Structure and Hydrogenation Properties of a HfNbTiVZr High-Entropy Alloy

Hydrogen-Stabilized ScYNdGd Medium-Entropy Alloy for Hydrogen Storage

The research on the functional properties of medium- and high-entropy alloys (MEAs and HEAs) has been in the spotlight recently. Many significant discoveries have been made lately in hydrogen-based economy-related research where these alloys may be utilized in all of its key sectors: water electrolysis, hydrogen storage, and fuel cell applications. Despite the rapid development of MEAs and HEAs with the ability to reversibly absorb hydrogen, the research is limited to transition-metal-based alloys that crystallize in body-centered cubic solid solution or Laves phase structures. To date, no study has been devoted to the hydrogenation of rare-earth-element (REE)-based MEAs or HEAs, as well as to the alloys crystallizing in face-centered-cubic (FCC) or hexagonal-close-packed structures. Here, we elucidate the formation and hydrogen storage properties of REE-based ScYNdGd MEA. More specifically, we present the astounding stabilization of the single-phase FCC structure induced by the hydrogen absorption process. Moreover, the measured unprecedented high storage capacity of 2.5 H/M has been observed after hydrogenation conducted under mild conditions that proceeded without any phase transformation in the material. The studied MEA can be facilely activated, even after a long passivation time. The results of complementary measurements showed that the hydrogen desorption process proceeds in two steps. In the first, hydrogen is released from octahedral interstitial sites at relatively low temperatures. In the second, high-temperature process, it is associated with the desorption of hydrogen atoms stored in tetrahedral sites. The presented results may impact future research of a novel group of REE-based MEAs and HEAs with adaptable hydrogen storage properties and a broad scope of possible applications.

High-entropy oxides as photocatalysts for organic conversion

A strategy involving organic photocatalytic conversion using hydrothermal synthesis of high-entropy oxide (HEO) (CoCuZnMnNa)Ox nanoparticles was developed. Under mild conditions, HEO nanoparticles were driven by visible light to achieve ideal yields and selectivity in sulfide oxidative coupling reactions and benzimidazole cyclization reactions, with a wide substrate range. This study is expected to contribute to the use of high-entropy oxides in organic photocatalysis.

Reactive HiTUS TiNbVTaZrHf-Nx Coatings: Structure, Composition and Mechanical Properties

High entropy metal sub-lattice stabilized nitride coatings based on multicomponent refractory transition metals (TM = Ti, Nb, V, Ta, Zr, Hf) are promising candidates for extreme conditions due to their high thermal, mechanical, and corrosion properties. The aims of the current work included the investigations of the possibilities of the novel High Target Utilization Sputtering (HiTUS) technique applied to reactive sputtering of TiNbVTaZrHf-xN coatings from the viewpoints of hysteresis behavior during reactive sputtering as well as the structure, composition, stoichiometry, and mechanical properties of the resulting coatings. With increasing nitrogen content, coating structures varied from amorphous in metallic alloy coatings to textured nano-columnar fcc structures. Despite certain deviations of TM from equiatomic concentrations, homogeneous solid solutions corresponding to single-phase multicomponent nitride analogous to high entropy stabilized compounds were obtained. Mechanical properties were found to be proportional to nitrogen content. The highest hardness H_IT ~ 33 GPa and indentation modulus E_IT ~ 400 GPa were found in a slightly sub-stoichiometric (~42 at% nitrogen) composition. H_IT/E_IT and limited pillar split measurements suggested that these coatings exhibit low fracture toughness (around 1 MPa.m^1/2). The work confirmed that reactive HiTUS is suitable for the preparation of multicomponent nitrides with the control of their stoichiometry and mechanical properties only via nitrogen additions.

Bibliometric Mapping of Literature on High-Entropy/Multicomponent Alloys and Systematic Review of Emerging Applications

High-entropy/multicomponent alloy (HEA/MCA) has received significant research attention in the last decade. There is a dearth of data-driven works dedicated to assessing and visualizing the HEA/MCA literature from a global perspective. To this end, we present the first bibliometric literature analysis of more than 3500 HEA/MCA articles, published between 2004 and 2021, in the Scopus database. We identify the most prolific authors, their collaborators, institutions, and most prominent research outlet. Co-occurrence networks of keywords are mapped and analyzed. A steep rise in research outputs is observed from 2013, when the number of annual publications doubled the previous years. The top five preferred research outlets include Journal of Alloys and Compounds, Materials Science and Engineering A, Scripta Materialia, Intermetallics, and Acta Materialia. Most of these publications emanate from researchers and institutions within China, USA, and Germany, although international scientific collaboration among them is lacking. Research gaps and future research directions are proposed, based on co-occurrence frequencies of author keywords. Finally, a brief systematic review of emerging applications, covering hydrogen storage, additive manufacturing, catalysis, and superconductivity, is undertaken. This work provides an important comprehensive reference guide for researchers to deepen their knowledge of the field and pursue new research directions.

