Evolution of Microstructure, Microchemistry and Coercivity in 2:17 Type Sm–Co Magnets with Heat Treatment

A systematic study has been undertaken to understand the evolution of microstructure, microchemistry, and coercivity of sintered Sm(Co bal Cu 0.06 Fe 0.015 Zr 0.027 ) 6.4 magnets with heat treatment using magnetometry, transmission electron microscopy, Lorentz microscopy, and nanoprobe chemical analysis. In general, the homogenized and quenched Sm(Co bal Cu 0.06 Fe 0.015 Zr 0.027 ) 6.4 magnets have a featureless microstructure with the 2: 17 hexagonal structure. During isothermal aging at 700–850°C, the 1:5 nuclei precipitate and then coalesce and start forming the cellular structure with 2: 17 rhombohedral cells surrounded by 1: 5 hexagonal cell boundaries. Uniform cellular and lamellar structures are formed after 2 h of isothermal aging, and both the cell size and density of lamella phase slightly increase with longer aging. Nanoprobe chemical analysis shows that the Cu content in 1: 5 cell boundaries increases during the slow cooling to lower temperatures, reaching a maximum value around 500°C, which is consistent with the development of coercivity. Also the Cu content in the triple cell boundary junctions is twice as much as the amount at the regular cell boundaries regardless of cell size and boundary width. Lorentz microscopy indicates that the triple cell boundary junctions may play a major role in domain wall pinning.