Presentation Information
[P1-24]Influence of the Nd-rich phase when processing Nd-Fe-B through the Hydrogen Ductilisation Process (HyDP)
*Patrick Powell1, Oliver Peter Brooks1, Viktoria Kozak1, Safiyah Hussain1, Allan Walton1 (1. University of Birmingham (UK))
Keywords:
HDDR Processing,HyDP,NdFeB,Nd-rich
Conventional sintered powder processing of NdFeB permanent magnets, which accounts for ~85% of global production [1], results in a 25% loss of material [2] primarily during the machining stage. Additionally, powder processing is highly susceptible to oxidisation, limiting grain size in the final magnet. Grains produced are ~5mm, far above the single magnetic domain size of 300nm [3]. This is part of the reason that the highest producible BHmax is ~437 kJ m-3, below the theoretical limit of 504 kJm-3, and means that more magnetic material is required to meet performance standards. These issues are pressing given the criticality of neodymium, and the skyrocketing demand of these magnets for use within net-zero technologies. An alternative processing route is the Hydrogen Ductilisation Process (HyDP) [4]. Waste is reduced by producing ductile disproportionated NdFeB, allowing the material to be formed into shape during processing [5] and removing the need for machining. The material remains solid throughout processing, reducing the risk of rapid oxidisation and allowing for grains of <1mm to be produced, closer to the single magnetic domain size. The Nd-rich phase has considerable influence over each stage of HyDP, impacting the reaction mechanics and microstructure of the disproportionated material. It also redistributes during recombination, leaving cavities in the final material which negatively impact the remanence. Considering the need to reduce Nd demand, this work explores three NdFeB alloys with different Nd content and the impact this has on HyDP.
The stages of HyDP investigated were disproportionation, deformation and recombination. Three compositions were investigated - Nd2.07Fe14B1.04, Nd2.20Fe14B1.05 and Nd2.31Fe14B1.06 - each containing increasing amounts of intergranular Nd-rich phase. Fine lamella rods are initially present in the microstructure of disproportionated Nd2Fe14B, but extending the disproportionation reaction time leads to the formation of spherical hydrides through Ostwald ripening. This microstructure is necessary for deformation, and the ripening is driven by internal stresses in the material. The composition Nd2.31Fe14B1.06 exhibited the least Ostwald ripening, suggesting that the intergranular Nd-rich hydride phase absorbs some residual stress. During deformation, higher Nd content resulted in increased yield stress and reduced ductility, attributed to less Ostwald ripening and an increased amount of fine lamella in the disproportionated microstructure [7]. When recombined through vacuum and slow recombination, material with higher Nd content exhibited more recombination through both methods. Slow recombination produced grains of size 130nm–1.8mm, but resulted in limited amounts of recombination. By comparison, vacuum recombination exhibited grain sizes in the range 220nm-20mm, and greater amounts of overall recombination.
Whilst a lower neodymium content produces more ductility in the disproportionated microstructure, it hinders the amount of recombination which can occur. This negatively impacts the coercivity of the material, as the presence of free iron in the remaining disproportionated regions reduces the coercivity. Further work needs to be done on improving the recombination parameters to ensure that full recombination can occur throughout the samples, to achieve the best coercivity, whilst continuing to produce submicron grains. However, this study suggests that higher neodymium content is necessary for processing through HyDP.
[1] M. Zhu et al., Chin. J. Eng., 22(5) p.37 (2020)
[2] T. Horikawa et al., J. Alloys. Compd., 408–412 p.1386 (2006)
[3] J.D. Livingston, J. Appl. Phys., 57(8) p.4137 (1985)
[4] I.R. Harris et al, US 2018/0190428 A1 (2018)
[5] O.P. Brooks et al., Acta Mater., 155 p.268 (2018)
[6] A.J. Williams et al., J. Alloys Compd., 232(1–2) p.L22–L26 (1996)
[7] O.P. Brooks et al., 27th Int. Workshop in REPM, Birmingham, UK. (2023)
The stages of HyDP investigated were disproportionation, deformation and recombination. Three compositions were investigated - Nd2.07Fe14B1.04, Nd2.20Fe14B1.05 and Nd2.31Fe14B1.06 - each containing increasing amounts of intergranular Nd-rich phase. Fine lamella rods are initially present in the microstructure of disproportionated Nd2Fe14B, but extending the disproportionation reaction time leads to the formation of spherical hydrides through Ostwald ripening. This microstructure is necessary for deformation, and the ripening is driven by internal stresses in the material. The composition Nd2.31Fe14B1.06 exhibited the least Ostwald ripening, suggesting that the intergranular Nd-rich hydride phase absorbs some residual stress. During deformation, higher Nd content resulted in increased yield stress and reduced ductility, attributed to less Ostwald ripening and an increased amount of fine lamella in the disproportionated microstructure [7]. When recombined through vacuum and slow recombination, material with higher Nd content exhibited more recombination through both methods. Slow recombination produced grains of size 130nm–1.8mm, but resulted in limited amounts of recombination. By comparison, vacuum recombination exhibited grain sizes in the range 220nm-20mm, and greater amounts of overall recombination.
Whilst a lower neodymium content produces more ductility in the disproportionated microstructure, it hinders the amount of recombination which can occur. This negatively impacts the coercivity of the material, as the presence of free iron in the remaining disproportionated regions reduces the coercivity. Further work needs to be done on improving the recombination parameters to ensure that full recombination can occur throughout the samples, to achieve the best coercivity, whilst continuing to produce submicron grains. However, this study suggests that higher neodymium content is necessary for processing through HyDP.
[1] M. Zhu et al., Chin. J. Eng., 22(5) p.37 (2020)
[2] T. Horikawa et al., J. Alloys. Compd., 408–412 p.1386 (2006)
[3] J.D. Livingston, J. Appl. Phys., 57(8) p.4137 (1985)
[4] I.R. Harris et al, US 2018/0190428 A1 (2018)
[5] O.P. Brooks et al., Acta Mater., 155 p.268 (2018)
[6] A.J. Williams et al., J. Alloys Compd., 232(1–2) p.L22–L26 (1996)
[7] O.P. Brooks et al., 27th Int. Workshop in REPM, Birmingham, UK. (2023)