Coercivity enhancement of low rare earth Nd–Fe–B sintered magnets by optimizing microstructure

•Magnetic performance in near-stoichiometric Nd2Fe14B magnets is investigated.•A high coercivity 16.4 kOe is achieved by optimizing the microstructure.•Both morphology and composition are varied in RE-rich grain boundary phases.•Modified RE-rich phase and refined grain size lead to a coercivity enhancement.

Magnetic performance of Nd–Fe–B sintered magnets, especially the coercivity is highly sensitive to microstructures extrinsically. To enhance coercivity of the near-stoichiometric Nd2Fe14B magnet (Nd, Pr)12.5FebalB6.1 (at%), the microstructure is optimized by varying sintering condition. At lower sintering temperatures, full densification cannot be reached even prolonging the sintering time, the coupling between adjacent 2:14:1 phase grains results in a low coercivity. At higher sintering temperatures, the grains of 2:14:1 phase grow up quickly, also deteriorating the coercivity. At an optimum sintering temperature of 1050 °C, the magnet reaches a full densification with most 2:14:1 phase grains of 4–5 μm, showing a high coercivity of 16.4 kOe. The formation of smooth and continuous grain boundary RE-rich phase reduces the magnetic interactions and the grain refinement hinders nucleation of reversed domains at low fields, respectively.