# Thermal stability and hydrogen storage properties of Hf0.75Ti0.25NbVZr and TiNbVZr high entropy alloys

Maria Moussa, Christophe Cona, Jacques Huot, Jean-Louis Bobet

► To cite this version:

Maria Moussa, Christophe Cona, Jacques Huot, Jean-Louis Bobet. Thermal stability and hydrogen storage properties of Hf0.75Ti0.25NbVZr and TiNbVZr high entropy alloys. Journal of Alloys and Compounds Communications, 2025, 7, pp.100089. 10.1016/j.jacomc.2025.100089. hal-05376807

HAL Id: hal-05376807

https://hal.science/hal-05376807v1

Submitted on 21 Nov 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /></figure>

# Thermal stability and hydrogen storage properties of Hf<sub>0.75</sub>Ti<sub>0.25</sub>NbVZr and TiNbVZr high entropy alloys

M. Moussa<sup>a,b</sup>, C. Cona<sup>a</sup>, J. Huot<sup>b</sup>, J.-L. Bobet<sup>a,*</sup>

## ARTICLE INFO

**Keywords:**

High entropy alloys

Arc melting

Heat-treatment

Crystal structure

Microstructure

Hydrogen storage

## ABSTRACT

This study examines the thermal stability and hydrogen storage performance of Hf<sub>0.75</sub>Ti<sub>0.25</sub>NbVZr and TiNbVZr high-entropy alloys (HEAs). Heat treatments were conducted at 1300 °C for 24 and 48 h and at 600 °C for 1 and 2 months. X-ray diffraction (XRD) analysis revealed that Hf<sub>0.75</sub>Ti<sub>0.25</sub>NbVZr transitioned from a mixture of BCC and C15 Laves phases to a single BCC phase at 1300 °C. At 600 °C, a combination of BCC, C14 Laves, and HCP phases was detected, with phase compositions remaining stable over time. Hydrogen absorption tests showed that heat-treated samples required activation at 250 °C. After 2 months at 600 °C, Hf<sub>0.75</sub>Ti<sub>0.25</sub>NbVZr alloy had a hydrogen uptake of 0.95 wt% while TiNbVZr treated at 1300 °C for 24 h absorbed 1.10 wt% of hydrogen. In contrast, the as-cast alloys absorbed 2.05 wt% and 2.7 wt%, respectively, at room temperature. Hydrogenation induced phase transformations, notably causing significant expansion in the C14 Laves phase. The highest hydrogen absorption was observed in the as-synthesized arc-melted alloys, which outperformed heat-treated samples in storage capacity. These findings demonstrate that heat treatment lowers hydrogen storage efficiency due to phase transformations and reduced BCC phase content.

## 1. Introduction

High-entropy alloys (HEAs) are a novel class of materials composed of multiple principal elements, typically a minimum of four principal elements where each element has a concentration of 5–35 at%. Their unique composition results in exceptional properties, including good ductility at room temperature [1,2], excellent mechanical properties at elevated temperatures [3], improved oxidation resistance [4] and enhanced hydrogen storage capabilities [5–7]. These features make HEAs a promising candidate for advanced structural and energy applications.

When it comes to crystal structure, these alloys are known to crystallize in simple crystal structures such as: Body Centered Cubic – BCC [8,9], Face Centered Cubic (i.e. cubic close-packed) - FCC [10,11] and Hexagonal Close-Packed - HCP [1]. However, Laves phases can also sometimes be observed (mainly hexagonal C14 and cubic C15 and less frequently hexagonal C36) [10–14].

However, some HEA solidifying as single-phase solid solutions may experience phase transformations when exposed to heat treatments that may result in different microstructure and properties [15–17]. For example, Zhang *et al.* [15] highlighted the microstructural evolution of

Co-20Fe-18Cr-19Mn alloy synthesised via laser-directed energy deposition, transitioning from an HCP structure to an FCC single-phase after heat treatment. Fang *et al.* [16] investigated the microstructural evolution of ZrNbTaHfx (x = 0.1–1.0) alloys showing that the as-cast alloys exhibits a dendritic structure with BCC and few percent of HCP phases. While annealing at 1773 K induces element diffusion, phase coarsening, and HCP accumulation at grain boundaries, significantly enhancing the mechanical properties. Nagarjuna *et al.* [17] examined the effect of heat treatment at elevated temperatures on CrFeNiTiV. The authors concluded that heat treatment has led to phase coarsening and improved phase distribution.

Additionally, among the studies dedicated to HEA, several reports tackled the stability of the equiatomic NbTiVZr alloys [18,19]. For example, Senkov *et al.* [18] examined the phase stability of NbTiVZr-based alloys showing that NbTiVZr maintains a single-phase BCC structure after homogenization annealing at 1200–1400 °C. Yurchenko *et al.* [19] studied the microstructural evolution and phase stability of Al<sub>x</sub>NbTiVZr (x = 0, 0.5, 1, 1.5) high entropy alloys fabricated by vacuum arc melting and annealed at 1200 °C. The NbTiVZr alloy exhibited a single BCC phase after annealing, while Al-containing alloys formed additional phases such as C14 Laves and Zr<sub>2</sub>Al-type phases.

<footnote><sup>a</sup> Bordeaux University, ICMCB, CNRS, UMR 5026, Pessac F-33600, France</footnote>

<footnote><sup>b</sup> Hydrogen Research Institute, Université du Québec à Trois-Rivières, 3351 Bd des Forges, Trois-Rivières, QC G9A 5H7, Canada</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: jean-louis.bobet@icmbc.cnrs.fr (J.-L. Bobet).</footnote>
In a previous work [20], we have investigated the effect of two different synthesis methods (arc melting and induction melting) and the effect of Hf substitution to Ti on crystal structure and microstructure evolution of the $Hf_{1-x}Ti_xNbVZr$ HEA. Experiments results were compared to thermodynamic calculations using Calphad method. We concluded that the cooling rate have a considerable effect on both the microstructure and the crystal structures of the alloys (e.g. dendritic microstructure observed in arc melting are replaced by an acicular microstructure upon induction melting). In addition, substituting Hf for Ti stabilized the BCC phase, whereas phase separation was enhanced with higher Hf concentration for both synthesis methods.

For the present study, we selected two compositions (e.g. $Hf_{0.75}Ti_{0.25}NbVZr$ and $TiNbVZr$) from the series of $Hf_{1-x}Ti_xNbVZr$ (e.g. $x = 0.25$ and $x = 1$) alloys, to deeper evaluate their phase stability at different temperature. The study of these compositions will allow us to explore one multiphase composition (e.g. $Hf_{0.75}Ti_{0.25}NbVZr$ consisting of a BCC and C15 Laves phases) and one single-phase composition (e.g. $TiNbVZr$ consisting of a single BCC phase). After heat treatment, the microstructure was investigated by Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD). Hydrogen absorption of the different heat-treated alloys are also studied and discussed.

