Phase transition in 2:17-type Sm-Co-Fe-Cu-Zr magnets

Abstract

The 2:17-type Sm-Co-Fe-Cu-Zr alloys are the strongest high temperature permanent magnets and have been widely applied in advanced industries. However, the concurrent recrystallization and precipitation that dominate the formation of their unique cellular nanostructure as well as the strong hard magnetism are still far from being clear. The main obstacle lies in the non-consensus on the 2:17H to 2:17R transformation, which has been deemed disorder-order (diffusive) or displacive types in the past. Herein, we investigated the concurrent recrystallization and precipitation in the supersaturated hexagonal Sm-Co-Fe-Cu-Zr alloys through detailed TEM investigations, where the recrystallization, growth of recrystallized subgrains (cells) and precipitates stem from the gradual formation and dissociation of defects, including basal stacking faults (SFs), vacancies and excess interstitial atoms. The diffusion-controlled glides of -type partial dislocations associated with the SFs not only transform the matrix from the mixture of 1:7H and 2:17H to Sm-depleted 2:17R cells but also provide continuous diffusion channels to reduce the point defects to form the Sm-enriched 1:5H cell boundary precipitates and Zr-enriched 1:3R platelets. It indicates a diffusion-controlled displacive phase transformation mechanism, characterized by the composition-dependent 2:17R’ intermediate phase (with one faulting basal layer in 2:17R lattice) due to incomplete basal slip and incomplete solute partitioning. A basal slip model is proposed to explain the formation and dissociation of defects along with the stacking period change and the simultaneous formation of continuous atomic diffusion channels. Combined HR-TEM and Lorentz-TEM investigations showed that 2:17R’ intermediate phase distributed at cell edges leads to repulsive domain-wall-pinning, unlike the 1:5H cell boundary precipitates that yield attractive domain-wall-pinning. Further magnetic measurements indicated that reducing the volume fraction of 2:17R’ phase can enhance the coercivity and squareness factor.