Origin of the quasi-periodic coarse grain regions in hot-deformed Nd-Fe-B magnets

Abstract

Quasi-periodic coarse grain regions (CGRs) are inevitable in hot-deformed Nd-Fe-B magnets, which are extremely harmful to magnetic properties, especially coercivity. However, the reasons for their formation are not yet fully understood. In this work, we systematically study grain growth behaviors in different regions of nanocrystalline and amorphous ribbons during the heat treatment and try to figure out the relationship between the grain growth and the CGRs of hot-deformed (HDed) magnets. The results indicates that coarse grains (CGs) strongly depend on the temperature and tend to form on the wheel-contacted side (WS) in the two kinds of melt-spun ribbons at the high temperature. However, the formation mechanisms of CGs share obvious differences. The CGs in nanocrystalline ribbons originate from the abnormal growth of fine grains on WS due to the high surface energy and abundant Nd-rich phases. As comparison, the CGs in amorphous ribbons result from the high temperature growth of large-sized grains caused by the low nucleation rate and high growth rate on the WS. Careful microstructural characterization of the HDed magnets reveal that the CGs are also located at the WS of the initial ribbons and the formation mechanisms of the CGs in HDed magnets follows the nanocrystalline and amorphous ribbons. This work provides theoretical guidance for the suppression of CGRs in HDed magnets.