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 DO = 1.05 x l0-7 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 x 10-6fl exp （-2.27 eV/kT） mE. s-1 and D2 - 8.36 x 10-6 exp （-4.81 eV/kT） m2. s-l, re- spectively. 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.
