Mechanical properties and microstructure of Nd-Fe-B magnet diffused by low melting point alloys

•Grain boundary alloying improves the bending strength of sintered Nd-Fe-B magnets.•Internal stress changes the fracture mechanism.•Transgranular fracture is detrimental to bending strength. Commercial Nd-Fe-B sintered magnet was processed by grain boundary diffusing Pr-Cu, Pr-Al, and Pr-Tb-Cu alloys, respectively. The coercivity increased from the original 13.78 kOe to 14.3 kOe, 16.51 kOe and 21.62 kOe, respectively. The bending strength increased from original 290.13 MPa to 315.63 MPa by diffusing Pr-Cu alloy, however, decreased to 266.83 MPa and 268.10 MPa by diffusing Pr-Al and Pr-Tb-Cu alloys, respectively. Microstructure analysis confirmed that Cu was enriched at grain boundaries after diffusion treatment. However, Tb and Al atoms diffused into the main phase and replaced some Nd and Fe atoms. The substitution between atoms results in lattice distortion of main phase, resulting in internal stress, which reduces the bending strength of sintered Nd-Fe-B magnets.