Phase Evolution and Deuterium Storage Properties of TiVNbZrCr High-Entropy Alloy: A Temperature-Resolved Synchrotron X-Ray Diffraction Study ^†

Abstract

TiVNbZrCr high-entropy intermetallic alloy was investigated as a deuterium storage material using gravimetric sorption measurements, thermogravimetric analysis, and temperature-resolved synchrotron X-ray diffraction during deuterium desorption. The as-prepared alloy had an experimentally determined composition of Ti₁₇V₁₉Zr₁₉Nb₂₂Cr₂₃ and a density of 6.59 g·cm⁻³. Empirical alloy-design parameters indicate that the alloy is not a single-phase bcc solid solution, but rather a compositionally complex intermetallic alloy. The calculated hydrogen-affinity descriptors suggest a strong thermodynamic driving force for deuteride formation. Under 5 MPa D₂, the alloy absorbed 3.28 wt.% D, corresponding to D/M = 1.1. After ex situ deuteration, additional diffraction reflections were indexed using tetragonal deuteride reference structures corresponding to ZrV₂D_2.35 and TiD₂, while the Cr-rich bcc phase remained comparatively stable. Thermal desorption released 2.28 wt.% D up to 600 °C in three partially overlapping steps. These results demonstrate that deuterium storage in TiVNbZrCr is governed by phase-selective deuteride formation and decomposition rather than by homogeneous bcc lattice expansion.

1. Introduction

Hydrogen is widely considered an essential energy carrier for future low-carbon energy systems; however, its efficient, safe and reversible storage remains a major materials challenge. Conventional compressed and liquefied hydrogen technologies require either high pressures or cryogenic temperatures, whereas solid-state storage in metal hydrides offers the possibility of high volumetric density and improved operational safety [1,2]. For practical applications, hydrogen storage materials must combine high gravimetric and volumetric capacity with fast absorption/desorption kinetics, moderate thermodynamic stability of the hydride phase, long-term cycling stability and acceptable cost [1,2,3]. Despite decades of research on intermetallic hydrides, including AB₅-, AB₂-, AB- and bcc-type alloys, the simultaneous optimization of capacity, reversibility and desorption temperature remains difficult [2,3].

In recent years, medium- and high-entropy alloys (MEAs/HEAs), more generally referred to as multi-principal-element alloys, have emerged as a promising platform for solid-state hydrogen storage [3,4,5]. Their compositional complexity provides a large number of chemically distinct local environments and interstitial sites, which may be used to tune hydrogen affinity, lattice expansion, phase stability and hydride formation pathways. Early work on the refractory TiVZrNbHf alloy demonstrated that high-entropy alloys can absorb exceptionally high amounts of hydrogen, with reported H/M ratios exceeding those of many conventional hydride-forming alloys [6]. Subsequent studies showed that the hydrogen storage performance of such alloys is highly sensitive to elemental composition, valence electron concentration, atomic-size mismatch, phase constitution and local lattice distortion [7,8,9,10]. Thus, the concept of high configurational entropy alone is insufficient to predict hydrogen storage properties, and detailed structural analysis during hydrogenation is required.

Among refractory multi-principal-element alloys, Ti–V–Nb-based systems are particularly attractive because Ti, V, Nb, and Zr are strong hydride-forming elements, whereas Cr can be used to modify phase stability, hydride thermodynamics, and desorption behavior [11]. TiVZrNb-based alloys have been reported to crystallize as bcc solid solutions and to form tetragonal or fcc-related hydride/deuteride phases upon hydrogenation or deuteration [8]. Similarly, Ti–V–Nb–Cr alloys can reach H/M ratios close to 2 with fast absorption kinetics; however, the desorption of hydrogen from stable hydrides often requires elevated temperatures [10,12]. The Ti_0.3V_0.25Cr_0.1Zr_0.1Nb_0.25 alloy, investigated by Bouzidi et al., rapidly absorbs hydrogen at room temperature up to approximately 2.0 H/M and transforms through a two-step bcc alloy → bcc monohydride → fcc dihydride reaction, as confirmed by synchrotron X-ray and neutron diffraction [13]. These results demonstrate the importance of diffraction methods for identifying transient hydride phases that cannot be reliably resolved from ex situ measurements alone.

