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 × 10−7 exp (−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 × 10−6 f1 exp (−2.27 eV/kT) m2 s−1 and D2 ≈ 8.36 × 10−6 exp (−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.