Formation mechanism of Tb-rich shell in grain boundary diffusion processed Nd–Fe–B sintered magnets

We propose a new mechanism for the formation of Tb-rich shell in the Nd–Fe–B sintered magnet that was subjected to the grain boundary diffusion (GBD) process using TbH2. Tb-enriched liquid films form along grain boundaries during the GBD process, which migrate by chemically induced liquid film migration (CILFM) to form Tb-rich shells behind. As a result, the Tb-rich shell form asymmetrically along a grain boundary with an abrupt concentration change at a core/shell interface with a gradual decrease of Tb concentration toward a grain boundary. These microstructure features can be reproduced with phase-field simulations.