Although many hydrogen-storage HEAs are designed to form disordered bcc solid solutions, increasing evidence suggests that multiphase and intermetallic high-entropy alloys can also exhibit favorable hydrogen storage behavior [13,14,15]. Intermetallic phases, especially Laves-type and related complex structures, may provide ordered sublattices, chemically heterogeneous interstitial sites and phase-boundary networks that promote hydrogen uptake or facilitate activation. This is particularly relevant for Ti–Zr–V–Nb–Cr compositions, where empirical phase-selection parameters such as the atomic-size mismatch Δ, mixing enthalpy ΔH_mix and valence electron concentration VEC do not necessarily place the alloy in the single-phase solid-solution region. Instead, the combination of large atomic-size mismatch and negative mixing enthalpy promotes the formation of intermetallic compounds or multiphase microstructures [4,5,14]. Therefore, the TiVNbZrCr alloy should not be considered merely as a conventional single-phase HEA, but rather as a compositionally complex intermetallic alloy whose hydrogen/deuterium storage properties are governed by phase evolution during gas–solid reaction.

The present work follows our preliminary study [16] and focuses deeply on the phase evolution and deuterium storage behavior of the TiVNbZrCr alloy using synchrotron X-ray diffraction. Deuterium absorption was investigated under controlled D₂ pressure in order to follow the structural transformations of the alloy during deuteration and subsequent heating. The study aims to identify the crystalline phases present in the as-prepared state, determine the deuteride phases formed during D₂ uptake, and correlate the observed phase transformations with the measured storage and desorption capacities. By combining deuterium sorption measurements, thermogravimetric desorption analysis and synchrotron diffraction, this work provides insight into the role of intermetallic phase constitution in the storage performance of Ti–V–Nb–Zr–Cr high-entropy intermetallic alloys.

2. Materials and Methods

2.1. Alloy Design

The investigated alloy was designed as an equiatomic TiVNbZrCr composition, combining early transition metals with high affinity to hydrogen/deuterium, namely Ti, V, Nb and Zr, with Cr as an alloying element affecting the phase stability and hydride/deuteride thermodynamics. Although TiVNbZrCr belongs to the family of compositionally complex multi-principal-element alloys, the alloy investigated in this work is not treated as a single-phase bcc high-entropy solid solution, but rather as a high-entropy intermetallic alloy. This classification is supported by its experimentally observed phase constitution and by the calculated empirical phase-selection parameters.

The thermodynamic and empirical parameters used for alloy classification were calculated from the nominal equiatomic composition. The configurational mixing entropy, atomic-size mismatch, mixing enthalpy and valence electron concentration were calculated using the following relationships:

$$\Delta S_{\mathrm{m}\mathrm{i}\mathrm{x}}=-R\sum _i{c}_i\mathrm{l}\mathrm{n}{c}_i$$

$$\delta =100\sqrt{\sum _i{c}_i{\left(1{\displaystyle \frac{r_i}{\overline{r}}}\right)}^2}$$

$$\overline{r}=\sum _i{c}_i{r}_i$$

$$\Delta H_{\mathrm{m}\mathrm{i}\mathrm{x}}=\sum _{i\ne j}4\Delta H_{ij}^{\mathrm{m}\mathrm{i}\mathrm{x}}{c}_i{c}_j$$

$$VEC=\sum _i{c}_i(VEC{)}_i$$

where $c_i$, $r_i$ and ${\left(VEC\right)}_i$ are the atomic fraction, atomic radius and valence electron concentration of the $i$-th element, respectively, $\overline{r}$ is the average atomic radius, $R$ is the universal gas constant, and $\Delta H_{ij}^{\mathrm{m}\mathrm{i}\mathrm{x}}$ is the binary mixing enthalpy of the corresponding equiatomic binary system. The binary mixing enthalpies and atomic parameters were taken from literature data commonly used for empirical phase prediction in high-entropy alloys [5,14,17]. The calculated parameters were used only as a qualitative guide, since phase formation in Ti–V–Nb–Zr–Cr alloys can be strongly affected by solidification segregation and by the formation of chemically ordered intermetallic phases.

