Novel Pathways for Solid-State Hydrogen Storage and Hydrogen-Mediated Microstructure Evolution

Hydrogen will play a crucial role in establishing a carbon-neutral economy, but the increased usage of hydrogen also necessitates efficient, cheap, and safe storage options. In this regard, the reversible absorption of hydrogen in metal hydrides poses an alternative to classic storage options such as high pressure and liquefaction. However, several issues remain, including sluggish initial absorption, the need for activation, and poor mechanical stability. Thus, this work attempted to overcome these limitations by preparing novel nanocomposites and nanoporous materials by combining methods of severe plastic deformation, specifically high-pressure torsion, and selective phase dissolution. The two metal hydride systems selected for this approach were the intermetallic compound FeTi and the high-entropy alloy TiVZrNbHf, along with their respective composites with Cu. After outlining the concepts and state-of-the-art of both metal hydrides and severe plastic deformation, this work presents the developed fabrication routes, the resulting functional materials, and the underlying mechanisms governing processing and properties. An in-depth investigation into the FeTi¿Cu composite materials revealed the mechanisms governing microstructure evolution during high-pressure torsion. A self-reinforcing refinement process is proposed, originating from the temperature- and grain size-dependent strain rate-sensitive deformation behavior, which is essential for overcoming detrimental deformation localization during severe plastic deformation. Investigations of chemical complex materials, such as TiVZrNbHf and TiVZrNbHf¿Cu, suggest a significant influence of hydrogen on the stability of these systems. In particular, severe plastic deformation of composites with and without hydrogen revealed a hydrogen-induced suppression of mechanical alloying, demonstrating a novel approach to steer microstructure evolution and achieve otherwise unattainable material states. Based on the FeTi¿Cu nanocomposites, selective phase dissolution enabled the preparation of nanoporous FeTi with tailorable structural sizes, allowing for a deep dive into the associated structure-properties relationships of this system. The investigation revealed the suppression of hydride formation by a confinement effect in nanoporous FeTi, but also a way to overcome such confinement by coarsening of the porous structure. Altogether, this work challenged the limits of controlled microstructure evolution of multi-phase materials during severe plastic deformation, and probed the limits of hydride formation in nanoscale confined systems. It becomes clear that targeted and controlled microstructure evolution is essential for advancing metal hydrides, but that, vice versa, hydrides and hydrogen can be used to tailor microstructure evolution itself. This work contributes to the fundamental understanding of both the field of severe plastic deformation and metal-hydrogen systems, highlighting synergies at the intersection of the respective fields.