Mechanism of anomalous α-Fe formation from stoichiometric Sm2Fe17 jet-milled powder during post-pulverization annealing

Abstract Our research group developed a novel low-oxygen powder metallurgy process and succeeded in producing anisotropic Sm2Fe17N3 sintered magnets from finely pulverized high-performance powder without coercivity degradation, but the maximum energy product of the magnets was disappointingly low compared to the excellent performance of the powder. The performance degradation was attributed to some type of surface damage, and more specifically, poor crystallinity and randomly oriented nanocrystalline grains at the particle surfaces caused by the milling process. In this study, the current authors focused on Sm2Fe17 fine pulverized powder instead of Sm2Fe17N3 and explored the possibility of damage recovery to remove those microstructural defects by post-milling annealing/nitridation treatments. This treatment improved crystallinity to some extent but failed to remove the randomized nanocrystalline grains completely. In addition, it was found that α-Fe forms upon annealing of stoichiometric Sm2Fe17, which is inconsistent with the Sm-Fe binary phase diagram at a glance. The mechanism of this unexpected α-Fe formation was investigated by detailed microstructural analyses using transmission electron microscopy and three dimensional atom probe tomography.