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Browsing by Author "Bai, Yu"

Now showing 1 - 4 of 4
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    A correlation between grain boundary character and deformation twin nucleation mechanism in coarse-grained high-Mn austenitic steel
    Hung, Chang-Yu; Bai, Yu; Shimokawa, Tomotsugu; Tsuji, Nobuhiro; Murayama, Mitsuhiro (Nature Research, 2021-04-19)
    In polycrystalline materials, grain boundaries are known to be a critical microstructural component controlling material’s mechanical properties, and their characters such as misorientation and crystallographic boundary planes would also influence the dislocation dynamics. Nevertheless, many of generally used mechanistic models for deformation twin nucleation in fcc metal do not take considerable care of the role of grain boundary characters. Here, we experimentally reveal that deformation twin nucleation occurs at an annealing twin (Σ3{111}) boundary in a high-Mn austenitic steel when dislocation pile-up at Σ3{111} boundary produced a local stress exceeding the twining stress, while no obvious local stress concentration was required at relatively high-energy grain boundaries such as Σ21 or Σ31. A periodic contrast reversal associated with a sequential stacking faults emission from Σ3{111} boundary was observed by in-situ transmission electron microscopy (TEM) deformation experiments, proving the successive layer-by-layer stacking fault emission was the deformation twin nucleation mechanism, different from the previously reported observations in the high-Mn steels. Since this is also true for the observed high Σ-value boundaries in this study, our observation demonstrates the practical importance of taking grain boundary characters into account to understand the deformation twin nucleation mechanism besides well-known factors such as stacking fault energy and grain size.
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    Investigating the dislocation reactions on sigma 3{111} twin boundary during deformation twin nucleation process in an ultrafine-grained high-manganese steel
    Hung, Chang-Yu; Shimokawa, Tomotsugu; Bai, Yu; Tsuji, Nobuhiro; Murayama, Mitsuhiro (Nature Portfolio, 2021-09-29)
    Some of ultrafine-grained (UFG) metals including UFG twinning induced plasticity (TWIP) steels have been found to overcome the paradox of strength and ductility in metals benefiting from their unique deformation modes. Here, this study provides insights into the atomistic process of deformation twin nucleation at Σ3{111} twin boundaries, the dominant type of grain boundary in this UFG high manganese TWIP steel. In response to the applied tensile stresses, grain boundary sliding takes place which changes the structure of coherent Σ3{111} twin boundary from atomistically smooth to partly defective. High resolution transmission electron microscopy demonstrates that the formation of disconnection on Σ3{111} twin boundaries is associated with the motion of Shockley partial dislocations on the boundaries. The twin boundary disconnections act as preferential nucleation sites for deformation twin that is a characteristic difference from the coarse-grained counterpart, and is likely correlated with the lethargy of grain interior dislocation activities, frequently seen in UFG metals. The deformation twin nucleation behavior will be discussed based on in-situ TEM deformation experiments and nanoscale strain distribution analyses results.
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    SCHEV Open Virginia Advisory Committee (OVAC) Webinar Series Part II: Open Education: Student Success and Faculty Autonomy
    Rebar, Beverly; Soholt, Chris; Bai, Yu; Brown, Anne M.; Thomas, Judith; Davis, Preston (2020-10-23)
    SCHEV Open Virginia Advisory Committee (OVAC) Webinar Series Part II: Open Education: Student Success and Faculty Autonomy Fall 2020 ESL Composition II Handbook Chris Soholt and Yu Bai, Northern Virginia Community College This presentation showcases the ESL Composition II Handbook developed for ESL 41 at the Northern Virginia Community College (NOVA) Loudoun Campus. The Handbook is being piloted in Fall 2020. This presentation includes both the instructor and the students' feedback to the Handbook and insights into the development of OER. We hope that our work will encourage our fellow faculty to embrace, adapt and create OER to reduce the cost for students and to improve the quality of education by making it more personalized and adaptive. Using Open Science Framework (OSF) and GitHub to Promote Student Training and Research Transparency Anne M. Brown, Virginia Tech Open access practices can be a cornerstone of undergraduate research training to encourage best practices with data and research reproducibility. Our research lab utilizes platforms such as GitHub and the Open Science Framework (OSF) consistently and with a structure to train and promote research outcomes and products. In using these tools and introducing them to students early, we are promoting a culture of research training and reproducibility in our students, while also documenting and providing all workflows and tutorials that our students utilize in an open way. This approach provides a digital footprint of student work, strengthening their portfolio and recognition in the field, and making our research and training more transparent. This talk discusses the creation and organization of a research lab centered OSF and GitHub page and how it is used by students and researchers. Privacy and Surveillance in Digital Courseware Judith Thomas, University of Virginia Much digital courseware, including “inclusive access” products, pose a threat to the future of open education. Purporting to address the textbook affordability crisis, publishers have devised automatic billing models for products that gather a lot of student data, which is then put to various uses, including product development and learning analytics. Students must agree to privacy terms in order to access the materials, and have no say in how their data is gathered, analyzed, and used. In this talk we look at a few privacy notices from major vendors and discuss the ethical implications of this type of data capture.
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    Unique transition of yielding mechanism and unexpected activation of deformation twinning in ultrafine grained Fe-31Mn-3Al-3Si alloy
    Bai, Yu; Kitamura, Hiroki; Gao, Si; Tian, Yanzhong; Park, Nokeun; Park, Myeong-heom H.; Adachi, Hiroki; Shibata, Akinobu; Sato, Masugu; Murayama, Mitsuhiro; Tsuji, Nobuhiro (Nature Portfolio, 2021-08-05)
    Tensile mechanical properties of fully recrystallized TWIP steel specimens having various grain sizes (d) ranging from 0.79 μm to 85.6 μm were investigated. It was confirmed that the UFG specimens having the mean grain sizes of 1.5 μm or smaller abnormally showed discontinuous yielding characterized by a clear yield-drop while the specimens having grain sizes larger than 2.4 μm showed normal continuous yielding. In-situ synchrotron radiation XRD showed dislocation density around yield-drop in the UFG specimen quickly increased. ECCI observations revealed the nucleation of deformation twins and stacking faults from grain boundaries in the UFG specimen around yielding. Although it had been conventionally reported that the grain refinement suppresses deformation twinning in FCC metals and alloys, the number density of deformation twins in the 0.79 μm grain-sized specimen was much higher than that in the specimens with grain sizes of 4.5 μm and 15.4 μm. The unusual change of yielding behavior from continuous to discontinuous manner by grain refinement could be understood on the basis of limited number of free dislocations in each ultrafine grain. The results indicated that the scarcity of free dislocations in the recrystallized UFG specimens changed the deformation and twinning mechanisms in the TWIP steel.