Since its discovery by Sagawa in 1984, the Nd-Fe-B magnet has been one of the most sought- permanent magnetic materials for many applications due to its exceptional combination of magnetic properties. The deterioration of coercivity of Nd-Fe-B magnet at high temperatures is an issue for automotive applications. Nb doping in melt-spun Nd-Fe-B has shown a significant effect on enhancing the coercivity. The formation of NbFeB phase is attributed to be the reason for the coercivity enhancement in Nb containing Nd-Fe-B magnet as it pins the domain walls and restricts the grain growth of Nd2Fe14B grains. Grain boundary diffusion process (GBDP) involving low-melting eutectic alloys is the most notable and effective approach for enhancing the coercivity. This seminar presents the role of Nb and Nd-Cu grain boundary diffusion on the magnetic properties of Nd-Fe-Nb-B melt-spun ribbons. Grain boundary diffusion process has been performed on the commercial Nb contained Nd-Fe-B melt-spun ribbons by using Nd-Cu low melting eutectic. The coercivity has enhanced from 1.2T to 2.1T after GBDP. The coercivity at 150 °C also improved to 0.92T from 0.6T. To understand the mechanism of coercivity enhancement, detailed microstructural analysis was performed. The grain size estimated from the Transmission electron microscopy (TEM) bright field images is 23 ± 1.5 nm and 26 ± 2.3 nm for the as-prepared and GBDP samples respectively. There is no significant grain size difference between the as-prepared and GBDP samples, unlike earlier reports wherein a significant grain growth has been reported for similar annealing conditions during grain boundary diffusion process which affects the coercivity. The three-dimensional atom probe (3DAP) analysis revealed the presence of nano sized Nb clusters which are responsible for the grain growth inhibition during diffusion process. TEM and 3DAP analysis indicate that the segregation of Nd and Cu along the grain boundaries magnetically decouples the Nd2Fe14B grains as evident from the Henkel plot studies in the GBDP sample. Thus, the combined effect of Nb and Nd-Cu segregation has led to the huge enhancement in the coercivity which makes this material a potential candidate for the high temperature automotive applications.

Reference:
“Enhancing the coercivity of Nd-Cu-diffused Nd-Fe-B permanent magnets by Nb-assisted grain boundary pinning,” M. B. Siva Kumar, D. Prabhu, M. Sadhasivam, B. Manjusha, N. Chandrasekaran, K. G. Pradeep, G. Sundararajan, R. Gopalan, Materials Real Money Rummy Letters, 10:12 (2022) 780-787, DOI: 10.1080/21663831.2022.2104139