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.