Microstructure and magnetic property optimization for Co-rich dual-main-phase Nd-(Fe, Co)-B sintered magnet

High-Co-containing RE-(Fe, Co)-B (RE, rare earth) sintered magnets by dual-main-phase route (DMP, sintering the mixture of Co-free and Co-containing RE ₂Fe ₁₄ B powders) were developed in this work, which effectively suppresses the formation of Co-rich intergranular phases and significantly increases the Co-substitution level in Nd-Fe-B magnets. Magnetic properties of B _r = 13.74 kGs, H _cj = 13.09 kOe, and (BH) _max = 44.32 MGOe were obtained even when 35 at. % Co substitutes for Fe in the dual-main-phase magnets. Phase components analysis indicated that the typical RE(Fe, Co) ₂ and RE(Fe, Co) ₄ B were detected until the relative concentration of Co up to 35 at. %, which are commonly conceived as harmful to magnetic properties. Microstructural analysis revealed the distribution of each intergranular phase at grain boundaries and the inhomogeneous distribution of RE in the core and shell of the grain. Compared to the addition of Co through alloying, introducing Co by mixing RE-(Fe, Co)-B and RE-Fe-B allows a metallurgical reaction between Co-rich intergranular phases and 2:14:1 matrix phase, accompanied by dramatical elemental interdiffusion during the sintering, thus the formation of Co-rich intergranular phases can be suppressed to a certain extent. These efforts provide another possibility to obtain Nd-Fe-B magnets with higher Co content and achieve better thermal stability.