Formation of Zr-rich BCC phase and its relation on the hydrogen storage properties of TiVNbZr high entropy alloy

Abstract

Due to the large difference in atomic size between Zr and Nb, TiVNbZr high entropy alloy possesses greater lattice distortion. The change of Zr content can affect the stability of hydrides, which can have an important impact on the hydrogen storage performance. In order to explore the effect of Zr/Nb on hydrogen storage properties, Ti32.5V27.5Nb40-xZrx (x = 7.5, 10, 12.5, 15) high entropy alloys composed of BCC phase are designed. It is found that with the increase of Zr/Nb, the dendritic microstructures first disappear and then gradually become coarse. The Zr-rich BCC phase and the Nb-rich BCC phase appear in the interdendritic and dendritic regions, respectively. Ti32.5V27.5Nb25Zr15 shows excellent activation performance, which is related to the Zr-rich BCC phase with the most catalytic phase on the surface. The hydrogen storage performance of low Zr/Nb composition is better at low temperature, and that of high Zr/Nb composition becomes better after rising to high temperature. This is because with the change of Zr/Nb, low temperature can promote the hydrogen absorption reaction and high temperature can increase the diffusion rate of hydrogen atoms in the interstitial positions of the alloy. Ti32.5V27.5Nb32.5Zr7.5 achieves the best hydrogen absorption capacity of 1.71 wt % at 150 °C. All components undergo a transition from the BCC phase to the FCC phase during the hydrogen absorption process, and then change to a single-phase BCC structure after dehydrogenation. With the increase of Zr content, the first step desorption temperature raises in turn, owing to the stronger thermal stability of hydride.