THE EFFECT OF THERMAL STABILIZATION ON THE COMPOSITION, MICROSTRUCTURE AND MAGNETIC PROPERTIES OF Nd-Fe-B SINTERED MAGNETS

Sintered Nd-Fe-B magnets have found a wide range of uses, especially in the modern motor and generator industry. The stability of magnetic properties at working temperature for the long term is very important factor for these applications. Thermal stabilization is achieved by exposing magnets to elevated temperature that is at, or slightly above, the maximum temperature expected in service. This pre-treatment reduces in-use performance drop to a minimum. The present paper deals with the effect of thermal stabilization at 160 degrees C on the concentration changes of alloying elements, microstructure and magnetic properties of sintered Nd-Fe-B magnets, commercial type N33EH. The concentration profiles of samples in the initial state and after 2-and 48-hours of heat treatment were investigated using the line SEM-EDX microanalysis and X-ray element maps. It was found out that 48-hours of annealing led to the concentration equalization of the observed elements at grain boundaries and in matrix grains, where the Nd content at grain boundaries decreased by approx. 3 at.%. In the structure of all samples, the matrix of the Nd2Fe14B phase was identified by means of the atomic concentration ratio of iron and rare earth metals (REM - Nd, Dy, Pr, Tb). The studies showed the presence of a number of oxygen containing phases with very high content of REM at triple junctions of grains and grain boundaries. Grain size of the matrix phase Nd2Fe14B was evaluated using the image analysis (software AnalySIS) and it ranged from 5 to 10 mu m. Magnetic properties of all samples were determined at room temperature, as well as after the thermal cycling at 160 degrees C using hystergraph.