Normal and abnormal grain growth in fine-grained Nd-Fe-B sintered magnets prepared from He jet milled powders
Fine-grained, heavy rare earth free Nd-Fe-B sintered magnets were prepared from He jet milled powders with an average particle size of 1.5 µm by low temperature sintering at 920 °C or 980 °C. A coercivity of >1600 kA/m was achieved for an average grain size of 1.68 µm. Transmission electron microscopy showed that the distribution and composition of intergranular and grain boundary junction phases was similar to that in conventionally processed magnets. Microstructural analysis on different length scales revealed the occurrence of abnormal grain growth, which is unexpected for sintering temperatures below 1000 °C. A larger area fraction of abnormal grains was observed in the sample sintered at 920 °C compared to that sintered at 980 °C. Microtexture investigation showed a better crystallographic alignment of the abnormal grains compared to the fine-grained matrix, which is explained by a size dependent alignment of the powder particles during magnetic field alignment prior to sintering. Slightly larger particles in the initial powder show a better alignment and will act as nucleation sites for abnormal grain growth. Magneto-optical Kerr investigations confirmed the lower switching field of the abnormal grains compared to the fine-grained matrix. The demagnetisation curve of the sample sintered at 920 °C showed reduced rectangularity and this was attributed to a cooperative effect of the larger fraction of abnormal grains with low switching field and, as a minor effect, a reduced degree of crystallographic texture in this sample compared to the material sintered at 980 °C, which did not show the reduced rectangularity of the demagnetisation curve.