Quasi-periodic layer structure of die-upset NdFeB magnets

Isotropic magnets were prepared from melt-spun powders at different hot pressing temperatures from 550 to 700 °C, then upset into fully dense anisotropic magnets at the same die-upsetting temperature of 850 °C. Die-upset magnets had the characteristics of inhomogeneous microstructure, including well-aligned grains structure and nonaligned grains layers transverse to press direction, which was quasi-periodic layer structure with a total length of 5–15 μm. Nonaligned grains layers were mainly made of large grains and had higher Nd content. To clearly understand the formation of layer structure, the microstructure of isotropic precursors with different hot pressing temperatures and their subsequent die-upset magnets was investigated. A new interpretation for the formation of layer structure was proposed in this paper: the layer structure was correlated to the original ribbon interface which was divided into three types based on the contact forms. Because of the incomplete contact of neighboring ribbons, concentration of stress occurred in the contacted points and the Nd-rich phase was squeezed into interspaces at high temperature under stress. Due to the release of interfacial energy and the fluidity of enough Nd-rich liquid phases, the nonaligned layers with large grains formed both in hot compaction and subsequent hot deformation process. The layer structure affected the magnetic properties of die-upset magnets. With increase of the hot pressing temperature, the nonaligned grains layers became thicker, and the magnetic performance of die-upset magnets decreased. It was necessary to reduce the thickness of large grains layers for the preparation of high-performance die-upset magnets.