Magnetic properties and microstructures of hydrogenation-disproportionation-desorption-recombination processed Nd-Fe-B powders by grain boundary diffusion of Nd-Cu-Al

The relatively low coercivity of the anisotropic bonded Nd-Fe-B powders prepared by hydrogenation-disproportionation-desorption-recombination (HDDR) method hinders their application in high-tech fields, such as new energy vehicles and robots. The low melting point Nd ₈₀Cu ₁₀Al ₁₀ (wt.%) ternary alloy is employed as the source for grain boundary diffusion to enhance the coercivity of the HDDR powders. The effect of the content and diffusion temperature on the magnetic properties is systematically investigated. It is found that the coercivity is enhanced after grain boundary diffusion. The coercivity increases monoclinically from 1026.0 to 1301.0 kA m ⁻¹ with the addition of 2% ∼ 6% (wt.%) diffusion source at 820 °C. Through the initial magnetization curve, the S-shape indicates that the coercivity mechanism is more inclined to pinning type. Meanwhile, the derivation curve shifts towards higher value gradually, indicating that the pinning field increases due to the grain boundary diffusion. Correspondingly, TEM micrographs show that the content of the grain boundary phase is increased, and the grain boundary is widened. In the range of 770-870℃, the coercivity increases at first and then decrease as the temperature exceeds 820 °C. The maximum of the coercivity of the HDDR powders of Hcj = 1301.0 kA m ⁻¹ is obtained with the addition of 6 wt% Nd-Cu-Al, diffusion treated at 820 °C.