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1. 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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2. 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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3. Visualization and validation of twin nucleation and early-stage growth in magnesium

   https://doi.org/10.1038/s41467-021-27591-z  

   Jiang, Lin; Gong, Mingyu; Wang, Jian; Pan, Zhiliang; Wang, Xin; Zhang, Dalong; Wang, Y. Morris; Ciston, Jim; Minor, Andrew M.; Xu, Mingjie; et al (December 2022, Nature Communications)

   Abstract The abrupt occurrence of twinning when Mg is deformed leads to a highly anisotropic response, making it too unreliable for structural use and too unpredictable for observation. Here, we describe an in-situ transmission electron microscopy experiment on Mg crystals with strategically designed geometries for visualization of a long-proposed but unverified twinning mechanism. Combining with atomistic simulations and topological analysis, we conclude that twin nucleation occurs through a pure-shuffle mechanism that requires prismatic-basal transformations. Also, we verified a crystal geometry dependent twin growth mechanism, that is the early-stage growth associated with instability of plasticity flow, which can be dominated either by slower movement of prismatic-basal boundary steps, or by faster glide-shuffle along the twinning plane. The fundamental understanding of twinning provides a pathway to understand deformation from a scientific standpoint and the microstructure design principles to engineer metals with enhanced behavior from a technological standpoint. 

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4. (2021) Accounting for the Effects of Dislocation Climb Mediated Flow in Mg Alloy ZK10 Sheet. In: Miller V.M., Maier P., Jordon J.B., Neelameggham N.R. (eds) . The Minerals, Metals & Materials Series. Springer, Cham. https://doi.org/10.1007/978-3-030-65528-0_6

   https://doi.org/10.1007/978-3-030-65528-0_6  

   Ritzo, Michael A.; Agnew, Sean R. (February 2021, Magnesium Technology 2021)

   Miller, Victoria M.; Maier, Petra; Jordon, J. Brian; Neelameggham, Neel R. (Ed.)

   Tensile samples of an Mg alloy ZK10 sheet were tested at a range of temperatures and strain rates designed to rather evenly probe a range of Zener Hollomon parameter values, from ln(Z) ≈ 15 (10–4 s−1 and 623 K) up to ln(Z) ≈ 50 (10–3 s−1 and 300 K). In contrast with more commonly examined Mg alloy AZ31B sheet material, ZK10 sheet material shows modest strain anisotropy (r-value) at low temperatures for both 45° (r45 ≈ 1.2) and TD (rTD ≈ 1.4) sample orientations, despite showing evidence of significant prismatic slip of dislocations, which often leads to high r-values at low temperatures. These low r-values become even lower (r45 ≈ 0.84 and rTD ≈ 0.89) at high temperatures. These behaviors are hypothesized to occur due to a distinct initial texture and deformation mechanism activity, which includes a modest level of tensile twinning and  slip at both room and elevated temperature. A version of the viscoplastic self-consistent (VPSC) code, which accounts for the kinematics of dislocation climb, is used to simulate the behavior of a textured Mg alloy ZK10 sheet reveals that both the glide of pyramidal  dislocations and the climb of basal < a > dislocations are required to describe the behavior at elevated temperatures. 

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5. An Investigation into the Role of Dislocation Climb During Intermediate Temperature Flow of Mg Alloys

   https://doi.org/10.1007/978-3-030-36647-6_19  

   Ritzo, Michael A; Bhattacharyya, Jishnu J; Lebensohn, Ricardo A; Agnew, Sean R (January 2020, Magnesium Technology 2020)

   Textured Mg alloy sheet samples were tensile tested parallel to the transverse direction, at Zener–Hollomon parameter values ranging from Z ~ 50 at room temperature and 10−3 s−1 down to Z ~ 18 at 350 °C and 10−5 s−1. At high Z, the samples exhibit strong texture evolution indicative of significant prismatic slip of dislocations with  Burgers vectors. Correspondingly, the plastic anisotropy is high, r ~ 4. At low Z, the texture evolution is minimal and the response is nearly isotropic, r ~ 1. Previously, it has been asserted that the high ductility and low plastic anisotropy observed at low Z conditions is due to enhanced activity of non-basal slip modes, including prismatic slip of  dislocations and pyramidal slip of  and  dislocations. The present results call this understanding into question and suggest that the enhanced ductility is more closely associated with the climb of  dislocations. 

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