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. [Display omitted] •Origins of high coercivity in a Ti-containing Nd-Fe-B sintered magnet are clarified.•Near-complete grain isolation by thick GB phases enhances coercivity.•The role of R6(Fe,Ga)14 phase formed after annealing on the coercivity is studied.•Formation of TiB2 phase provides conditions for formation of R6(Fe,Ga)14 phase.