Mechanical alloying of Nd-Fe-B based magnetic nanocomposites

Abstract

Nanocomposite magnets are of great interest because they are currently the best performing permanent magnets. The magnetic properties of magnetic materials are greatly determined by their microstructure. Different preparation methods have been studied to optimize the microstructure of nanocomposite magnets. Previous work has shown that mechanical milling is an effective technique to produce Nd2Fe14B/α-Fe nanocomposite magnets. The conventional preparation technique for producing Nd2Fe14B/α-Fe nanocomposite magnets involves melt spinning followed by annealing of the amorphous ribbons to induce crystallization. However, due to the non-uniform and coarse Nd2Fe14B grains, the values of the coercivity and remanence are usually not optimum. Mechanical milling prior to annealing creates numerous α-Fe nanocrystals which serve as nucleation sites for Nd2Fe14B. The maximum energy product of nanocomposite magnets which uses partially nanocrystalline powders as precursors is expected to have superior magnetic properties compared to one that is prepared using conventional techniques involving annealing of melt spun ribbons. The process of mechanical milling of melt spun Nd-Fe-B based hard magnets and nanocomposites is investigated in this project. Melt spun Nd-Fe-B ribbons are subjected to different milling duration and intensity. The progress of the mechanical milling process was monitored by Reitveld refinement analysis of the XRD (x-ray diffraction) data. When milled, α-Fe crystal will form from the initially amorphous state. It was observed through Rietveld analysis that with decreasing milling speeds, the time needed to reach steady state increases. It was also observed that the grain size at steady state increases with milling intensity.