Hydrogen Affinity

In addition to the empirical phase-selection parameters, the hydrogen affinity of the alloy was also evaluated. Hydrogen affinity describes the thermodynamic tendency of an alloy to dissolve hydrogen in interstitial sites and subsequently form an interstitial hydride/deuteride phase. In the present work, this tendency was estimated using the semi-empirical approach proposed by Griessen and co-workers and later applied to high-entropy alloy hydrides [18,19]. This approach considers two thermodynamic quantities: the enthalpy of hydrogen solution at infinite dilution, $\Delta H_{\mathrm{\infty }}$, and the enthalpy of formation of the concentrated hydride, $\Delta H_f$. The former reflects the initial interaction between isolated hydrogen atoms and the metallic lattice, whereas the latter approximates the thermodynamic stability of the fully developed hydride phase.

For a multicomponent alloy, both quantities were calculated by a linear rule-of-mixtures approximation over the metallic sublattice:

$$\begin{array}{c}\Delta H_{\mathrm{\infty }}={\displaystyle \sum _i}{c}_i\Delta H_{\mathrm{\infty },i}\end{array}$$

where $c_i$ is the atomic fraction of the $i$-th alloying element and $\Delta H_{\mathrm{\infty },i}$ is the enthalpy of hydrogen solution at infinite dilution in the corresponding pure metal. Similarly, the enthalpy of formation of the concentrated hydride was calculated as:

$$\begin{array}{c}\Delta H_f={\displaystyle \sum _i}{c}_i\Delta H_{f,i}\end{array}$$

where $\Delta H_{f,i}$ is the enthalpy of formation of the concentrated hydride of the $i$-th pure metal. In this approximation, more negative values of $\Delta H_{\mathrm{\infty }}$ and $\Delta H_f$ indicate a stronger thermodynamic driving force for hydrogen/deuterium uptake and hydride/deuteride formation, whereas positive or weakly negative values suggest limited affinity to hydrogen.

2.2. Alloy Preparation

The TiVNbZrCr alloy was prepared by arc melting from pure elemental pieces of Ti, V, Nb, Zr and Cr under a high-purity argon atmosphere. The starting elements had purities Ti (99.99%), V (99. 5%), Nb (99.8%), Zr (99.5%), Cr (99.99%). Prior to melting, the arc-melting chamber was repeatedly evacuated and backfilled with argon in order to minimize oxygen contamination. A Ti getter was melted before alloy synthesis to further reduce the residual oxygen partial pressure in the chamber.

Due to the high melting point of Nb, Ti and Nb were first pre-melted to form a binary Ti–Nb precursor button. Subsequently, V, Zr and Cr were added and the complete TiVNbZrCr alloy was melted. The alloy button was re-melted five times and turned over between consecutive melting steps to improve chemical homogeneity. After solidification, the alloy button was visually inspected for macroscopic cracks and inhomogeneities. The mass loss after arc melting was below 1 wt.%, indicating negligible evaporation of the constituent elements during synthesis.

For diffraction, deuterium sorption and thermal desorption experiments, the alloy button was mechanically crushed and pulverized. The powder fraction below 45 μm was separated by sieving under an inert atmosphere and used for further experiments. Powder preparation under inert conditions was applied to reduce surface oxidation before deuterium exposure.

2.3. Density and Chemical Characterization

The density of the alloy was determined by the Archimedes method with precise laboratory scales, the Kern ABT 120-4M fitted with the specialized ABT-A01 adapter for density determination (KERN & SOHN GmbH, Balingen, Germany). The measured density was 6.59 g·cm⁻³.

The chemical composition was determined using energy-dispersive X-ray spectroscopy (EDS) on a Jeol JSM 7000F scanning electron microscope (JEOL Ltd., Akishima, Tokyo, Japan), equipped with energy-dispersive X-ray spectroscopy (EDS). Metallographic cross-sections were prepared from the bulk alloy by standard grinding and polishing procedures. The final polishing step was performed using colloidal silica. EDS measurements were carried out at low 50× magnification and data were taken from ~5 mm² area. The average experimentally determined composition was Ti₁₇V₁₉Zr₁₉Nb₂₂Cr₂₃, which is close to the nominal equiatomic composition within 3 at.%.

