High resolution electron microscope study of Sm(Co, Fe, Cu, Zr)7.5magnets

High resolution electron microscopy of a set of Sm(Co.725Fe.20Cu.055Zr.02)7.5magnets has been used to characterize the structure and the size of the various micro-phases present. Our investigations of the samples, which were sintered at 1215°C, subsequently isothermally aged in steps of 50° at temperatures between 700°C and 900°C and step aged at 400°C, revealed changes in the shape and size of the rhomboid cellular microstructure with aging temperature. Lattice fringe images show the coherent precipitation of the cell boundary phase and of the cell interior phases. The platelet phase, consisting of a hexagonal 2:17 -layer, and twin boundaries have been characterized within the cell interiors. It is shown that the intrinsic coercive force, being determined by the domain wall pinning at the continuous cell boundary phase, is correlated with the morphology of the precipitation structure and depends on the heat treatment of the magnets.