# Influence of hydrogen on the growth kinetics of Al3Zr precipitates in an Al-Zr alloy

Omar Boukir, Kaveh Edalati, Fabien Cuvilly, Shivam Dangwal, Xavier Sauvage

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Omar Boukir, Kaveh Edalati, Fabien Cuvilly, Shivam Dangwal, Xavier Sauvage. Influence of hydrogen on the growth kinetics of Al3Zr precipitates in an Al-Zr alloy. Scripta Materialia, 2025, 265, pp.116722. 10.1016/j.scriptamat.2025.116722. hal-05135461

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# Influence of hydrogen on the growth kinetics of Al₃Zr precipitates in an Al-Zr alloy

Omar Boukir<sup>a, *</sup> ◆, Kaveh Edalati<sup>b, c ◆</sup>, Fabien Cuvilly<sup>a ◆</sup>, Shivam Dangwal<sup>b, c ◆</sup>,
Xavier Sauvage<sup>a, *</sup> ◆

ARTICLE INFO

Keywords:

Hydrogen

Diffusion

Growth kinetics

Aluminium alloy

Vacancy

A B S T R A C T

The influence of hydrogen on the precipitation kinetics of L1₂-Al₃Zr ordered precipitates in an industrial Al-Zr alloy was investigated using transmission electron microscopy (TEM) and atom probe tomography (APT). Aging at 375 °C under 10 MPa of hydrogen led to a significant acceleration of Al₃Zr precipitate growth, a larger mean precipitate size compared to aging in air, and an enhanced decomposition of the supersaturated solid solution. Fitting experimental data with a coarsening model suggests that hydrogen increases the diffusion coefficient of Zr by a factor of two, likely due to hydrogen-vacancy interactions promoting the mobility of Zr.

Al-Zr alloys containing Al₃Zr precipitates are particularly suited to applications requiring both high thermal stability and mechanical strength [1,2], such as the manufacturing of electrical cables and the design of lightweight structural components [3,4]. L1₂-Al₃Zr ordered precipitates, resulting from the decomposition of supersaturated solid solutions, have a cubic structure and are coherent with the aluminium matrix (lattice misfit: 0.75 % [1]). Due to the low mobility of Zr, these precipitates exhibit a relatively small growth rate [5,6]. Earlier studies have shown that the ideal temperature range to achieve a fine distribution of nanoscaled precipitates in the Al-Zr system is in a range of 350 to 375 °C [2]. Nevertheless, Al₃Zr precipitates are metastable and it has been shown that their dissolution starts at approximately 475 °C, above which they transform into the equilibrium D₀₂₃ ordered structure [7]. This equilibrium phase nucleates heterogeneously, typically on dislocations or low-angle grain boundaries, or appears via the formation of antiphase boundaries within the coherent L1₂ precipitates, as described by Knipling et al. [7].

Hydrogen, a highly mobile element, can enter aluminum alloys during processing or service. Hydrogen insertion can occur either in the form of atomic or molecular hydrogen (H or H₂), typically during a corrosion reaction or in H₂ atmosphere. It involves a three steps sequence: physical adsorption, chemical adsorption, and dissolution [8, 9]. Once inside the material, hydrogen interacts strongly with defects, such as grain boundaries, and dislocations, that are diffusion pathways

[8], but also with vacancies [10–13]. Hydrogen-vacancy complexes may affect the dislocation mobility [14–18] and H-vacancy interactions influence both the vacancy mobility and concentration. Thus, hydrogen may influence the diffusion mechanism of solutes in Al alloys [18,19], leading for example to a significant delay in the formation of Guinier-Preston zones in the Al-Cu system [18].

Hydrogen exhibits a strong affinity with Zr (enthalpy of solution for hydrogen in α-Zr: -0.46 eV [20]) suggesting that hydrogen could significantly impact the nucleation and growth of L1₂-Al₃Zr precipitates. In this context, this work aims to investigate the influence of hydrogen on the precipitation kinetics in an industrial Al-Zr alloy. The structure, size, distribution and volume fraction of precipitates grown under a hydrogen environment were measured by transmission electron microscopy (TEM) and atom probe tomography (APT), and compared to that of precipitates grown in air.

