Magnetic monitoring of the disproportionation process in Nd-Fe-B-based materials

The hydrogen disproportionation reaction was investigated for three Nd - Fe - B-based alloys - Nd14.9Dy1.0Fe76.1B8.0, Nd15.2Dy1.0Fe74.8B8.3Zr0.7 and Nd15.4Dy1.0Fe73.3B8.3Zr2.0 - using a modified vibrating-sample magnetometer. The addition of zirconium was found to substantially slow down the rate of the disproportionation reaction. A more surprising result, however, was the presence of a two-stage reaction for the Zr-free material, which prevents us from interpreting the disproportionation reaction in terms of the Johnson - Mehl - Avrami - Kolmogorov (JMAK) theory of phase transformations. Our isothermal experiments at 650 degrees C resulted in very low values in the range 0.6 - 0.7 for the Avrami transformation constant. These low values also suggest that the standard JMAK model is inappropriate for describing the process of disproportionation in these alloys, and an extension to the JMAK theory that takes into account a critical radius for stable iron nuclei is required to explain these low values. (C) 2004 Elsevier B. V. All rights reserved.