Textured NdFeB HDDR magnets produced by die-upsetting and backward extrusion

Abstract

The hydrogenation disproportionation desorption recombination (HDDR) process was applied to produce isotropic, submicron powder in 500 g batches in a specially designed HDDR reactor. The hot pressing characteristics of the material were determined and it was shown that the material has an excellent stability against grain growth. The coercivity of 1000-1080 is almost constant over a hot pressing temperature range of 700-, making the material highly suitable for subsequent hot deformation. Die-upset HDDR magnets were prepared in order to study the basic deformation behaviour. A remanence of 1.13 T in the axial direction and a coercivity of were achieved. Similar properties were obtained for the backward extruded magnets produced at and only a small decrease in alignment along the axial direction of the ring was found. Grain sizes were very uniform and on the submicron scale. Platelet-shaped grains were observed in the die-upset magnets. The formation of interaction domains, along the axial and radial directions for the die-upset and backward extruded magnets respectively, were established by high-resolution Kerr microscopy. The high degree of texture in the hot deformed HDDR magnets was also confirmed by Lorentz microscopy revealing continuous equispaced domains extending over the entire thinned sample with only small directional variations.