Growth of defects around grain boundary and evolution of cellular structure during isothermal aging process in 2:17-type SmCo permanent magnets

Abstract

The cellular structure consisted of cell boundary phase (1:5H phase) and cell phase (2:17R phase) is crucial for 2:17-type SmCo magnets to obtain high performance. In this study, the growth of defects around grain boundary and the evolution of cellular structure during isothermal aging process are systematically investigated. Further analysis determines the effects of these microstructural changes on magnetic properties. With the isothermal aging time increasing from 0.5 to 12 h, the width of 1:5H phase-free zones (PFZs) around the grain boundary expands from approximately 100 to 200 nm. Simultaneously, the cellular structure inside the grain becomes more complete, and the Cu content in the 1:5H phase is increased. However, as the isothermal aging time further extends to 36 h, accompanied by the formation of substantial amounts of Cu-rich phases along the grain boundaries, the width of PFZs around the grain boundary increases to approximately 300 nm. Moreover, the width of the 1:5H phase widens, and it even disappears, leading to incomplete cellular structure and a low Cu content in the 1:5H phase. The broad PFZs and incomplete cellular structure negatively impacts the coercivity and squareness of the magnet. Finally, an optimal performance of Br = 10.8 kG, Hcj = 29.7 kOe, (BH)max = 26.7 MGOe, and SF = 49.9 % is obtained for the magnet aged 12 h. These findings indicate that the defects around the grain boundary can be effectively controlled and form complete cellular structure by adjusting the isothermal aging time, which may provide an important clue for developing high-performance and low-cost 2:17-type SmCo magnets.