Research on the microstructure evolution and segregation characteristics of Sm25Co48.5Fe19Cu4.8Zr2.7 alloy with different cooling rates during directional solidification

The 2:17-type SmCo permanent magnet shows pronounced microstructural segregation during the early smelting stage, which severely compromises ingot uniformity and significantly affects product quality. Therefore, a clear understanding of the phase evaluation in 2:17-type SmCo alloys depends on detailed insight into their solidification characteristics. To address this limitation, the current employed directional solidification approach is used for investigating the solidification characteristics of the Sm 25 Co 48.5 Fe 19 Cu 4.8 Zr 2.7 alloy. Initially, the precipitation phase was characterized by 180 °C/min, and the influence of cooling rate on dendrite spacing and segregation was subsequently analyzed, leading to the identification of the phase solidification sequence. The experimental findings identified the 2:17R, 1:5H, 1:7H, and 1:3R as the primary phase constituents of the alloy at 180 °C/min cooling rate. Increasing the cooling rate subsequently decreased both the primary dendrite arm spacing (PDAS) and the secondary dendrite arm spacing (SDAS), decreasing from 142 and 60.12 μm at 120 °C/min, to 102 and 15.76 μm at 300 °C/min. The fitting equations for PDAS and SDAS at different cooling rates can be expressed as and respectively. The Sm, Zr, and Cu elements preferentially concentrated in interdendritic regions and are depleted in dendritic cores, demonstrating a strong positive segregation. However, the Co and Fe were enriched in interdendritic regions, demonstrating negative segregation, At 180 °C/min cooling rate, the sample solidifies in the following sequence: L → L + 2:17R → L + 2:17R + 1:3R → L + 2:17R + 1:5H + 1:3R → L + 2:17R + 1:3R + 1:5H + 1:7H. The research offers valuable guidance for understanding SmCo alloy solidification, improving the alloy uniformity while achieving high-performance magnets.