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1. On the intrusion-like co-zone twin-twin structure: an in situ observation

   https://doi.org/https://doi.org/10.1016/j.matlet.2020.129140  

   Culbertson, D.; Yu, Q.; Jiang, Y. (March 2021, Materials letters)

   null (Ed.)

   An in situ optical microscopy combined with ex situ electron backscatter diffraction testing was applied to a pristine single-crystal magnesium specimen under monotonic tension along the c-axis. An intrusion-like co-zone twin-twin structure is observed for the first time at the micron scale. In situ observation reveals that the intrusion-like twin-twin structure consists of multiple twin-twin boundaries (TTBs) and incoherent twin boundaries (I-CTBs) following energetically favorable formation sequences. The initial interaction results in the impinging TTBI and the acute-angle TTBA. In the local junction region on the obtuse angle side, the impinging twinning dislocations (TDs) further deposit near TTBI due to the preferred local twinning shear stress, leading to the incoherent curve of the impinging twin boundary adjacent to TTBI. Shortly after, the barrier twin boundary on the obtuse angle side migrates and encompasses the incoherent impinging twin boundary. The combination of sequential TTBI, TTBA, and I-CTBs formed locally on the obtuse angle side shapes the final configuration of the intrusion-like twin-twin structure at the micron scale. 

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2. In situ TEM observations and molecular dynamics simulations of deformation defect activities in Mg via nanoindentation

   https://doi.org/10.1016/j.jma.2023.08.013  

   Lai, Yi-Cheng; Ying, Yubin; Yadav, Digvijay; Guerrero, Jose; Hu, Yong-Jie; Xie, Kelvin Y. (December 2023, Journal of Magnesium and Alloys)

   In this work, we performed in situ nanoindentation in TEM to capture the real-time 〈c + a〉 dislocation and twinning activities in pure Mg during loading and unloading. We demonstrated that the screw component of 〈c + a〉 dislocations glides continuously, while the edge components rapidly become sessile during loading. The twin tip propagation is intermittent, whereas the twin boundary migration is more continuous. During unloading, we observed the elastic strain relaxation causes both 〈c + a〉 dislocation retraction and detwinning. Moreover, we note that the plastic zone comprised of 〈c + a〉 dislocations in Mg is well-defined, which contrasts with the diffused plastic zones observed in face-centered cubic metals under the nanoindentation impressions. Additionally, molecular dynamics simulations were performed to study the formation and evolution of deformation-induced crystallographic defects at the early stages of indentation. We observed that, in addition to 〈a〉 dislocations, the I1 stacking fault bounded with a 〈1/2c+p〉 Frank loop can be generated from the plastic zone ahead of the indenter, and potentially serve as a nucleation source for abundant 〈c + a〉 dislocations observed experimentally. These new findings are anticipated to provide new knowledge on the deformation mechanisms of Mg, which are difficult to obtain through conventional ex situ approaches. These observations may serve as a baseline for simulation work that investigate the dynamics of 〈c + a〉 dislocation slip and twinning in Mg and alloys. 

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3. Accommodation and formation of {-1012} twins in Mg-Y alloys

   https://doi.org/https://doi.org/10.1016/j.actamat.2020.116514  

   Yaddanapudi, Krishna; Leu, Brandon; M, Arul Kumar; Wang, Xin; Schoenung, Julie M; Lavernia, Enrique J; Rupert, Timothy J; Beyerlein, Irene J; Mahajan, Subhash (February 2021, Acta materialia)

   Schuh, Christopher A (Ed.)

   The {-1012} tensile twins terminating inside the grains of a deformed Mg-Y alloy were investigated by transmission electron microscopy. The crystallographic features of terminating twins and associated slip structures were quantified and correlated. The local stresses developed at a terminating {-1012} twin were computed using crystal plasticity simulations in order to interpret the observed slip patterns. Results indicate that both basal and matrix glide were involved in accommodating the plastic stresses developed in the vicinity of terminating twins. Along the twin boundary, the defect contrast consistent with that of lattice dislocations and twinning partials was observed. Based on these observations, a dislocation reaction is proposed that establishes an interrelationship between the observed matrix glide and {-1012} twinning in Mg-Y alloys. 

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4. 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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5. Structural characteristics of irrational Type-II Twin interfaces

   https://doi.org/10.1016/j.ijplas.2024.104016  

   Mohammed, Ahmed_Sameer Khan; Sehitoglu, Huseyin (July 2024, International Journal of Plasticity)

   The Type-II Twin Boundary (TB) is a critical interface in functional materials whose irrational Miller-index identity has recently drawn significant research interest. This study establishes general structural characteristics of the Type-II twin interface, utilizing TBs in Shape Memory Alloys (SMAs) - TiPd, TiPt, and AuCd - as study targets. It is shown how the irrational identity of each TB is explained by the Terrace-Disconnection (T-D) structural topology. It is proposed that the terrace is the rational-plane nearest to the irrational TB in the reciprocal space, having in- tegral Miller-indices of least magnitude. Crystallographic-registry on this terrace requires non- trivial coherence-strains. A novel kinematic-origin of the coherence-strain is proposed, coming directly from a transformation of the classical twinning deformation-gradient. This trans- formation revealed that the classical twinning-shear partitions into the coherence-strain and a new metric termed the “terrace-shear” . It is shown that the magnitude of shear relating the twin- structure to the matrix is the terrace-shear and not the twinning-shear, contrary to classical un- derstanding. Furthermore, the Burgers vector of the twinning disconnection is shown to be related directly to the terrace-shear. The energy of each Type-II interface is determined from ab initio Density Functional Theory (DFT) calculations. It is shown that the energy-minimal atomic- structure on the terrace requires determination of a “lattice-offset” that is non-trivial and un- known apriori. In summary, this study expounds on T-D topological structure of Type-II twin interfaces, establishing methods to identify rational terraces, coherence strains, ab initio planar TB energies and revealing a unique partitioning of the twinning-shear exhibited by this class of interfaces. 

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