Mechanical processing and thermal stability of the equiatomic high entropy alloy TiVZrNbHf under vacuum and hydrogen pressure

This study investigates the potential of nanostructuring the equiatomic high entropy alloy TiVZrNbHf by high-pressure torsion to improve its already promising hydrogen absorption properties. The detailed microstructural analysis of the material after processing demonstrates that a homogenous single-phase nanocrystalline structure can be obtained despite shear band development. Due to the metastable character of many high entropy alloys, this analysis was complemented by investigating the thermal stability of the alloy under both vacuum and hydrogen pressure. For the latter, the material was characterized via in situ X-ray diffraction during hydrogen charging at 500 °C, giving a detailed insight into the phase evolution during initial absorption and subsequent cycling. These experiments evidenced the inherent metastability of TiVZrNbHf, which resulted in its decomposition into a bcc, hcp, and C14 Laves phase under both vacuum and hydrogen atmospheres. Despite decomposition, the material retained its nanocrystalline structure under hydrogen pressure, presumably due to hydride formation, while significant grain growth occurred under vacuum. These findings deepen the understanding of the deformation and hydrogen charging behavior of this promising high entropy alloy, suggesting an approach for engineering such alloys for enhanced stability and performance, particularly in solid-state hydrogen storage applications.