Composition-dependent hydrogen diffusion in Fe-Cr alloys: A first-principles and ab initio molecular dynamics...

Hydrogen diffusion plays a key role in hydrogen-metal interactions and is closely linked to embrittlement in steels. In iron-based alloys, the influence of local atomic environments on hydrogen diffusion is well recognized, whereas the role of alloy composition remains unclear. Fe-Cr binary alloys, therefore, provide a simple and well-defined model system to isolate the effect of Cr on hydrogen diffusion in iron lattices. In this work, hydrogen diffusion in Fe-Cr alloys is investigated using first-principles calculations based on density functional theory. Ab initio molecular dynamics simulations are further employed to evaluate the influence of Cr concentration on hydrogen diffusion coefficients. The results show that hydrogen diffusion in Fe-Cr alloys is strongly suppressed compared with bcc Fe. This suppression is primarily attributed to higher migration energy barriers resulting from strong Fe-Cr interactions and more compact local atomic packing. Charge transfer analysis demonstrates that hydrogen behaves as an electron acceptor, and the amount of charge transferred is inversely related to the migration barrier. In the disordered solid-solution models, the hydrogen diffusion coefficient decreases with increasing Cr content in the low-Cr range, whereas in the ordered alloy models it exhibits a non-monotonic dependence on composition, with a minimum near 25 at. % Cr. These results reveal the electronic and structural origins of composition-dependent hydrogen diffusion in Fe-Cr alloys at the atomic scale.