2.4. Deuterium Absorption Measurements

Deuterium absorption was measured gravimetrically using a magnetic suspension balance (IsoSORP series, TA Instruments, New Castle, DE, USA), which can operate at pressures up to 50 MPa with a measurement accuracy of 0.05%. Approximately 5.078 g of the TiVNbZrCr powder was placed into the reaction chamber. The sample was measured according to the following protocol:

• Approximately 5.078 g of powder alloy was placed into the reaction chamber of the magnetic suspension balance. The system was then sealed and evacuated to a rotary pump vacuum < 0.02 bar (2 kPa).
• The alloy was activated by exposure to high-purity D₂ gas (99.89%, Linde GmbH, Pullach, Germany) of pressure 0.1 MPa and heated to 200 °C for 2 h.
• In the next step, the D₂ pressure was increased to 5 MPa (50 bar), and the sample was cooled down to room temperature. The cooling and stabilization process lasted approximately 1200 min (20 h). During this stage, the mass increase in the sample caused by D₂ absorption was continuously monitored.
• The deuterium absorption capacity of the sample was subsequently measured at a constant D₂ pressure of 5 MPa (50 bar) during heating up to 200 °C, where a rapid increase in sample mass was observed. The sample was kept under deuterium until the absorption curve reached a stable plateau. The absorbed deuterium content was calculated from the mass increase after buoyancy correction and expressed both as wt.% D and as the deuterium-to-metal atomic ratio D/M.

The deuterium content was calculated according to:

$$w_{\mathrm{D}}={\displaystyle \frac{m_{\mathrm{D}}}{m_{\mathrm{a}\mathrm{l}\mathrm{l}\mathrm{o}\mathrm{y}}}}\times 100$$

where $m_{\mathrm{D}}$ is the mass of absorbed deuterium and $m_{\mathrm{a}\mathrm{l}\mathrm{l}\mathrm{o}\mathrm{y}}$ is the initial mass of the alloy. The D/M ratio was calculated using the average molar mass of the experimentally determined alloy composition:

$$D/M={\displaystyle \frac{n_{\mathrm{D}}}{n_{\mathrm{M}}}}={\displaystyle \frac{\frac{m_{\mathrm{D}}}{M_{\mathrm{D}}}}{\frac{m_{\mathrm{a}\mathrm{l}\mathrm{l}\mathrm{o}\mathrm{y}}}{{\overline{M}}_{\mathrm{a}\mathrm{l}\mathrm{l}\mathrm{o}\mathrm{y}}}}},$$

where $n_{\mathrm{D}}$ is the amount of absorbed deuterium atoms, $n_{\mathrm{M}}$ is the total amount of metal atoms in the alloy, $M_{\mathrm{D}}$ is the molar mass of deuterium atom, and ${\overline{M}}_{\mathrm{a}\mathrm{l}\mathrm{l}\mathrm{o}\mathrm{y}}$ is the average molar mass of the alloy calculated from the experimentally determined chemical composition:

$${\overline{M}}_{\mathrm{a}\mathrm{l}\mathrm{l}\mathrm{o}\mathrm{y}}=\sum _i{c}_i{M}_i$$

where $c_i$ and $M_i$ are the atomic fraction and molar mass of the $i$-th metallic element, respectively. Buoyancy corrections were applied using standard procedures for high-pressure gravimetric gas sorption measurements [20].

2.5. Synchrotron X-Ray Diffraction

The synchrotron X-ray diffraction experiments were performed at ambient pressure and not under a high-pressure D₂ atmosphere. First, the as-prepared TiVNbZrCr powder was measured by high-energy synchrotron X-ray diffraction. In a separate experiment, the powder was deuterated ex situ under 5 MPa D₂ in the magnetic suspension balance, recovered after the absorption experiment, and subsequently measured by synchrotron X-ray diffraction under ambient-pressure conditions. Temperature-resolved synchrotron diffraction of the deuterated powder was then performed during heating in order to follow structural changes associated with deuterium desorption. Therefore, no diffraction patterns were collected during the absorption step under high-pressure D₂ gas.

