Microtwin evolution and grain boundary phase formation in iron-rich 2:17-type Sm-Co magnets: The effect of iron content

Abstract

Iron-rich 2:17-type Sm-Co magnets are expected to possess a high energy product due to their elevated saturation magnetization. However, achieving high energy in manufactured magnets poses a challenge, and the mechanism by which the microstructure is affected by the iron content remains unclear. Here, we investigate the microstructure of 2:17-type Sm-Co magnets with varying iron contents in both single solid-solution precursors and aged magnets. Our study indicates that a higher iron content results in microtwin structures with a lower dislocation density in the 1:7H precursor phase, leading to an increased energy barrier and decreased driving force during the subsequent phase transformation. As a result, the 1:7H phase does not completely decompose, causing an increase in the intermediate phases. Furthermore, the impact of iron content on the 1:7H phase causes the aged magnets to have insufficient cell boundaries and massive grain boundary phases. Our study provides an understanding on the iron-rich 2:17-type Sm-Co magnets through the manipulation of the free energies and the defects in the solid-solution precursors.