Grain boundary diffusion of different rare earth elements in Nd-Fe-B sintered magnets by experiment and FEM simulation

In the present work, we explore the influence of a surface-bulk coercivity gradient in Nd-Fe-B magnets produced by the Grain Boundary Diffusion Process (GBDP) on the overall coercivity. In our systematic and comprehensive study we diffused four different rare earth elements (Dy, Tb, Ce and Gd) in two different kinds of commercial Nd-Fe-B magnets, one very Dy-lean and one Dy-rich. By means of cutting the magnets into thin slices we obtain lateral coercivity profiles, from which diffusion constants are extracted. We find that in both magnets Tb diffuses significantly faster than Dy. The diffusion is generally slower in the Dy-lean magnet, which is attributed to the different chemistry and a smaller grain size. Ce diffuses slightly slower than Dy and the overall coercivity decrease is similar for Ce and Gd. With scanning electron microscopy it is revealed that, contrary to the magnets diffused with the heavy rare earths, the microstructure in the magnets treated with Ce show no (Nd,Ce)-Fe-B shells in the surface regions. While not of practical importance this allows some interesting insights into the metallurgy of (Nd,Ce)-Fe-B system. High-resolution scanning transmission electron microscopy coupled with electron probe microanalysis show the nano-scale distribution of Tb around the grain boundaries located in the bulk of the magnet. Finally, a simple model for the magnetization reversal in grain boundary diffusion processed gradient Nd-Fe-B magnets was developed and implemented into a FEM software. Our calculated demagnetization curves correspond very well for the Dy and Tb samples, but deviate significantly for Ce and Gd.

Coercivity changes in Nd-Fe-B permanent magnets after diffusion of several rare earth elements and resulting microstructures.434