The influence of residual nitrogen element on microstructure and magnetic properties of sintered Nd-Fe-B magnet

The preparation procedures for sintered Nd-Fe-B magnets typically involve several key steps: alloy casting, powdering, green compacting, sintering and tempering. As the procedures of jet milling and pressing are conducted in nitrogen (N2) atmosphere, nitrogen element is inevitably introduced into powders and remains within magnet. To elucidate the influences of the jet milling and pressing processes in the residual amount, distribution and form of nitrogen element in both powder and magnet, Nd-Fe-B strip casts were jet milled in N2 and Ar atmosphere, respectively, and then pressed concurrently under N2 atmosphere. The results indicated that the nitrogen element in powder primarily originated from the N2 jet milling process and was incorporated into hcpNd phase. Furthermore, the nitrogen content in magnet was found to be higher than that in powder, indicating that the nitrogen element was also introduced during pressing process in N2 atmosphere. Additionally, after sintering, the near-spherical c-Nd2(N, O)3 particles were formed and distributed at grain boundaries of N2magnet. As the Nd element at grain boundaries was consumed to from the c-Nd2(N, O)3 phase, the thin Nd-rich layers acting as the exchange-decoupling effect were reduced, resulting in the lower coercivity for the N2-magnet than that for the Ar-magnet.