In-situ STEM study on thermally induced phase transformation of magnetic (Nd0.75Ce0.25)2Fe14B ribbons

Studying the phase transformation and microstructure evolution of magnetic Nd-Ce-Fe-B nanocrystalline melt-spun ribbon is of paramount significance in developing this important class of magnetic materials. In this work, the thermal-induced phase transformation of (Nd0.75Ce0.25)2Fe14B nanocrystalline sample was revealed via in-situ heating scanning transmission electron microscopy (STEM) experiment (RT-750 ℃). Starting from a single-phase (Nd0.75Ce0.25)2Fe14B nanocrystalline matrix at room temperature (RT), grain boundary (GB) diffusion behavior was investigated at an atomic scale. Our result shows that the rare earth elements accumulate at GB area from 200 °C and form a uniform thin amorphous layer between grains. The decomposition of the primary phase was observed at about 350 ℃ and crystalized as BCC-Fe, Fe2B, and (Nd0.75Ce0.25)2O3 phases driven by thermal activation at a higher temperature. This work provides the essential information of phase transformation and microstructure evolution of NdCeFeB-based magnets at high temperatures and new insights into thermal treatment designing and thermal stability considerations.