Desorption–recombination kinetics and mechanism of a mechanically milled nano-structured Nd16Fe76B8 alloy during vacuum annealing

Abstract

The process of mechanically activated disproportionation by ball milling in hydrogen and subsequent desorption–recombination has been proved to be a promising method to produce nanocrystalline NdFeB-type magnets. Aiming to install higher magnetic properties of NdFeB-type magnet, it is crucial to select properly and optimize the desorption–recombination processing temperature and time to achieve a fully recombined microstructure with homogeneous nano-sized Nd2Fe14B grains. Thus, the desorption–recombination kinetics of the as-disproportionated Nd16Fe76B8 (atomic ratio) alloy during vacuum annealing was experimentally investigated using Mössbauer effect, X-ray diffraction and transmission electron microscopy. The effect of processing time and temperature on the desorption–recombination kinetics to form Nd2Fe14B was elucidated and the underlying mechanisms were discussed by referring to the thermal energy input intensity and the Gibbs free energy change for the reaction. The optimum magnetic properties were achieved by vacuum annealing at 780°C for 30min due to a fully recombination microstructure with uniform grains of 50–55nm in average size. Furthermore, by applying chemical equation for Mössbauer spectra, the transformed fraction of Nd2Fe14B phase during desorption–recombination process was determined, which increased as f=1-exp(−ktn) with n falling in the range of 0.91–0.97. The kinetic equation, the rate constant, the reaction order exponent, and the activation energy for the desorption–recombination reaction to form Nd2Fe14B during vacuum annealing were obtained by fitting the experimental data with the JMAK theory, and thus the kinetics of desorption–recombination can be quantified and evaluated.