Improvement of magnetic performance of hot-deformed Nd-Fe-B magnets by secondary deformation process after Nd-Cu eutectic diffusion

To optimize the volume fraction of Nd-rich phase and c-axis alignment of Nd2Fe14B grains, a secondary deformation (SD, 20-50% thickness reduction) has been applied to the hot-deformed Nd-Fe-B magnets after Nd70Cu30 eutectic diffusion process. Although the SD50 sample showed the highest maximum energy product (BH)(max)) of 411 kJ/m(3) at 298 K, its (BH)max was strongly limited by its low coercivity of 0.3 Tat 400 K. Both the SD20 and SD35 samples processed a highest (BH)max of 262 kJ/m(3) at 400 K. The temperature dependence of magnetic properties of the hot-deformed, the diffusion processed and the SD samples was discussed based on the microstructure characterization and magnetic measurement. Temperature coefficient of coercivity was found to be more negative as the area fraction of Nd-rich phase decreased. The apparent dependence of coercivity on grain size is stronger in the present work than previously reported. This is due to the corresponding change in the area fraction of Nd-rich phase in SD samples. A schematic model of an optimized microstructure of hot-deformed magnet after diffusion and secondary processing is proposed. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.