Hydrogen disproportionation by reactive milling and recombination of Nd 2(Fe 1− xCo x) 14B alloys

Abstract

Stoichiometric Nd 2(Fe 1− x Co x ) 14B alloys ( x=0, 0.25, 0.5, 0.75 and 1) have been disproportionated into NdH 2+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 Nd 2Co 14B alloy. This reaction is not possible via the conventional hydrogenation disproportionation desorption and recombination (HDDR) process. Grain sizes of disproportionated and recombined Nd 2(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 Nd 2Co 14B alloy shows on average slightly smaller grain sizes than the Nd 2Fe 14B compound. A more effective exchange coupling leading to enhanced remanences, possibly due to the slightly smaller grain size, has been observed for Nd 2Co 14B powders recombined at 600–700°C.