ELECTRON-MICROSCOPY OF GRAIN-BOUNDARIES - AN APPLICATION TO RE-FE-B (RE=PR OR ND) MAGNETIC-MATERIALS

To study the grain boundaries of the technologically important RE(13.75)Fe(80.25)B(6) (RE = Pr or Nd) permanent magnets, we developed a new method using small-angle dark-field imaging based on inner-potential differences at the grain boundaries, combined with high-resolution electron microscopy and nanoscale chemical analysis. In both the Pr- and Nd-based melt-quenched die-upset magnets we consistently observed a very thin Fe-rich amorphized intergranular phase at the boundaries, and this finding may have an impact on our understanding of the coercivity mechanism of these materials. Although the value of the method was demonstrated for transition metal-rare earth intermetallics, it may also be applied to grain-boundary studies in other materials.