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1. 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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2. Deformation-induced HCP phase transformation of CrCoNiSi0.3 medium-entropy alloy under high strain rate tension

   https://doi.org/10.1063/5.0202924  

   Chang, Hui; Zhang, Tuanwei; Qiao, Junwei; Liaw, Peter K; Jiao, Zhiming; Li, Zhiqiang; Quan, Long; Wang, Zhihua (April 2024, Applied Physics Letters)

   The yield strength of a CrCoNiSi0.3 medium-entropy alloy is significantly increased from 450 MPa (quasi-static, 0.001 s−1) to 1600 MPa (at a strain rate of 5000 s−1) under dynamic tension, with a considerable ductility of 60%. The high strain-rate sensitivity (SRS) of strength and work hardening is obtained, and the strength SRS reaches 0.408. The dominant deformation mechanisms are abundant multiple-twinning, increasing fractions of deformation twins and phase transformation from face-centered-cubic to hexagonal-close-packed (HCP) phases with a strain rate. A universal dislocation-hardened constitutive model considering the evolution of the twin and HCP transformation is established to predict the flow stress and microstructure evolution. 

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3. 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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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. The evolution of deformation twinning microstructures in random face-centered cubic solid solutions

   https://doi.org/10.1063/5.0135538  

   Jagatramka, Ritesh; Ahmed, Junaid; Daly, Matthew (February 2023, Journal of Applied Physics)

   The varied atomic arrangements in face-centered cubic (FCC) solid solutions introduce atomic-scale fluctuations to their energy landscapes that influence the operation of dislocation-mediated deformation mechanisms. These effects are particularly pronounced in concentrated systems, which are of considerable interest to the community. Here, we examine the effect of local fluctuations in planar fault energies on the evolution of deformation twinning microstructures in randomly arranged FCC solid solutions. Our approach leverages the kinetic Monte Carlo (kMC) method to provide kinetically weighted predictions for competition between two processes: deformation twin nucleation and deformation twin thickening. The kinetic barriers underpinning each process are drawn from the statistics of planar fault energies, which are locally sampled using molecular statics methods. kMC results show an increase in the fault number densities of solid solutions relative to a homogenized reference, which is found to be driven by the fluctuations in planar fault energies. Based on kMC relations, an effective barrier model is derived to predict the competition between deformation twinning nucleation and thickening processes under a fluctuating planar fault energy landscape. A key result from this model is a measurement of the length-scale over which the influence of local fluctuations in planar fault energies diminish and nucleation/thickening-dominated behaviors converge to bulk predictions. More broadly, the tools developed in this study enable examination of the influence of chemistry and length-scale on the evolution of deformation twinning mechanisms in FCC solid solutions. 

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