Identifications of smco SmCo₅ and smSm_{n +} co₁Co_{5n − 1}-type phases in 2:17-type Sm-Co-Fe-Cu-Zr permanent magnets

Both SmCo₅ and Sm_{n +1}Co_{5 n − 1}-type phases were identified in the Sm₂₅Co_42.9Fe_23.5Cu_5.6Zr_3.0 (wt.%) commercial sintered magnet even at the solution-treated state, with the 1:5H cell boundary phase occupying pyramidal planes, the 1:3R (n = 2) thin platelets occupying some basal planes, and the mixture of 1:3R, 2:7R (n = 3) and 5:19R (n = 4) phases distributing at grain boundaries. The mixed Sm_{n + 1}Co_{5 n − 1}-type phases at grain boundaries act as primary sinks and lead to sparse 1:5H-precipitates surrounding them after isothermal aging. It clearly reveals that the 1:5H cell boundaries are formed at the early decomposition stage and why they are rare at grain boundaries in the aged magnets.