Presentation Information
[O1-3]Diffusion of RE-rich alloys into NdFeB magnetic material produced via the hydrogen ductilisation process (HyDP)
*Oliver Peter Brooks1, Patrick Powell1, Enbang Ti1, David Brown1, Allan Walton1 (1. University of Birmingham (UK))
Keywords:
NdFeB,Hydrogen Processing,Diffusion,Hydrogen Ductilisation
The majority of NdFeB magnets are produced via the conventional sintering process1. Whilst this process produces magnets with exemplary properties it requires complex processing of highly pyrophoric powders, which limits acheivable grain sizes. Moreover, the materials produced are extremely brittle and must be machined to size, which produces significant waste2 and reduces achievable magnet shapes.
Recently, researchers at the University of Birmingham have been developing a novel processing technology for NdFeB alloys, termed the hydrogen ductilisation process (HyDP)3,4. HyDP uses high temperature (>650°C) hydrogen processing to transform the brittle Nd2Fe14B matrix phase into a bulk solid, ductile mixture of NdH2, Fe2B, and α-Fe via solid hydrogenation disproportionation5. This mixture can be deformed at room temperature and then heated in a vacuum to desorb the hydrogen, causing the constituents to recombine into submicron Nd2Fe14B grains. Thus, it may be possible to create NdFeB magnets from cast alloys using HyDP, without powder processing, and greater ability to shape.
Whilst HyDP has many advantages, several challenges must be overcome for it to be a viable processing route. One key challenge is limiting the effect of cavitation occurring from recombination6. This phenomenon occurs during the recombination step of solid-HDDR processing6, and is due to a redistribution of liquid Nd-rich phase into newly formed grain boundaries, creating cavities that lower density and magnetic properties. These cavities can be reduced by lowering the rare earth (RE) content of the starting alloy, and thus the volume of Nd-rich phase. However, this reduction in Nd-rich phase leads to incomplete recombination and reduced magnetic properties7.
For sintered magnets, the grain boundary diffusion process is used to diffuse heavy RE elements into the magnet to improve magnetic properties8. Recent studies have shown similar diffusion processes can be used eutectic RE alloys and HDDR powders, to boost magnetic properties9. Thus, it may be possible to diffuse in additional RE to HyDP material using eutectic RE alloys, both filling the cavitation and improving magnetic properties.
This work utilises melt spun binary/ternary alloys (consisting of Nd, Dy, Cu, Al, Ga) and a dip coating method to diffuse REs into HyDP materials. Diffusion treatments have been performed at temperatures of 550-800°C, showing that REs will diffuse into the cavitation. However, at higher temperatures grain growth occurs, lowering magnetic performance.
1. Sagawa, M. & Une, Y. The status of sintered NdFeB magnets. in Modern Permanent Magnets 135–168 (Woodhead Publishing, 2022). doi:10.1016/B978-0-323-88658-1.00010-8.
2. Kumari, A. & Sahu, S. K. A comprehensive review on recycling of critical raw materials from spent neodymium iron boron (NdFeB) magnet. Separation and Purification Technology vol. 317 Preprint at https://doi.org/10.1016/j.seppur.2023.123527 (2023).
3. Brooks, O., Walton, A., Zhou, W. & Harris, I. R. The Hydrogen Ductilisation Process (HyDP) for shaping NdFeB magnets. J Alloys Compd 703, 538–547 (2017).
4. Harris, I. R., Walton, A. & Brooks, O. P. Magnet Production. Preprint at (2018).
5. Gutfleisch, O., Verdier, M. & Harris, I. R. Kinetic studies on solid-HDDR processes in Nd-Fe-B-type alloys. J Appl Phys 76, 6256–6258 (1994).
6. Brooks, O. P., Walton, A., Zhou, W., Brown, D. & Harris, I. R. Complete ductility in NdFeB-type alloys using the Hydrogen Ductilisation Process (HyDP). Acta Mater 155, (2018).
7. Brooks, O. et al. The Influence of the Disproportionated Microstructure on the Hydrogen Ductilisation Process (HyDP) for NdFeB Alloys. in The 27th international workshop on REPMs (Birmingham, 2023).