## 2. Materials and methods

The as-cast high entropy alloys with composition $Hf_{1-x}Ti_xNbVZr$ ($x = 0.25$ and $1$) were prepared by arc melting under an argon atmosphere in order to avoid any oxidation of the material. The raw elements were purchased from Alfa Aesar® and had the following purities: Ti (99.9 %), Hf (99.9 %), V (99.7 %), Zr (99.5 %), and Nb (99.9 %). For each sample, raw elements were weighed in the right proportion and melted together. The samples were turned over 5 times to achieve a homogeneous distribution of the constituent elements. The cast pellets were thereafter heat-treated at 1300 °C in a sealed tantalum tube (under ultra pure argon atmosphere) for 24 h and 48 h, and at 600 °C in a sealed quartz tube (under $10^{-1}$ mbar primary vacuum) for 1 month and 2 months. The cooling of the pellet to room temperature was carried out by water quenching. The heat-treatment temperature were chosen based on previous CALPHAD calculations. It was established that for the compositions $Hf_{0.75}Ti_{0.25}NbVZr$ and $TiVZrNb$, heat-treatment at 1300 °C would result in a single BCC phase, while at temperatures below 700 °C, two BCC phases are expected [20].

Before the hydrogenation measurements, the heat-treated alloys were hand-crushed under air using a hardened steel mortar and pestle. The hydrogenation of all the alloys were measured on a homemade Sievert's type apparatus. The first absorption (activation) was conducted at 250 °C under 20 bars of hydrogen (as no absorption was observed at room temperature).

The X-ray diffraction patterns were acquired using a Bruker D8 Focus X-ray or Brucker D8.1 diffractometer with Cu Kα radiation. For the X-ray diffraction, the as-cast alloy was reduced into powder using a hardened stainless-steel mortar and pestle. The crystallographic parameters (lattice parameters, crystallite size) were evaluated by Rietveld refinement of the X-ray diffraction patterns using TOPAS software [21]. To model the amorphous contribution, a broad peak was manually added using the "Add Peak" function in TOPAS, corresponding to a pseudo-Voigt profile. This approach is not based on a crystallographic structure or an internal standard, but rather on an empirical adjustment to account for the diffuse scattering. For distinguishing the different phases, scanning electron microscopy (SEM) was performed using a Tescan Vega3-SB SEM in backscattered electron mode. For the EDS measurements, an average of five measurements was taken for each microstructure contrast.

## 3. Results and discussion

### 3.1. $Hf_{0.75}Ti_{0.25}NbVZr$ ($x = 0.25$): crystal structure

The crystal structure of the arc-melted alloy $Hf_{0.75}Ti_{0.25}NbVZr$ ($x = 0.25$) (hereinafter referred as "as-synthesized"), have already been reported in detail in a previous article [20]. To better support our discussion, the corresponding X-Ray Diffraction pattern, the crystal structure parameters and the abundance of each phase as determined by Rietveld's analysis are provided in the supplement file (Figure S1 and Table S1, respectively). It was found that the synthesized alloy displayed a BCC phase ($a_{BCC} = 3.372 (1) Å$, and a cubic C15 phase ($a_{C15} = 7.428 (6) Å$).

After heat-treatment at 1300 °C for 24 h and 48 h, the crystal structure was investigated by XRD. The XRD diffraction pattern are shown in Fig. 1. The heat-treated alloys for 24 h or 48 h both crystallizes with a BCC structure having very similar lattice parameter ($a_{BCC,24 h} = 3.373 (1) Å$ and $a_{BCC,48 h} = 3.375 (1) Å$) (Table 1) and also close to that of the as-synthesized sample ($a_{BCC, as-synthesized} = 3.372 (1) Å$). However, no C15-type Laves phase peaks could be detected (the small peak at 26° can be attributed to carbon contamination during the sample preparation). Consequently, heat-treatment for 24 h or 48 h does not affect the lattice parameter of the BCC structure, but results in the disappearance of the C15-type Laves phase. In addition, a few non-indexed peaks of very low intensity are present between 30°–40° and 60°–70° (2θ). It is important to note that these peaks are detectable exclusively following the heat-treatment at elevated temperature. It is perhaps the signature of some traces of oxidation during the heat treatment.

For the heat-treated alloys at 600 °C (for 1 and 2 months), the XRD diffraction patterns are illustrated in Fig. 2 accompanied by detailed crystallographic parameters and phase abundances in Table 2. For these samples, the main phase is the C14-type Laves phase (featuring two non-equivalent atomic positions for the B atom (Wyckoff positions 2a and 6h) that coexists with a BCC phase and an HCP phase.

For the alloy treated at 600 °C for 1 month, the abundance of the C14-type Laves phase is 51 %, that of the BCC structure is 27 % and for the HCP structure, the abundance is 22 % (e.g. Fig. 2 and Table 2).

Regarding the alloy heat-treated at 600 °C for 2 months, the abundance of BCC and C14 phases increases (27–32 % and 51–54 % respectively) compared to the alloy heat-treated for 1 month, while that of the HCP structure decreases (from 22 % to 14 %).

In both cases, the lattice parameters of the three phases (i.e. BCC, C14 and HCP) remain almost unchanged.

The above results could indicate that the $Hf_{0.75}Ti_{0.25}NbVZr$ alloy synthesized by arc melting is not in a stable state. As demonstrated before [20], heat treatment can significantly affect the phases distribution within a sample. According to our experiments, it seems that heat treatment at 1300 °C lead to more stable system than the heat treatment at 600 °C.

### 3.2. $Hf_{0.75}Ti_{0.25}NbVZr$ ($x = 0.25$): microstructure

The microstructure and the chemical compositions of the as-synthesized alloy were previously reported in detail [20]. They are presented in the supplementary file (Figure S2 and Table S2). For the alloy thermally treated at 1300 °C (for 24 h and 48 h), Back Scattered Electron (BS) micrographs and Energy-Dispersive Spectroscopy (EDS) analyses were performed and are presented in Fig. 3 and Table 3 respectively.

For the as-synthesized alloy, based on scanning electron microscopy and chemical composition results (Figure S2 and Table S2) two phases were observed: a dendritic phase with a composition close to the nominal alloy identified as BCC, and a V-enriched dark region identified as the C15 Laves phase.

The microstructure of the alloy heat-treated at 1300 °C for 24 h (Fig. 3(a)) is different from that of the one treated for 48 h (Fig. 3(b)).
<figure><img src="image_3.png" /><figcaption>Fig. 1. XRD (X-Ray Diffraction) patterns of the Hf<sub>0.75</sub>Ti<sub>0.25</sub>NbVZr (x = 0.25) alloy heat-treated at 1300 °C for (a) 24 h and (b) 48 h.</figcaption></figure>

<caption>**Table 1**</caption>

Crystal structure parameters of Hf<sub>0.75</sub>Ti<sub>0.25</sub>NbVZr (x = 0.25) alloy heat-treated at 1300 °C for 24 h and 48 h. The error on the last significant digit is indicated between brackets.

