The alloys with composition around Sm11.5Co58.4Fe22.5Cu5.3Zr2.4 (at%) present hard magnet properties when their microstructure consists of a network of cells based on Sm2(Co, Fe)17, limited by boundaries based on Sm(Co, Cu)5 and cut by platelets based on (Sm, Zr)Co3. The genesis of this microstructure is studied by transmission electron microscopy of heat‐treated specimens. The fractions and compositions of the phases formed are evaluated from atomic balance equations. The microstructure forms by nucleation growth of ordered R2:17 cells in the 1:7 matrix, then by nucleation of 1:3 platelets on the twin boundaries in R2:17. The processes are likely limited by diffusion.

Extended x-ray-absorption fine-structure spectroscopy (EXAFS) has been used to determine the position of Zr within the unit cell of Sm2Co17. Induction-melted Sm2Co17:Zr ternary alloys, aged at 1180 °C, then quenched, consisted of intimately mixed H2:17 and R2:17 having Zr in solid solution as well some regions of R2:17 that were poor in Zr. EXAFS spectroscopy of these specimens indicates that the most probable position for Zr is a site having 2 Sm near-neighbor atoms and 11 Co atoms distributed over three different interatomic distances. This is consistent with a direct substitution of Zr for Co in the Co site in the mixed planes (12j in P63/mmc or 18f in R3m).

Despite the research devoted to new compounds, the Sm-Co-Cu-Fe-Zr alloys with the 2:17 type are, as yet, the only hard magnets to be used above 200 °C. However, these materials are not optimized because their coercivity mechanism is not clearly understood. Their typical microstructure consists of a network of 2:17 cells separated by 1:5 boundaries. Their coercivity is ascribed to the domain wall pinning by the 1:5 cell boundaries. The compositions of the 1:5, 2:17 phases during the cell microstructure genesis were recently evaluated for the alloy Sm(Co0.66Fe0.25Cu0.06Zr0.03)7.6. In the present work, the coercivity behavior of this alloy is analyzed from the comparison of the domain wall energies γ 1:5, γ 2:17. Sintered samples, heat-treated in various conditions, are characterized by transmission electron microscopy and hysteresis curves. γ 2:17 and γ 1:5 are calculated from measured and published data of magnetization and anisotropy. The different treatments generate similar cell microstructures but two ranges of coercivity (about 600 and 2200 K A m−1). In the two cases, γ 1:5 is larger than γ 2:17: the cell boundaries appear as repelling energy barriers rather than pinning sites. The existence of a disturbed anisotropy layer along the 1:5/2:17 interface is proposed to explain the higher coercivity state. The evaluation of Hc using the usual models is attempted.

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