The investigated alloy (composition (wt %): 0.24 % Zr, 0.094 % Fe, 0.07 % Si, 0.0013 % Ni) was solutionized at 620 °C during 288 h. Samples (8 mm in diameter, 1 mm thick) were aged at 375 °C up to 170 h in air or in H₂ environment. To promote the insertion of hydrogen into the sample during aging, a hydrogen pressure of 10 MPa was applied. In such conditions (375 °C, 10 MPa of H₂), the solubility of hydrogen is about 5 atomic ppm [21]. Considering the diffusion coefficient of hydrogen in aluminum D<sub>H</sub> [12], the diffusion length (L = √6D<sub>H</sub> t, with t is the time) is larger than a few mm for one hour. Thus, the hydrogen

<footnote><sup>a</sup> Univ Rouen Normandie, CNRS, INSA Rouen Normandie, Groupe de Physique des Matériaux UMR 6634, F-76000 Rouen, France</footnote>

<footnote><sup>b</sup> WPI, International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, Fukuoka 819-0395, Japan</footnote>

<footnote><sup>c</sup> Department of Automotive Science, Graduate School of Integrated Frontier Sciences, Kyushu University, Fukuoka 819-0395, Japan</footnote>

<footnote>* Corresponding authors.</footnote>

<footnote>E-mail addresses: omar.boukir@univ-rouen.fr (O. Boukir), xavier.sauvage@univ-rouen.fr (X. Sauvage).</footnote>
concentration in aged samples quickly reaches equilibrium. To ensure that other aging parameters were similar, aging under both conditions (air and H₂) were performed in the same Sieverts-type apparatus (Suzuki Shokan Co. Ltd., Japan). Prior to aging, all samples were polished to remove oxide layers that could hinder hydrogen insertion during aging.

TEM samples were prepared by electropolishing using a solution of 2/3 methanol and 1/3 nitric acid at -30 °C with a voltage of 17 V. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) observations were made using a JEOL® ARM 200F microscope, equipped with a Cs-probe corrector and a Shottcky field emitter, operating at 200 kV. The acquisition parameters for HAADF-STEM imaging were as follows: probe diameter of 0.14 nm, objective aperture semi-angle of 24–27 mrad and detector half-collection angle ranging from 50 to 180 mrad. The analyzed surface area for each condition was about 10 µm² to ensure a representative assessment of the bulk material. Precipitate diameters were measured from HAADF-STEM images using the Feret diameter method [22]. The uncertainty was expressed as twice the standard deviation (2σ) of the measured values.

Zr concentrations in the matrix and in precipitates were measured by APT. A LEAP® 5000 XS instrument was used with the following operating conditions: electric pulse frequency of 500 kHz, detection rate of 0.2 %, pulse fraction of 20 %, and a specimen temperature of 60 K. The 3D reconstruction of analyzed volumes was performed using the IVAS® software suite, followed by data processing with the GPM_3dSAT software (IDDN.FR.001.430017.000. S.P.2020.000.10000).

TEM observations of the different aging states are summarized in Fig. 1. After 48 h, at 375 °C in air, a low density of homogeneously distributed nanoscale spherical Al₃Zr precipitates with an L1₂ structure is exhibited (Fig. 1a), the mean diameter is 5.6 ± 1.2 nm. These observations are consistent with previous studies conducted on this system [7,

23,24]. When the aging time is extended to 96 h and 170 h, as shown in Fig. 1b and 1c, the Al₃Zr precipitates significantly grow, reaching an average diameter of 8.0 ± 1.5 nm and 11.7 ± 2.1 nm, respectively.

TEM observations of samples aged in similar conditions but in H₂ atmosphere (Fig. 1d, 1e, 1f) show that Al₃Zr precipitates have nucleated and grown, with a similar morphology (relatively spherical) but they are systematically larger (8.4 ± 2.0, 11.5 ± 1.5 and 21.0 ± 2.0 nm after 48 h, 96 h and 170 h respectively). For greater clarity, precipitate size distributions for both aging conditions are displayed in Fig. 2, confirming that the whole distributions are shifted toward higher values under hydrogen. Thus, the growth of Al₃Zr precipitates seems significantly faster under hydrogen than under air.