<table><thead><tr><th>Alloy heat-treated at 1300 °C for:</th><th>Phases</th><th>Lattice parameter (Å)</th><th>Cell volume (Å³)</th></tr></thead><tbody><tr><td>24 h</td><td>BCC</td><td>3.373 (1)</td><td>38.38 (2)</td></tr><tr><td>48 h</td><td>BCC</td><td>3.375 (1)</td><td>38.41 (1)</td></tr></tbody></table>

The black regions present for the alloy heat-treated for 24 h are richer in V than the alloy heat-treated for 48 h, indicating that V has a strong tendency to diffuse towards the “gray phase”. As vanadium is the smallest atom in our HEA considering the metallic radii of the elements [22] (Ti = 1.46 Å, Hf = 1.58 Å, V = 1.36 Å, Zr = 1.60 Å and Nb = 1.45 Å), his diffusion is easier. Moreover, extending the heat treatment to 48 h facilitates the grain growth of second-phase.

According to the XRD patterns of these two samples, the matrix corresponds to the BCC structure. However, the “black” region cannot be attributed to a C15-type Laves phase like in the case of the as-synthesized alloy, as it is not detected by XRD, likely due to being a minor phase below the detection limit of the technique.

### 3.2.1. Heat-treated alloy at 600 °C

Heat-treatment at 600 °C during one and two months change drastically the microstructure (Fig. 4(a) and (b)) in comparison to both the as-synthesized alloy and the heat-treated at 1300 °C.

Following the heat-treatment at 600 °C (for 1 or 2 months), the microstructure consists of a gray matrix with a composition close to the nominal composition, a dark region rich in V, and a white region (bright dendrites) rich in Zr. The XRD results (Table 2) show that a mixture of BCC, HCP and C14 structures are formed for this alloy (more detailed SEM images in BSE mode are shown in the supplement file Figure S3 (a) and (b)). These crystal structures could be correlated with the chemical compositions of the phases obtained by chemical EDS analysis (Table 4) as follows: the regions identified as gray, black, and white (Fig. 4) were quantified using ImageJ software, resulting in proportions of 38 %, 47 %, and 15 %, respectively. The BCC crystal structure, with an estimated abundance of 27–32 % (Table 2), closely aligns with the proportion of the gray phase, suggesting that BCC can be attributed to this region.

The measured area by ImageJ for this white region was 14 % (NB: as the density of all the phases are very close (ranging from 6.92 to 7.20 g/cm³) the surface and the weight/volume partition were considered as equivalent). Therefore, this white region can be attributed to the HCP phase. Although, This HCP phase, exhibiting cell parameters (a = 3.211 (1) Å and c = 5.104 (2) Å) aligns closely with an HCP solid solution

<figure><img src="image_4.png" /><figcaption>Fig. 2. XRD (X-Ray Diffraction) pattern of the Hf<sub>0.75</sub>Ti<sub>0.25</sub>NbVZr (x = 0.25) alloy heat-treated at 600 °C for (a) 1 month and (b) 2 months.</figcaption></figure>
<caption>**Table 2**</caption>

Crystal structure parameters of the Hf<sub>0.75</sub>Ti<sub>0.25</sub>NbVZr (x = 0.25) alloy heat-treated at 600 °C for 1 and 2 months. The errors on the last significant digit are indicated between brackets.

<table><thead><tr><th>Heat-treated at 600 °C</th><th>Phases</th><th>Lattice parameter (Å)</th><th>Cell volume (Å<sup>3</sup>)</th></tr></thead><tbody><tr><td rowspan="4">1 month</td><td>C14 (51 % ± 3)</td><td>a = 5.356 (1)<br>c = 8.652 (1)</td><td>214.9 (1)</td></tr><tr><td>BCC (27 % ± 3)</td><td>3.400 (1)</td><td>39.31 (3)</td></tr><tr><td>HCP (22 % ± 2)</td><td>a = 3.211(1)<br>c = 5.104 (2)</td><td>45.58 (3)</td></tr><tr><td>C14 (54 % ± 3)</td><td>a = 5.354 (1)<br>c = 8.641 (3)</td><td>214.5 (1)</td></tr><tr><td rowspan="3">2 months</td><td>BCC (32 % ± 3)</td><td>3.425 (1)</td><td>40.18 (3)</td></tr><tr><td>HCP (14 % ± 1)</td><td>a = 3.214 (1)<br>c = 5.111 (2)</td><td>45.71 (2)</td></tr></tbody></table>

dominated by Zr and Hf as the expected values for a close packed for Hf<sub>0.5</sub>Zr<sub>0.5</sub> structure are: a = 3.206 Å and c = 5.087 Å. The white phase in the sample contains heavy elements, primarily hafnium (18 %) and zirconium (29 %) and can be associated with an HCP phase of approximate composition 18Hf-6Ti-22Nb-25V-29Zr.

Finally, the dark region, with a measured abundance of 48 % by ImageJ, is consistent with the C14 Laves phase (51–54 % by XRD refinement, i.e. Table 2). Once again surface and volume/weight partition were considered as equivalent (for the same reason as previously mentioned in the previous paragraph).

From the results presented up to this point, we can conclude that the Hf<sub>0.75</sub>Ti<sub>0.25</sub>NbVZr alloy heat-treated at 1300 °C for 48 h shows almost the same structure and the same morphology as for the same alloy heat-treated at 1300 °C for 24 h (i.e., BCC and C15 phase mixture). Nevertheless, the presence of several unidentified peaks can be observed for the alloy that had been heat-treated at 1300 °C for 48 h and for this reason, in the following the one heat-treated for 24 h will be preferentially used. Heat-treatment at 600 °C for 1 or 2 months results in the formation of a three-phase mixture (BCC, C14 and HCP), with the duration of heat treatment only affecting the relative quantities of the three phases. This clearly highlights the metastability of these HEA as already reported in the work of King *et al.* [23] and Munitz *et al.* [24].

### 3.3. Hydrogen absorption properties

The Hf<sub>0.75</sub>Ti<sub>0.25</sub>NbVZr alloy heat-treated at (i) 1300 °C for 24 h and (ii) 600 °C for 2 months has been selected for hydrogen absorption. This selection was influenced by the presence of several unidentified peaks in the alloy that had been heat-treated at 1300 °C for 48 h. Additionally, we chose the alloy heat-treated at 600 °C for 2 months, as the longer

heat treatment duration can suggest that it is more likely to be closer to an equilibrium state.

The kinetics of first hydrogenation (also called activation) of the Hf<sub>0.75</sub>Ti<sub>0.25</sub>NbVZr alloy heat-treated at (i) 1300 °C for 24 h and (ii) 600 °C for 2 months have been studied and presented in Fig. 5(a). The as-synthesized alloy activated at room temperature (as reported in a previous article [25]) is also presented in Fig. 5(b) for comparison.

Activation of the heat-treated samples was carried out at 250 °C under 20 bars of hydrogen, as no activation took place at room temperature after 24 h of exposure to hydrogen. The alloy treated at 600 °C absorbed 0.95 wt%, and the alloy treated at 1300 °C absorbed 1.10 wt %. The activation of the as-synthesized (Hf<sub>0.75</sub>Ti<sub>0.25</sub>NbVZr) alloy occurs at room temperature, whereas the heat-treated alloys require a temperature of 250 °C for first absorption. This need for a higher activation temperature is likely due to alterations of both the phase composition and the abundance induced by the heat treatment.

