Effect of carbon on the coercivity and microstructure in fine-grained Nd–Fe–B sintered magnet

We have investigated the effect of carbon on the coercivity and microstructure in fine-grained Nd–Fe–B sintered magnets fabricated by the pressless sintering method. The coercivity of the sample with the carbon content of 730ppm (low-C) was 1.59T while that of the sample with 1500ppm (high-C) was 1....
Ausführliche Beschreibung

We have investigated the effect of carbon on the coercivity and microstructure in fine-grained Nd–Fe–B sintered magnets fabricated by the pressless sintering method. The coercivity of the sample with the carbon content of 730ppm (low-C) was 1.59T while that of the sample with 1500ppm (high-C) was 1.44T in the as-sintered state. The low-C sample exhibited a larger coercivity increase by a post-sinter annealing, reaching the highest coercivity of 1.85T, while the high-C sample reached a lower coercivity of 1.54T. Detailed microstructure investigations using scanning electron microscopy, scanning transmission electron microscopy and atom probe tomography revealed that the high carbon content resulted in the formation of a Nd-carbide with a tetragonal structure and the reduction in the volume fraction of an α-Nd phase at triple junctions. This in turn decreased the Nd+Pr concentration in thin Nd-rich grain boundary phase, resulting in the lower coercivity. Ausführliche Beschreibung