Temperature-resolved synchrotron X-ray diffraction was performed in transmission (Debye–Scherrer) geometry at the P21.1 undulator beamline located at the electron storage ring PETRA III (storing electron of energy: 6.0 GeV and current: 100 mA, operated in top-up mode) at DESY (Hamburg, Germany). The following setup was applied: transmission (Debye–Scherrer) geometry; monochromatized high energy X-ray beam of photon energy ~ 103.06 keV (λ = 0.1203 Å) to obtain high quality diffraction patterns up to the magnitude of the scattering vector Q_max = 4πsin (θ)/λ = 18 Å⁻¹; beam cross-section on the sample was ~ 0.5 mm × 0.5 mm; a fast 2D image plate detector Perkin Elmer XRD 1621 (2048 pixels × 2048 pixels, size of a pixel: 200 μm × 200 μm (Varex Imaging, Salt Lake City, UT, USA)) to record diffracted X-rays [21].

The recorded 2D XRD patterns were integrated into one-dimensional I(Q) using the Fit2D software(Version: V12.077) [22] from which the X-ray total structure factors S(Q) were obtained, based on the Faber–Ziman approach [23]. Phase identification was performed by the MATCH! software(Version: 3.8.1.143 64-bit) [24] using the crystallographic open database (COD) [25].

For visualization of the phase evolution, sequential diffraction patterns were plotted as temperature-resolved two-dimensional intensity maps. In addition, a structural-change parameter was calculated from the integrated absolute difference between consecutive X-ray total structure factors according to:

$$\sum \left[S(Q{)}_T-S(Q{)}_{T-5K}\right]$$

where $S(Q{)}_T$ and $S(Q{)}_{T-5K}$ are the X-ray total structure factors obtained at two consecutive temperature steps. Absolute values, rather than squared differences, were used. The parameter was calculated over the same Q range and on the same Q grid for all patterns after standard normalization of the total structure factors according to the Faber–Ziman formalism. The resulting quantity was used as a relative indicator of structural change between consecutive temperature steps and was not intended as an absolute thermodynamic or kinetic parameter.

2.6. Thermal Desorption Analysis

After deuteration, the deuterated TiVNbZrCr powder was subjected to thermogravimetric analysis (TGA) in order to determine the amount of reversibly released deuterium and the temperature range of desorption. The measurements were performed using the simultaneous thermal analyzer Netsch Jupiter STA 449-F1 analyzer (Selb, Germany), under flowing high-purity Ar. The sample was heated from room temperature to 600 °C at a heating rate of 10 K·min⁻¹.

3. Results and Discussion

3.1. Alloy Classification, Composition and Deuterium Affinity

The experimentally determined composition of the investigated alloy was Ti₁₇V₁₉Zr₁₉Nb₂₂Cr₂₃, which is close to the nominal equiatomic TiVNbZrCr composition within 3 at.%. The measured density was 6.59 g·cm⁻³. The calculated alloy design parameters and deuterium storage data are summarized in

Table 1. Experimentally determined composition, phase classification, density, deuterium storage capacity and calculated alloy-design parameters of the TiVNbZrCr alloy. IM denotes intermetallic alloy; ΔH_mix is the mixing enthalpy; Δ is the atomic-size mismatch; VEC is the valence electron concentration; ΔH_∞ and ΔH_f denote the enthalpy of hydrogen solution at infinite dilution and the formation enthalpy of the concentrated hydride, respectively.

Sample (EDX Composition) [at.%]	Phase	Density [g·cm−3]	Absorbed D [wt.%] (D/M)	Desorbed D [wt.%]	ΔHmix [kJ·mol−1]	Δ [%]	VEC	ΔH∞ [kJ·mol−1]	ΔHf [kJ·mol−1]
TiVNbZrCr (Ti17V19Nb22Zr19Cr23)	IM	6.59	3.28 (1.1)	2.28	−4.99	8.8	4.8	−28.8	−48.4

. The alloy is classified as an intermetallic (IM) compositionally complex alloy rather than as a single-phase bcc high-entropy solid solution. This interpretation is supported by the relatively high atomic size mismatch Δ = 8.8%, the negative mixing enthalpy ΔH_mix = −4.99 kJ·mol⁻¹ and the experimentally observed multiphase diffraction response.

The ΔH_mix–Δ phase-selection map shown in Figure 1 places TiVNbZrCr outside the region usually associated with saturated single-phase solid solutions and close to the intermetallic-compound field. This position is consistent with the large atomic-size contrast between Zr/Nb and Cr/V/Ti, and with the tendency of Zr- and V-containing refractory systems to form chemically ordered or Laves-related structures [15]. The VEC value of 4.8 indicates that bcc-related refractory metal-rich phases can still be present; however, the empirical parameters do not support a purely disordered bcc solid solution as the only equilibrium or as-solidified state.

