Regulating fracture behaviors and mechanical properties of sintered Nd–Fe–B magnet by reconstructing grain boundaries

In this paper, the fracture behaviors and interface stresses of sintered Nd–Fe–B magnets were investigated. It was revealed that cracks propagated along the interfaces of Nd₂Fe₁₄B/Nd₂Fe₁₄B, Nd₂Fe₁₄B/Nd-rich phases and within Nd-rich phases. The misfits between 2:14:1 and Nd-rich phases were quantitatively determined, reflecting the concentration of interface stresses in magnet. Grain boundaries of magnet were reconstructed by diffusing Tb-containing and Tb-free sources, respectively. The Tb-rich shells at extensive layers of 2:14:1 grains caused lattice distortion, i.e., internal stresses, resulting in the crack mode changing from intergranular to transgranular in near surfaces of Tb-containing diffused magnets. Therefore, the bending strengths were deteriorated although the coercivities were apparently enhanced. In the case of Tb-free Pr–Cu diffused magnet, the Pr and Cu elements were enriched at grain boundaries, and the misfits between 2:14:1 and Nd-rich phases were declined, lowering interface stresses and strengthening grain boundaries. The mechanical and magnetic properties of Pr–Cu diffused magnet were simultaneously improved.

Graphical abstract