A study on the Nd-rich phase evolution in the Nd–Fe–B sintered magnet and its mechanism during post-sintering annealing

We investigated the microstructural evolution and the phase transformation of the RE-rich (Nd–Dy–O) phase, the Cu-rich (Nd–Cu–Co–O) triple junction phase (TJP), the mixed TJP, and their grain boundary phase (GBP) in the sintered Nd–Fe–B base magnet as a function of the post-sintering annealing (PSA) steps. The round-shaped RE-rich phase and triangular-shaped Cu-rich TJP have an fcc-NdO (a = 0.499 nm) type structure in the as-sintered state. These phases are gradually transformed into the hexagonal Nd2O3 (h-Nd2O3, a = 0.383 and c = 0.600 nm) type structure during the 1st and 2nd PSA steps. The Cu-enriched TJP, which contains a large amount of Cu (about 40 at.%), was transformed into the cubic Nd2O3 (C-Nd2O3, a = 1.108 nm) type structure after the 2nd PSA, suggesting that Cu-enrichment could be a trigger for the formation of the meta-stable C-Nd2O3 phase. The h-Nd2O3 GBP, formed from the Cu-rich phase and the mixed TJP in the as-sintered magnet, was also transformed into Cu-enriched C-Nd2O3, which is embedded in the amorphous matrix, during the 2nd PSA. The formation of the C-Nd2O3 TJP and the GBP after the 2nd PSA is the major factor involved in enhancing the coercivity (from 21.7 to 29.7 kOe). The mechanism of the phase evolution and the coercivity enhancement is discussed based on these results.