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(13.7 5)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 d ifferences at the grain boundaries, combined with high-resolution elec tron microscopy and nanoscale chemical analysis. In both the Pr- and N d-based melt-quenched die-upset magnets we consistently observed a ver y thin Fe-rich amorphized intergranular phase at the boundaries, and t his finding may have an impact on our understanding of the coercivity mechanism of these materials. Although the value of the method was dem onstrated for transition metal-rare earth intermetallics, it may also be applied to grain-boundary studies in other materials.