Refractory medium/high-entropy alloys (M/HEAs) are emerging as promising alternative materials for hydrogen storage and hydrogen combustion engines due to their favorable thermodynamic and kinetic conditions for hydrogen accommodation (for the former) and promising high-temperature mechanical properties (for the latter). A better understanding of hydrogen-metal interactions is necessary to advance the development of this material class, thus helping leverage hydrogen-based applications. Here we reveal the microstructural evolution of a TiNbZr MEA by in-situ synchrotron high-energy X-ray diffraction (HEXRD) during hydrogenation in pure H₂ gas at atmospheric pressure. At 500 °C, dissolved hydrogen atoms gradually expand the crystal lattice isotropically, and the body-centered cubic crystal remains stable up to a hydrogen concentration of ∼46.4 at.%. The thermodynamics of hydrogen accommodation associated with experimental observations in the crystal lattice is elucidated using density functional theory (DFT). The calculations suggest that tetrahedral interstitial sites are the thermodynamically favorable positions for hydrogen accommodation in both cases (i) for a single hydrogen in the special quasirandom structure supercell and (ii) at a high hydrogen concentration (∼45.4 at.%). In the latter case, hydrogen interstitials are randomly distributed on the tetrahedral sites. Upon cooling, it is observed that the body-centered cubic lattice transforms to a body-centered tetragonal structure. The DFT calculations show that this change is related to the ordering distribution of hydrogen interstitials within the TiNbZr lattice. By combining in-situ HEXRD experiments and DFT calculations, the study provides fundamental insights into hydrogen accommodation in the interstitial positions and its impact on the lattice symmetry in TiNbZr MEA.

The temperature-dependent intrinsic stacking fault Gibbs energy is computed based on highly converged density-functional-theory (DFT) calculations for the three prototype face-centered cubic metals Al, Cu, and Ni. All relevant temperature-dependent contributions are considered including electronic, vibrational, magnetic, and explicit anharmonic Gibbs energy contributions as well as coupling terms employing state-of-the-art statistical sampling techniques. Particular emphasis is put on a careful comparison of different theoretical concepts to derive the stacking fault energy such as the axial-next-nearest-neighbor-Ising (ANNNI) model or the vacuum-slab approach. Our theoretical results are compared with an extensive set of previous theoretical and experimental data. Large uncertainties in the experimental data highlight the necessity of complementary parameter-free calculations. Specifically, the temperature dependence is experimentally unknown and poorly described by thermodynamic databases. Whereas calphad derived data shows an increase of the stacking fault energy with temperature for two of the systems (Cu and Ni), our results predict a decrease for all studied systems. For Ni, the temperature induced change is in fact so strong that in the temperature interval relevant for super-alloy applications the stacking fault energy falls below one third of the low temperature value. Such large changes clearly call for a revision of the stacking fault energy when modeling or designing alloys based on such elements.

Quantum-mechanical calculations are used to determine the temperature dependence of the Gibbs energy of vacancy formation in nickel. Existing data reveal a discrepancy between the high-temperature estimates from experiments and low-temperature approximations from density functional theory. Our finite-temperature calculations - which include the effects of magnetism and fully interacting phonon vibrations - demonstrate that this discrepancy is mostly caused by the previously neglected explicit anharmonic contribution.