Optimization of the post-sintering annealing condition for the high Cu content Nd-Fe-B sintered magnet

We investigated the changes in microstructure and magnetic properties of Nd–Fe–B magnets as a function of Cu content (Cu addition in the range of 0.2–0.5 at. %). We have focused on the phase-transformation temperature of the Nd-rich phases in dependence of the Cu content. The coercivity of the magnets decreased (from 28.7 kOe to 27.1 kOe) on increasing Cu content under identical post-sintering annealing (PSA) conditions. The Cu-enriched C-Nd₂O₃ triple junction phase (TJP) and grain boundary phase (GBP) were observed in the 0.2 at. % Cu-containing magnet. However, in the 0.5 at. % Cu-containing magnet, the Cu-enriched TJP and GBP appeared in the form of the h-Nd₂O₃ phase that could be transformed into the C-Nd₂O₃ phase by a modified 1st-PSA temperature. The coercivity of the 0.5 at. % Cu-containing magnet was noticeably enhanced (from 27.1 kOe to 29.4 kOe).