The equilibrium D0₂₃ phase was never detected in any of the studied conditions. However, elongated precipitates containing Fe, Si, and Zr, have been occasionally observed in samples aged in air and hydrogen (see Appendix A). They are consistent with the d-phase reported by Dorin et al. [25]. Due to their extremely low number density, the influence of hydrogen on the growth of these precipitates could not be addressed statistically.

Al₃Zr precipitates were analyzed by APT at different aging steps (Fig. 3). They contain about 25 % at Zr (see Fig. 3d and Table 1) as expected for the Al₃Zr stoichiometric phase, and they exhibit sizes consistent with TEM observations. Both matrix and precipitate compositions extracted from these APT data are reported in Table 1. The small amount of Zr detected in the matrix is above the equilibrium solubility limit of 0.004 ± 0.0004 at % (estimated from Thermo-Calc® at T = 375 °C with TTAL6 database). This indicates that the system has not yet reached equilibrium and is still in the growth regime for all considered states. Zr concentrations in the matrix are systematically lower in samples aged under hydrogen than in air, confirming that hydrogen promotes the

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** HAADF-STEM images of the Al-Zr alloy aged in air (a, b, c) and in hydrogen (d, e, f) during 48 h (a, d), 96 h (b, e) and 170 h (c, f). All images were recorded in ⟨100⟩ zone axis, excepted b) and f) in ⟨110⟩ zone axis. Insets are high-resolution images showing the L1₂ ordered structure of Al₃Zr precipitates.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Fig. 2. Al₃Zr precipitates size histogram, measured from HRSTEM images of samples aged under air and hydrogen. During 48 h (a), 96 h (b), and 170 h (c).</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 3. 3D reconstructions of volumes analyzed by APT in the Al-Zr alloy after aging (40 × 40 × 40 nm³), (a) 48 h in air, (b) 48 h in hydrogen, (c) 96 h in air. Al atoms are plotted in gray and Zr atoms in orange. Data were filtered to display only Zr atoms in Zr rich regions (threshold 10 at %) to highlight Al₃Zr precipitates. (d) Concentration profile through the Al₃Zr precipitate of (a) (thickness of sampling volume is 0.4 nm).</figcaption></figure>

<caption>Table 1</caption>

Zr concentration (at %) in precipitates and matrix from APT data (the total number of atoms collected for each state is over 60 million). precipitate volume fraction (estimated from Eq. (2) using compositions measured by APT), and number density (dₚ) of Al₃Zr precipitates calculated from Eq. (1).

<table><thead><tr><th>Aging conditions</th><th>Precipitates (at % Zr)</th><th>Matrix (at % Zr)</th><th>f<sub>v</sub> (%)</th><th>d<sub>p</sub>(10<sup>22</sup> m<sup>-3</sup>)</th></tr></thead><tbody><tr><td>48 h /Air</td><td>23.9 ± 3.4</td><td>0.047 ± 0.011</td><td>0.11 ± 0.02</td><td>0.9 ± 0.34</td></tr><tr><td>48 h /H<sub>2</sub></td><td>24.4 ± 3.2</td><td>0.028 ± 0.013</td><td>0.19 ± 0.02</td><td>0.6 ± 0.4</td></tr><tr><td>96 h /Air</td><td>25.6 ± 3.1</td><td>0.035 ± 10<sup>-4</sup></td><td>0.16 ± 0.0004</td><td>0.6 ± 0.33</td></tr></tbody></table>

decomposition of the Zr supersaturated solid solution.

Several hypotheses could be raised to account for this faster decomposition of the supersaturated solid solution under hydrogen. First, hydrogen may influence the atomic mobility of Zr, which is controlled by the vacancy concentration and mobility. Some studies have demonstrated that hydrogen may indeed significantly reduce the

vacancy formation enthalpy in metallic alloys [26,27]. Besides, the high pressure (10 MPa) may also affect both the formation enthalpy and the migration enthalpy of vacancies [28]. Furthermore, the possible influence of hydrogen on the nucleation rate of Al₃Zr precipitates should also be considered.

