- Email: [email protected]
Structure and magnetic properties of Alnico8 alloy thermo-magnetically treated under a 10 T magnetic field
Intermetallics 119 (2020) 106691
Contents lists available at ScienceDirect
Intermetallics journal homepage: http://www.elsevier.com/locate/intermet
Structure and magnetic properties of Alnico8 alloy thermo-magnetically treated under a 10 T magnetic field X.Y. Sun a, b, C.L. Chen a, M.Y. Ma a, L. Yang a, L.X. Lv a, S. Atroshenko c, W.Z. Shao a, b, L. Zhen a, * a
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, 150001, China National Key Laboratory for Precision Hot Processing of Metals, Harbin Institute of Technology, Harbin, 150001, China c School of Mathematics and Mechanics, Saint-Petersburg State University, Saint-Petersburg, 199034, Russia b
A R T I C L E I N F O
A B S T R A C T
Keywords: Alnico8 alloy High magnetic field Structure Magnetic properties
The structure and compositions of spinodal phases in Alnico8 alloy isothermally treated at 830 � C under external magnetic field (Hext) up to 10 T were investigated by transmission electron microscopy (TEM). Under ultra-high magnetic field (10 T), the spinodal structure of Alnico8 alloy presents interesting diversity for the shapes of α1 phase particles, namely, Π-shape, H-shape and Ш-shape apart from regular rod-shape. Composition analysis shows that the Fe was mainly distributed in the α1 phase. The Co content in the α1 phase was lower than that in the α2 phase as evidenced by the point analysis results, and Co content inα1 phase of the alloy treated without magnetic field was higher than that of the alloy treated with a magnetic field. In contrast, the Ni, Al, Cu and Ti were concentrated in the α2 phase. The average hyperfine field of the alloy treated without magnetic field was about 301.3 kOe. While the alloy treated under magnetic field, it was slightly decreased, which was about 292.0 and 296.1 kOe under 0.7 and 10 T, respectively. Magnetic properties of the alloy treated under a 10 T magnetic field were the best, which were about 1.09T, 396Oe and 9.5 kJ/m3 for Br, Hc and (BH)max respectively. Especially for the coercivity, which was mainly contributed by the anisotropy of the α1 phase induced by the high magnetic field.
1. Introduction Alnico alloy was firstly discovered in 1931, and can be used in some high-technological fields, especially on some extreme environment such as space field and nuclear industry due to their excellent comprehensive performance (such as high Curie temperature and good thermal stabil ity) [1–3]. Recently, Alnico alloys were further studied and developed again due to these rare-earth-free magnets (the high cost of rare-earth magnets) [4–7]. The spinodal structure of the alloys has been system atically characterized by the advanced analytical facilities, such as atom probe tomography (APT) [7,8], high-resolution transmission electron microscope (HRTEM), Lorentz transmission electron microscope (LTEM) [7], high angle annular dark-field scanning transmission elec tron microscopy (HAADF-STEM), etc [9–13], aiming to understand how the chemistry and processing affect its magnetic properties. Spinodal decomposition will occur during the thermomagnetic treatment (TMT) stage, and the external magnetic field will significantly affect the mag netic properties and spinodal structure of these kinds of alloys. The external magnetic field was used to enhance the magnetic properties for
these magnetic materials, such as Alnico alloys and Fe–Cr–Co alloys [14–16], because the anisotropy of magnetic α1 phase particles in these alloys was improved during the TMT stage under the condition of the external magnetic field. Many researches have been focused on the improvement of micro alloying and processing of the alloy to increase the magnetic properties [6,10,15]. Compared with the rare-earth magnets (such as NdFeB and SmCo magnets), the coercivity of the Alnico alloys is much low. Usually, the coercivity is less than 2000 Oe, which leads its maxim magnetic energy less than 80 kJ/m3 [17]. In previous research, the Hext. intensity applied during the TMT processing was usually less than 3T [6,9]. In this work, the high magnetic field up to 10T was applied during the isothermal TMT processing. The aim is to find whether the high mag netic field could obviously improve its magnetic properties, and to analysis how the high magnetic field affects the spinodal structure and magnetic properties of the alloy during isothermal TMT processing. Alnico8 alloy was isothermally treated at 830 � C (In order to get good magnetic properties, the Alnico8 alloy was usually TMT around 830 � C, and then multi-step aged [14]) under different Hext. intensities,
* Corresponding author. E-mail address: [email protected] (L. Zhen). https://doi.org/10.1016/j.intermet.2019.106691 Received 16 September 2019; Received in revised form 28 December 2019; Accepted 31 December 2019 Available online 23 January 2020 0966-9795/© 2020 Elsevier Ltd. All rights reserved.
