Origin and inhibition on quasi-periodic coarse grain regions in hot-deformed Nd-Fe-B magnets

Hot-deformed Nd-Fe-B magnets with nanocrystalline grains are promising contenders for high-performance permanent magnets. However, the quasi-periodic coarse grain regions at ribbon interfaces counteract the positive effects on magnetic properties to some extent. Up to now, the ambiguous formation mechanism of coarse grain regions impedes the development of advanced process to obtain uniform microstructure and excellent properties in hot-deformed Nd-Fe-B magnets. To address this issue, systematical studies have been carried out on the grain growth behaviors from melt-spun ribbons to bulk hot-deformed magnets. In this work, we have confirmed that abnormal grain growth occurred in the wheel-side of melt-spun ribbons under high temperature of hot deformation process, due to the ultra-fine grain size with high activation and lack of isolation by Nd-rich phase. Correspondingly, low-temperature heat treatment was applied on melt-spun ribbons to optimize the distribution of grain size, resulting in the significant inhibition on coarse grain regions in hot-deformed magnets.