Grain-boundary diffusion was studied in ultra-fine grained Nd2Fe14B-based permanent magnets, below and above the melting transition of the Nd-enriched intergranular phase, by using radiotracer techniques and 59Fe. The product, δDgb, of the interface diffusion coefficient and the interface thickness exhibited a substantial increase above the intergranular melting transition. By assuming a volume self-diffusivity like that in α-Fe, an analysis performed within the framework of type-B grain-boundary diffusion kinetics revealed an Arrhenius-type behavior:

δDgb (m3/s) = 1.53 x 10-11exp[-1.74(eV)/kT]

below the intergranular melting transition. Similar values of δDgb were observed for ultra-fine grained Nd-Fe-B with a reduced Nd excess in the grain boundaries.

M.Eggersmann, F.Ye, S.Herth, O.Gutfleisch, R.Würschum: Interface Science, 2001, 9[3-4], 337-41