IN-SITU NEUTRON-DIFFRACTION STUDY OF THE HDDR PROCESS OF ND-FE-B MAGNETS

High coercive isotropic Nd-Fe-B powders can be obtained using the nati on-disproportionation-desorption-recombination (HDDR) process. The Nd- Fe-B technological alloy is made of several phases; among them the mai n one is Nd2Fe14B but with important additions of an eutectic composit ion rich in neodymium (Nd-rich) and of NdFe4B4. In situ neutron diffra ction experiments have been performed on different alloys and Nd-rich compositions. It was possible to follow solid-gas reactions and to rec ord in situ the different steps of the HDDR process. and more particul arly, the behaviour of the intergranular phase under vacuum and under hydrogen. Experiments performed on Nd2Fe14B11 during the hydruration p rocess have evidenced the presence of hydrides with composition varyin g with temperature up to the decomposition of the phase in neodymium h ydride and iron. Structural refinements of the phases appearing during the process have been performed and a mechanism for the HDDR process is proposed. (C) 1998 Elsevier Science B.V. All rights reserved.