The Role of Microstructure on the Performance of Sintered Nd-Fe-B Magnets

A combination of electron microscopy analysis and micromagnetic modelling was used to study the microstructure role on the performance of commercial sintered Nd-Fe-B magnets that are used in wind power units. These magnets were observed to be composed of Nd2Fe14B phase grains with different Dy content, size distribution and anisotropy orientation. The grains were surrounded by intergranular phases with different Fe/rare-earth ratios. The magnetic characterisation of these samples revealed that it is possible for Dy-free samples to have higher coercivities than Dy-containing samples with the control of the microstructure. Micromagnetic modelling revealed that decreasing the angular dispersion of Nd2Fe14B grain anisotropy provides the most significant increase to coercivity. For example, decreasing this angular dispersion from 30° to 1° increased the coercivity by 25% of its initial values. In addition, the presence of intergranular phases with high Fe/rare-earth ratios (Fe-like) led to the generation of reversed domain walls at low applied field. The formation of non- ferromagnetic intergranular phases (low Fe/rare-earth) led to 8% increase in coercivity; however, it led to step-wise hysteresis loops. On the other hand, the effect of grain size distribution on coercivity was found to be less significant compared to previous features. Micromagnetic modelling of grain boundary diffused samples revealed that the coercivity may increase with increasing the thickness and Dy concentration of Dy-rich shells. In addition, the higher rare-earth content at the intergranular regions of these samples was revealed to be beneficial to coercivity. Finally, the possibility of twinning in main phase grains showed a detrimental effect on the coercivity. The formation of non-ferromagnetic intergranular phases may reduce the effect of twinned grains and increase coercivity but no significant increase in remanence and maximum energy density was observed. These insights could be useful to produce sintered Nd-Fe-B magnets with higher coercivity and lower heavy rare-earth content.