Origins of inhomogeneous microstructure of hot-deformed nanocrystalline cylindrical Nd-Fe-B magnets and its effects on magnetization behaviors

Abstract

To apply bulk hot-deformed magnets into commercial fields, it is quite necessary to deeply understand the origins and evolution of their inhomogeneous microstructure. In this work, the microstructure of hot-deformed nanocrystalline cylindrical Nd-Fe-B magnets with a dimension of ∼24.8 mm in diameter and ∼7.0 mm in height along the radial direction was investigated in order to correlate them with the inhomogeneous magnetic properties. The remanence was 1.42 T, 1.37 T, and 1.30 T for the samples taken from the center (sample P3), subcenter (sample P2), and edge (sample P1) of the hot-deformed magnet, respectively. Scanning electron microscopy images showed from the P3 to P1 the interval of strip-shaped Nd-rich layers increased. Meanwhile, the perpendicularity between the pressure direction and the Nd-rich layers deteriorated for the samples from the P3 to P1. Well oriented platelet-shaped grains with c-axis parallel to the pressure direction were observed in the P3 sample, and the orientation gradually deteriorated with the position changing from the center to the edge. It was also found that large Nd-rich phase aggregation areas occurred in the edge sample. The maximum value of Henkel plot decreased gradually for the samples from the P3 to P1, which was caused by the thickened grain boundary observed by transmission electron microscopy. The openness of recoil loops increased gradually from the center to the edge due to the aggregation of the Nd-rich phase. The inhomogeneous microstructure shows distinct effects on magnetic properties, therefore changing the magnetization behavior along the radial direction of hot-deformed magnets.