Hydrogen uptake and hydride formation in Al$_x$CoCrFeNi high-entropy alloys: First-principles, universal-potential, and experimental study

Abstract

Hydrogen uptake in complex multicomponent alloys, including high-entropy alloys (HEAs), governs both hydrogen storage capacity and resistance to hydrogen-induced degradation. We combine high-pressure experiments, density-functional theory (DFT), and a GRACE universal interatomic potential to investigate hydrogen absorption in Al$_{0.3}$CoCrFeNi and Al$_3$CoCrFeNi HEAs. In H$_2$ as a pressure-transmitting medium, the FCC Al$_{0.3}$CoCrFeNi alloy forms hydrides at ambient temperature above 3 GPa, whereas the Al-rich B2 Al$_3$CoCrFeNi alloy shows no evidence of hydride formation even upon heating at pressures up to 50 GPa. Experiments and calculations show that aluminum suppresses hydrogen uptake by increasing solution energies and destabilizing interstitial sites. The universal potential, employed in the calculations and pretrained on large DFT databases, closely reproduces DFT energetics and demonstrates transferability from the dilute limit to the hydride-forming regime. Simulations further disentangle the roles of local ordering, volume changes, composition, and crystal structure. Overall, our results indicate that hydrogen solubility in Al-containing HEAs is governed primarily by composition, with Al-driven B2 ordering as a strong secondary effect.

Figures (5)

• Figure 1: Energy–volume curves for Al$_x$CoCrFeNi with (a) $x=0.3$ and (b) $x=3.0$ for 216-atom supercell models. Predictions using the GRACE foundational potential (blue) and DFT calculations (black) are presented for the BCC and FCC phases. For Al$_3$CoCrFeNi, the B2 phase is also included. Symbols denote the calculated data, and lines show the fits using the Vinet EOS.
• Figure 2: Comparison of hydrogen solution energies obtained from DFT and the GRACE foundational potential for FCC Al$_{0.3}$CoCrFeNi and the B2-ordered Al$_3$CoCrFeNi using 64-atom supercells. Each point corresponds to a unique interstitial configuration. The black dashed line represents the correlation with a mean signed difference of $\Delta\mu = -72$ meV, as determined by linear regression, indicating a systematic difference in the hydrogen chemical potential.
• Figure 3: (a) Compressibility data obtained from DAC and LVP experiments for FCC Al$_{0.3}$CoCrFeNi. The samples collected in DACs were surrounded by either H2 or Ne pressure-transmitting medium. Symbols denote experimental measurements. The dotted vertical lines indicate the pressure range in which two FCC phases, with lower and higher volumes, respectively, coexist. The data associated with solid triangles was collected in a DAC on compression. LVP data (open circles) were collected at room temperature during compression and then at high temperature (closed circle). Solid and dashed lines show pressure–volume curves of the hydrogen-free parent alloy and of the ideal hydride with H/M = 1, respectively, obtained from DFT and GRACE calculations for comparison. (b) Corresponding DAC measurements for B2 Al$_3$CoCrFeNi in H$_2$ and Ne, demonstrating the absence of hydride formation up to 50 GPa. GRACE and DFT calculated EOS curves for the hydrogen-free parent alloys are shown for reference.
• Figure 4: Histograms of hydrogen solution energies for interstitial octahedral and tetrahedral sites in FCC Al$_{0.3}$CoCrFeNi (upper part) and B2 Al$_3$CoCrFeNi (lower part) obtained using the GRACE foundational potential. The bar height represents the number of sites per metal atom. Semi-transparent bars indicate dynamically unstable configurations in which the hydrogen atom relaxes from its initial site to a more stable one.
• Figure 5: Solution energies of hydrogen for tetrahedral (orange) and octahedral (blue) interstitial sites across different structures, compositions, and volumes of the Al-containing HEAs, obtained using the GRACE foundational potential. Each state is shown as a distribution of site-resolved energies, with mean values indicated by outlined markers: blue squares for tetrahedral sites and orange circles for octahedral sites. Arrows denote the corresponding change in chemical order, volume, structure, or Al content. The black-dashed line marks zero-solution energy. The stable reference states (B2 Al$_3$CoCrFeNi and FCC Al$_{0.3}$CoCrFeNi) are highlighted.