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 °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 {0001} basal planes and {011¯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.