Thermal stability and hydrogen storage properties of Hf0.75Ti0.25NbVZr and TiNbVZr high entropy alloys

This study examines the thermal stability and hydrogen storage performance of Hf 0.75 Ti 0.25 NbVZr and TiNbVZr high-entropy alloys (HEAs). Heat treatments were conducted at 1300°C for 24 and 48 hours and at 600°C for 1 and 2 months. X-ray diffraction (XRD) analysis revealed that Hf 0.75 Ti 0.25 NbVZr transitioned from a mixture of BCC and C15 Laves phases to a single BCC phase at 1300°C. At 600°C, a combination of BCC, C14 Laves, and HCP phases was detected, with phase compositions remaining stable over time. Hydrogen absorption tests showed that heat-treated samples required activation at 250°C. After 2 months at 600°C, Hf 0.75 Ti 0.25 NbVZr alloy had a hydrogen uptake of 0.95 wt.% while TiNbVZr treated at 1300°C for 24 hours absorbed 1.10 wt.% of hydrogen. In contrast, the as-cast alloys absorbed 2.05 wt.% and 2.7 wt.%, respectively, at room temperature. Hydrogenation induced phase transformations, notably causing significant expansion in the C14 Laves phase. The highest hydrogen absorption was observed in the as-synthesized arc-melted alloys, which outperformed heat-treated samples in storage capacity. These findings demonstrate that heat treatment lowers hydrogen storage efficiency due to phase transformations and reduced BCC phase content. • The as-cast Hf₀.₇₅Ti₀.₂₅NbVZr transitions from a BCC + C15 Laves mixture to a single BCC phase at 1300°C, while 600°C treatments yield BCC, C14 Laves, and HCP phases. • The phase composition at 600°C remains stable over time, with distinct phase formations influencing hydrogen storage performance. • Heat-treated samples require activation at 250°C; after 2 months at 600°C, Hf₀.₇₅Ti₀.₂₅NbVZr absorbs 0.95 wt.% of H₂, while TiNbVZr (1300°C, 24 h) absorbs 1.10 wt.%. • As-cast alloys exhibit superior hydrogen uptake (2.05 wt.% and 2.7 wt.% at room temperature), outperforming heat-treated alloys. • Heat treatment reduces hydrogen storage capacity by decreasing BCC phase content and promoting phase transformations.