A measurement of the precipitate number density directly from TEM images, is not reliable due to the small number of precipitates that were observed. However, considering that Al₃Zr precipitates are spherical, the number density of precipitates per unit volume dₚ can be determined using the following equation:

$$d_p = \frac{f_v}{V_p} \qquad (1)$$

where Vₚ is the mean volume of Al₃Zr precipitates (Vₚ = 4πr³/3) where r is the mean radius measured from TEM data). The volume fraction of Al₃Zr precipitates f_v can then be estimated based on the mass balance [29], assuming that atoms are exclusively shared between the matrix and Al₃Zr precipitates:

$$C_0 = C_m(1 - f_v) + C_p f_v \qquad (2)$$

where C₀ is the initial atomic fraction of Zr (0.07 at %), Cₚ is the mean concentration of Zr in precipitates, and Cₘ is the Zr concentration in the matrix. For all samples, the number density of Al₃Zr precipitates is relatively similar (Table 1), indicating that the nucleation rate is not significantly affected by hydrogen. Since the nucleation barrier for homogeneous precipitation is mostly dependent on the precipitate-matrix interfacial energy, it can be concluded that hydrogen does not significantly affect this interfacial energy. Then, different growth rates are more likely attributed to a change in Zr atomic mobility.

The mean radius of Al₃Zr precipitates measured from HAADF images was plotted as a function of time in Fig. 4. The growth rate for the sample aged in air is consistent with data reported by Souza et al. [30] for a similar alloy (Al-0.22Zr wt %) aged at 377 °C. Assuming that precipitates exhibit a simple spherical growth morphology, the experimental radius values can be fitted using a classical coarsening model as [24,31]:

$$R(t) = \left( R_0^2 + 2D \frac{C_0 - C_e}{C_p - C_e} t \right)^{\frac{1}{2}} \qquad (3)$$

where D is the diffusion coefficient, C_e is the Zr concentration in matrix at equilibrium (C_e= 0.004 at %, estimated from Thermo-Calc® at 375 °C with TTAL6 database) and R is the precipitate radius. The diffusion coefficient of zirconium in aluminum at a given temperature can be calculated through classically known equation as:

$$D = D_0 \exp\left(-\frac{Q}{RT}\right) \qquad (4)$$

where D₀ is the solute pre-exponential factor, Q is the activation energy, R is the gas constant, and T is the temperature. From Eq. (3), the theoretical growth rate of Al₃Zr precipitates was plotted (Fig. 4), using the parameters listed in Table 2. Even if this model assumes that the matrix
<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Growth rate of Al₃Zr precipitate. The red and blue dots are TEM experimental data for the material aged under air and under hydrogen, respectively. The solid black line was plotted from Eq. (6) with D given in Table 2. The dashed black line was plotted from Eq. (6) to fit the growth rate under hydrogen with a diffusion coefficient (D) twice larger than under air. Error bars are twice the standard deviation (2σ).</figcaption></figure>

<table><caption>**Table 2**
Parameters of Eq. (3 & 4), from [3,31,32].</caption><thead><tr><td>Parameter</td><td>Value</td></tr></thead><tbody><tr><td>R₀ (nm)</td><td>0.8</td></tr><tr><td>D₀ (m²/s)</td><td>5.4 × 10⁻³</td></tr><tr><td>Q (kJ/mol)</td><td>222</td></tr><tr><td>T (°C)</td><td>375 °C</td></tr></tbody></table>

composition remains constant (growth of a single precipitate in an infinite media), it fits relatively well experimental data.

As suggested by Souza et al. [30] the Zr concentration decrease in the matrix during precipitate growth (see Table 1) has a negligible effect on the growth rate in the early stage of the process. A numerical integration of Eq. (3) taking into account the change of the Zr concentration in the matrix (see Appendix B), shows indeed that there is no significant difference up to about 100 h of aging. Then, the simple model of Eq. (3) can be used to quantify the variation in Zr mobility induced by hydrogen. Experimental data for precipitates grown under hydrogen could be fit with a diffusion coefficient twice larger than in air (Fig. 4), indicating that hydrogen increases the mean diffusion coefficient of Zr by a factor of two. Some discrepancy between experimental data and the theoretical fit appears for longer aging time (170 h) which might be due to a change in precipitate morphology when they become relatively larger (Fig. 2f).