The calculated hydrogen-affinity descriptors further suggest that the alloy should possess a strong thermodynamic driving force for deuterium uptake. The enthalpy of hydrogen solution at infinite dilution, ΔH_∞ = −28.8 kJ·mol⁻¹, and the formation enthalpy of the concentrated hydride, ΔH_f = −48.4 kJ·mol⁻¹, are both negative. These values reflect the dominant contribution of Ti, V, Nb and Zr, all of which are hydride-forming elements. The role of Cr is different: it reduces the average hydrogen affinity and may contribute to the persistence of a Cr-rich bcc phase during deuteration.

3.2. Deuterium Absorption Behavior

The deuterium absorption behavior measured by magnetic suspension balance is shown in Figure 2. During the initial activation step at 0.1 MPa D₂, heating to 200 °C resulted in a limited but detectable uptake. This indicates that surface activation and/or the first stage of deuterium dissolution can occur at low deuterium pressure, but the amount of absorbed deuterium remains limited under these conditions. After increasing the D₂ pressure to 5 MPa and cooling the sample to room temperature, the deuterium content increased slowly and reached approximately 1.0 wt.% D after about 1200 min. Thus, the alloy is able to absorb deuterium at room temperature, but the kinetics are relatively slow after the initial activation step.

A markedly different response was observed during subsequent heating under 5 MPa D₂. When the temperature approached approximately 200 °C, the sample mass increased rapidly and reached a stable plateau corresponding to 3.28 wt.% D. Using the experimentally determined average molar mass of the alloy, this value corresponds to D/M = 1.1. The high uptake demonstrates that the intermetallic TiVNbZrCr alloy has a substantial deuterium storage capacity, but efficient filling of the available interstitial sites requires both elevated pressure and moderate heating. The sharp increase in capacity during heating suggests that deuteration is not governed only by a continuous solid-solution mechanism, but is coupled with a structural transformation to deuteride phases.

3.3. Phase Constitution Before and After Deuteration

The synchrotron X-ray diffraction data reveal pronounced differences between the as-prepared and deuterated states (Figure 3 and Figure 4). In the as-prepared state, the temperature-resolved diffraction patterns are dominated by a set of relatively stable reflections, indicating that the initial intermetallic alloy does not undergo major structural changes during heating in the absence of deuterium. In contrast, the deuterated state exhibits additional diffraction maxima and a more complex temperature dependence, confirming the formation of deuteride phases and their progressive decomposition during heating.

The phase identification of the as-prepared alloy (Figure 4) indicates the coexistence of at least three crystalline contributions. The first contribution can be indexed as a bcc Ti-rich phase with space group Im-3m and lattice parameter a = 3.29 Å (COD ID: 9008554). The second contribution corresponds to an Fd-3m intermetallic phase with a = 7.28 Å, indexed here as a CrVZr-type phase (COD ID: 1524329). The third contribution is a bcc Cr-rich phase with space group Im-3m and a = 2.87 Å (COD ID: 5000220). The simultaneous presence of these phases confirms that TiVNbZrCr does not behave as a single-phase bcc high-entropy solid solution. Instead, it solidifies as a compositionally complex intermetallic alloy with bcc-related and ordered/intermetallic components.

After deuteration, the diffraction pattern changes substantially. The bcc Ti-rich and CrVZr-type reflections are replaced or strongly modified by additional Bragg peaks that can be indexed using tetragonal deuteride reference structures corresponding to ZrV₂D_2.35 with space group I41/amd and lattice parameters a = 5.44 Å and c = 7.71 Å (COD ID: 1531582), and TiD₂ with space group I4/mmm and lattice parameters a = 3.40 Å and c = 4.10 Å (COD ID: 2310984). In contrast, the bcc Cr-rich phase remains visible after deuteration, suggesting that it has a significantly lower deuterium affinity than the Ti-, Zr-, V- and Nb-rich parts of the alloy. The deuterium storage process is therefore phase selective: deuteride formation occurs preferentially in hydride-forming regions, whereas the Cr-rich phase acts mainly as a weakly absorbing or nearly inert structural component.

