Grain boundary infiltration in HDDR Nd2Fe14B magnets

Abstract

Nd-Fe-B based permanent magnets show the largest maximum energy product (BH)max compared to other types of permanent magnets which make them the material of choice for energy conversion and mobility applications used in wind turbines and hybrid/electric vehicles [1]. For high-temperature applications, heavy rare earth elements such as Dy or Tb are added to the base alloy to increase the anisotropy field and thus the coercivity [2]. As annual production rates of Dy and Tb are only 1.4% and 0.1% of that of Nd, intense research has been devoted to reduce the amount of Dy in (Nd,Dy)-Fe-B based high coercivity permanent magnets. Promising approaches to enhance coercivity are diffusion of HRE [3] and the eutectic grain boundary diffusion process in Nd-Fe-B-based magnets [4]. Hydrogenation Disproportionation Desorption Recombination (HDDR) process allows through reversible hydrogen absorption and desorption to significantly refine the microstructure in intermetallic alloys [5]. Moreover, the crystallographic texture during the entire process can be maintained, leading to ultrafine-grained, anisotropic Nd2Fe14B magnets [6]. Therefore, HDDR processed powders were chosen for the present approach to enhance coercivity by grain boundary infiltration. In this work, we study the influence of the grain boundary diffusion process of various low melting eutectic alloys on the coercivity of HDDR processed Nd-Fe-B powders. Different alloys with compositions close to the eutectic melting points such as Nd70Cu30, Nd90Al10, Nd80Ga15Cu5, Nd62Fe14Ga20Cu4, and Nd60Tb10Cu30 containing rare earth and additional elements were synthesized and mixed with the HDDR Nd-Fe-B powders. The grain boundary diffusion process is investigated systematically by varying annealing time, temperature, and the ratio of grain boundary phase for different alloys. The resultant microstructure, phase constitution, and magnetic properties are thoroughly characterized. [1] O. Gutfleisch, M.A. Willard, E. Brück, C.H. Chen, S.G. Sankar, J.P. Liu, Magnetic materials and devices for the 21st century: Stronger, lighter, and more energy efficient, Adv. Mater. 23 (2011) 821–842. [2] K. Hono, H. Sepehri-Amin, Strategy for high-coercivity Nd-Fe-B magnets, Scr. Mater. 67 (2012) 530–535. [3] S. Sawatzki, I. Dirba, H. Wendrock, L. Schultz, O. Gutfleisch, Diffusion processes in hot-deformed Nd-Fe-B magnets with DyF3 additions, J. Magn. Magn. Mater. 358–359 (2014) 163–169. [4] L. Liu, H. Sepehri-Amin, T. Ohkubo, M. Yano, A. Kato, T. Shoji, K. Hono, Coercivity enhancement of hot-deformed Nd-Fe-B magnets by the eutectic grain boundary diffusion process, J. Alloys Compd. 666 (2016) 432–439. [5] O. Gutfleisch, I.R. Harris, Fundamental and practical aspects of the HDDR process, J. Phys. D Appl. Phys. 29 (1996) 2255e2265. [6] O. Gutfleisch, K. Khlopkov, A. Teresiak, K.H. Müller, G. Drazic, C. Mishima, Y. Honkura, Memory of texture during HDDR processing of NdFeB, IEEE Trans. Magn. 39 (2003) 2926e2931.