Presentation Information
[P2-62]Mn-Al-C efficient powder nanostructuring by fast cryogenic milling
*Jorge Vergara Vergara Ortega1,2, Cyrus Zamani1, Andres Martin-Cid1 (1. IMDEA nanociencia (Spain), 2. Universidad Complutense de Madrid (Spain))
Keywords:
MnAlC,cryomilling,microstructuring,rare-earth free magnet
Using magnetic materials is of paramount importance in developing environmentally sustainable technologies, including efficient cooling systems and electrical generators. The increasing demand for permanent magnets, particularly in the domains of electric vehicles and wind turbines, has given rise to concerns regarding the extraction of rare earth elements REEs, such as Nd, Dy, and Tb, which is an environmentally detrimental process. Furthermore, heavy REEs, which are essential for optimal magnet performance at elevated temperatures, are becoming increasingly scarce. To address these issues, research is focusing on RE-free alternatives, such as MnAl-based compounds, with an estimated (BH)max of 12 MGOe [1] at room temperature and a density of 5.2 g/cm3 compared to 7.6 g/cm3 of Nd2Fe14B, which can offer an alternative for medium performance and application at a reduced cost. [2]
In this work, the differences between two different processing methods for the coercivity development of τ-MnAlC, i.e. flash milling [3] and cryomilling [4]. The precursor material is an industrially produced MnAlC powder, with a phase purity of almost 100 percent τ-MnAlC, provided by Less Common Metals LCM. The precursor powder was further processed to obtain a reduced particle size of under 300 micrometers, used for cryomilling, and under 90 micrometers, used for flash-milling. Experiments involving cryomilling were conducted over a range of timeframes between 15 seconds and 120 seconds and flash-milling experiments ranged from 30 up to 840 seconds.
A maximum coercivity of 0.43 T was achieved for the powder cryomilled for 90 s, representing a substantial increase compared to the maximum 0.32 T obtained for the 840 s flash-milled powder. However, this increase in coercivity is accompanied by a decrease in magnetization, making it inappropriate for permanent magnet applications.
To improve the overall magnetic properties of the milled powder, an annealing process is necessary. Differential scanning calorimetry DSC measurements show an exothermic peak around 427 degrees Celsius, which can be related to the formation of the τ-MnAlC phase. A recrystallization study was done based on this temperature, obtaining the best overall magnetic properties by annealing the powders at 500 degrees Celsius for 10 minutes. Scherrer equation was used to calculate the crystallite size, showing a different growing trend for each of the milling methods.
After the annealing process, the MnAlC powder partially recovers the magnetization, achieving a remanence of 42 Am2kg-1 for the cryomilled samples, in comparison to 36.7 Am2kg-1 for the flash-milling samples Fig 1, while keeping a coercivity of 0.21 T and 0.23 T respectively.
In this work, we have shown how the cryomilling method presents itself as an alternative processing route to high-energy ball milling techniques, such as the flash-milling method, for the development of coercivity in τ-MnAlC-based alloys in short times and obtaining promising magnetic properties.
Acknowledgements
The authors would like to acknowledge the financial support provided by the European Union's Horizon 2020 research and innovation programme, through the project entitled PASSENGER (grant agreement No. 101003914). In addition, the authors would like to express their gratitude to Less Common Metals for their generous contribution of the samples, which were provided free of charge.
References
[1] C. Munoz-Rodriguez et al., "Fabrication of bulk τ MnAl–C magnets by hot-pressing from ε-phase gas-atomized and milled powder", J. Alloys Compd. 847 2020 156361-156361
[2] Y. Jia et al., "On the ε → τ Phase Transformation and Twinning in L10-MnAl alloys", Acta Mater. 232 2022 117892-117892
[3] J. Rial et al., "Application of a novel flash-milling procedure for coercivity development in nanocrystalline MnAl permanent magnet powders", J. Phys. D Appl. Phys. 50 2017 105004
[4] E.J. Lavernia, B.Q. Han, J.M. Schoenung, “Cryomilled nanostructured materials: processing and properties”, Mater. Sci. Eng. A 493 2008 207–214
In this work, the differences between two different processing methods for the coercivity development of τ-MnAlC, i.e. flash milling [3] and cryomilling [4]. The precursor material is an industrially produced MnAlC powder, with a phase purity of almost 100 percent τ-MnAlC, provided by Less Common Metals LCM. The precursor powder was further processed to obtain a reduced particle size of under 300 micrometers, used for cryomilling, and under 90 micrometers, used for flash-milling. Experiments involving cryomilling were conducted over a range of timeframes between 15 seconds and 120 seconds and flash-milling experiments ranged from 30 up to 840 seconds.
A maximum coercivity of 0.43 T was achieved for the powder cryomilled for 90 s, representing a substantial increase compared to the maximum 0.32 T obtained for the 840 s flash-milled powder. However, this increase in coercivity is accompanied by a decrease in magnetization, making it inappropriate for permanent magnet applications.
To improve the overall magnetic properties of the milled powder, an annealing process is necessary. Differential scanning calorimetry DSC measurements show an exothermic peak around 427 degrees Celsius, which can be related to the formation of the τ-MnAlC phase. A recrystallization study was done based on this temperature, obtaining the best overall magnetic properties by annealing the powders at 500 degrees Celsius for 10 minutes. Scherrer equation was used to calculate the crystallite size, showing a different growing trend for each of the milling methods.
After the annealing process, the MnAlC powder partially recovers the magnetization, achieving a remanence of 42 Am2kg-1 for the cryomilled samples, in comparison to 36.7 Am2kg-1 for the flash-milling samples Fig 1, while keeping a coercivity of 0.21 T and 0.23 T respectively.
In this work, we have shown how the cryomilling method presents itself as an alternative processing route to high-energy ball milling techniques, such as the flash-milling method, for the development of coercivity in τ-MnAlC-based alloys in short times and obtaining promising magnetic properties.
Acknowledgements
The authors would like to acknowledge the financial support provided by the European Union's Horizon 2020 research and innovation programme, through the project entitled PASSENGER (grant agreement No. 101003914). In addition, the authors would like to express their gratitude to Less Common Metals for their generous contribution of the samples, which were provided free of charge.
References
[1] C. Munoz-Rodriguez et al., "Fabrication of bulk τ MnAl–C magnets by hot-pressing from ε-phase gas-atomized and milled powder", J. Alloys Compd. 847 2020 156361-156361
[2] Y. Jia et al., "On the ε → τ Phase Transformation and Twinning in L10-MnAl alloys", Acta Mater. 232 2022 117892-117892
[3] J. Rial et al., "Application of a novel flash-milling procedure for coercivity development in nanocrystalline MnAl permanent magnet powders", J. Phys. D Appl. Phys. 50 2017 105004
[4] E.J. Lavernia, B.Q. Han, J.M. Schoenung, “Cryomilled nanostructured materials: processing and properties”, Mater. Sci. Eng. A 493 2008 207–214
