The effects of Dy content and post-sintering heat-treatment condition on the magnetic properties of sintered $Nd_{15-x}Dy_xFe_77B_8$ magnets (x=0.05, 1.0 and 1. 5) have been investigated. The intrinsic coercivity($_iH_c$) of $Nd_{15-x}Dy_xFe_{77}B_8$ magnets increased with increasing Dy content(x) at a rate of about 3.5kOe/at%Dy, while the remanence($B_r$) and maximum energy product($(BH)_max$) decreased slowly due to the antiferromagnetic coupling between Dy and Fe moments. All the $Nd_{15-x}Dy_xFe_{77}B_8$ magnets showed maximum $_iH_c$ values for the post-sintering het-treatment temperature of 600℃. On the other hand, the highest $B_r$ and $(BH)_max$ were obtained when the magnets were heat treated at 650℃
Microstructural examinations revealed that the sintered $Nd_{15-x}Dy_xFe_{77}B_8$ magnets were composed of the hard magnetic $(Nd,Dy)_2\,Fe_{14}B$, B-rich and Nd-rich phases. The crystal structure and stoichiometry of the former two intermetallic compounds did not change with Dy content and heat treatment condition. But two kinds of Nd-rich phases, with different crystal structures and compositions were observed according to the heat-treatment condition as well as Dy content. Auger electron spectroscopy showed that the fcc Nd-rich phase contained a considerable amount of oxygen(20at%) while the composition of dhcp Nd-rich phase was close to pure Nd. It has been also found that the dhcp Ndrich phase plays an important role in the coercivity and the magnets with increased amount of the dhcp phase exhibit the high coercivity values.
