TEM STUDY OF INHOMOGENEITIES IN ND-FE-B SINTERED MAGNETS

Nd-Fe-B sintered magnets with a nominal composition Nd18.8Fe73.6B6.9Al 0.7 and Nd19Fe69.8B6.4Al4.8 were prepared by conventional powder metal lurgy processes and post-sintering heat treatment. Microstructure stud ies we by transmission electron microscopy (TEM) using Philips EM300 a nd Hitachi (200 kV). Specimens for TEM were prepared by electropolishi ng and ion thinning. The structure of these magnets contains following inhomogeneities: nanocrystalline phase on grain boundaries and interf aces, grain boundaries without isolation of the Nd-rich phase, interfa ce irregularities or anomalous strain contrast, low angle boundaries a nd coincidence boundaries between Nd2Fe14B phase and coherent interfac es. It was observed that higher Al concentration improves magnetic pro perties by allowing grain boundary phase to form equal thickness layer without stress concentration on the interface.