The gravimetric hydrogen content of the as-cast alloy (Fig. 5(b)) is higher than the values recorded during activation of the two heat-treated samples. However, since the activations were not carried out at the same temperature, it is difficult to compare these results. For this reason, the activation of the as-synthesis alloy was also carried out at 250 °C, as shown in Fig. 5(c).

The as-synthesized alloy activates at 250 °C absorbs only 1.18 wt% compared to 2.05 wt% for the activation at room temperature. This capacity is close to that of the alloy heat-treated at 1300 °C (1.10 wt%). This result confirms that the activation of the alloy heat-treated at 1300 °C or 600 °C is probably hindered by the equilibrium pressure at 250 °C, leading to incomplete hydrogen uptake. Nevertheless, for all the studied compounds, the activation occurs after an incubation period of a few minutes and the maximum absorption capacity is reached quickly (within approximately one minute).

It is worth to note that the absorption at 250 °C is probably not the full absorption and the sample continued to absorb hydrogen during cooling from 250 °C to room temperature and so the real maximum

<caption>**Table 3**</caption>

Chemical composition (in at%, EDS measurements) of Hf<sub>0.75</sub>Ti<sub>0.25</sub>NbVZr (x = 0.25) heat-treated at 1300 °C for 24 h and 48 h. Accuracy is around 1–2 at % for all measurements,*assumed to be the BCC phase according to phase abundance obtained from previous XRD data.

<table><thead><tr><th>Heat-treatment conditions</th><th>Phases</th><th>Hf</th><th>Ti</th><th>Nb</th><th>V</th><th>Zr</th></tr></thead><tbody><tr><td rowspan="2">24 h at 1300 °C</td><td>Nominal composition (at.%)</td><td>18.75</td><td>6.25</td><td>25</td><td>25</td><td>25</td></tr><tr><td>Bright (BCC*)</td><td>17</td><td>7</td><td>26</td><td>26</td><td>24</td></tr><tr><td rowspan="3">48 h at 1300 °C</td><td>Dark</td><td>13</td><td>2</td><td>16</td><td>55</td><td>14</td></tr><tr><td>Bright (BCC*)</td><td>16</td><td>8</td><td>28</td><td>26</td><td>21</td></tr><tr><td>Dark</td><td>17</td><td>4</td><td>18</td><td>34</td><td>27</td></tr></tbody></table>

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** SEM (Scanning Electron Microscopy) images in backscattered (BSE) mode of Hf<sub>0.75</sub>Ti<sub>0.25</sub>NbVZr (x = 0.25) alloy heat treated at 1300 °C for (a) 24 h and (b) 48 h.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Fig. 4.** SEM (Scanning Electron Microscopy) images in backscattered (BSE) mode of the x = 0.25 alloy heat-treated at 600 °C for (a) one month and (b) two months.</figcaption></figure>

<caption>**Table 4**</caption>

Chemical composition (in at%, EDS measurements) of Hf<sub>0.75</sub>Ti<sub>0.25</sub>NbVZr (x = 0.25) heat-treated at 600 °C for 1 and 2 months. Accuracy is around 1–2 at % for all measurements.

<table><thead><tr><th>Heat-treatment conditions</th><th>Phases</th><th>Hf</th><th>Ti</th><th>Nb</th><th>V</th><th>Zr</th></tr></thead><tbody><tr><td rowspan="4">1 month at 600 °C</td><td>Nominal composition (at%)</td><td>18.75</td><td>6.25</td><td>25</td><td>25</td><td>25</td></tr><tr><td>Gray matrix (BCC)</td><td>14</td><td>6</td><td>24</td><td>26</td><td>26</td></tr><tr><td>Dark regions (C14 Laves)</td><td>17</td><td>3</td><td>19</td><td>42</td><td>21</td></tr><tr><td>Bright dendrites (HCP)</td><td>17</td><td>7</td><td>22</td><td>25</td><td>28</td></tr><tr><td rowspan="3">2 months at 600 °C</td><td>Gray matrix (BCC)</td><td>16</td><td>7</td><td>27</td><td>26</td><td>24</td></tr><tr><td>Dark regions (C14 Laves)</td><td>14</td><td>3</td><td>20</td><td>42</td><td>21</td></tr><tr><td>Bright dendrites (HCP)</td><td>18</td><td>6</td><td>22</td><td>24</td><td>30</td></tr></tbody></table>

hydrogen capacity is higher than the one reported in Fig. 5.

The XRD diffraction patterns of the hydrides after activation at 250 °C are shown in Fig. 6, with the crystallographic parameters and phase abundances detailed in Table 5.

The hydride of the Hf<sub>0.75</sub>Ti<sub>0.25</sub>NbVZr alloy heat-treated at 1300 °C crystallizes in a single FCC crystal structure. In the diffractogram of this hydride, a broad peak is observed between 30° and 40° (2θ), which can be identified as a poorly crystallized phase with an abundance of 13 % (named amorphous in Table 5). As reported by Sleiman and Huot [26], this phenomenon can be explained by the combination of rapid hydrogen absorption kinetics and the high processing temperature. These conditions kinetically prevent the material from mechanically adapting to the new crystalline structure, resulting in the formation of an amorphous or poorly crystallized phase.

After hydrogenation, the Hf<sub>0.75</sub>Ti<sub>0.25</sub>NbVZr sample treated at 600 °C exhibits a mixture of one BCT phase and one C14 hexagonal phase. The HCP phase present before hydrogen sorption disappears after hydrogenation. The C14 phase having a hexagonal structure retains an abundance of 52 % in the hydrided state, closely matching its abundance before hydrogenation of 54 % (see Table 2). This phase experiences a significant unit cell volume expansion from 214.50 Å<sup>3</sup> (i.e., V/Metallic atoms = 17.87) to 259.46 Å<sup>3</sup> (i.e. V/Metallic atoms = 21.62) upon hydrogenation. For this phase, an estimation of the hydrogen content can be done by assuming a volume of hydrogen atom of 2.7 Å<sup>3</sup> [27]. The amount of hydrogen retained in this phase is estimated to be around 1.38H/M. For the BCC and HCP solid solutions, they reorganize during hydrogenation to form a single BCT phase. This BCT phase exhibits a cell volume per metallic atom of 25.56 Å<sup>3</sup>, which is comparable to that of the fully hydrogenated FCC phases, with a cell volume per metallic atom of approximately 24.49 Å<sup>3</sup>. For the BCC phase, a unit cell volume increase

of 9.95 Å<sup>3</sup> is recorded (from 40.18 to 50.13 Å<sup>3</sup>), while for the HCP phase, the volume increases by 5.4 Å<sup>3</sup> (from 45.71 Å<sup>3</sup> to 51.13 Å<sup>3</sup>). By assuming a volume of hydrogen atom of 2.7 Å<sup>3</sup> the amount of hydrogen is estimated to be around 2H/M for the BCC phase and 1H/M for the HCP phase.

