Temperature-dependent Dy diffusion processes in Nd-Fe-B permanent magnets
Nd-Fe-B permanent magnets have been coated with 0.6 wt.% dysprosium and annealed at various temperatures to study the impact of the temperature-dependent Dy diffusion processes on both the magnetic properties and the microstructure. When optimum annealing conditions are applied the Dy processed magnets with initial coercivity of similar to 1100 kA m(-1) yield coercivity increases which can exceed 400 kA m(-1) without a significant reduction of the remanent magnetic polarization. The improved stability against opposing magnetic fields can be observed up to a depth of similar to 3 mm along the diffusion direction, restricting the application of the Dy diffusion process to either thin magnets or magnets with tailored coercivity gradients. While in the proximity of the Dy-coated surface, each grain has a Dy-enriched shell with a Dy content of similar to 6 at.%; the Dy concentration decreases exponentially to similar to 1.8 at.% after a diffusion depth of 400 mu m and to similar to 1 at.% after a diffusion depth of 1500 mu m, as was found with wavelength dispersive X-ray spectroscopy and scanning transmission electron microscopy-energy dispersive X-ray spectroscopy, respectively. In the vicinity of the Dy-coated surface, the mechanism of the Dy-shell formation is attributed to the melting/solidification of a heavy-rare-earth-rich intermediate phase during high-temperature annealing. This is based on the observation that a constant Dy concentration over the width of the shells was found. Also an epitaxial relation between the Dy-poor core and the Dy-rich shell was observed by electron backscattered diffraction, which is supported by results obtained with Kerr microscopy.