Hydrogen Sorption Properties of a Novel Refractory Ti-V-Zr-Nb-Mo High Entropy Alloy

High entropy alloys belong to a new and promising class of functional materials for solid-state hydrogen storage. In this context, a novel single-phase body centered cubic (bcc) high entropy alloy Ti0.30V0.25Zr0.10Nb0.25Mo0.10 was prepared. The physicochemical and hydrogen sorption properties have been determined by both laboratory and large-scale facilities. This alloy can quickly absorb hydrogen up to 2.0 H/M (2.8 wt.%) at room temperature and forms a face centered cubic (fcc) hydride, as proven by synchrotron X-ray diffraction. The Pressure–Composition Isotherm and in situ neutron diffraction during hydrogen/deuterium desorption reaction suggest that the alloy experiences a reversible single step phase transition (bcc↔fcc). PDF analysis from X-ray total scattering data points out that the hydride phase possesses an average fcc structure with random atoms distribution and small lattice distortion. Despite an initial small fading of the capacity, the alloy withstands 20 absorption/desorption cycling without phase decomposition, as demonstrated by kinetic measurements coupled with X-ray diffraction and microstructural study by SEM-EDS. Moreover, the complete hydrogen absorption occurs in less than 30 s at room temperature and the kinetic improves during cycling.

Microstructure and Hydrogen Storage Properties of the Multiphase Ti0.3V0.3Mn0.2Fe0.1Ni0.1 Alloy

The hydrogen storage properties of a multi-component alloy of composition Ti0.3V0.3Mn0.2Fe0.1Ni0.1 were investigated. The alloy was synthesized by arc melting and mechanical alloying, resulting in different microstructures. It was found that the as-cast alloy is multiphase, with a main C14 Laves phase matrix along with a BCC phase and a small amount of Ti2Fe-type phase. The maximum hydrogen storage capacity of the alloy was 1.6 wt.%. We found that the air-exposed samples had the same capacity as the as-cast sample but with a longer incubation time. Synthesis by mechanical alloying for five hours resulted in an alloy with only BCC structure. The hydrogen capacity of the milled alloy was 1.2 wt.%, lower than the as-cast one. The effect of ball milling of the as-cast alloy was also studied. Ball milling for five hours produced a BCC structure similar to the one obtained by milling the raw materials for the same time.

High-Pressure Torsion for Synthesis of High-Entropy Alloys

High-pressure torsion (HPT) is widely used not only as a severe plastic deformation (SPD) method to produce ultrafine-grained metals but also as a mechanical alloying technique to synthesize different alloys. In recent years, there have been several attempts to synthesize functional high-entropy alloys using the HPT method. In this paper, the application of HPT to synthesize high-entropy materials including metallic alloys, hydrides, oxides and oxynitrides for enhanced mechanical and hydrogen storage properties, photocatalytic hydrogen production and high light absorbance is reviewed.

Effects of the Chromium Content in (TiVNb)100−xCrx Body-Centered Cubic High Entropy Alloys Designed for Hydrogen Storage Applications

In this paper, we report an investigation of adding a non-hydride forming element in the multicomponent Ti-V-Nb-M system. By the Calculation of Phase Diagrams approach (CALPHAD), the thermodynamic phase stability of the TiVNbT (T = Cr, Mn, Fe, Co, and Ni) was investigated, and Cr was selected as the fourth alloying element due its high tendency to stabilize body-centered cubic solid solutions (BCC). The (TiVNb)100−xCrx alloys (with x = 15, 25, and 35 at.% Cr) were synthesized by arc-melting. The structural characterization reveals that the three alloys were composed of a major BCC phase, which agrees with the thermodynamic calculations. The three alloys absorb hydrogen at room temperature without any activation treatment, achieving a hydrogen uptake of about H/M = 2. The Pressure-Composition-Isotherms curves (PCI) has shown that increasing the Cr amount increases the equilibrium pressures, indicating that tunable H storage properties can be achieved by controlling the alloys’ Cr content.

How 10 at% Al Addition in the Ti-V-Zr-Nb High-Entropy Alloy Changes Hydrogen Sorption Properties

Al_0.10Ti_0.30V_0.25Zr_0.10Nb_0.25 was prepared to evaluate the effect of 10% aluminum into the previously reported quaternary alloy, Ti_0.325V_0.275Zr_0.125Nb_0.275. The as-cast quinary alloy formed a single-phase body centered cubic solid solution and transformed into a body centered tetragonal after hydrogenation. The alloy had a storage capacity of 1.6 H/M (2.6 wt.%) with fast absorption kinetics at room temperature, reaching full capacity within the first 10 min. The major improvements of Al addition (10%) were related to the desorption and cycling properties of the material. The temperature for hydrogen release was significantly decreased by around 100 °C, and the quinary alloy showed superior cycling stability and higher reversible storage capacity than its quaternary counterpart, 94% and 85% of their respective initial capacity, after 20 hydrogenation cycles without phase decomposition.