Hydrogen absorption is strongly influenced by the composition and abundance of phases. Thermal treatment is an effective method for modifying phase composition, distribution, and crystallographic parameters. As-synthesized samples contain metastable phases, and their hydrogen sorption properties may be linked to the metastability of these secondary phases. Therefore, for hydrogen storage applications, the optimal synthesis method for the Hf<sub>0.75</sub>Ti<sub>0.25</sub>NbVZr alloy is arc melting without any subsequent thermal treatment as this alloy can be activated at room temperature and absorbed more hydrogen than the thermally treated ones.

### 3.4. TiNbVZr (x = 1): crystal structure

The results discussed previously for the Hf<sub>0.75</sub>Ti<sub>0.25</sub>NbVZr alloy demonstrate that the crystalline structure and microstructure are negligibly affected by the duration of thermal treatment. Consequently, for the TiNbVZr alloy (i.e., x = 1), the effect of treatment duration was not considered. Experiments were only conducted at 1300 °C for 48 h and at 600 °C for 1 month.

The X-ray diffraction patterns of the as-synthesized TiNbVZr alloy have already been reported in detail in a previous work [20] and presented in the supplement file (Figure S4 and Table S3). The alloy crystallized with a single BCC phase (a = 3.311 Å). The diffraction patterns after heat-treatment at 1300 °C for 48 h are shown in Fig. 7, with the corresponding crystallographic parameters summarized in Table 6.

The alloy heat-treated at 1300 °C crystallizes with a BCC structure (exhibiting a lattice parameter similar to that of the as-cast alloy) and a BCT structure. Additionally, low-intensity, unindexed peaks are observed on the diffraction pattern (at 38° and 42°).

Regarding the alloy heat-treated at 600 °C for 1 month, the diffraction patterns and corresponding analyses are presented in Fig. 8 and Table 7 respectively.

For the alloy heat-treated at 600 °C, three phases are observed: two with a BCC structures (a<sub>BCC1</sub> = 3.271 (1) Å and a<sub>BCC2</sub> = 3.464 (2) Å) and one with a C15 Laves structure. These results are rather consistent with the observations of King *et al.* [23], who reported the decomposition of the BCC phase in TiNbVZr alloys into BCC, HCP, and Laves phases after heat-treatment at 700 °C for 100 h. In addition, one could notice that the lattice parameters of both BCC1 and BCC2 are respectively slightly smaller and highly bigger than the one of the as cast BCC. Nevertheless, considering the relative amount of each phase, it is possible to calculate an average radius (e.g. (3.271 *0.59) + (3.464 *0.22) = 3.32 Å) which is very close to the one of the as cast BCC (e.g. 3.311 Å). Therefore, it is suggesting the reorganization of the single BCC phase into two distinct
<figure><img src="image_7.png" /><figcaption>Fig. 5. Activation curves (a) at 250 °C for Hf<sub>0.75</sub>Ti<sub>0.25</sub>NbVZr alloy heat-treated at 600 °C and 1300 °C, for the alloy synthesized by arc melting (b) activation curve at room temperature, and (c) activation curve at 250 °C. Note that time is displayed in logarithmic scale.</figcaption></figure>

BCC phases.

### 3.5. TiNbVZr ($x = 1$): microstructure

Backscattered electron (BSE) micrographs and energy-dispersive

spectroscopy (EDS) analyses for the alloy thermally treated at 1300 °C for 48 h were performed and are presented in Fig. 9 and Table 8.

The microstructure of the alloy heat-treated at 1300 °C for 48 h shows only a single phase, corroborating findings by Senkov et al. [18], who reported the stability of the TiNbVZr alloy after thermal treatment at 1400 °C. This implies that the compositions of both the BCC and the BCT phases (i.e. Table 6) are very similar to each other, which suggests that the system is quite metastable. The dark regions highlighted in Fig. 9, correspond to voids likely created during alloy solidification.

For the alloy thermally treated at 600 °C for 1 month, the BSE micrographs and EDS analyses are presented in Fig. 10, and Table 9.

The alloy thermally treated at a lower temperature (600 °C, Fig. 10) displays “dark” regions slightly enriched in V and “bright” regions slightly enriched in Zr (i.e., Table 9). The chemical composition of these phases, determined by EDS, correlates with the crystalline structures deduced from diffraction analyses: The “bright gray” regions are associated with the BCC2 structure (a = 3.464 (2) Å), while the “dark” regions correspond to the BCC1 structure (a = 3.271 (1) Å). This correlation arises because the Zr-enriched regions (bright regions) have a higher mean atomic number Z compared to the V-enriched regions (dark regions), which are depleted in Zr. No regions could be attributed to the C15 phase, which could potentially indicate that the C15 phase has a composition similar to one of the BCC phases. However, if we calculate the average radius according to the chemical composition of both BCC phases, we obtain 1.46 Å and 1.47 Å. This very small difference (about 0.72 %) cannot explain the rather large difference in cell parameters (a<sub>BCC1</sub> = 3.271 (1) Å, a<sub>BCC2</sub> = 3.464 (2) Å). Therefore, this means that the BCC phase having the smallest parameter is more compact than the one having the larger cell parameter (a<sub>BCC1</sub> = 3.271 (1) Å, a<sub>BCC2</sub> = 3.464 (2) Å). This fact is corroborated by the micro deformation value (0.14 % for the BCC1 and 0.70 % for the BCC2). It can indicate that the two phases have different thermodynamic stability.

These findings align reasonably well with the conclusions of Senkov et al. [18], who reported the presence of three BCC phases with different lattice parameters after thermal treatment of the TiNbVZr alloy at 1000 °C. In their study, one phase was enriched in Zr, another was enriched in V and Nb, and a third phase (not observed in our study) exhibited a composition close to the nominal alloy composition.

### 3.6. TiNbVZr ($x = 1$): hydrogen absorption properties

The hydrogen absorption properties were evaluated at 250 °C under a hydrogen pressure of 20 bar, as no absorption was observed after 24 h at room temperature. The results, shown in Fig. 11, indicate that the TiNbVZr alloy heat-treated at 600 °C for 1 month and at 1300 °C for 48 h achieved maximum hydrogen absorption capacities of 1.40 wt% and 1.30 wt%, respectively.

The as-cast TiNbVZr alloy absorbs hydrogen at room temperature, while heat-treated alloys require activation at 250 °C. The hydrogen storage capacity of the as-cast alloy activated at room temperature or 250 °C exceeds that of heat-treated alloys. These results are likely due to microstructural changes or phase transformations induced during the heat-treatment process. For all studied compounds, activation is achieved after an incubation period of several minutes, but after, the reaction kinetics are rather fast.

Following hydrogenation, the hydrides were analyzed by XRD. Fig. 12 shows the XRD diffraction pattern of the hydrides activated at 250 °C, with crystal structure parameters and phase abundances are listed in Table 10.

