Hydrogen storage in TiVCr(Fe,Co)(Zr,Ta) multi-phase high-entropy alloys

High-entropy alloys (HEAs) have a great potential in hydrogen storage applications. Developing an alloy showing remarkable hydrogen sorption capacity, close to ambient temperature without activating is a significant challenge for solid-state hydrogen storage. The present investigation was conducted to develop HEAs to satisfy these requirements. Accordingly, four novel equiatomic TiVCrFeTa, TiVCrFeZr, TiVCrCoTa and TiVCrCoZr HEAs were designed, fabricated and characterized to address their capability for the hydrogen storage application. Alloy design was accomplished based on empirical relations and thermodynamic calculations in order to obtain a microstructure containing both BCC and Laves phases using elements with different affinity to hydrogen. The thermodynamic calculations through CALPHAD predicted the presence of BCC/B2 phase together with C14 and C15 Laves phases in all designed alloys which was in good agreement with experimental analyses. Studies on hydrogen storage properties revealed that all alloys, except for TiVCrFeZr, are able to absorb hydrogen at 294 K and 30 bar without any activation process at a short incubation time. The results revealed that after activation, TiVCrFeZr and TiVCrCoZr alloys containing high volume fraction of Laves phase (similar to 40%) displayed the highest absorption capacity, with 2.3 and 1.6 wt% of hydrogen, respectively, at 294 K and 30 bar. In addition, the PCT curves proposed formation of solid solution of hydrides in TiVCrFeTa and TiVCrCoTa alloys at room temperature, however, TiVCrFeZr and TiVCrCoZr alloys provide a plateau region illustrating typical transition during hydrogen absorption. This study is a step forward to understanding necessities for developing advanced materials for the hydrogen storage.