Coercivity improvement of hot-deformed Nd-Fe-B magnets by stress-induced Pr-Cu eutectic diffusion

Low-melting eutectic diffusion has emerged as a practical approach to enhance the coercivity of hot deformed Nd-Fe-B magnets. Herein, we report a high-efficient stress-induced eutectic diffusion method to develop high-coercivity hot-deformed Nd-Fe-B magnets. In this method, fast Pr-Cu eutectic diffusion is accomplished with the hot-pressed and hot-deformed processes simultaneously. The coercivity of diffusion-processed magnets is substantially increased by about 33% with a slight remanence loss. The applied stress effectively controls the distribution of Pr-Cu eutectic and relieves the remanence loss. Microstructure analysis confirms that the Pr-Cu concentration gradient constructed in hot-pressed precursors effectively suppresses the near-surface grain growth during the hot-deformation process and reduces the lateral/longitudinal ratio of platelet-shaped grains. The formation of thick and continuous Pr-rich grain boundaries weakens the exchange coupling between neighboring grains, offering more pinning sites for domain wall displacement. Partially Pr-substitution improves the locally magnetically hardening of 2:14:1 main phases. The optimized structural and magnetic characterizations hinder the nucleation and propagation of reversed magnetic domains at low magnetic fields, which is beneficial to coercivity enhancement. These findings clarify the characteristics of the stress-induced diffusion method and give a new insight into the structural modulation for Nd-Fe-B materials. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.