Microstructure Control of Nd-rich Triple Junction and Grain Boundary Phases for Development of High-Performance Nd-Fe-B Permanent Magnets

We report on the technique for controlling the microstructure of Nd-rich triple junction and grain boundary phases, which is a key for developing the high-performance Nd-Fe-B permanent magnets. Through a systematic observation on the structural changes in the Nd-rich phases of the Cu-doped magnets during annealing, the structure of meta-stable Nd-oxide required to achieve high coercivity was revealed, and its formation mechanism was clarified. As a result, the annealing process that can stabilize the meta-stable Nd-oxide in the Cu-doped magnets could be developed, thereby improving the coercivity of the magnets further. In addition, by understanding the chemical and structural weak points of the Cu-doped magnets through SEM, EPMA, and HR-TEM analyses, the metal element doping process that can effectively complement the microstructural weakness of Cu-doped magnets could be developed. As a result, the coercivity of Cu-doped magnets was successfully improved with a slight remanence reduction. Based on the results from the current microstructure characterizations, a guideline for developing microstructure control technique on the structure and chemistry of the Nd-rich triple junction and grain boundary phases will be proposed to achieve high coercivity in Nd-Fe-B permanent magnets while minimizing the use of high-cost rare-earth elements.