8. Liu, Z., He, J. & Ramanujan, R. V. Significant progress of grain boundary diffusion process for cost-effective rare earth permanent magnets: A review. Mater Des 209, 110004 (2021). Dirba, I. et al. Grain boundary infiltration in HDDR processed Nd2Fe14B magnets. J Alloys Compd 930, 167411 (2023).
Recently, researchers at the University of Birmingham have been developing a novel processing technology for NdFeB alloys, termed the hydrogen ductilisation process (HyDP)3,4. HyDP uses high temperature (>650°C) hydrogen processing to transform the brittle Nd2Fe14B matrix phase into a bulk solid, ductile mixture of NdH2, Fe2B, and α-Fe via solid hydrogenation disproportionation5. This mixture can be deformed at room temperature and then heated in a vacuum to desorb the hydrogen, causing the constituents to recombine into submicron Nd2Fe14B grains. Thus, it may be possible to create NdFeB magnets from cast alloys using HyDP, without powder processing, and greater ability to shape.
Whilst HyDP has many advantages, several challenges must be overcome for it to be a viable processing route. One key challenge is limiting the effect of cavitation occurring from recombination6. This phenomenon occurs during the recombination step of solid-HDDR processing6, and is due to a redistribution of liquid Nd-rich phase into newly formed grain boundaries, creating cavities that lower density and magnetic properties. These cavities can be reduced by lowering the rare earth (RE) content of the starting alloy, and thus the volume of Nd-rich phase. However, this reduction in Nd-rich phase leads to incomplete recombination and reduced magnetic properties7.
For sintered magnets, the grain boundary diffusion process is used to diffuse heavy RE elements into the magnet to improve magnetic properties8. Recent studies have shown similar diffusion processes can be used eutectic RE alloys and HDDR powders, to boost magnetic properties9. Thus, it may be possible to diffuse in additional RE to HyDP material using eutectic RE alloys, both filling the cavitation and improving magnetic properties.
This work utilises melt spun binary/ternary alloys (consisting of Nd, Dy, Cu, Al, Ga) and a dip coating method to diffuse REs into HyDP materials. Diffusion treatments have been performed at temperatures of 550-800°C, showing that REs will diffuse into the cavitation. However, at higher temperatures grain growth occurs, lowering magnetic performance.
1. Sagawa, M. & Une, Y. The status of sintered NdFeB magnets. in Modern Permanent Magnets 135–168 (Woodhead Publishing, 2022). doi:10.1016/B978-0-323-88658-1.00010-8.
2. Kumari, A. & Sahu, S. K. A comprehensive review on recycling of critical raw materials from spent neodymium iron boron (NdFeB) magnet. Separation and Purification Technology vol. 317 Preprint at https://doi.org/10.1016/j.seppur.2023.123527 (2023).
3. Brooks, O., Walton, A., Zhou, W. & Harris, I. R. The Hydrogen Ductilisation Process (HyDP) for shaping NdFeB magnets. J Alloys Compd 703, 538–547 (2017).
4. Harris, I. R., Walton, A. & Brooks, O. P. Magnet Production. Preprint at (2018).
5. Gutfleisch, O., Verdier, M. & Harris, I. R. Kinetic studies on solid-HDDR processes in Nd-Fe-B-type alloys. J Appl Phys 76, 6256–6258 (1994).
6. Brooks, O. P., Walton, A., Zhou, W., Brown, D. & Harris, I. R. Complete ductility in NdFeB-type alloys using the Hydrogen Ductilisation Process (HyDP). Acta Mater 155, (2018).
7. Brooks, O. et al. The Influence of the Disproportionated Microstructure on the Hydrogen Ductilisation Process (HyDP) for NdFeB Alloys. in The 27th international workshop on REPMs (Birmingham, 2023).
8. Liu, Z., He, J. & Ramanujan, R. V. Significant progress of grain boundary diffusion process for cost-effective rare earth permanent magnets: A review. Mater Des 209, 110004 (2021). Dirba, I. et al. Grain boundary infiltration in HDDR processed Nd2Fe14B magnets. J Alloys Compd 930, 167411 (2023).