Hydrogen Accommodation in the TiZrNbHfTa High Entropy Alloy.

The body-centred cubic high entropy alloy (HEA) TiZrNbHfTa has been of particular interest as a hydrogen storage medium as it offers a maximum storage capacity that could feasibly replace compounds that contain rare earth elements. We report that the TiZrNbHfTa structure and its hydrides (TiZrNbHfTa)H_0.4-2.0, as well as the BCC to BCT to FCC phase transformation that results from increased hydrogen concentration, have successfully been modelled as special quasi-random structures (SQS) with density functional theory calculations employed to analyse key thermodynamic processes such as vacancy formation and hydrogen solution energies. Local environments, the nearest neighbour lattice atoms surrounding an interstitial, were observed to effect the hydrogen solution energy of a given site and resulted in a wide distribution of energies throughout the structures. By considering the diverse hydrogen solution energies for the various environments in the HEA and the temperature dependence of configurational and vibrational entropy terms, a model predicting the decomposition of the hydrides has been produced. Accounting for interstitials bound to vacancies in the structure, the formation of which were found to be promoted by the presence of hydrogen interstitials – which lowered the energy barrier of vacancy formation by 0.16 eV, identified a second major decomposition peak at an elevated temperature, in agreement with recent findings. The introduction of vacancies in the local environment was found to result in interstitials adopting octahedral positions upon relaxation, in contrast to systems where vacancies were not present and tetrahedral sites were favourably occupied. This observation, and the hydride decomposition model, provides a mechanistic basis for experimentally observed behaviour and provides a comprehensive understanding of hydrogen absorption and desorption in the TiZrNbHfTa HEA.