Hydrogen disproportionation by reactive milling and recombination of Nd-2(Fe1-xCox)(14)B alloys

Stoichiometric Nd-2(Fe1-xCox)(14)B alloys (x = 0, 0.25, 0.5, 0.75 and I)hav e been disproportionated into NdH2+delta and bcc-(Fe,Co) (0 less than or eq ual tox less than or equal to0.75) or fee-Co (x = 1), respectively, by mill ing in hydrogen at enhanced temperatures. Reactive milling leads to the dis proportionation of the thermodynamically very stable Nd2Co14B alloy. This r eaction is not possible via the conventional hydrogenation disproportionati on desorption and recombination (HDDR) process. Grain sizes of disproportio nated and recombined Nd-2(Fe,Co)(14)B materials were found to be <10 nm and 40-50 nm, respectively - approximately an order of magnitude smaller than those of conventional-HDDR processed alloys. The recombined Nd2Co14B alloy shows on average slightly smaller grain sizes than the Nd2Fe14B compound. A more effective exchange coupling leading to enhanced remanences, possibly due to the slightly smaller grain size, has been observed for Nd2Co14B powd ers recombined at 600-700<degrees>C. (C) 2000 Acta Metallurgica Inc. Publis hed by Elsevier Science Ltd. All rights reserved.