Further enhancing the coercivity of Nd-Tb-Fe-B sintered magnets by Dy grain boundary diffusion

•Every 1 wt% Tb doping led to about 0.37 T linear increase in the coercivity.•The coercivity increased by 0.7 T by Dy GBD for magnets with different Tb content.•Network enriched with Dy formed after Dy GBD and became clearer after annealing.•The coercivity increased by 0.3 T after post diffusion annealing.•The optimization of Nd-rich GB phase during annealing process maintained after GBD.