Desorption-recombination and microstructure change of a disproportionated nanostructured NdFeB alloy

The desorption-recombination behavior of a mechanically disproportionated, nanostructured Nd₁₂ Fe₈₂B₆ alloy was investigated by differential scanning calorimetry and thermogravimetric analysis. The microstructure change due to desorption-recombination treatments at various temperatures was characterized by x-ray diffraction, Mössbauer study, and transmission electron microscopy observation. The results show that the hydrogen desorption of the as-disproportionated alloy occurs in two stages: (i) the partial dehydriding of the Nd hydride from Nd₂H₅ to NdH₂, as well as the desorption of the hydrogen absorbed/adsorbed at sites of crystal defects but not in the form of Nd hydride, at temperatures between 180 and 460°C; and (ii) the complete dehydriding of the Nd hydride from NdH₂ to Nd at temperatures between 630 and 780°C. The recombination of α-Fe with Fe₂B and Nd to form Nd₂Fe₁₄B occurs following the dehydriding of NdH₂ to Nd, and it acts as the controlling step for the whole desorption-recombination process. The kinetics of both the desorption-recombination reaction and the growth of the newly formed Nd₂Fe₁₄B grains accelerate with increasing temperature. For a fixed annealing time of 30 min, the optimal processing temperature seems to be 760°C, which gives rise to a fully recombined Nd₂Fe₁₄B-α-Fe nanocomposite microstructure with Nd₂Fe₁₄B and α-Fe phases of 25-30 nm in average size.