Bonded Sm–Fe–(Ta)–N materials produced via attritor milling and HDDR

Abstract

Abstract An alloy of composition Sm 13.8 Fe 82.2 Ta 4.0 was compared with a more conventional Sm 13.7 Fe 86.3 binary material in order to assess Ta as a potential additive for SmFe-based permanent magnet materials. The optimum conditions necessary to provide the highest coercivities using only the HDDR process, and for the HDDR process combined with a pre-attritor milling stage were investigated. The coercivities obtained after using the HDDR process and subsequent nitriding were 680 kA/m for the SmFeTaN and 360 kA/m for the SmFeN samples. Coercivities of 1010 kA/m for SmFeN and 1280 kA/m for SmFeTaN were achieved by reducing the particle size to about 5 μm with attritor milling prior to the HDDR process. The magnetic results were compared with the phase composition, determined using scanning electron microscopy (SEM) and X-ray diffraction (XRD) and the grain size of the recombined material (SEM). The better coercivities obtained with the Ta-containing sample were found to be due to the presence of a much smaller amount of α-Fe, a soft magnetic phase, with a TaFe 2 phase forming in its place. The attritor milling prior to the HDDR treatment improves the magnetic properties because of the small particle size, produced by attritor milling prior to the HDDR process, which physically prevents the growth of large grains, with their consequent very negative effect on the coercivity.