Browsing by Author "Sun, W"

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    Austenite formation kinetics from multicomponent cementite-ferrite aggregates by in situ neutron powder diffraction
    (Australian Institute of Nuclear Science and Engineering (AINSE), 2020-11-11) Wu, Y; Wang, L; Sun, W; Styles, MJ; Studer, AJ; Brechet, Y; Hutchinson, C
    The development of third generation advanced high strength steels (AHSS) as the next generation sheet steel grade is driven by the automotive industry. The key processing step is called ‘intercritical annealing’ at temperatures in the region of the ferrite and austenite two-phase field. The transformed austenite during intercritical annealing will be retained at a metastable state in the final microstructure. Controlling the fraction and chemistry of austenite and resulting mechanical properties is critical for many AHSS. The kinetics of austenite formation depend sensitively on the initial microstructure and annealing conditions. In this talk, we will present detailed kinetic studies of austenite formation from cementite-ferrite aggregate in a range of AHSS grades via in situ neutron powder diffraction at WOMBAT. The quantitative phase analyses highlight that the saturation of transformation kinetics in relation to global equilibrium is affected by the competition between different interface migration. Depending on the relative contribution of cementite dissolution in respect to migrating interface of austenite/ferrite, the incomplete dissolution of enveloped cementite limited by slow diffusion in austenite could result in austenite plateauing below equilibrium, while fast dissolution of matrix cementite could result in austenite plateau above equilibrium. Both contributions need to be considered and modelled to describe the austenite formation kinetics. The experimental and computational work in this contribution would guide future processing and alloy design of AHSS.
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    Nitrogen-rich molybdenum nitride synthesized in a crucible under air
    (American Chemical Society, 2024-03-05) Demura, M; Nagao, M; Lee, CH; Goto, Y; Nambu, Y; Avdeev, M; Masubuchi, Y; Mitsudome, T; Sun, W; Tadanaga, K; Miura, A
    The triple bond in N2 is significantly stronger than the double bond in O2, meaning that synthesizing nitrogen-rich nitrides typically requires activated nitrogen precursors, such as ammonia, plasma-cracked atomic nitrogen, or high-pressure N2. Here, we report a synthesis of nitrogen-rich nitrides under ambient pressure and atmosphere. Using Na2MoO4 and dicyandiamide precursors, we synthesized nitrogen-rich γ-Mo2N3 in an alumina crucible under an ambient atmosphere, heated in a box furnace between 500 and 600 °C. Byproducts of this metathesis reaction include volatile gases and solid Na(OCN), which can be washed away with water. X-ray diffraction and neutron diffraction showed Mo2N3 with a rock salt structure having cation vacancies, with no oxygen incorporation, in contrast to the more common nitrogen-poor rock salt Mo2N with anion vacancies. Moreover, an increase in temperature to 700 °C resulted in molybdenum oxynitride, Mo0.84N0.72O0.27. This work illustrates the potential for dicyandiamide as an ambient-temperature metathesis precursor for an increased effective nitrogen chemical potential under ambient conditions. The classical experimental setting often used for solid-state oxide synthesis, therefore, has the potential to expand the nitride chemistry. © 2024 American Chemical Society.