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

Stoichiometric Nd₂(Fe_1−xCo_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 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.