The hydride of the TiNbVZr alloy treated at 1300 °C (having a BCC and a BCT structure before hydrogenation) crystallizes in an FCC and BCT crystal structure upon hydrogenation. Most probably, the BCC absorbs hydrogen to form an FCC while the BCT remains unchanged. By assuming a volume of hydrogen atom of 2.7 Å³, the amount of hydrogen in the FCC is estimated to be 1.67 H/M (2.43 wt%). The BCT phase before hydrogenation having a cell volume of 47.13 Å³ (i.e. V/Z = 11.8)
<figure><img src="image_8.png" /><figcaption>**Fig. 6.** X-ray diffraction patterns of Hf<sub>0.75</sub>Ti<sub>0.25</sub>NbVZr alloy hydrides after activation at 250 °C under 20 bars of hydrogen (a) Treated at 1300 °C for 24 h, (b) treated at 600 °C for 2 months, (c) as-synthesized. The lower red line in each diffractogram represents the residuals from the Rietveld refinement.</figcaption></figure>

<caption>**Table 5**</caption>

Crystal structure parameters of Hf<sub>0.75</sub>Ti<sub>0.25</sub>NbVZr alloy hydrides in the as-synthesized state and after thermal treatment, following activation at 250 °C under 20 bars of hydrogen. Errors in the last significant digit are indicated in parentheses.

<table><thead><tr><th>Preparation method</th><th>Phases</th><th>Lattice parameter</th><th>Cell volume</th></tr></thead><tbody><tr><td rowspan="2">As-synthesized<br>activated at 20 bars of H<sub>2</sub> at<br>250 °C</td><td>FCC (88 %<br>± 5)</td><td>4.606 (3)</td><td>97.76 (3)</td></tr><tr><td>C15 (11 %<br>± 2)</td><td>8.016 (2)</td><td>513 (5)</td></tr><tr><td rowspan="2">Heat-treated at 1300 °C for<br>24 h<br>activated at 20 bars of H<sub>2</sub> at<br>250 °C</td><td>FCC (87 %<br>± 5)</td><td>4.609 (2)</td><td>97.92 (15)</td></tr><tr><td>13 % amorphous</td><td></td><td></td></tr><tr><td rowspan="2">Heat-treated at 600 °C for 2<br>months<br>activated at 20 bars of H<sub>2</sub> at<br>250 °C</td><td>C14 (52 %<br>± 7)</td><td>a = 5.687 (1)<br>c = 9.265 (2)</td><td>259.46 (7)</td></tr><tr><td>BCT (48 %<br>± 8)</td><td>a = 3.348 (1)<br>c = 4.560 (3)</td><td>51.13 (5)</td></tr></tbody></table>

remains unchanged after hydrogenation (47.42 Å³, i.e., V/Z = 11.8), demonstrating no hydrogen is absorbed in this phase.

After hydrogenation, the TiNbVZr alloy treated at 600 °C exhibits a mixture of one BCT phase, one FCC phase and one C15 Laves phase. The C15 Laves phase retains an abundance of 22 % in the hydried state, closely matching its initial abundance of 19 % in the as-synthesized state. Upon hydrogenation this phase experiences a significant unit

<figure><img src="image_9.png" /><figcaption>**Fig. 7.** X-ray diffraction pattern of the TiVZrNb alloy heat-treated at 1300 °C for 48 h.</figcaption></figure>

cell volume expansion from 423.34 Å³ (i.e., V/Z = 17.6) to 510.72 Å³ (i.e., V/Z = 21.30). For this C15 Laves phase and by assuming a volume of hydrogen atom of 2.7 Å³ the amount of hydrogen retained in this phase
<caption>**Table 6**</caption>

Crystal structure parameters of heat-treated at 1300 °C for 48 h TiNbVZr (x = 1) alloy. The error on the last significant digit is indicated between brackets.

<table><thead><tr><th>Preparation method</th><th>Phases</th><th>Lattice parameter (Å)</th><th>Cell volume (Å³)</th></tr></thead><tbody><tr><td rowspan="3">Heat-treated at 1300 °C for 48 h</td><td>BCC (54 % ± 5)</td><td>3.314 (1)</td><td>36.39 (3)</td></tr><tr><td>BCT (46 % ± 4)</td><td>a = 3.221 (1)<br>c = 4.545 (2)</td><td>47.13 (3)</td></tr><tr><td colspan="3">Not identified (unknown)</td></tr></tbody></table>

<figure><img src="image_10.png" /><figcaption>**Fig. 8.** X-ray diffraction pattern of the TiVZrNb alloy heat-treated at 600 °C for 1 month.</figcaption></figure>

<caption>**Table 7**</caption>

Crystal structure parameters of heat-treated at 600 °C for 1 month TiNbVZr (x = 1) alloy. The error on the last significant digit is indicated between brackets.

<table><thead><tr><th>Preparation method</th><th>Phases</th><th>Lattice parameter (Å)</th><th>Cell volume (Å³)</th></tr></thead><tbody><tr><td rowspan="3">Heat-treated at 600 °C for 1 month</td><td>BCC1 (59 % ± 7)</td><td>3.271 (1)</td><td>36.39 (3)</td></tr><tr><td>BCC2 (22 % ± 4)</td><td>3.464 (2)</td><td>41.55 (7)</td></tr><tr><td>C15 (19 % ± 2)</td><td>7.509 (7)</td><td>423.3 (2)</td></tr></tbody></table>

is estimated to be around 1.35 H/M. For the two BCC solid solutions, they reorganize during hydrogenation to form one BCT phase and an FCC phase. Since it is difficult to determine which BCC phase crystallizes in FCC or BCT, no hydrogen absorption estimation is done for the FCC or BCT phases.

For hydrogen storage applications, the optimal synthesis route for the TiNbVZr alloy is arc melting without subsequent heat-treatment, as this method enables room-temperature activation and results in higher hydrogen absorption compared to heat-treated alloys.

## 4. Conclusion

This study explores the microstructure, crystalline structure, and hydrogenation behavior of heat-treated high-entropy alloys (HEAs) with compositions Hf₀.75Ti₀.25NbVZr (x = 0.25) and TiNbVZr (x = 1).

<figure><img src="image_11.png" /><figcaption>**Fig. 9.** SEM (Scanning Electron Microscopy) images in backscattered (BSE) mode of the x = 1 alloy heat-treated at 1300 °C for 48 h.</figcaption></figure>

<caption>**Table 8**</caption>

Chemical composition (in at%) of the phases in the studied heat-treated alloy at 1300 °C for 48 h. The accuracy is about 1–2 at% for all the measurements.

<table><thead><tr><th>Heat-treatment conditions</th><th>Phases</th><th>Ti</th><th>Nb</th><th>V</th><th>Zr</th></tr></thead><tbody><tr><td rowspan="2">48 h at 1300 °C</td><td>Nominal composition (at %)</td><td>25</td><td>25</td><td>25</td><td>25</td></tr><tr><td>Gray matrix (BCC)</td><td>25</td><td>25</td><td>27</td><td>23</td></tr></tbody></table>

<figure><img src="image_12.png" /><figcaption>**Fig. 10.** SEM (Scanning Electron Microscopy) images in backscattered (BSE) mode of the x = 1 alloy heat-treated at 600 °C for 1 month.</figcaption></figure>

<caption>**Table 9**</caption>

Chemical composition (in at%) of the phases in the studied heat-treated alloy at 600 °C for 1 month. The accuracy is about 1–2 at% for all the measurements.

