Cu-assisted Tb in situ grain boundary diffusion Nd-Fe-B magnet by high pressure torsion: magnetic properties and microstructure

Heavy rare earth Tb with assistance of low melting point metal Cu was introduced in situ for preparing grain boundary diffusion (GBD) Nd-Fe-B magnets by using high pressure torsion. The addition of Cu element is beneficial to save the amount of Tb, lower the heat treatment temperature, shorten the heat treatment time, as well as obtain better magnetic properties. The intrinsic coercivity ( H cj ) of the Tb–Cu magnet significantly increases from ‌8.27 kOe‌ (0% Tb–Cu) to ‌18.85 kOe‌ (3 % Tb–Cu), corresponding to a ‌128 % enhancement‌ in magnetic performance. Compared with 5 % Tb-doped magnets, H cj increases by 36 % and the maximum energy product (( BH ) max ) by 20 %, while the degradation of remanence ( B r ) is suppressed. In situ GBD of Tb–Cu forms a continuous, elongated RE-rich phase at the micrometer scale, rather than the typical core-shell structure, indicating that Tb diffuses into the matrix phase and forms (Nd,Tb) 2 Fe 14 B phase. Staggered distribution of 2:14:1 phase and (RE)-rich phase is observed at nanoscale. The specific magnetic phase structure enhances intergranular demagnetization coupling effect, which is the main factor for the enhancement of coercivity.