STUDY OF DESORBED HYDROGEN-DECREPITATED ANISOTROPIC ND-FE-B POWDER USING X-RAY-DIFFRACTION

The intrinsic magnetic coercivity (H-ci) of Nd-Fe-B-based permanent ma gnet material is profoundly affected by hydrogen absorbed during the h ydrogen decrepitation (HD) process for producing anisotropic powders f rom bull: anisotropic hot-deformed MAGNEQUENCH (MQ) magnets. Hydrogen (H) content and x-ray diffraction measurements clarify the effects of H and desorption temperature (Td) on the intrinsic magnetic anisotropy (IMA) of the Nd2Fe14B-type phase and-the nature of the intergranular phases, both of which are crucial for high H-ci. The Nd-rich intergran ular phase disproportionates during HD, initially forming a microcryst alline Nd-hydride phase, possibly Nd2H5. For T-d less than or equal to 220 degrees C, H remains in the Nd2Fe14B-type phase, severely degradi ng the IMA, which causes a low H-ci. For 220 degrees C less than or eq ual to T-d less than or equal to 250 degrees C, enough H desorbs from the Nd2Fe14B-type phase and the IMA recovers its large prehydrided val ue, and the microstructure supports a high H(ci)less than or equal to 10 kOe in spite of the H disproportionated intergranular phase. Only f or T-d>250 degrees C is H-ci degraded by the microstructure, correspon ding to further H desorption and the microcrystalline Nd-hydride phase becoming well-crystallized NdH2. The NdH2 phase decomposes with conti nued H desorption and at T-d>580 degrees C recombines to re-form the N d-rich intergranular phase of prehydrided MQ material. H is completely desorbed above 580 degrees C and H-ci>11 kOe, nearly that of the preh ydrided MQ magnets.