It should be noted that these formulae are used here as crystallographic reference labels for phase indexing. The exact deuterium stoichiometry in the present multiphase compositionally complex alloy cannot be determined from X-ray diffraction alone and would require deuterium-sensitive methods such as neutron diffraction, nuclear reaction analysis, thermal desorption spectroscopy coupled with mass spectrometry, or related techniques.

Within the Zr–V deuteride reference structure, deuterium atoms are expected to occupy interstitial sites within the metallic framework. Owing to the chemically complex local environment of the TiVNbZrCr alloy, several non-equivalent interstitial environments with different local metal–deuterium bonding strengths may be present. However, the present synchrotron X-ray diffraction data do not allow individual deuterium sites, site occupancies, or possible local ordering of deuterium atoms to be resolved.

3.4. Thermal Desorption and Structural Transformation Sequence

The thermal stability of the deuterated alloy was evaluated by thermogravimetric analysis (TGA) and correlated with the structural-change parameter obtained from consecutive synchrotron total-scattering structure factors (Figure 5). The total mass loss up to 600 °C was approximately 2.28 wt.% D. Compared with the maximum absorbed amount of 3.28 wt.% D, this indicates that not all deuterium taken up during high-pressure absorption was released in the TGA experiment as a single fully reversible fraction. Part of the deuterium may have been released during depressurization and sample handling before TGA, while another part may remain strongly bound or below the sensitivity of the mass-loss evaluation. Therefore, the difference between the maximum absorbed deuterium content and the deuterium amount released during TGA should not be assigned to a single mechanism. Rather, it may reflect both partial deuterium loss during depressurization and sample handling prior to TGA, and the presence of more strongly bound residual deuterium remaining in stable Ti/Zr/V-containing deuteride environments. The relative contribution of these effects cannot be quantified from the present data alone.

The desorption curve can be divided into three partially overlapping regions. The first mass-loss step starts at approximately 150–160 °C and reaches about 0.60 wt.% D by approximately 250 °C. This step coincides with the strongest peak in the structural-change parameter, indicating that the onset of deuterium release is accompanied by a significant rearrangement or destabilization of deuteride-related diffraction features. The second step occurs between approximately 250 and 350 °C and contributes an additional mass loss of about 0.71 wt.% D. In this interval, the diffraction response indicates continued phase evolution rather than simple thermal contraction. The third, broader desorption region extends to 600 °C and brings the total released amount to 2.28 wt.% D. The structural-change parameter shows another pronounced contribution at higher temperature, around 430–470 °C, which is consistent with the decomposition of more stable Ti-, Zr-, and V-containing deuteride phases.

3.5. Phase-Transformation Pathway and Deuterium Storage Mechanism in TiVNbZrCr

The combined gravimetric, thermal and diffraction results show that deuterium storage in TiVNbZrCr is controlled by phase evolution in a chemically heterogeneous intermetallic alloy. The alloy design parameters predict strong deuterium affinity, and the absorption experiment confirms a high total deuterium uptake of 3.28 wt.% D. However, the diffraction data demonstrate that this uptake is not associated with a simple expansion of a single bcc lattice. Instead, deuteration produces tetragonal deuteride phases, indexed using reference structures corresponding mainly to ZrV₂D_2.35 and TiD₂, while the Cr-rich bcc phase remains comparatively stable. This behavior is consistent with the lower deuterium affinity of Cr compared with Ti, Zr, Nb, and V. The persistence of the Cr-rich bcc phase during deuteration and subsequent heating suggests that deuterium is preferentially captured by the Ti-, Zr-, V-, and Nb-rich regions, whereas the Cr-rich phase behaves mainly as a weakly absorbing or nearly inert structural component.

This behavior differs from the idealized concept of a single-phase bcc high-entropy alloy undergoing a homogeneous bcc-to-hydride transformation. In the present alloy, the intermetallic nature appears to be essential: ordered or chemically segregated regions provide high-affinity sites for deuteride formation, while Cr-rich regions reduce the overall hydride stability and may help preserve structural continuity during deuteration and desorption. The relatively high desorption temperatures indicate that the formed deuteride phases are thermodynamically stable; therefore, the alloy is not yet optimized for low-temperature reversible storage. Nevertheless, the results demonstrate that compositionally complex intermetallic alloys can achieve substantial deuterium uptake and that temperature-resolved synchrotron diffraction is necessary to resolve the sequence of deuteride formation and decomposition.

