A novel two-step grain boundary diffusion process using TaF5 and Pr70Cu15Al10Ga5 for realizing high-coercivity in Nd-Fe-B-sintered magnets without use of heavy rare-earth

To achieve high-coercivity in Nd-Fe-B-sintered magnets without relying on the use of heavy rare-earth (HRE), developing an HRE-free grain-boundary-diffusion-process (GBDP) using the light rare-earth, Pr, is highly desired. The key factor for achieving high-coercivity via Pr-GBDP is to increase the Pr-concentration of Pr-rich shell by reducing its thickness, and this can be realized by inhibiting the chemically induced liquid film migration (CILFM) that occurs to form the shell. Herein, for the first time, we report achievement of high-coercivity of 2.35 T without using HRE by developing CILFM-inhibited two-step GBDP that uses TaF₅ to form a intergranular precipitate (PPT) and Pr₇₀Cu₁₅Al₁₀Ga₅ to form a Pr-rich shell in the 1st- and 2nd-steps, respectively. Due to the formation of hexagonal-TaB₂ intergranular PPT during the 1st-GBDP, the CILFM is inhibited during the 2nd-GBDP, thereby reducing the grain size and forming the thinner shell with higher Pr-concentration in the magnets. As a result, the μ₀H_c of two-step GBDP magnets (2.35 T) is considerably higher than that of magnets GBD-treated with Pr₇₀Cu₁₅Al₁₀Ga₅ alone (1.85 T). A micromagnetic simulation shows that the nucleation field at the interface between the 2-14-1 grain and Nd-rich phase in two-step GBDP magnets increases by such a thinner and higher Pr-concentration shell. Furthermore, due to the CILFM inhibition, the number of Pr atoms consumed for the shell formation near the magnet surface reduces in the two-step GBDP magnets, resulting in an increased GBD-depth of Pr, and this is another contributor for realizing a high-coercivity in magnets via the HRE-free two-step GBDP.