Thermal stability of coercivity in grain boundary modified anisotropic hot-deformed Nd-Fe-B magnets

Development of high coercivity Dy-free Nd-Fe-B permanent magnets has been the center of research interest in permanent magnet community during past several years. The main approach to enhance the coercivity has been microstructure modifications by grain size reduction and grain boundary composition/structure modifications. Based on the empirical graph of coercivity versus grain size, a room temperature coercivity of higher than 2.5 T is expected for ~300 nm grain sized Nd-Fe-B magnets. Hence, anisotropic hot-deformed Nd-Fe-B magnets, comprised of nearly single domain sized platelet shaped Nd₂Fe₁₄B grains, are good candidates to achieve this high coercivity. However, the room temperature coercivity of hot-deformed Nd-Fe-B magnets is limited to ~1.8 T. Our microstructure studies and micromagnetic simulations showed that the high concentration of ferromagnetic elements in the intergranular phase is the main reason for the low coercivity. We have shown that the Fe and Co content in the intergranular phase can be reduced by the diffusion of RE-TM (RE = Nd, Pr, TM = Cu, Al) low temperature eutectic alloy along grain boundaries (GBs). We have demonstrated a high coercivity of 2.3 T by the diffusion of Nd₇₀Cu₃₀ alloy into the GBs of hot-deformed Nd-Fe-B magnets. In addition, we reported even a much higher coercivity of 2.6 T in the Pr-Cu diffusion processed hot-deformed Nd-Fe-B magnets. However, the thermal stability of coercivity is degraded in the Pr-Cu diffusion processed magnet compared to that of Nd-Cu diffusion processed magnet. In this talk, by applying multi-scale microstructure characterizations and micromagnetic simulations, correlation of temperature coefficient of coercivity and microstructure of grain boundary modified hot-deformed Nd-Fe-B magnets are discussed.