From a materials design perspective, TiVNbZrCr represents a useful model system for understanding the balance between deuterium affinity and reversibility in refractory multi-principal-element alloys. Increasing the fraction of Ti-, V-, Nb-, and Zr-rich hydride-forming environments enhances the storage capacity, whereas excessive hydride stability shifts desorption to higher temperatures. Future optimization should therefore focus on controlling the amount, chemistry, and connectivity of the intermetallic and bcc-related phases in order to reduce deuteride stability while maintaining high absorption capacity and favorable kinetics. In this respect, a possible compositional strategy may involve increasing the relative fraction of V, and possibly Cr, while reducing the amount of strongly hydride-forming Zr-rich environments. Such tuning could decrease the average deuterium binding strength and lower the desorption temperature, while preserving sufficient deuterium uptake capacity and acceptable absorption kinetics.

4. Conclusions

The present work investigated the phase evolution and deuterium storage behavior of TiVNbZrCr high-entropy intermetallic alloy using gravimetric deuterium absorption, thermogravimetric desorption analysis and synchrotron X-ray diffraction. The most important findings can be summarized as follows:

•

The prepared TiVNbZrCr alloy exhibits an experimentally determined composition of Ti₁₇V₁₉Zr₁₉Nb₂₂Cr₂₃, close to the nominal equiatomic composition, and a measured density of 6.59 g.cm⁻³. The calculated alloy-design parameters indicate that TiVNbZrCr should not be interpreted as a simple single-phase bcc high-entropy solid solution. Instead, it behaves as a compositionally complex intermetallic alloy. The calculated hydrogen-affinity descriptors confirm a strong thermodynamic tendency toward deuterium uptake and deuteride formation, mainly due to the presence of Ti, V, Nb and Zr.

•

Gravimetric absorption measurements showed that the alloy absorbs deuterium slowly at room temperature under 5 MPa D₂, reaching approximately 1.0 wt.% D after about 1200 min. During subsequent heating to 200 °C under the same pressure, the deuterium uptake increased rapidly to 3.28 wt.% D, corresponding to D/M = 1.1. Because deuterium has approximately twice the atomic mass of hydrogen, the gravimetric capacity expressed as wt.% D cannot be directly compared with wt.% H values reported in hydrogen-storage literature. Therefore, the isotope-independent D/M ratio is also reported. The value D/M = 1.1 corresponds to the same atomic occupancy as H/M = 1.1 and is therefore the appropriate parameter for comparison with hydrogen absorption data.

•

Synchrotron X-ray diffraction confirmed that the as-prepared alloy is multiphase, containing a bcc Ti-rich phase, an Fd-3m CrVZr-type intermetallic phase and a bcc Cr-rich phase. After deuteration, the diffraction pattern changed substantially, and the newly appearing reflections could be indexed using tetragonal deuteride reference structures corresponding to ZrV₂D_2.35 and TiD₂.

•

Thermal desorption analysis showed a total mass loss of 2.28 wt.% D up to 600 °C. Deuterium release occurred in three partially overlapping regions: approximately 0.60 wt.% D below about 250 °C, an additional 0.71 wt.% D between about 250 and 350 °C, and the remaining fraction gradually up to 600 °C. The correlation between TGA and the structural-change parameter derived from synchrotron data shows that deuterium release is accompanied by structural rearrangements and decomposition of deuteride phases rather than by simple thermal desorption from a uniform solid solution.

Presented research confirms that deuterium storage in TiVNbZrCr is phase selective and controlled by the transformation of hydride-forming intermetallic regions into stable deuteride phases, while the Cr-rich bcc phase acts mainly as a weakly absorbing or nearly inert structural component. Although TiVNbZrCr exhibits a high total deuterium capacity, the relatively high desorption temperature indicates that the formed deuterides are thermodynamically stable. Future optimization should focus on tuning the chemistry and volume fraction of intermetallic and bcc-related phases to reduce deuteride stability while preserving high uptake capacity and favorable absorption kinetics.