Effect of non-rare-earth element enrichment on in-the-column secondary electron image contrast of Nd-rich phases in Nd-Fe-B sintered magnets

Abstract

In-the-column secondary electron (SE) imaging has demonstrated an important role in identifying various Nd-rich phases in Nd-Fe-B sintered magnets at a microscale field of view. It is well known that modifying the microstructure of Nd-Fe-B sintered magnets by adding non-rear-earth (NRE) elements dissolved into the intergranular Nd-rich phases during post-sinter annealing can effectively increase the magnet coercivity. However, the effect of NRE element enrichment on in-the-column SE image contrast of Nd-rich phases has not received much attention. In this study, we have combined a focused ion/electron dual beam system (FIB/SEM) and a scanning/transmission electron microscope (S/TEM) to investigate the in-the-column SE image contrast, structure, and element distribution of intergranular Nd-rich phases in commercial heavy-rare-earth (HRE) free Nd-Fe-B sintered magnets. In-lens SE imaging revealed that Cu, Co, or Al-enriched neodymium oxides with fcc, Ia3¯-type, or amorphous structures are brighter than the Nd2Fe14B matrix at an accelerating voltage of 2 kV and a working distance of 4 mm. Bright stripes inside the intergranular fcc NdOx were also visible in high-resolution through-the-column SE imaging, reflecting the nanoscale composition variation with Co or Cu enrichment. Contrarily, at the same imaging conditions, the nearly pure fcc NdOx and Ia3¯-Nd2O3 exhibit darker than the Nd2Fe14B matrix in the in-lens SE image. It has been demonstrated that the in-lens SE image contrast of Nd-rich phases with respect to the Nd2Fe14B matrix remains constant by systematically adjusting the accelerating voltage from 2 kV to 5 kV and the working distance from 4 mm to 6 mm. Our research results suggest that low-voltage in-the-column SE imaging could be used to track element diffusion in triple junction and grain boundary Nd-rich phases of Nd-Fe-B sintered magnets during post-sinter annealing by in-situ MEMS heating in SEM.