Presentation Information
[O3-3]Anisotropic spherical NdFeB powder obtained by hydrogenation, disproportionation, desorption, and recombination (HDDR) of a gas atomized powder
BLANCA LUNA CHECA FERNANDEZ1, DIEGO MONZON1, PABLO ORTEGA RUIZ1, GABRIELA SARRIEGUI1, NEREA BURGOS1, *JOSE MANUEL MARTIN1, VALENTINA ZHUKOVA2, ARKADY ZHUKOV2 (1. CEIT-Basque Research and Technology Alliance (BRTA), Manuel Lardizabal 15, 20018 Donostia/San Sebastián (Spain), 2. Department of Advanced Polymers and Material, University of the Basque Country, UPV/EHU, Manuel Lardizabal 3, 20018 Donostia/San Sebastián (Spain))
Keywords:
HDDR process,Gas atomization,NdFeB powder,Anisotropy,Recycling
The HDDR process is a heat treatment that allows obtaining anisotropic NdFeB powders with ultrafine grain size (~300 nm) [1]. Therefore, these powders have both high coercivity and remanence. Conventional HDDR process involves the disproportionation of Nd2Fe14B with H2 at temperatures between 700 and 900 ºC to form NdHx, α-Fe, and Fe2B. The subsequent desorption of the hydrogen produces the recombination reaction. The development of the anisotropic microstructure depends strongly on the reaction rates for disproportionation and recombination [2]. This observation has led to the formulation of a dynamic HDDR (d-HDDR) process in which the reaction rates are controlled through the H2 pressure. A significant research effort has been devoted to clarifying the mechanism by which the crystallographic texture is developed. The shared view is that the c-axis of the recombined Nd2Fe14B ultrafine grains is parallel to the c-axis of the parent grains when high anisotropy is obtained [3]. Several phases have been proposed as intermediaries that memorize and transfer the texture from the parent to the child grains, including metastable tetragonal Fe3B, α-Fe [4], and Fe2B [2].
Our objective is to develop a recycled spherical, anisotropic powder suitable for injection molding and, hence, to make bonded magnets. The spherical shape is desirable since it reduces the feedstock viscosity during injection. A direct recycling route for End-of-Life (EoL) sintered magnets using gas atomization and the d-HDDR process is being studied with this purpose.
The initial powder was produced from sintered NdFeB scrap via gas atomization with Ar. Next, the powder was subjected to a conventional d-HDDR cycle. The particles <20 μm were separated by sieving and characterized in detail. Figure 1a displays the original microstructure of the powder, which is isotropic with a grain size <3 μm [5]. Figures 1b and 1c illustrate the ultrafine microstructure after d-HDDR using the Inverse Pole Figure (IPF) and the Pole Figure (PF) of a representative single particle, respectively. The analysis of several particles showed that the ultrafine grains with similar crystallographic orientation formed 3-4 colonies per particle and that the average degree of orientation (<cosθ>) ranged from 0.6 to 0.8. It is important to highlight as well that the <cosθ> obtained inside the individual colonies ranged from 0.94 to 0.97, which fixes the upper limit attainable by HDDR. The hysteresis loop of the powder, displayed in Figure 1d, was measured by VSM on the hard and easy axis. The degree of orientation obtained from the magnetic measurement was <cosθ>=Mr/Ms=0.87/1.34=0.65, which agrees with the OIM values.
It is obvious that the final powder is anisotropic. By comparing Figure1a with 1b, we realize that there are 3-4 colonies per particle, while there were tens or hundreds of grains in a particle of the original gas atomized powder. This suggests that the ultrafine grains of a single colony are not coming from the same parent grain, but from many with different crystallographic orientation. In other words, it is not mandatory that the parent grain texture is memorized in any intermediate phase. These powders are anisotropic because colony size is comparable to particle size. A more careful control of the d-HDDR conditions should allow reducing the rates of disproportionation and recombination, enlarging colony size and, hence, the degree of anisotropy.
Acknowledgments
This work has received funding from EU under grant agreement No. 101138767 (HARMONY project).
References
[1] S. Sugimoto and D. Book, in: Y. Liu, D. Sellmyer, D. Shindo (Eds.), Handb. Adv. Magn. Mater. SE - 23, Springer US, 2006: pp. 977–1007.
[2] Y. Honkura et Al., J. Magn. Magn. Mater. 290–291 (2005) 1282–1285.
[3] T.-H. Kim et Al., Scr. Mater. 115 (2016) 6–9.
[4] T. Horikawa et Al., AIP Adv. 9 (2019). https://doi.org/10.1063/1.5079953.
[5] G. Sarriegui et Al., Mater. Charact. 187 (2022) 111824.
Our objective is to develop a recycled spherical, anisotropic powder suitable for injection molding and, hence, to make bonded magnets. The spherical shape is desirable since it reduces the feedstock viscosity during injection. A direct recycling route for End-of-Life (EoL) sintered magnets using gas atomization and the d-HDDR process is being studied with this purpose.
The initial powder was produced from sintered NdFeB scrap via gas atomization with Ar. Next, the powder was subjected to a conventional d-HDDR cycle. The particles <20 μm were separated by sieving and characterized in detail. Figure 1a displays the original microstructure of the powder, which is isotropic with a grain size <3 μm [5]. Figures 1b and 1c illustrate the ultrafine microstructure after d-HDDR using the Inverse Pole Figure (IPF) and the Pole Figure (PF) of a representative single particle, respectively. The analysis of several particles showed that the ultrafine grains with similar crystallographic orientation formed 3-4 colonies per particle and that the average degree of orientation (<cosθ>) ranged from 0.6 to 0.8. It is important to highlight as well that the <cosθ> obtained inside the individual colonies ranged from 0.94 to 0.97, which fixes the upper limit attainable by HDDR. The hysteresis loop of the powder, displayed in Figure 1d, was measured by VSM on the hard and easy axis. The degree of orientation obtained from the magnetic measurement was <cosθ>=Mr/Ms=0.87/1.34=0.65, which agrees with the OIM values.
It is obvious that the final powder is anisotropic. By comparing Figure1a with 1b, we realize that there are 3-4 colonies per particle, while there were tens or hundreds of grains in a particle of the original gas atomized powder. This suggests that the ultrafine grains of a single colony are not coming from the same parent grain, but from many with different crystallographic orientation. In other words, it is not mandatory that the parent grain texture is memorized in any intermediate phase. These powders are anisotropic because colony size is comparable to particle size. A more careful control of the d-HDDR conditions should allow reducing the rates of disproportionation and recombination, enlarging colony size and, hence, the degree of anisotropy.
Acknowledgments
This work has received funding from EU under grant agreement No. 101138767 (HARMONY project).
References
[1] S. Sugimoto and D. Book, in: Y. Liu, D. Sellmyer, D. Shindo (Eds.), Handb. Adv. Magn. Mater. SE - 23, Springer US, 2006: pp. 977–1007.
[2] Y. Honkura et Al., J. Magn. Magn. Mater. 290–291 (2005) 1282–1285.
[3] T.-H. Kim et Al., Scr. Mater. 115 (2016) 6–9.
[4] T. Horikawa et Al., AIP Adv. 9 (2019). https://doi.org/10.1063/1.5079953.
[5] G. Sarriegui et Al., Mater. Charact. 187 (2022) 111824.
