PUB - Publikationen an der Universität Bielefeld

Grain-boundary (Gb) diffusion was studied in ultrafine grained Nd2Fe14B-based permanent magnets below and above the melting transition of the Nd-enriched intergranular phase using the radiotracer technique with the isotope Fe-59. The product deltaD(Gb) of interface diffusion coefficient and interface thickness shows a substantial increase above the intergranular melting transition. Assuming a volume self-diffusivity as in alpha-Fe, an analysis in the framework of grain-boundary diffusion kinetic of type B yields an Arrhenius-type behaviour deltaD(Gb) = 1.53 x 10(-11) exp(-1.74 eV/kT) m(3) s(-1) below the intergranular melting transition. Similar values deltaD(Gb) are observed for ultrafine grained Nd-Fe-B with reduced Nd excess in the grain boundaries. The diffusion characteristics are compared with the kinetics of the hot-deformation which is of technical relevance for the processing of high-performance permanent magnets.