Microstructure evolution of hot-deformed SmCo-based nanocomposites induced by thermo-mechanical processing

Abstract

• The two-stage phase transition occurred successively, accompanied by grain growth. • Texture formed in 1:5H phase, not in 2:17R phase in SmCo-based nanocomposites. • Proposed a new strategy for forming c-axis textures in nanocomposites. Nanocomposite permanent magnets have ultra-high theoretical magnetic energy products, due to coupling of the soft/hard magnetic phases, inciting strict microstructural requirements. In this study, the microstructure evolution, including the phase transition, morphological changes, and texture formation, of hot-deformed SmCo-based nanocomposites under thermal–stress–strain coupling was characterized to determine a possible strategy for achieving high performance. The SmCo 5 /α-Fe nanocomposites precursor contained fine and dispersed Sm(Fe, Co) 5 and Fe-Co grains and exhibited a two-stage phase transformation accompanied by grain growth. In the early stage of deformation at relatively low temperature, the adjacent Sm(Co, Fe) 5 and Fe-Co phase formed the Sm 2 (Co, Fe) 17 -H phase, which was stable only with small grain sizes. In the high-temperature deformation stage, the Sm 2 (Co, Fe) 17 -H phase transformed into the Sm 2 (Co, Fe) 17 -R phase with large grain sizes. In addition, the strong c -axis texture formed in the Sm(Co, Fe) 5 phase but not in the Sm 2 (Co, Fe) 17 -R phase. Subsequently, the phase transition process and texture formation mechanism were systematically analyzed by transmission electron microscopy. The initiation of a slip system and/or preferential grain growth explained the formation of texture under the action of uniform stress and strain and assisted by dispersed Sm-rich nanograins. The Sm 2 (Co, Fe) 17 -R grains with poor orientations and large grain sizes did not achieve magnetic hardening, which also damage the magnetic properties. According to the results of this work, we also presented a new strategy to prepare high-performance SmCo-based nanocomposites magnets.