The increase in the diffusion coefficient of Zr could be due to an increase in vacancy concentration, which results from a decrease in vacancy formation energy, or from an enhancement in vacancy mobility [33]. According to the thermodynamic model of vacancy diffusion, the solute diffusivity is related to the vacancy formation and migration energies ($E_f$, $E_m$) through the following equation:

$$D_{Zr} = a^2 v \exp\left(\frac{S_f + S_m}{k}\right) \exp\left(-\frac{E_f + E_m}{kT}\right) \quad (5)$$

where a is the jump length, v is the vacancy jump frequency, and $S_f$ and $S_m$ refers to the vacancy formation and migration entropy, respectively. An increase in the enthalpies of formation and migration generally leads to a decrease in the diffusion coefficient. Pressure is also known to influence vacancy formation and migration in FCC systems [28]. This effect on vacancies can, in turn, alter the solute diffusion. Since the two samples were aged under different pressures (10 MPa for hydrogen and 0.1 MPa for air), the influence of pressure should be considered at first. Extrapolation of the data of Connétable et al. [28] between 0.1 and 10

MPa, indicates that the change in enthalpy is negligible (0.0008 eV for vacancy formation and 0.0002 eV for vacancy migration), leading to a 2 % decrease in the diffusion coefficient of Zr under 10 MPa. Thus, the accelerated growth of Al₃Zr precipitates cannot be attributed to the high pressure.

X-ray diffraction studies have shown that hydrogen decreases the vacancy formation energy [34,35]. Ab initio calculations have also demonstrated that hydrogen can influence vacancy mobility. In Cu and Pd, hydrogen enhances vacancy mobility [36], while in Ni, it significantly slows it down [37]. In the case of chemical or gaseous hydrogen loading, hydrogen forms a vacancy-hydrogen complex at the surface of the samples and diffuses into the material, which is suggested by a hydrogen-vacancy ratio close to one, often observed in aluminum [12, 38]. Such a complex may have a significant impact on the equilibrium vacancy concentration.

Besides, As mentioned earlier, Zr binds strongly to hydrogen [20], which itself has a high affinity for vacancies [16]. Since Zr exhibits limited interaction with vacancies in Al alloys compared to other solutes [39], this suggests that hydrogen-vacancy interactions could modify the Zr mobility that is driven by a vacancy mechanism.

Assuming that hydrogen influences only $E_m$ and $E_f$, without affecting the exchange probability between Zr and vacancies, as observed in the Al-Cu system [18], the variation in vacancy migration energy and formation energy can be estimated from the change in the diffusion coefficient using Eq. (5), which leads to:

$$\Delta E_f + \Delta E_m = -KT \ln \left( \frac{D_{Zr}^H}{D_{Zr}^A} \right) \quad (6)$$

where $D_{Zr}^H$ and $D_{Zr}^{Air}$ are the diffusion coefficients of Zr during aging under hydrogen and air, respectively. With $D_{Zr}^H \approx 2 \times D_{Zr}^{Air}$, it can be estimated that the reduction of the vacancy migration or formation energies is about 0.04 eV.

In summary, hydrogen was found to accelerate the growth of Al₃Zr precipitates. The measured Zr concentration in the matrix, examined by APT, was systematically lower in samples aged in hydrogen, confirming an enhanced decomposition of the supersaturated solid solution. Based on classical coarsening models, hydrogen increases the diffusion coefficient of Zr by approximately a factor of two. This enhancement is attributed to hydrogen-vacancy interactions, which reduce vacancy formation or migration energies by approximately 0.04 eV. These findings provide new insights into the role of hydrogen in precipitation kinetics in Al-Zr alloys. Further investigations, including ab initio
calculations and in situ hydrogen charging experiments, are envisioned
to refine the understanding of the mechanisms involved.

CRediT authorship contribution statement

**Omar Boukir:** Writing – original draft, Visualization, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Kaveh Edalati:** Writing – review & editing, Methodology, Investigation. **Fabien Cuvilly:** Investigation. **Shivam Dangwal:** Investigation. **Xavier Sauvage:** Writing – review & editing, Writing – original draft, Validation, Supervision, Resources, Project administration, Methodology, Funding acquisition, Conceptualization.

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.

Acknowledgements

Guillaume Hachet is gratefully acknowledged for fruitful discussions.
This work was partially supported by the CNRS federation IRMA - FR
3095 and the Region Normandy.

Supplementary materials

Supplementary material associated with this article can be found, in
the online version, at doi:10.1016/j.scriptamat.2025.116722.

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