<p>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.</p> Graphical abstract <p></p>