Intergranular Melting of Ultrafine Grained Nd2Fe14B Studied by Means of Radiotracer Diffusion

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 ⁵⁹Fe. The product D _Gb of interface diffusion coefficient and interface thickness shows a substantial increase above the intergranular melting transition. Assuming a volume self-diffusivity as in -Fe, an analysis in the framework of grain-boundary diffusion kinetic of type B yields an Arrhenius-type behaviour D _Gb = 1.53 × 10^–11 exp(–1.74 eV/kT) m³ s^–1 below the intergranular melting transition. Similar values D _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.