Atomic-scale oxidation of a Sm2Co17-type magnet

Oxidation induced irredeemable magnetic loss is of key concern in the high temperature applications of Sm2Co17-type permanent magnets. Herein, the atomic-scale oxidation mechanism of a Sm2Co17 magnet is unveiled using aberration-corrected transmission electron microscopy. Heating at 500 degrees C in air, the oxidation scale growth and energy product reduction of the magnets undergo a two-stage process. Due to the formation of a transition oxidation zone between the internal oxidation zone (IOZ) and matrix, the stage-I (t < 24 h) exhibits a 7-10 times faster oxidation rate than stage-II (t >= 24 h). Besides, it is found that the anisotropic phase distribution in the parent magnet strongly affects the oxidation behavior. The {0 0 01} basal planes and {0 1 <(1)over bar>1} pyramidal planes act as the preferential oxygen diffusion pathways at the magnet side and top surfaces, respectively. This results in the non-uniform oxidation, i.e., the oxidation scale at the cylindrical sides of the magnet is 1.4-2 times thicker than that at the top. Oxygen penetration along basal or pyramidal planes firstly induces the oxidation of 1:3R Z-plates and Cu depletion from the 1:5H boundaries in the magnet. Then the 1:5H and 2:17R phases are decomposed into Sm, CoFe and Cu metal lamellae, which finally evolve into the IOZ with nano-oxides, oxygen-enriched CoFe and Cu particles inside. This work sheds light on the atomic-scale oxidation behavior of Sm2Co17-type magnets. (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.