A Density Functional Theory Study of the Hydrogen Absorption in High Entropy Alloy TiZrHfMoNb.

The high entropy alloy is promising for hydrogen storage, especially in regard to its adjustable hydrogen storage properties. Despite several experimental investigations, there still lacks a detailed atomic-level understanding of the hydrogenation process. In this study, based on first-principles calculations, the hydrogen behaviors and microstructural evolution in high entropy alloy TiZrHfMoNb during the hydrogen absorption are investigated systematically. At low hydrogen content, hydrogen atoms prefer to occupy the octahedral interstitial sites of the BCC phase, which is different from that in BCC pure metals; when the hydrogen content reaches 1.08 wt %, the BCC TiZrHfMoNb hydrides transform into FCC phase, and hydrogen atoms are more favorable to occupy the tetrahedral interstitial sites. Further radial distribution function (RDF) analysis indicates that the enhanced disorder of <hydrogen-hydrogen> bonds and decreased lattice distortion of the metal structure destabilize the BCC TiZrHfMoNb hydride and eventually induce the BCC → FCC phase transformation, which is quite different from that in conventional alloys; the difference originates from the severe lattice distortion in high entropy alloy. The phonon spectra of different TiZrHfMoNb hydrides show that the hydride with a H/M ratio of 2 dynamically has a stable lattice, corresponding to a hydrogen storage capacity of 1.94 wt %. The present study demonstrates that the high entropy alloys have unique hydrogen absorption ability, which may advance the related experimental and theoretical studies.