Hydrogenation-disproportionation processes in Nd14.01Hf0.08Fe78.91B6.99 alloys

RODRÍGUEZ TORRES, C.E.; SACCONE, F.D.; SÁNCHEZ, F.H.

MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING

ELSEVIER SCIENCE SA

Año: 2000 vol. 292 p. 104 - 111

0921-5093

The kinetics of the reactions undergone by the system Nd14.01Hf0.08Fe78.91B6.99 submitted to thermal treatments under H2 atmosphere were studied in detail. Constant number of moles, constant pressure (P≈119 kPa), isothermal or high heating rate experiments were performed at temperatures between 240 and 980 °C on samples of approximately 200 mg. The state after each treatment was characterized by Moessbauer effect spectroscopy and X-ray diffraction. Three temperature regions were identified. In the first one (T≤580 °C) two processes were observed: the formation of Nd-hydride at grain boundaries (2.6 H/Nd) and a solid solution Nd2Fe14BHx in the matrix phase. Activation energy of 36.20±0.02 kJ mol-1 for absorption process was determined. In the second temperature region (680 °C≤T≤800 °C) two processes of H2 absorption were found: (1) φ-Nd2Fe14B hydrogenation and formation of grain border Nd-hydride, and (2) disproportionation regime. For T≥800 °C disproportionation rates are slower for increasing temperatures, the φ-phase incorporates less than 0.6 H/Nd and a strong dependence with the experimental conditions was found in this temperature range. Between 800 and 875 °C the disproportionation reaction is fast but saturates, leaving only a small non-disproportionated fraction. This may constitute the best temperature interval for the hydrogenation-disproportionation processes.