<table><thead><tr><th>Heat-treatment Conditions</th><th>Phases</th><th>Ti</th><th>Nb</th><th>V</th><th>Zr</th></tr></thead><tbody><tr><td rowspan="3">1 month at 600 °C</td><td>Nominal composition (at %)</td><td>25</td><td>25</td><td>25</td><td>25</td></tr><tr><td>Dark gray matrix (BCC 1)</td><td>25</td><td>25</td><td>28</td><td>22</td></tr><tr><td>Bright gray matrix (BCC 2)</td><td>23</td><td>23</td><td>26</td><td>28</td></tr></tbody></table>

1) The as-cast Hf₀.75Ti₀.25NbVZr alloy exhibits a mixture of BCC and C15 Laves phases, which transform into a single-phase BCC structure after heat-treatment at 1300 °C or a combination of BCC, HCP, and
<figure><img src="image_13.png" /><figcaption>**Fig. 11.** Activation curves under 20 bar of hydrogen and (a) at 250 °C for the TiNbVZr alloy heat-treated at 600 °C and 1300 °C, (b) at room temperature for the as-cast TiNbVZr alloy (c) at 250 °C for the as-cast TiNbVZr alloy.</figcaption></figure>

C14 Laves phases following treatment at 600 °C. Phase composition remains unaffected by treatment duration, with similar results obtained after treatments of 24 h or 48 h at 1300 °C and 1 or 2 months at 600 °C.

2) Hydrogenation studies reveal that the Hf<sub>0.75</sub>Ti<sub>0.25</sub>NbVZr alloy is best synthesized through arc melting without subsequent heat treatment. Unlike heat-treated alloys, which absorbs hydrogen at 250 °C, the arc-melted alloy absorbs hydrogen at room temperature and exhibits

<figure><img src="image_14.png" /><figcaption>**Fig. 12.** X-ray diffraction patterns of TiNbVZr hydrides after activation at 250 °C under 20 bars of hydrogen (a) heat-treated alloy at 1300 °C for 48 h, (b) heat-treated alloy at 600 °C for 1 month, (c) As-synthesized.</figcaption></figure>

a significantly higher storage capacity (2.2 wt%) compared to alloys heat-treated at 1300 °C (1.1 wt%) and 600 °C (0.95 wt%).

3) The TiNbVZr (x = 1) alloy, synthesized by arc melting, crystallizes in a BCC structure. Heat-treatment at 1300 °C results in a mixture of BCC and BCT phases, while treatment at 600 °C results in three phases: two BCC phases with distinct lattice parameters and a C15 Laves phase. As observed with Hf<sub>0.75</sub>Ti<sub>0.25</sub>NbVZr, the arc-melted
<caption>**Table 10**</caption>

Crystal structure parameters of TiNbVZr hydrides in the as-synthesized state and after heat-treatment, following activation at 250 °C under 20 bars of hydrogen. The error on the last significant digit is indicated between brackets.

<table><thead><tr><th>Preparation method</th><th>Phases</th><th>Lattice parameter (Å)</th><th>Cell volume (Å³)</th></tr></thead><tbody><tr><td>As-synthesized<br>activated at 20 bars of H₂<br>at 250 °C</td><td>FCC</td><td>4.545 (1)</td><td>93.91 (4)</td></tr><tr><td>Heat-treated at 1300 °C<br>for 48 h<br>activated at 20 bars of H₂<br>at 250 °C</td><td>FCC (55 %<br>± 2)<br>BCT (45 %<br>± 3)</td><td>4.495 (1)<br>a = 3.226 (2)<br>c = 4.557 (6)</td><td>90.80 (7)<br>47.42 (8)</td></tr><tr><td>Heat-treated at 600 °C for<br>1 month<br>activated at 20 bars of H₂<br>at 250 °C</td><td>BCT (44 %<br>± 5)<br>FCC (34 %<br>± 4)<br>C15 (22 %<br>± 2)</td><td>a = 3.396 (1)<br>c = 4.381 (5)<br>4.493 (1)</td><td>50.54 (7)<br>90.67 (2)</td></tr></tbody></table>

TiNbVZr as-synthesized alloy exhibits the best hydrogen storage performance (2.7 wt% for the as-synthesized alloy vs. 1.4 wt% and 1.30 wt% for the alloy heat-tread at 600 °C and 1300 °C).

4) For heat-treated alloys, hydrogen activation requires elevated temperatures, likely due to heat-induced changes in phase composition. Heat-treatment significantly reduces hydrogen storage capacity, attributed to a decreased abundance of the BCC phase and modifications in phase composition, distribution, and crystallographic parameters.

### CRediT authorship contribution statement

**M. Moussa:** Methodology, Formal analysis, Investigation, Writing – original draft, Visualization, Validation, Project administration. **C. Cona:** Methodology, Formal analysis, Investigation. **J. Huot:** Conceptualization, Validation, Resources, Writing – review & editing, Project administration, Supervision, Funding acquisition. **J.-L. Bobet:** Conceptualization, Validation, Resources, Writing – review & editing, Project administration, Supervision, Funding acquisition.

### Funding

This research was funded by the Region Nouvelle Aquitaine and the Canadian government.

### Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgments

The authors thank the Region Nouvelle Aquitaine and the Canadian government for financial support to this work through a PhD grant. The authors also thank the ANR through the ANR-22-PEHY-0007 for some financial support.

### Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jacomc.2025.100089.

### Data availability

Data available by contacting the authors.

### References

[1] O.N. Senkov, J.M. Scott, S.V. Senkova, D.B. Miracle, C.F. Woodward, Microstructure and room temperature properties of a high-entropy TaNbHfZrTi alloy, J. Alloy. Compd. 509 (20) (2011) 6043–6048, https://doi.org/10.1016/j.jallcom.2011.02.171.

[2] É. Fazakas, V. Zadorozhnyy, L.K. Varga, A. Inoue, D.V. Louzguine-Luzgin, Fuyang Tian, L. Vitos, Experimental and theoretical study of Ti20Zr20Hf20Nb20X20 (X=V or Cr) refractory high-entropy alloys, Int. J. Refract. Met. Hard Mater. 47 (2014) 131–138, https://doi.org/10.1016/j.jirmhm.2014.07.009.

[3] O.N. Senkov, G.B. Wilks, J.M. Scott, D.B. Miracle, Mechanical properties of Nb25Mo25Ta25W25 and V20Nb20Mo20Ta20W20 refractory high entropy alloys, Intermetallics 19 (5) (2011) 698–706, https://doi.org/10.1016/j.intermet.2011.01.004.

[4] Kai-Chi Lo, Hideyuki Murakami, Jien-Wei Yeh, An-Chou Yeh, Oxidation behaviour of a novel refractory high entropy alloy at elevated temperatures, Intermetallics 119 (2020) 106711, https://doi.org/10.1016/j.intermet.2020.106711.

