Coercivity and its thermal stability of Nd-Fe-B hot-deformed magnets enhanced by the eutectic grain boundary diffusion process

Eutectic grain boundary diffusion process was applied to Nd-Fe-B hot-deformed magnet using Nd(6)0Tb(20)Cu(20) alloy, which resulted in a large coercivity enhancement from 0.87T to 2.57T with a relatively small decrease in remanent magnetization from 1.50T to 1.38 T. Improved temperature coefficient of coercivity from -0.493 parts per thousand C-1 to -0.328 parts per thousand C-1 led to a high coercivity of 1.47 Tat 150 degrees C. The partial formation of Tb-rich shell on the surface of platelet shaped Nd2Fe14B grains while maintaining their ultra-fine grain size is the reasons for the substantial enhancement of the coercivity. Micromagnetic simulations suggested that a higher coercivity can be obtained when Tb-rich shell covers the c-plane surface interface of the grains than that covering the side surface interfaces. Improvement of the thermal stability of coercivity was found to be due to the exchange decoupling of Nd2Fe14B grains and the formation of (Nd,Tb)(2)Fe14B shell. In the frame of Kronmuller equation and based on the micromagnetic simulations, the improvement of the thermal stability of coercivity is attributed to the decrease of N-eff and increase of alpha induced by exchange decoupling of grains, as well as the additional decrease of N-eff induced by the formation of high-H-a shell. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.