Role of Vacuum Diffusing Temperature and Time on Improving Hysteresis Characteristics of Grain Boundary Diffused NdFeB Permanent Magnets by Dy-metal Vapor Sorption

This study investigates the impact of vacuum grain boundary diffusion (GBD) conditions on sintered NdFeB permanent magnets using Dy-vapor sorption. The study analyzed variables such as vacuum diffusing treatment temperature (T), time (t), and diffusion depth (d). In a 4.5 mm-thick cube-shaped magnet, Dy diffusion increased intrinsic coercivity from 13.13 kOe to 20.56 kOe (56.6 %) with only a 1.5 % reduction in remanence for a modified two-stage grain boundary diffused magnet (940 °C/8 h + 950 °C/0.5 h), achieving commercial G52SH grade. This enhancement exceeds previous GBD studies on non-rare earth compounds or alloys. FE-EPMA analysis attributed the increased coercivity to a continuous Dy-rich region within Nd2Fe14B grains and a thicker core-shell structure forming the (Nd,Dy)2Fe14B phase, which suppresses reversed domain formation. Glow discharge optical emission spectrometer (GDOES) analysis revealed a 2.57 times increase in peak Dy content and a 1.9 times increase in deeper Dy content at 10 μm from the magnet surface compared to the original process. These findings indicate that Dy vapor sorption, combined with a modified two-stage vacuum heat treatment, enhances both the concentration and depth of Dy grain boundary diffusion. Furthermore, the resulting magnet has a clean surface without additional cleaning, beneficial for producing low-cost, high-performance NdFeB magnets.