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Kim Y.,Korea Institute of Science and Technology | Kim Y.,Center for Iron and Steel Research | Reed D.,University of Birmingham | Lee Y.-S.,Korea Institute of Science and Technology | And 4 more authors.
Journal of Alloys and Compounds | Year: 2010

Hydrogen desorption/absorption cycling properties of Ca(BH4)2 + MgH2 mixture having 9.1 wt% of theoretical hydrogen capacity have been investigated. The hydrogenation reaction sequence starting from the dehydrogenated product CaH2 + CaB6 + Mg was thoroughly followed using X-ray diffraction, and that can be summarized as (i) Mg + H2 → MgH2, (ii) 4CaH2 + 3MgH2 → Ca4Mg3H14, (iii) Ca4Mg3H14 + 2CaB6 + 20 H2 → 6Ca(BH4)2 + 3MgH2. The steps (i) and (ii) proceeded rather fast, and the total amount of reabsorbed hydrogen was governed by the step (iii). The kinetics of the step (iii) was critically affected by the temperature of dehydrogenation/hydrogenation reaction, which we mainly attributed to the microstructure of the dehydrogenated product such as particle size. We indirectly proved our hypothesis by showing that inserting high-energy ball-milling process during hydrogenation is more effective than simply increasing the reaction time. Optimal reversibility was achieved from the sample which was dehydrogenated at 673 K and hydrogenated at 623 K. © 2009 Elsevier B.V. All rights reserved.

Ha S.,Center for Iron and Steel Research | Kim S.-J.,Center for Iron and Steel Research | Hong S.,Center for Iron and Steel Research | Yim C.-D.,Korea Institute of Material Science | And 4 more authors.
Materials Letters | Year: 2010

This paper suggests a novel method for improving the ductility of magnesium alloy sheets using a laser scanning treatment combined with a defocusing technique. The crystallographic orientation on both surface regions of the AZ31B magnesium alloy sheet processed using this method was changed from a strong basal texture to an almost random texture. The laser-scanned magnesium alloy sheet showed enhanced tensile elongation of up to 50% with a similar a tensile strength. © 2009 Elsevier B.V. All rights reserved.

Ahn T.-H.,Center for Iron and Steel Research | Oh C.-S.,Korea Institute of Materials Science | Kim D.H.,Center for Iron and Steel Research | Oh K.H.,Center for Iron and Steel Research | And 3 more authors.
Scripta Materialia | Year: 2010

Strain-induced martensitic transformation of metastable austenite was investigated by nanoindentation of individual austenite grains in multi-phase steel. A cross-section prepared through one of these indented regions using focused ion beam milling was examined by transmission electron microscopy. The presence of martensite underneath the indent indicates that the pop-ins observed on the load-displacement curve during nanoindentation correspond to the onset of strain-induced martensitic transformation. The pop-ins can be understood as resulting from the selection of a favorable martensite variant during nanoindentation. © 2010 Acta Materialia Inc.

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