Hydrogen disproportionation by reactive milling and recombination of Nd2(Fe1−xCox)14B alloys Original Research Article

Stoichiometric Nd2(Fe1−xCox)14B alloys (x=0, 0.25, 0.5, 0.75 and 1) have been disproportionated into NdH2+δ and bcc–(Fe,Co) (0≤x≤0.75) or fcc–Co (x=1), respectively, by milling in hydrogen at enhanced temperatures. Reactive milling leads to the disproportionation of the thermodynamically very stable Nd2Co14B alloy. This reaction is not possible via the conventional hydrogenation disproportionation desorption and recombination (HDDR) process. Grain sizes of disproportionated and recombined Nd2(Fe,Co)14B 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 powders recombined at 600–700°C.