Influence of Ti addition on microstructure and magnetic properties of a heavy-rare-earth-free Nd-Fe-B sintered magnet

We investigated the influence of Ti addition on the coercivity and microstructure of Ga-doped R-Fe-B (R = Nd + Pr) based sintered magnets. Detailed microstructure observations of the 0.5 at.%Ti-doped sample with the highest coercivity of 2.38 T revealed a near-complete grain isolation through the formation of thick (0.5–1.0 μm) grain boundary (GB) phases such as the R6(Fe,Ga)14 and the double hexagonal close-packed (DHCP) Nd. This suggests that these GB phases are non-ferromagnetic and decouple intergrain exchange. The formation of the R6(Fe,Ga)14 phase is found to be triggered by the formation of hexagonal TiB2 at high temperatures, which leads to a B-lean composition of GB phases that fall into a narrow compositional region, in which the R6(Fe,Ga)14 forms during post-sinter annealing at intermediate temperatures. Despite the high coercivity, the interface between the R6(Fe,Ga)14 and R2Fe14B phases in the post-sinter annealed 0.5 at.%Ti sample exhibits zig-zag morphology which is expected to deteriorate the coercivity with local demagnetizing field. Thus, further efforts to improve interfacial roughness should lead to further improve coercivity.