Microstructure analysis of Ti-doped sintered Nd-Fe-B magnets

Abstract

A systematic microstructure analysis has been carried out for the as-sintered and annealed Ti-doped sintered Nd-Fe-B magnets. Upon annealing, grain boundaries become more obvious and show higher Nd content and lower Fe content, leading to the improvement of coercivity. Meanwhile, the amount of Nd6Fe13Ga increases and that of Nd2Fe14B decreases, leading to a slight loss of remanence. Many rod-shaped TiB2 precipitates are found both in the as-sintered and annealed samples with most of them located at grain boundaries. There is no significant difference between the as-sintered and annealed sample in the distribution, grain shape and size of TiB2. Atomic-scale characterization of TiB2 and its neighboring area reveals the existence of an amorphous layer surrounding TiB2 with the thickness of 3.5 ± 2.5 nm. The amorphous layer is measured and analyzed to be non-ferromagnetic with obvious enrichment of Nd and Pr. Its existence is speculated to be the result of strong cohesion of Ti and B, leaving a B-poor region around TiB2 difficult to form stable crystals. The existence of TiB2 and the surrounding (Nd,Pr)-rich amorphous layer in the Ti-doped magnets is beneficial for preventing the propagation of reversed magnetic domains and improving coercivity.