Influence of cooling rate on microstructural and magnetic properties of 2:17-type Sm-Co permanent magnets

Various specialized fields widely employ 2:17-type Sm-Co permanent magnets; however, achieving consistency and stability of the magnets through mass production presents challenges. We have found that the squareness of the magnets obtained varies significantly owing to the different placements of the green compacts in vacuum sintering furnaces, which results in different cooling rates after solution treatment. In this study, microscopic structural and microchemical analyses revealed that the enrichment of Cu at the cell boundaries is hindered in magnets with slow cooling rates after solution treatment, and the inconsistency in the types and numbers of defects present after the solution treatment leads to variations in the recrystallization process. Notably, the 2:17 R' phase near the cell boundaries was observed to partition the cells. Furthermore, three different types of Zr-rich 1:3 R Zr-plates were observed, and micromagnetic simulations revealed that the junction of the Zr-plates and 2:17 R' phases became a nucleation site for reverse magnetization, leading to the spreading of reverse magnetization in the 2:17 R cell. The 2:17 R' phase can lead to a reduction in the density of the 1:5 H phase, which weakens the pinning strength and results in poor coercivity and squareness of the magnet. These discoveries have deepened the understanding of the coercivity mechanism of 2:17-type Sm-Co magnets, and they provide a new path for the advancement of manufacturing technology to improve the squareness of magnets.