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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. Tenacious turds from the Upper Cretaceous Smoky Hill Member, Niobrara Chalk, Kansas

   Jacquet, Sarah M; Selly, Tara (June 2024, Papers on Paleontology)

   The Smoky Hill Member of the Niobrara Chalk (Late Coniacian to Early Santonian), Kansas is renowned for its diverse assemblage of bromalites including coprolites, enterospirae, and even rare regurgitates. Producers of these fecal products are atributed to sharks, teleost fish and marine reptles based on their variously spiraled and non-spiraled morphologies, containing a range of invertebrate and vertebrate inclusions. Traditonally, examinaton of these fossils has necessitated sub-sampling via consumptve techniques like disaggregaton or dissoluton on either portons or the entrety of the specimen. Three-dimensional imaging techniques such as X-ray tomographic microscopy (µCT) offer a non-destructve alternatve to reveal both macroscopic and microscopic inclusions. Due to the minimal diagenetc alteraton of specimens from this locality, µCT imaging and segmentaton facilitates the extracton of structural and taphonomic informaton potentally obscured by physical extracton methods. This study employes non-destructve methods to explore the diversity of gross morphotypes represented by coprolites and a possible regurgitate from this member alongside their internal structure and inclusions. Preliminary results from segmented specimens offer insights into the taphonomic atributes of the coprolites and their ability to preserve exceptonally delicate structures, with remains of vertebral columns stll partally artculated. Lightly to non-mineralized inclusions, possibly crustaceans and scale remains, represent a hidden component of the assemblage rarely preserved otherwise. Virtual renders also enable quanttatve analysis of the inclusions with respect to the degrees of fragmentaton, the orientaton and alignment of boney inclusions relatve to the longitudinal axis of the specimen, and the relatve proportons of bone, pore space, and phosphatc matrix. This work offers a rare glimpse into the feeding, digestve, and excretory behaviors of producers whilst simultaneously capturing unique paleoecological and paleoenvironmental informaton. 

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3. Synergistic Thermal and Electron Wind Force-Assisted Annealing for Extremely High-Density Defect Mitigation

   https://doi.org/10.3390/ma17133188  

   Rahman, Md Hafijur; Todaro, Sarah; Waryoba, Daudi; Haque, Aman (July 2024, Materials)

   This study investigates the effectiveness of combined thermal and athermal stimuli in mitigating the extremely high-density nature of dislocation networks in the form of low-angle grain boundaries in FeCrAl alloy. Electron wind force, generated from very low duty cycle and high current density pulses, was used as the athermal stimulus. The electron wind force stimulus alone was unable to remove the residual stress (80% low-angle grain boundaries) due to cold rolling to 25% thickness reduction. When the duty cycle was increased to allow average temperature of 100 °C, the specimen could be effectively annealed in 1 min at a current density of 3300 A/mm2. In comparison, conventional thermal annealing requires at least 750 °C and 1.5 h. For specimens with 50% thickness reduction (85% low-angle grain boundaries), the electron wind force was again unable to anneal the defects even at 3300 A/mm2 current density and average temperature of 100 °C. Intriguingly, allowing average concurrent temperature of 200 °C eliminated almost all the low-angle grain boundaries at a current density of 700 A/mm2, even lower than that required for the 25% thickness reduced specimens. Comprehensive electron and X-ray diffraction evidence show that alloys with extremely high defect density can be effectively annealed in less than a minute at approximately 200 °C, offering a substantial improvement over conventional high-temperature annealing. 

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4. Resolving the effects of 2-D versus 3-D grain measurements on apatite (U–Th) ∕ He age data and reproducibility

   https://doi.org/10.5194/gchron-1-17-2019  

   Cooperdock, Emily H.; Ketcham, Richard A.; Stockli, Daniel F. (January 2019, Geochronology)

   Abstract. (U–Th) ∕ He thermochronometry relies on the accurate andprecise quantification of individual grain volume and surface area, whichare used to calculate mass, alpha ejection (FT) correction, equivalentsphere radius (ESR), and ultimately isotope concentrations and age. The vastmajority of studies use 2-D or 3-D microscope dimension measurements and anidealized grain shape to calculate these parameters, and a long-standingquestion is how much uncertainty these assumptions contribute to observedintra-sample age dispersion and accuracy. Here we compare the results forvolume, surface area, grain mass, ESR, and FT correction derived from2-D microscope and 3-D X-ray computed tomography (CT) length and width datafor > 100 apatite grains. We analyzed apatite grains from twosamples that exhibited a variety of crystal habits, some with inclusions. Wealso present 83 new apatite (U–Th) ∕ He ages to assess the influence of 2-D versus 3-D FT correction on sample age precision and effective uranium(eU). The data illustrate that the 2-D approach systematically overestimatesgrain volumes and surface areas by 20 %–25 %, impacting the estimates formass, eU, and ESR – important parameters with implications for interpretingage scatter and inverse modeling. FT factors calculated from 2-D and 3-Dmeasurements differ by ∼2 %. This variation, however, haseffectively no impact on reducing intra-sample age reproducibility, even onsmall aliquot samples (e.g., four grains). We also present a grain-mountingprocedure for X-ray CT scanning that can allow hundreds of grains to be scannedin a single session and new software capabilities for 3-D FT andFT-based ESR calculations that are robust for relatively low-resolutionCT data, which together enable efficient and cost-effective CT-basedcharacterization. 

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5. Synthesis of pigeonites for spectroscopic studies

   https://doi.org/10.2138/am-2019-6869CCBYNCND  

   Lindsley, D. H.; Nekvasil, H.; Glotch, T. D. (January 2019, The American mineralogist)

   Pigeonite (P21/c clinopyroxene) crystallizes in various terrestrial and extraterrestrial rocks. However, because it breaks down (“inverts”) in slowly cooled rocks, bulk natural samples of pigeonite from coarse-grained rocks are not available. We have synthesized eight samples of pigeonite with compositions of Wo8 and Wo10 [where Wo (mol%) = 100Ca/(Ca+Mg+Fe2+)] and X ranging from 20 to 60 [where X = 100Fe2+/(Mg+Fe2+)]. These samples are suitable for spectroscopic and other studies that require bulk samples. Because of relatively fine grain size (mainly 5–50 mm) and slight grain-to grain variation in composition, they are generally not suitable for studies requiring individual crystals. We will make samples available for appropriate investigations, especially if the techniques used are non-destructive and the samples can be returned after use. 

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