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 (N 2 ) 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 N 2 and Ar atmosphere, respectively, and then pressed concurrently under N 2 atmosphere. The results indicated that the nitrogen element in powder primarily originated from the N 2 jet milling process and was incorporated into hcp-Nd 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 N 2 atmosphere. Additionally, after sintering, the near-spherical c-Nd 2 (N, O) 3 particles were formed and distributed at grain boundaries of N 2 -magnet. As the Nd element at grain boundaries was consumed to from the c-Nd 2 (N, O) 3 phase, the thin Nd-rich layers acting as the exchange-decoupling effect were reduced, resulting in the lower coercivity for the N 2 -magnet than that for the Ar-magnet. • The nitrogen element in the powder primarily originated from the N 2 jet milling process and was incorporated into hcp-Nd phase. • The nitrogen content in the magnet was higher than in the powder, indicating that nitrogen was also introduced when pressing in the N 2 atmosphere. • The c-Nd 2 (N, O) 3 phase was prominently formed and distributed at the triple junction regions of the magnet.