TEM study of inhomogeneities in Nd-Fe-B sintered magnets

Nd-Fe-B sintered magnets with a nominal composition Nd/sub 18.8/Fe/sub 73.6/B/sub 6.9/Al/sub 0.7/ and Nd/sub 19/Fe/sub 69.8/B/sub 6.4/Al/sub 4.5/ were prepared by conventional powder metallurgy processes and post-sintering heat treatment. Microstructure studies were performed by transmission electron microscopy (TEM) using Philips EM300 and Hitachi (200 kV). Specimens for TEM were prepared by electropolishing and ion thinning. The structure of these magnets contains following inhomogeneities nanocrystalline phase on grain boundaries and interfaces, grain boundaries without isolation of the Nd-rich phase, interface irregularities or anomalous strain contrast, low angle boundaries and coincidence boundaries between Nd/sub 2/Fe/sub 14/B phase and coherent interfaces. It was observed that higher Al concentration improves magnetic properties by allowing grain boundary phase to form equal thickness layer without stress concentration on the interface.<
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