Improved (BH)max and squareness in diffusion-processed hot-deformed Nd-Fe-B magnets by self-diffusion process under pressure

Abstract

To recover the remanence (or maximum energy product ((BH)max)) and squareness with high coercivity in diffusion-processed hot-deformed Nd-Fe-B magnets, new efforts have been made in this work. Annealing at a selected temperature of 800 ℃ under pressure was applied to the hot-deformed magnets diffused by the Nd70Cu30 or Nd70Cu15Ga15 alloy. After this process, the thick powder boundary phase dissolved in the matrix and diffused into the intergranular regions. The c-axis misalignment of Nd2Fe14B grains near the diffusion surface was significantly improved, leading to increased remanence by 0.10–0.18 T and (BH)max by 47–85 kJ/m3 in the two diffusion-processed magnets. The distribution of the powder boundary phase, the proportions of the intergranular Nd-rich phase, the c-axis alignment of grains and the grain sizes from the diffusion surface to the center of the magnets tend to be relatively uniform. The squareness factor of the demagnetization curve increased from 0.89 to 0.95 for the Nd70Cu30-diffused magnet and from 0.85 to 0.93 for the Nd70Cu15Ga15-diffused magnet after the process, returning to the level of the as-deformed magnet. Microstructure analysis and micromagnetic simulations indicate that the improved squareness factor is due to the homogeneous magnetization reversal process caused by the homogeneous microstructure.