Dependence of magnetic, phase-transformation and microstructural characteristics on the Cu content of Nd-Fe-B sintered magnet

We have investigated the changes in the magnetic, phase-transformation and microstructural properties of Nd-Fe-B sintered magnets as a function of Cu content (0.2-0.5 at.%). The coercivity decreased (28.7 → 27.1 kOe) with increasing Cu content without changing the remanence of the magnets under the normal post-sintering annealing (PSA) conditions. Phase-transformation temperature changes of the magnet were observed as the Cu content of the magnet was increased. In the 0.2 at.% Cu magnet, the triple junction phase (TJP) and grain boundary phase (GBP), composed of the Cu-enriched C-Nd₂O ₃ phase, were formed. However, in the 0.5 at.% Cu magnet, the TJP and GBP were composed of the h-Nd₂O₃ phase. By considering the dependence of Cu content on the phase transformation, we have modified the 1st-PSA temperature to recover the coercivity. Through our newly established PSA conditions, the TJP and GBP in the 0.5 at.% Cu magnet were formed as the C-Nd₂O₃ phase. As a result, the coercivity of the magnet was noticeably enhanced (27.1 → 29.4 kOe). The reasons for the coercivity deterioration in the high-Cu-content magnet were clarified. The detailed mechanism of the microstructural and magnetic property improvements induced by the modified 1st-PSA condition is discussed. The critical role of Cu in the microstructural changes of Nd-rich TJP and GBP during the PSA is also analyzed based on these results.