Formation of the Cellular Structure in the Sm2Co17-type Permanent Magnets: A Phase-Field Study

The cellular structure with ordered rhombohedral (R) phase in the cells and disordered hexagonal (H) phase in the cell boundary is key to the high coercivity and large magnetic energy product of the Sm2Co17-type permanent magnets. However, a theoretical understanding on the formation of the cellular structure is still lacking. Here in this work, we developed a phase field model to simulate the precipitation of the ordered R phase from the disordered parent H phase during isothermal aging. The model takes into account the lattice misfit strain energy between the precipitate and parent phase as well as the six different R variants resulting from the ordering of the R phase, which reproduces the cellular structure similar to experimental observations. Analysis shows that the cell morphology on different crystallographic planes is explained by the minimization of the misfit strain energy. More importantly, it is found that the six R variants are essential in the formation of the cellular structure in which the H phase with volume fraction as low as 20% is interconnected to form the continuous cell boundary matrix. Specifically, reducing the number of R variants, or lowering the R antiphase boundary energy below 2 times of the R/H interfacial energy destabilizes the cellular structure. This work elucidates the formation mechanism of the cellular structure in the Sm2Co17-type permanent magnets. It could shed light on the designing of high-performance permanent magnets.