Maksim Koptsev

–—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.

Novel multicomponent Nb–Ni–Ti–Zr–Co alloys with 20–55 at.% Nb were synthesized from metal powders by arc melting. The resulting alloys consist primarily of Nb-rich and eutectic body-centered (BCC) phases. The content of the eutectic BCC phase is highest for an equimolar composition, while the content of the Nb-rich BCC phase increases with Nb content in the alloy. The content of secondary phases is the highest for the alloy with 32 at.% Nb. According to ab initio calculations, hydrogen occupies tetrahedral interstitial sites in the Nb-rich phase and octahedral sites in the eutectic BCC phase. For different Nb concentrations, hydrogen-binding energies were calculated. An increase in the Nb-rich phase leads to softening of multicomponent alloys. The alloys with 20 and 32 at.% Nb demonstrate high hydrogen permeability (1.05 and 0.96 × 10⁻⁸ molH₂m⁻¹s⁻¹Pa^−0.5, respectively) at 400 °C, making them promising for hydrogen purification membrane application. Multicomponent alloys with a high Nb content (55 at.%) have low resistance to hydrogen embrittlement.

In the present work, porous composites were fabricated from pure Al₂O₃ and mixed Ti₃AlC₂/Al₂O₃ powder by slip casting and sintering. The effect of sintering temperature and different composition ratio on microstructure, phase composition, porosity and gas permeation flux of the fabricated materials was investigated. The microstructure and phase composition of the samples were analyzed by scanning electron microscopy and X-ray diffraction, respectively. The gas permeation experiments were performed using pure hydrogen at 0.1–0.9 MPa pressure. It is shown that a decrease in sintering temperature from 1500 to 1350 °C results in an increase in hydrogen permeation flux of the alumina from 5 to 25 mol/(m² × s), which is due to higher pore size and overall porosity of the samples. Sintering of Ti₃AlC₂/Al₂O₃ powder mixtures leads to the formation of Al₂O₃, Al₂TiO₅ and TiO₂ phases as a result of oxidation of the Ti₃AlC₂ phase, resulting in an increased pore size in the composites compared with pure alumina. The open porosity of composites increases from 3.4 to 40% with an increasing Ti₃AlC₂/Al₂O₃ ratio from 1/10 to 1/2, respectively. The composites with the highest porosity (40%) had a maximum permeation flux of 200 mol/(m² × s). The changes in the bending strength of the alumina and composite samples, depending on the microstructure and porosity, were also discussed. The investigated composites are considered promising materials for hydrogen separation membrane supports.

In the present work, Al₂O₃/Ti₃AlC₂ composites with different composition ratios was fabricated. The composites were sintered at 1550°C from blanks produced by slip casting. The microstructure and phase composition of the samples were analysed by scanning electron microscopy and X-ray diffraction, respectively. Gas permeability tests were investigated using a gas reaction automated machine. It is shown that the porosity of composites increases with increasing Ti₃AlC₂ content in the raw material. The composite Ti₃AlC₂/Al₂O₃ (1/2) with the highest porosity (40 %) had a maximum hydrogen flow of 200 mol/(m²s).

Hydrogen separation membranes are one of the most promising technologies for hydrogen purification. The development of high-entropy alloys (HEAs) for hydrogen separation membranes is driven by a “cocktail effect” of elements with different hydrogen affinities to prevent hydride formation and retain high permeability due to the single-phase BCC structure. In this paper, equimolar and non-equimolar Nb-Ni-Ti-Zr-Co high entropy alloys were fabricated by arc melting. The microstructure and phase composition of the alloys were analyzed by scanning electron microscopy and X-ray diffraction, respectively. The hydrogen permeation experiments were performed at 300–500 °C and a hydrogen pressure of 4 bar. In order to estimate the effect of composition and lattice structure on hydrogen location and diffusivity in Nb-Ni-Ti-Zr-Co alloy, ab initio calculations of hydrogen binding energy were performed using virtual crystal approximation. It was found that Nb-enriched and near equimolar BCC phases were formed in Nb₂₀Ni₂₀Ti₂₀Zr₂₀Co₂₀ HEA while Nb-enriched BCC and B2-Ni(Ti, Zr) were formed in Nb₄₀Ni₂₅Ti₁₈Zr₁₂Co₅ alloy. Hydrogen permeability tests showed that Nb₂₀Ni₂₀Ti₂₀Zr₂₀Co₂₀ HEA shows lower activation energy and higher permeability at lower temperatures as well as higher resistance to hydrogen embrittlement compared to Nb₄₀Ni₂₅Ti₁₈Zr₁₂Co₅ alloy. The effect of composition, microstructure and hydrogen binding energies on permeability of the fabricated alloys was discussed.