Diffusion of niobium in α-iron

Diffusion coefficient of Nb-95 in alpha-iron has been determined in the temperature range between 823 and 1163 K by use of a serial sputter-microsectioning technique. The diffusion coefficient of niobium is about two times as large as the self-diffusion coefficient in alpha-iron. The temperature dependence of the diffusion coefficient, D, in the whole temperature range of alpha-iron across the Curie temperature (T-C = 1043 K) can be expressed by D = (1.40(-1.05)(+4.17)) x 10(-1) exp [-(299.7+/-12.7 kJmol(-1))(1+(0.061+/-0.007)s2/RT)] m(2)s(-1) where s is the ratio of the spontaneous magnetization at T K to that at 0 K. The factor 0.061 in the equation is smaller than 0.156 for the self-diffusion, showing that the magnetic effect on the diffusion of niobium in alpha-iron is smaller than that on the self-diffusion. The activation energy 299.7 kJmol(-1) for niobium diffusion in the paramagnetic iron is much higher than 250.6 kJmol(-1) for the self-diffusion. Atomic size effect is predominant in the activation energies for the diffusion of transition metal solutes in the paramagnetic alpha-iron.