A Mössbauer investigation of the homogenisation of Nd 11.8Fe 82.3− xNb xB 5.9 alloys

Abstract

The effect of niobium addition on the microstructure of Nd–Fe–Nb–B alloys (with 0, 2, 4 and 5 at.% Nb) has been investigated by transmission Mössbauer spectrometry and X-ray diffraction analyses. In the as-cast alloys, the Nb 26Fe 32B 42 (at.%) phase was characterised by a paramagnetic doublet, with an isomer shift IS=−0.04 mm s −1 relative to α-Fe, and a quadrupolar splitting QS=0.22 mm s −1. The contribution of the Nd 2Fe 17 phase was observed in the Mössbauer spectra of the alloys with 4 and 5 at.% Nb. Niobium additions allow a drastic reduction of the homogenisation times during annealing in the 800–1100°C temperature range, in agreement with previous studies. The kinetics of the disappearance of α-Fe in alloys with 2 at.% Nb were determined by Mössbauer spectrometry. The experimental curves were fitted with a simple theoretical model of growth of a spherical Nb–Fe–B precipitate in a spherical α-Fe matrix. The activation energy related to the α-Fe disappearance was estimated to be 112±16 kJ mol −1.