slips were the dominant plastic deformation mechanisms without involving persistent twinning-detwinning. At an identical strain amplitude, the fatigue life of the pre-twinned alloy was much lower than that of the as-extruded alloy. Fatigue cracks were mainly initiated on the prismatic or prismatic-basal slip bands in the parent grains. The material volume reduction of the parent grains in the pre-twinned alloy enhanced fatigue damage. Twin cracks were not observed. 

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3. Effect of initial texture on fatigue properties of extruded ZK60 magnesium alloy

   https://doi.org/10.1111/ffe.12792  

   Xiong, Y. ; Gong, X. ; Jiang, Y. ( February 2018 , Fatigue & Fracture of Engineering Materials & Structures)

   The effect of initial texture on cyclic deformation behavior of extruded ZK60 magnesium (Mg) alloy was experimentally investigated under strain‐controlled loading with the strain amplitudes at 4%, 1%, and 0.35%. The testing specimens were taken from extrusion direction (ED), transverse direction (TD), and a material precompressed to 9.4% along the ED (ED_−9.4%). At a high strain amplitude of 4%, the cyclic deformation modes of ED and ED_−9.4%specimens are similar, and they experience twinning exhaustion → slip and detwinning exhaustion → slip during each loading cycle. At a medium strain amplitude of 1%, twinning‐detwinning is involved in the cyclic deformation, but different deformation mechanisms were observed in the 3 different specimens. Partial twinning‐complete detwinning mode dominates the cyclic deformation in the ED specimen, while partial detwinning‐retwinning mode occurs in the ED_−9.4%specimen. For the TD specimen, both basal slip andtension twinning occur during cyclic deformation. At a low strain amplitude of 0.35%, dislocation slips dominate the deformation for the ED specimen with a few observable tension twins. For the ED_−9.4%specimen, initially twined texture increases the ductility of the material and enhances fatigue life as compared with the other 2 specimens.

    

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4. Rejuvenation of plasticity via deformation graining in magnesium

   https://doi.org/10.1038/s41467-022-28688-9  

   Liu, Bo-Yu ; Zhang, Zhen ; Liu, Fei ; Yang, Nan ; Li, Bin ; Chen, Peng ; Wang, Yu ; Peng, Jin-Hua ; Li, Ju ; Ma, En ; et al ( December 2022 , Nature Communications)

   Abstract Magnesium, the lightest structural metal, usually exhibits limited ambient plasticity when compressed along its crystallographic c -axis (the “hard” orientation of magnesium). Here we report large plasticity in c -axis compression of submicron magnesium single crystal achieved by a dual-stage deformation. We show that when the plastic flow gradually strain-hardens the magnesium crystal to gigapascal level, at which point dislocation mediated plasticity is nearly exhausted, the sample instantly pancakes without fracture, accompanying a conversion of the initial single crystal into multiple grains that roughly share a common rotation axis. Atomic-scale characterization, crystallographic analyses and molecular dynamics simulations indicate that the new grains can form via transformation of pyramidal to basal planes. We categorize this grain formation as “deformation graining”. The formation of new grains rejuvenates massive dislocation slip and deformation twinning to enable large plastic strains. 

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5. Local microstructure and micromechanical stress evolution during deformation twinning in hexagonal polycrystals

   https://doi.org/10.1557/jmr.2020.14  

   Kumar, Mariyappan Arul ; Beyerlein, Irene J. ( February 2020 , Journal of Materials Research)

   Deformation twinning is a prevalent plastic deformation mode in hexagonal close-packed (HCP) materials, such as magnesium, titanium, and zirconium, and their alloys. Experimental observations indicate that these twins occur heterogeneously across the polycrystalline microstructure during deformation. Morphological and crystallographic distribution of twins in a deformed microstructure, or the so-called twinning microstructure, significantly controls material deformation behavior, ductility, formability, and failure response. Understanding the development of the twinning microstructure at the grain scale can benefit design efforts to optimize microstructures of HCP materials for specific high-performance structural applications. This article reviews recent research efforts that aim to relate the polycrystalline microstructure with the development of its twinning microstructure through knowledge of local stress fields, specifically local stresses produced by twins and at twin/grain–boundary intersections on the formation and thickening of twins, twin transmission across grain boundaries, twin–twin junction formation, and secondary twinning. 

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