Thermal Stability of Hot-Pressed NdFeB Magnets with Dy-Fluoride Additions

摘要

In order to transfer the recently introduced grain boundary diffusion process (GBDP) from sintered to hot-pressed magnets commercially available melt-spun NdFeB powder has been coated with DyF_3 and subsequently hot compacted. Permagraph measurements show a strong dependency of coercivity as a function of the Dy-fraction which can be explained by a decomposition of DyF_3 with formation of (Dy,Nd)_2Fe_(14)B, Nd-fluoride and Dyoxide phases, respectively. Energy dispersive X-ray (EDX) and wavelength dispersive X-ray (WDX) elemental maps verified this feature. A low temperature heat treatment has been carried out at 600°C to promote the diffusion without inducing grain growth. It is found that annealing leads to an improved coercivity which can be attributed to additional formation of (Dy,Nd)_2Fe_(14)B. An enhanced fluorine diffusion along the flake boundaries revealed with EDX and WDX elemental maps supports this conclusion.