The diffusion of nitrogen into Nb-modified Sm 2Fe 17 powder

Abstract

Niobium-modified Sm 2Fe 17 powders were nitrided in pure nitrogen for a range of times and investigated using thermomagnetic analysis, scanning electron microscopy, optical microscopy and electron probe microanalysis (EPMA). Thermomagnetic analysis showed that, on nitriding the Curie point of the core increases continually until the whole particle has been uniformly nitrided. Such results could be explained either by the formation of a continuous solid solution of nitrogen or by the presence of just two compositions with the nitrogen-saturated shell expanding the lattice of the Sm 2Fe 17 core. A shell-core diffusion pattern was imaged using both optical microscopy and backscattered electron imaging. Comparing the EPMA profiles with the backscattered electron images, a diffusion layer between the shell and core with composition Sm 2Fe 17N x (0 < x < 2.7) was identified. This is consistent with the diffusion of nitrogen in the form of a continuous solid solution. The thickness of the nitrided shell varied considerably, not just from particle to particle, but also from different edges of the same particle. In some cases the difference was as large as a 1:10 ratio. Assuming an isotropic bulk diffusion mechanism propagating uniformly from the surface of the particle, such large differences cannot be explained reasonably by the non-central sectioning of the particles in question. Near the edges of some partially nitrided powder particles, the Sm 2Fe 17N 3-δ phase decomposed and this was observed as a mottled region consisting of α-Fe and samarium nitride. The soft magnetic α-Fe can be removed by subsequent zinc bonding, which develops a reaction layer ∼ 4 μm deep for the given processing parameters.