Study on the grain growth and the evolution of defects around grain boundaries during heat treatment process in Sm2Co17 permanent magnets

Abstract

The magnetic properties and thermal stability are associated with the grain size and defects around grain boundaries in Sm2Co17 permanent magnets. It is of scientific and practical interest to study the grain growth and the evolution of defects around grain boundaries during heat treatment process. In this study, the grain sizes after sintering, solution, isothermal aging and final aging are quantitatively characterized, and its influence on magnetic properties and flux losses are systematically analyzed. In addition, the Cu-rich and Zr-rich precipitates and Cu-lean regions around grain boundaries are carefully examined from sintering to final aging. The results show that solution treatment is the most effective approach to promote the grain growth, while the grain growth during isothermal aging and final aging is also non-negligible. The grain growth helps to reduce the grain boundaries and reduce the flux loss. This study provides a convenient and cost-effective approach (increasing the heat treatment time) to improve the thermal stability without significant sacrifice of magnetic properties. On the other hand, the Cu-lean regions and incomplete cellular nanostructures form during isothermal aging, while the Cu-rich and Zr-rich precipitates form during sintering. The formation of Cu-rich and Zr-rich precipitates mainly stems from the excess addition of Cu and Zr, which is indispensable to improve the working temperature.