Substructural State of the High-Entropy Alloy AlCoNiTiNb under Hydrogen Saturation and Heating

–—The work investigates the structure and substructural properties of the high-entropy alloy (HEA) AlCoNiTiNb saturated with hydrogen using X-ray diffraction analysis and positron annihilation spectroscopy (PAS). The study examines the changes in hydrogen-induced defects in HEAs during heating in vacuum and hydrogen environments, as well as the influence of temperature and hydrogen content on the formation of various imperfections. Based on a database of crystallographic standards created by the USPEX algorithm, it was established that the structural state is determined by stable phases (STRUC-1, STRUC-2, and STRUC-3). These structures correspond to cells with parameters close to simple cubic lattices. The study revealed correlations between positron annihilation parameters (S and W) and defect concentration as a function of temperature and hydrogen saturation. The expected proportion of hydride phases in the AlCoNiTiNb alloy was estimated to be within 5–10%, with maximum hydrogen accumulation occurring in the temperature range of 200–500°C. Despite the formation of hydride phases, the material demonstrates low cyclic stability, which limits its long-term application in hydrogen energy.