Revealing High-Temperature Reduction Dynamics of High-Entropy Alloy Nanoparticles via In Situ Transmission Electron Microscopy

Understanding the behavior of high-entropy alloy (HEA) materials under hydrogen (H₂) environment is of utmost importance for their promising applications in structural materials, catalysis, and energy-related reactions. Herein, the reduction behavior of oxidized FeCoNiCuPt HEA nanoparticles (NPs) in atmospheric pressure H₂ environment was investigated by in situ gas-cell transmission electron microscopy (TEM). The reduction reaction front was maintained at the external surface of the oxide. During reduction, the oxide layer expanded and transformed into porous structures where oxidized Cu was fully reduced to Cu NPs while Fe, Co, and Ni remained in the oxidized form. In situ chemical analysis showed that the expansion of the oxide layer resulted from the outward diffusion flux of all transition metals (Fe, Co, Ni, Cu). Revealing the H₂ reduction behavior of HEA NPs facilitates the development of advanced multicomponent alloys for applications targeting H₂ formation and storage, catalytic hydrogenation, and corrosion removal.

Elucidating the Effects of the Composition on Hydrogen Sorption in TiVZrNbHf-Based High-Entropy Alloys

A number of high-entropy alloys (HEAs) in the TiVZrNbHf system have been synthesized by arc melting and systematically evaluated for their hydrogen sorption characteristics. A total of 21 alloys with varying elemental compositions were investigated, and 17 of them form body-centered-cubic (bcc) solid solutions in the as-cast state. A total of 15 alloys form either face-centered-cubic (fcc) or body-centered-tetragonal (bct) hydrides after exposure to gaseous hydrogen with hydrogen per metal ratios (H/M) as high as 2.0. Linear trends are observed between the volumetric expansion per metal atom [(V/Z)_fcc/bct – (V/Z)_bcc/hcp]/(V/Z)_bcc/hcp with the valence electron concentration and average Pauling electronegativity (χ_p) of the alloys. However, no correlation was observed between the atomic size mismatch, δ, and any investigated hydrogen sorption property such as the maximum storage capacity or onset temperature for hydrogen release.

The effect of the composition on hydrogen sorption has been studied on high-entropy alloys based on TiVZrNbHf. Most alloys crystallize in body-centered-cubic solid solutions and form fluorite-type metal hydrides. The desorption behavior of three selected metal deuterides was studied with in situ neutron diffraction coupled with gravimetric analysis. It was found that when Zr is added to TiVNb, deuterium first jumps from tetrahedral interstitial sites to octahedral sites before leaving the structure.

TiVZrNb Multi-Principal-Element Alloy: Synthesis Optimization, Structural, and Hydrogen Sorption Properties

While the overwhelming number of papers on multi-principal-element alloys (MPEAs) focus on the mechanical and microstructural properties, there has been growing interest in these alloys as solid-state hydrogen stores. We report here the synthesis optimization, the physicochemical and the hydrogen sorption properties of Ti_0.325V_0.275Zr_0.125Nb_0.275. This alloy was prepared by two methods, high temperature arc melting and ball milling under Ar, and crystallizes into a single-phase bcc structure. This MPEA shows a single transition from the initial bcc phase to a final bct dihydride and a maximum uptake of 1.7 H/M (2.5 wt%). Interestingly, the bct dihydride phase can be directly obtained by reactive ball milling under hydrogen pressure. The hydrogen desorption properties of the hydrides obtained by hydrogenation of the alloy prepared by arc melting or ball milling and by reactive ball milling have been compared. The best hydrogen sorption properties are shown by the material prepared by reactive ball milling. Despite a fading of the capacity for the first cycles, the reversible capacity of the latter material stabilizes around 2 wt%. To complement the experimental approach, a theoretical investigation combining a random distribution technique and first principle calculation was done to estimate the stability of the hydride.

Elemental Distribution in CrNbTaTiW-C High Entropy Alloy Thin Films

The microstructure and distribution of the elements have been studied in thin films of a near-equimolar CrNbTaTiW high entropy alloy (HEA) and films with 8 at.% carbon added to the alloy. The films were deposited by magnetron sputtering at 300°C. X-ray diffraction shows that the near-equimolar metallic film crystallizes in a single-phase body centered cubic (bcc) structure with a strong (110) texture. However, more detailed analyses with transmission electron microscopy (TEM) and atom probe tomography (APT) show a strong segregation of Ti to the grain boundaries forming a very thin Ti-Cr rich interfacial layer. The effect can be explained by the large negative formation enthalpy of Ti-Cr compounds and shows that CrNbTaTiW is not a true HEA at lower temperatures. The addition of 8 at.% carbon leads to the formation of an amorphous structure, which can be explained by the limited solubility of carbon in bcc alloys. TEM energy-dispersive X-ray spectroscopy indicated that all metallic elements are randomly distributed in the film. The APT investigation, however, revealed that carbide-like clusters are present in the amorphous film.

