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1. Deviations from Theoretical Orientation Relationship Along Tensile Twin Boundaries in Magnesium

   https://doi.org/10.1007/978-3-030-36647-6_20  

   Leu, Brandon; Arul Kumar, M.; Liu, Y.; Beyerlein, I. J. (April 2020, Magnesium technology)

   Deformation twinning is a prevalent mode of plastic deformation in hexagonal close packed (HCP) magnesium. Twin domains are associated with significant lattice reorientation and localized shear. The theoretical misorientation angle for the most common 1012 tensile twin in magnesium is 86.3°. Through electron backscatter diffraction characterization of twinning microstructure, we show that the twin boundary misorientation at the twin tips is approximately 85°, and it is close to the theoretical value only along the central part of the twin. The variations in twin/matrix misorientation along the twin boundary control the twin thickening process by affecting the nucleation, glide of twinning partials, and migration of twinning facets. To understand this observation, we employ a 3D crystal plasticity model with explicit twinning. The model successfully captures the experimentally observed misorientation variation, and it reveals that the twin boundary misorientation variations are governed by the local plasticity that accommodates the characteristic twin shear. 

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2. 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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3. Critical Shape for the Growth of Grain Boundary Twin Embryos in Mg and Mg Alloys: Crystal Plasticity Modeling

   https://doi.org/10.3390/alloys1020013  

   Su, Yanqing; Arul Kumar, M.; Beyerlein, Irene J. (September 2022, Alloys)

   Application of polycrystalline hexagonal close packed (HCP) metals in engineering designs has been constrained by their anisotropic responses due to twinning and limited plasticity. In deformation, twins most often initiate at grain boundaries (GBs), and thicken and propagate across the grain. In this work, the GB twin embryos in Mg and Mg alloys, and the conditions that influence their propagation are investigated. Using a micromechanical crystal plasticity model, the role of embryo shape on the driving forces prevailing at the embryo boundaries that could support its expansion is studied. The modeled embryos are either planar, extending more in the shear direction than normal to the twin plane, or equiaxed. Results show that the thinner the embryo, the greater the driving forces for both thickening and forward propagation. Alloys with low prismatic-to-basal critical resolved shear stress (CRSS) ratios promote embryo thickening and large CRSS values for the slip mode that primarily accommodates the twin shear encourage propagation. The neighboring grains with orientations that enable local accommodation of the embryo twin shear by pyramidal slip promote forward propagation but have little effect on thickening. When two like embryos lie along the same GB, their paired interaction promotes forward propagation but hinders thickening. 

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4. Twinning stress of type I and type II deformation twins

   https://doi.org/10.1016/j.actamat.2019.07.004  

   Mullner, Peter (July 2019, Acta materialia)

   Recent reports on highly mobile type II twin boundaries challenge the established understanding of deformation twinning and motivate this study. We consider the motion of twin boundaries through the nucleation and growth of disconnection loops and develop a mechanism-based model for twin boundary motion in the framework of isotropic linear elasticity. While such mechanisms are well established for type I and compound twins, we demonstrate based on the elastic properties of crystals that type II twin boundaries propagate in a similar way. Nucleation of a type I twinning disconnection loop occurs in a discrete manner. In contrast, nucleation of a type II twinning disconnection loop occurs gradually with increasing Burgers vector. The gradual nucleation of a type II disconnection loop accounts for the higher mobility of type II twin boundaries compared with type I twin boundaries. We consider the homogeneous nucleation of a disconnection loop, which is adequate for twinning in shape memory alloys with a low-symmetry crystal lattice. For the magnetic shape memory alloy Ni-Mn-Ga, the model predicts twinning stresses of 0.33 MPa for type II twinning and 4.7 MPa for type I twinning. Over a wide temperature range, the twinning stress depends on temperature only through the temperature dependence of the elastic constants, in agreement with experimental results. 

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5. Shape Memory Alloys – Frontier Developments

   https://doi.org/10.1016/B978-0-12-822944-6.00058-X  

   Sehitoglu, Hüseyin; Anlas, Gunay; Mohammed, Ahmed_Sameer Khan (January 2023, Elsevier)

   Aliabadi, Ferri MH; Soboyejo, Winston Wole (Ed.)

   This article is mainly concerned with summarizing the recent discoveries in the deformation physics of shape memory materials with special emphasis on (1) the need to develop fundamental understanding of twinning, slip, shear/shuffle at atomic scales, martensite twin boundary topologies, atomic stacking and stable/metastable fault structures in shape memory materials, and (2) factors that result in performance degradation and accumulation of residual strains leading to fatigue and fracture. The fundamental understanding has benefitted from ab-initio modeling of atomic-electronic structures and the fault energetics of stable/metastable crystal structures. Especially, the slip phenomenon during nucleation of martensite needs further elaboration, as it exercises a strong influence on hysteresis, functional and mechanical degradation of the shape memory alloys Finally, we discourse on studies of fatigue and fracture of shape memory alloys from the literature and outline efforts to explain the complex experimental trends with respect to fatigue thresholds and nucleation. It is in the fatigue area where advancing the understanding of cycle by cycle accumulation of the irreversibilities, the strong orientation dependence of slip resistance (i.e., non-Schmid behavior), complex evolution of elastic moduli, strain sensitive evolution of interfaces with terrace-disconnection energy minimal nanostructures, and asymmetric stress-strain response will lead to the development of a comprehensive model. In the case of fracture, continuum models employ LEFM concepts and the results need corrections for martensite-induced tractions which are rather complex with localized variants. Overall, deeper scientific activities, with potential use of lattice scale theories and ab-initio based empirical atomic potentials, are paramount to advance the field to the next level. 

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