Grain boundary self-diffusion in high purity iron

The grain boundary self-diffusion coefficient Dgb in high purity iron was measured by a serial sectioning method with radioactive tracer 59Fe in the temperature range between 523 and 703 K using type-C kinetics. Furthermore, the grain boundary self-diffusivity δDgb was measured between 873 and 1173 K using type-B kinetics. Inserting the width of grain boundary δ = 0.5 nm into Dgb obtained by type-C kinetics, the temperature dependence of δDgb links smoothly with that obtained by type-B kinetics over the whole temperature range across the Curie temperature: δDgb=(6.35+6.21/-3.19)×10-15exp{-(55.7±6.1kJ mol-1)[1+(1.28±0.03)s2]/RT}m3s-1, where s is the ratio of spontaneous magnetization of bulk iron at T K to that at 0 K. The grain boundary self-diffusivity in iron obtained by the present work is much higher than those of previous authors. The activation energy for the grain boundary self-diffusion in the paramagnetic α-iron, 55.7 kJ mol−1, is only 0.22 of the activation energy for the volume diffusion. Furthermore, the factor 1.28 in the above equation is about eight times that of volume diffusion, showing that the influence of the magnetic transformation on the grain boundary diffusion is much larger than that on the volume diffusion.