Investigation of microstructural evolution in sintered NdFeB during the two-step annealing

Two-step annealing is an effective approach for enhancing coercivity of sintered NdFeB magnets. In-situ transmission electron microscopy (in-situ TEM) were utilized to figure out the microstructural evolution in two-step annealing. It is found that the grain boundaries of as-sintered sample present a zigzag morphology, while those subjected to 1st annealing become flatter. According to Gibbs-Thomson law, during 1st annealing, atoms at the solid-liquid front occurs the long-term diffusion driven by curvature gradients to repair grain boundary defects, enabling boundary straightening at micrometer scale. Furthermore, based on the prior step, the subsequent 2nd annealing achieves atomic-scale boundary straightening through localized atomic rearrangement. The real-time observations reveal stress alleviation at Nd2Fe14B/Nd2Fe14B and Nd2Fe14B/triple junction phases (TJP) interfaces, accompanied by crystallization of metastable amorphous phases. This demonstrated that the lattice distortion of main phase and system energy were both decreased, which recover the interfacial magnetic anisotropy. We believed that boundary straightening and stress release caused by two-step annealing synergistically lead to a huge coercivity increment compared to the as-sintered state.