Study on hydrogen absorption and surface properties of TiZrVNbCr high entropy alloy

Abstract In this study, we examined the microstructure, hydrogen absorption behavior, and native oxide layer of 20Ti–20Zr–20V–20Nb–20Cr high entropy alloy. It was revealed that the alloy consisted of two phases with the chemical composition of 31.1Ti-19.6Zr-16.6V-26.0Nb-6.7Cr (bright phase) and 9.1Ti-20.3Zr-23.4V-14.1Nb-33.1Cr (dark phase). Even though an oxide layer was formed on the surface of the alloy due to the exposure to an air atmosphere for 10 days, it could absorb hydrogen at a low hydrogen pressure without any thermal activation process. After the hydrogen absorption experiment, not only the hydrogen absorption as high as 1.77 wt% was achieved, but the phase transformation occurred; the bright phase transformed to dihydride and the dark one did monohydride. To clarify which phase mainly contributed to the hydrogen absorption behavior, ingots with chemical composition of the dark and bright phases were prepared separately. The hydrogen absorption occurred only for the dark phase ingot, not for the bright phase ingot, suggesting that only the oxide layer on the dark phase seems to have a reactivity with hydrogen. XPS results revealed that while the Cr element was not detected in the oxide film on the bright phase, the oxide film on the dark phase possessed the Cr element. Based on the results, the presence of the Cr element in the oxide would be ascribed to the high reactivity with hydrogen.