A DFT Study of Hydrogen Storage in High-Entropy Alloy TiZrHfScMo

In recent years, high-entropy alloys have been proposed as potential hydrogen storage materials. Despite a number of experimental efforts, there is a lack of theoretical understanding regarding the hydrogen absorption behavior of high-entropy alloys. In this work, the hydrogen storage properties of a new TiZrHfScMo high-entropy alloy are investigated. This material is synthesized successfully, and its structure is characterized as body-centered cubic. Based on density functional theory, the lattice constant, formation enthalpy, binding energy, and electronic properties of hydrogenated TiZrHfScMo are all calculated. The calculations reveal that the process of hydrogenation is an exothermic process, and the bonding between the hydrogen and metal elements are of covalent character. In the hydrogenated TiZrHfScMo, the Ti and Sc atoms lose electrons and Mo atoms gain electrons. As the H content increases, the <Ti–H> bonding is weakened, and the <Hf–H> and <Mo–H> bonding are strengthened. Our calculations demonstrate that the TiZrHfScMo high-entropy alloy is a promising hydrogen storage material, and different alloy elements play different roles in the hydrogen absorption process.

Influence of Deposition Temperature on the Phase Evolution of HfNbTiVZr High-Entropy Thin Films

In this study, we show that the phase formation of HfNbTiVZr high-entropy thin films is strongly influenced by the substrate temperature. Films deposited at room temperature exhibit an amorphous microstructure and are 6.5 GPa hard. With increasing substrate temperature (room temperature to 275 °C), a transition from an amorphous to a single-phased body-centred cubic (bcc) solid solution occurs, resulting in a hardness increase to 7.9 GPa. A higher deposition temperature (450 °C) leads to the formation of C14 or C15 Laves phase precipitates in the bcc matrix and a further enhancement of mechanical properties with a peak hardness value of 9.2 GPa. These results also show that thin films follow different phase formation pathways compared to HfNbTiVZr bulk alloys.

A Novel TiZrHfMoNb High-Entropy Alloy for Solar Thermal Energy Storage

An equiatomic TiZrHfMoNb high-entropy alloy (HEA) was developed as a solar thermal energy storage material due to its outstanding performance of hydrogen absorption. The TiZrHfMoNb alloy transforms from a body-centered cubic (BCC) structure to a face-centered cubic (FCC) structure during hydrogen absorption and can reversibly transform back to the BCC structure after hydrogen desorption. The theoretical calculations demonstrated that before hydrogenation, the BCC structure for the alloy has more stable energy than the FCC structure while the FCC structure is preferred after hydrogenation. The outstanding hydrogen absorption of the reversible single-phase transformation during the hydrogen absorption⁻desorption cycle improves the hydrogen recycling rate and the energy efficiency, which indicates that the TiZrHfMoNb alloy could be an excellent candidate for solar thermal energy storage.

Thermal Stability of the HfNbTiVZr High-Entropy Alloy

The multicomponent alloy HfNbTiVZr has been described as a single-phase high-entropy alloy (HEA) in the literature, although some authors have reported that additional phases can form during annealing. The thermal stability of this alloy has therefore been investigated with a combination of experimental annealing studies and thermodynamic calculations using the CALPHAD approach. The thermodynamic calculations show that a single-phase HEA is stable above about 830 °C. At lower temperatures, the most stable state is a phase mixture of bcc, hcp, and a cubic C15 Laves phase. Annealing experiments followed by quenching confirm the results from thermodynamic calculations with the exception of the Laves phase structure, which was identified as a hexagonal C14 type instead of the cubic C15 type. Limitations of the applied CALPHAD thermodynamic description of the system could be an explanation for this discrepancy. As-synthesized HfNbTiVZr alloys prepared by arc-melting form a single-phase bcc HEA at room temperature. In situ annealing studies of this alloy show that additional phases start to form above 600 °C. This indicates that the observed HEA is metastable at room temperature and stabilized by a slow kinetics during cooling. X-ray diffraction analyses using different cooling rates and annealing times show that the phase transformations in this HEA are slow and that completely different phase compositions can be obtained depending on the annealing procedure. In addition, it has been shown that the sample preparation method (mortar grinding, heat treatment, etc.) has a significant influence on the collected diffraction patterns and therefore on the phase identification and analysis.