Magnetic hardening studies in sintered Nd-Fe-B based magnets

The effects of small elemental substitutions and low temperature annealing on the coercivity and microstructure of Nd-Fe-B sintered magnets have been investigated. Results show that the 0.18 at% additions of Al, Cu, Mo, V and Sri into the Dy-free Nd-Fe-Co-B magnets am not effective for improving the coercivity. Adddion of 0.18 at% Ga was found to be the most effective for improving the coercivity in both the Nd-Fe-B and Nd-Fe-Co-B magnets without decreasing much the remanent magnetization. Microstructure studies did not reveal a noticeable difference in the gram size and structure morphology between the Ga-free and Ga-substituted magnets at the different annealing stages. However, Fe and especially Co concentrations in the intergranular Nd-rich phase were found to be markedly increased after the small Ga addition. ne Nd-rich phase in the triple gram boundary junctions was always found to extend along the gram boundaries after annealing taking the form of a thin grain boundary phase. The extended grain boundary phases became more and coarser in the sample annealed at 475degreesC (17 kOe), but decreased in the sample annealed at 600degreesC (1 kOe). The thin grain boundary phases were also observed by TEM and they are found to be effective for domain wall pinning, and possibly responsible for the development of coercivity in these magnets.