Effect of post-sinter annealing on the coercivity and microstructure of Nd–Fe–B permanent magnets

To understand the mechanism of the increase in coercivity caused by post-sinter annealing of Nd–Fe–B-based magnets, we have investigated the microstructures of commercial sintered magnets by high-resolution scanning electron microscopy, transmission electron microscopy and atom probe tomography. Continuous thin layers of a Nd-rich amorphous phase were found along the grain boundaries in the post-sinter annealed sample, the chemical composition of which was determined to be Nd30Fe45Cu24.1B0.9. A fine Cu-enriched shell was also confirmed in the Nd-rich phase grain, suggesting the Nd2Fe14B grains are completely enveloped by the Cu- and Nd-enriched layers. Furthermore, a lamellar microstructure of the Cu-enriched phase was confirmed in some Nd-rich phase grains. The mechanism of the coercivity increase caused by post-sinter annealing is discussed based on these characterization results.