Correlation between ordered solid solution and cellular structure of Sm2Co17 type magnets with high iron content

Abstract

The ordering transformation of solid solution and the insufficient 1:5H boundary phase in the final magnets challenge the realization of high performance in Sm2Co17 magnets with high iron content. The investigation correlates the ordering degree of solid solutions to the cellular structure of the final magnets with the composition of Sm(CobalFexCu0.06Zr0.02)7.8 in which x ranges from 0.30 to 0.36. The ordering tendency of solid solution increases with increasing iron content. Rietveld structure refinement demonstrates that increasing iron content reduces the occupancy of Sm at Co-Co dumbbell sites in 2:17R, as well as Zr solubility on Sm 6c sites, leading to higher ordering degree and less Z planar phase formation separately. Antiphase boundaries (APBs) arising from the different 2:17R variants with the same or different directions were frequently observed in solid solutions, which facilitate the nucleation and formation of 1:5H phase during aging treatment. With increasing Fe content, the width of nanotwins increases and thus the APB density decreases in the solid solution. Furthermore, overlapping zones appear in solid solutions with higher Fe content, which will retain and enlarge as the twins grow during aging treatment, inhibiting the formation of 1:5H boundary phase. In addition, the density of Z phase declines and more defective platelet phases occur in x = 0.36 magnets, which depress the element diffusion and re-distribution during slow cooling. The above correlation between solid solution and cellular structure well explain the structure origin for the deterioration of magnetic performance with higher Fe content, which may provide guidance for further performance improvement in Sm2Co17 type magnets with higher iron content.