Hydrogen Segregation at the Coherent α-Fe/V₄C₃ Interface: First-Principles Insights into the Role of Carbon Vacancies

Hydrogen trapping at carbide/matrix interfaces is important for improving the resistance of steels to hydrogen embrittlement. In this work, the segregation behavior of hydrogen at the coherent α-Fe/V₄C₃ interface was investigated by first-principles calculations. Representative hydrogen sites were considered systematically, including interstitial sites in the near-interface region, interfacial sites, and carbon-vacancy sites in V₄C₃. All of the sites examined are energetically favorable for hydrogen trapping, but the carbon vacancy inside V₄C₃ exhibits the strongest trapping tendency. Charge density, Bader charge, and density-of-states analyses indicate that hydrogen at this site gains more electrons and forms stronger interactions with neighboring V atoms, leading to enhanced stability. The behavior of H₂ at the internal carbon vacancy was also evaluated. After structural relaxation, the H₂ molecule dissociated into two separate H atoms, indicating that hydrogen is more stably trapped in atomic rather than molecular form. These findings reveal the crucial role of carbon vacancies in regulating hydrogen trapping at the α-Fe/V₄C₃ interface and provide atomic-scale insight into the hydrogen trapping mechanism of vanadium carbide precipitates in steels.