Grain boundary effect on the microstructure of solution-treated Fe-rich Sm-Co-Fe-Cu-Zr alloys

Grain boundaries with sparse 1:5H precipitates have been thought as the primary demagnetization sites of the pinning-controlled 2:17-type Sm-Co-Fe-Cu-Zr permanent magnets, leading to poor squareness as well as lower-than-ideal maximum energy product. To trace the underlying origin, here we revived their solution-treated precursors, focusing on the microstructure difference between grain boundaries and grain interiors. The transmission electron microscopy (TEM) investigations of two model magnets Sm₂₅Co_balFe_xCu_5.6Zr_3.0 (x = 19.5 and 23.5 wt%) reveal that both of them exhibit fine cellular nanostructure at the solution-treated state, with the cell interiors having 2:17R nanotwins and basal stacking faults (SFs) and the cell boundaries aggregated with defects or occupied by 1:5H nanoprecipitates. The grain boundary regions contain larger 2:17R nanovariants, less SFs and less defects-aggregated cell boundaries (DACBs) than the grain interiors, accompanied with few 1:5H nanoprecipitates constrained at the grain boundaries. The comparable results indicate that the early decomposition occurs preferably at the grain boundaries. The sparse DACBs at the grain boundary regions thus leave fewer channels to be occupied by the 1:5H precipitates than the grain interiors during the subsequent aging process. These findings help to understand why the grain boundaries of Sm-Co-Fe-Cu-Zr permanent magnets contain sparse 1:5H cell boundary phase.