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In this study, we investigated the microstructures, hydrogen absorption kinetics, and surface oxides of a Ti0.2Zr0.2Nb0.2V0.2Cr0.17Fe0.03 high entropy alloy (HEA). The prepared HEA had a dual phase microstructure consisting of body-centered cubic (BCC) phase (32.2Ti-13.1Zr-30.3Nb-18.6 V-5.8Cr) and face-centered cubic (FCC) phase (11.3Ti-19.6Zr-13.2Nb-24.7 V-25.7Cr-5.5Fe). The HEA ingot absorbed hydrogen under a hydrogen pressure of 5 bar at room temperature without any thermal activation process. After hydrogenation, the FCC and BCC phases were transformed to a monohydride and a dihydride phase, respectively. To examine the hydrogen absorption behavior of each phase, two ingots having the same compositions as the BCC and the FCC phases were separately prepared. Though the BCC phase ingot did not react with hydrogen, the FCC phase ingot absorbed hydrogen, which could result from the formation of a highly reactive oxide layer on the FCC phase ingot. From the X-ray photoelectron spectroscopy results of the two ingots, although the BCC phase ingot contained Cr, no Cr was detected in the oxide layer. In contrast, the oxide layer on the FCC phase ingot displayed a high Cr concentration, and it seems that the reactivity of the oxide layer with hydrogen could be improved by the presence of Cr in the surface oxides.

This study examines the insulation coating technology of Fe–Si–Cr powder via selective oxidation annealing, which oxidizes elements selectively by controlling the oxidation potential. The study calculated the oxidation driving force of Fe, Si, and Cr, and conducted a thermodynamic analysis of oxidation and reduction conditions according to temperature and oxidation potential. Based on the results, a selective oxidation annealing was performed in an atmosphere in which Fe is reduced and only Si and Cr are selectively oxidized. The oxidation potential was controlled through the partial pressure ratio of hydrogen and water vapor. The XPS analysis results confirmed that a Si and Cr complex oxide layer formed on the powder surface after the selective oxidation annealing. Afterward, withstanding voltages were analyzed to evaluate the insulation property. Then, the withstanding voltage of the powder applying the selective oxidation annealing increased significantly compared to that of the initial powder. Further analysis showed that the powder annealed in an air atmosphere had a significantly lower saturation magnetic flux density than the initial powder, while the powder applying the selective oxidation annealing had only a slightly reduced saturation magnetic flux density.