The HDDR behaviour of crystalline and amorphous rapidly quenched NdFeB

The disproportionation and recombination reactions in which Nd₂Fe₁₄B decomposes into αFe, Nd hydride and Fe₂B in hydrogen and then reforms under vacuum, have been investigated in melt-spun ribbon with the compositions Nd_13.1Fe_82.4B_4.5 and Nd₁₈Fe₇₆B₆. TPA and DTA investigations have shown that hydrogen absorption and desorption are more rapid in the amorphous Nd_13.1Fe_82.4B_4.5 ribbon cast at 30 m s^-1 than in the fully crystalline ribbon of this composition, spun at 10 m s^-1. Disproportionation was found to occur in two stages in 30 m s^-1 ribbon, the first stage in the range 400-500 °C associated with the amorphous phase, and the second stage in the range 500-700 °C due to the presence of crystalline Nd₂Fe₁₄B phase. By selecting ribbons with the smallest thicknesses, it was possible to investigate totally amorphous material on a Faraday balance, and these studies showed that, in this material, the disproportionation reaction was very rapid, and finished at 500 °C. This is in agreement with the observations on the two-phase material. In addition, DTA desorption measurements showed that recombination was more rapid and took place at a lower temperature in 30 m s^-1 ribbon disproportionated at 500 °C compared with that disproportionated at 800 °C, suggesting a coarsening of the disproportionated microstructure at the higher temperature.