[5] I.Kunce, Microstructure and hydrogen storage properties of a TiZrNbMoV high entropy alloy synthesized using laser engineered net shaping (LENS), Int. J. Hydrog. Energy 39 (2014) 9904–9910.

[6] Qiwen Lai, Mark Paskevicius, Drew A. Sheppard, Craig E. Buckley, Aaron W. Thornton, Matthew R. Hill, Qinfen Gu, et al., Hydrogen storage materials for mobile and stationary applications: current state of the art, ChemSusChem 8 (17) (2015) 2789–2825, https://doi.org/10.1002/cssc.201500231.

[7] P. Dantzer, Metal-hydride technology: a critical review, in: H. Wipf (Ed.), Hydrogen in Metals III. Topics in Applied Physics, Springer, Berlin, Heidelberg, 1997, https://doi.org/10.1007/BFb0103405.

[8] M. Sahlberg, D. Karlsson, C. Zlotea, et al., Superior hydrogen storage in high entropy alloys, Sci. Rep. 6 (2016) 36770, https://doi.org/10.1038/srep36770.

[9] Chien-Chang Juan, Ming-Hung Tsai, Che-Wei Tsai, Chun-Ming Lin, Woeri-Ren Wang, Chih-Chao Yang, Swe-Kai Chen, Su-Jien Lin, Jien-Wei Yeh, Enhanced mechanical properties of HfMoTaTiZr and HfMoNbTaTiZr refractory high-entropy alloys, Intermetallics 62 (2015) 76–83, https://doi.org/10.1016/j.intermet.2015.03.013.

[10] Sheng Guo, Qiang Hu, Chun Ng, C.T. Liu, More than entropy in high-entropy alloys: forming solid solutions or amorphous phase, Intermetallics 41 (2013) 96–103, https://doi.org/10.1016/j.intermet.2013.05.002.

[11] Yu-Liang Chen, Che-Wei Tsai, Chien-Chang Juan, Ming-Hao Chuang, Jien-Wei Yeh, Tsung-Shune Chin, Swe-Kai Chen, Amorphization of equimolar alloys with HCP elements during mechanical alloying, J. Alloy. Compd. 506 (1) (2010) 210–215, https://doi.org/10.1016/j.jallcom.2010.06.179.

[12] M.C. Gao, B. Zhang, S.M. Guo, et al., High-entropy alloys in hexagonal close-packed structure, Met. Mater. Trans. A 47 (2016) 3322–3332, https://doi.org/10.1007/s11661-015-3091-1.

[13] E.J. Pickering, N.G. Jones, High-entropy alloys: a critical assessment of their founding principles and future prospects, Int. Mater. Rev. 61 (3) (2016) 183–202, https://doi.org/10.1080/09506608.2016.1180020.

[14] F. Stein, M. Palm, G. Sauthoff, Structure and stability of Laves phases part II—structure type variations in binary and ternary systems, Intermetallics 13 (10) (2005) 1056–1074, https://doi.org/10.1016/j.intermet.2004.11.001.

[15] Wei Zhang, Minsu Park, Haeju Jo, Chanho Park, Je. In Lee, Wookjin Lee, Microstructure, mechanical properties, and shape memory behavior of laser-directed energy deposited Co–20Fe–18Cr–19Mn high-entropy alloy, Mater. Sci. Eng. A 917 (2024) 147421, https://doi.org/10.1016/j.msea.2024.147421.

[16] Liyang Fang, Chenran Xu, Yongkang Tan, Xiaoma Tao, Guanglong Xu, Yifang Ouyang, Yong Du, Composition control and heat treatment effect on characteristics of ZrNbTaHf refractory high entropy alloys, Int. J. Refract. Met. Hard Mater. 120 (2024) 106597, https://doi.org/10.1016/j.ijrmhm.2024.106597.

[17] Cheenepalli Nagarjuna, Sheetal Kumar Dewangan, Hansung Lee, Byungmin Ahn, Evolution of phase stability and structural properties in CrFeNiTiV high-entropy alloy under high-temperature heat treatment conditions, Mater. Sci. Eng. A 886 (2023) 145680, https://doi.org/10.1016/j.msea.2023.145680.

[18] O.N. Senkov, S. Rao, K.J. Chaput, C. Woodward, Compositional effect on microstructure and properties of NbTiZr-based complex concentrated alloys, Acta Mater. 151 (2018) 201–215, https://doi.org/10.1016/j.actamat.2018.03.065.

[19] Nikita Y. Yurchenko, Nikita D. Stepanov, Mikhail A. Tikhonovsky, Gennady A. Salishchev, Phase evolution of the Al~x~NbTiVZr (x = 0; 0.5; 1; 1.5) high entropy alloys, Metals 6 (12) (2016) 298, https://doi.org/10.3390/met6120298.

[20] Maria Moussa, Stéphane Gorsse, Jacques Huot, Jean Louis Bobet, Effect of the synthesis route on the microstructure of Hf~x~Ti~(1-x)~NbVZr refractory high-entropy alloys, Metals 13 (2) (2023) 343, https://doi.org/10.3390/met13020343.

[21] A. Bruker, Topas V3: general profile and structure analysis software for powder diffraction data—user’s manual; Bruker AXS, Karlsruhe; Coehlo AA (2007) TOPAS academic, Coelho Software, Brisbane, 2005.

[22] Charles Kittel, Paul McEuen, Introduction to Solid State Physics, John Wiley & Sons, 2018.

[23] D.J.M. King, S.T.Y. Cheung, S.A. Humphry-Baker, C. Parkin, A. Couet, M.B. Cortie, G.R. Lumpkin, S.C. Middleburgh, A.J. Knowles, High temperature, low neutron cross-section high-entropy alloys in the Nb-Ti-V-Zr system, Acta Mater. 166 (2019) 435–446, https://doi.org/10.1016/j.actamat.2019.01.006.

[24] A. Munitz, L. Meshi, M.J. Kaufman, Heat treatments’ effects on the microstructure and mechanical properties of an equiatomic Al-Cr-Fe-Mn-Ni high entropy alloy, Mater. Sci. Eng. A 689 (2017) 384–394, https://doi.org/10.1016/j.msea.2017.02.072.
[25] Maria Moussa, Lambert van Eijck, Jacques Huot, Jean Louis Bobet, Structure analysis (XRD and Neutrons) and hydrogen storage properties of Hf1-xTixNbVZr BCC high entropy alloys, J. Alloy. Compd. 1010 (2025) 177103, https://doi.org/10.1016/j.jallcom.2024.177103.

[26] Salma Sleiman, Jacques Huot, Effect of particle size, pressure and temperature on the activation process of hydrogen absorption in TiVZrHfNb high entropy alloy,

J. Alloy. Compd. 861 (2021) 158615, https://doi.org/10.1016/j.jallcom.2021.158615.

[27] Salma Sleiman, Jacques Huot, Microstructure and first hydrogenation properties of TiHfZrNb₁-xV₁+x Alloy for x = 0, 0.1, 0.2, 0.4, 0.6 and 1, Molecules 27 (3) (2022) 1054, https://doi.org/10.3390/molecules27031054.