Preparation-Dependent Microstructure and Hydrogen Storage in High-Entropy Alloys

High-entropy alloys (HEAs) have emerged as an important class of materials for solid-state hydrogen storage because their compositional complexity provides access to diverse phase constitutions, local lattice environments, and hydrogen-related responses. However, hydrogen-storage behavior in these alloys cannot be understood from composition alone. What ultimately governs performance is the microstructural state generated during preparation. This perspective examines HEAs from that standpoint, focusing on how different preparation routes produce distinct structural states and how those states determine hydrogen accommodation, diffusion, phase transformation, and reversibility. Arc melting and subsequent homogenization typically generate bulk refractory alloys with comparatively simple average phase constitution, whereas mechanical alloying and reactive ball milling produce defect-rich, fine-scale, and metastable non-equilibrium structures. Representative systems are discussed to show that even alloys with similar nominal compositions may follow different hydriding pathways once their structurally realized state changes. The article further evaluates the structural descriptors most often invoked in the field, including phase constitution, local lattice environment, grain size, defect density, interface density, chemical homogeneity, and processing history. It is argued that future progress will depend less on continued composition screening alone than on establishing more transferable microstructure-hydrogen-storage relationships across route-defined structural states.