Microstructural evolution of triple junction and grain boundary phases of a Nd-Fe-B sintered magnet by post-sintering annealing

The microstructural evolution of Nd-rich grain boundary phases (GBP) in connection with triple junction phases (TJP) during post-sintering annealing (PSA) was investigated. The Cu-rich TJP in the as-sintered sample was mostly a fcc-NdO phase but the GBP were a mixture of h-Nd₂O₃ and Nd phases. The fcc-NdO of the TJP in the as-sintered state gradually transformed to h-Nd₂O₃ during the first and the second PSA steps. However, it transformed to a C-Nd₂O₃ phase as both a massive form such as TJP and a thin GBP after the modified second PSA step. This suggests that the mechanism for the formation of metastable C-Nd ₂O₃ may not be solely the interface energy. In contrast, the mixture of h-Nd₂O₃ and Nd of the GBP in the as-sintered state gradually transformed to C-Nd₂O₃ which is embedded in the amorphous matrix as the PSA goes from the first to second or modified second PSA step. The formation of the C-Nd₂O₃ GBP with an amorphous phase is the main factor for increasing the coercivity (from 21.8 to 30.4 kOe) after the second or modified second PSA step.