Boron diffusion in bcc-Fe studied by first-principles calculations

The diffusion mechanism of boron in bcc-Fe has been studied by first-principles calculations. The diffusion coefficients of the interstitial mechanism, the B-monovacancy complex mechanism, and the B-divacancy complex mechanism have been calculated. The calculated diffusion coefficient of the interstitial mechanism is D₀ = 1.05× 10^-7exp(-0.75 eV/kT)m²· s^-1, while the diffusion coefficients of the B-monovacancy and the B-divacancy complex mechanisms are D₁ = 1.22× 10^-6f₁ exp(-2.27 eV/kT) m²· s^-1 and D₂ ≈ 8.36× 10^-6exp(-4.81 eV/kT)m²· s^-1, respectively. The results indicate that the dominant diffusion mechanism in bcc-Fe is the interstitial mechanism through an octahedral interstitial site instead of the complex mechanism. The calculated diffusion coefficient is in accordance with the reported experiment results measured in Fe-3%Si-B alloy (bcc structure). Since the non-equilibrium segregation of boron is based on the diffusion of the complexes as suggested by the theory, our calculation reasonably explains why the non-equilibrium segregation of boron is not observed in bcc-Fe in experiments.