X.Y. Sun et al.
Intermetallics 119 (2020) 106691
830 � C for different durations (up to 10 min) were adopted for the alloy without and with an external magnetic field (0.7 and 10 T), and the direction of magnetic field applied was parallel to the length of the sample. The details of heat treatment were shown in our previous work [18]. Phase identification of the alloy treated under different conditions were carried out using X-ray diffraction. Conventional 2θ-ω Bragg diffraction data were collected on a PANalyticalX’pert Pro MPD diffractometer. TEM thin foils were jet-polished and then observed on a FEI G2 F30 trans mission electron microscope at 300 kV. The composi tions of decomposed phases in Alnico8 alloy treated under different conditions were analyzed by HAADF STEM. The hyperfine structure of Alnico8 alloy treated under different Hext. intensities was also obtained €ssbauer spectrometry. Detailed experimental processing was by Mo described in detail in a previous study [19]. The magnetic properties of the samples were measured on NIM-200 magnetic properties test system at room temperature. 3. Results and discussion 3.1. XRD patterns Fig. 1. XRD patterns with 2θ ω scans for the Alnico8 alloy treated under different conditions (solid-solution state, isothermally treated at 830 � C for 10 min under 0T, 0.7T, 10T, respectively).
Fig. 1 shows 2θ-ω scan results of the Alnico8 alloy treated under different conditions. There are mainly three peaks for the alloy between 20� and 90� , the bcc (100), (110), and (200) diffraction peak, respec tively. After the alloy was solid solution treated at 1250 � C for 40 min, peaks were indexed to bcc α phase, and the percentage of peak area for the (110) diffraction peak was about 25.8%. While the alloy was isothermally treated at 830 � C for 10 min under different Hext. in tensities, the percentage of peak area for the (110) diffraction peak was decreased as the Hext. intensities increasing. The percentage of peak area for the (110) diffraction peak was about 10.5%, 3.6%, 3.4%, as the alloy treated under 0T, 0.7T, 10T, respectively. At same time, the (200) diffraction peak was slightly changed for the alloy treated under different conditions. The (200) diffraction peak was at 64.1 � 0.1� for the alloy with solid-solution state. When the alloy treated under 0T, 0.7T, 10T, the (200) diffraction peak was at 64.4 � 0.1� , 64.7 � 0.1� , 64.7 � 0.1� , respectively. When the alloy was isothermally treated at 830 � C, the α phase was spinodally decomposed into α1 phase and α2 phase, and the α1 phase would grow along the orientation of <001>. If there was an external magnetic field during spinodal decomposition, the
including zero magnetic field, low magnetic field (0.7 T) and high magnetic field (10 T). The morphology and compositions of spinodal structure were observed and measured by TEM and HAADF-STEM. The €ssbauer hyperfine structure of Alnico8 alloy was also investigated by Mo spectrometry. The relationship among the structure, composition and magnetic properties of the Alnico8 alloy was discussed in this work. 2. Experimental The compositions and local structure of decomposed phases in Alnico8 alloy (Fe-34.2Co-14.6Ni–7Al-6.1Ti-2.7Cu, wt%) treated under different Hext conditions were systemically studied. The Alnico8 alloy was isotropic crystal. Flake-like samples of 12 � 10 � 4 mm3 cut from the bulk were solid solution treated at 1250 � C for 40 min, and then quenched into iced brine. The same parameters of isothermal ageing at
Fig. 2. Bright-field TEM images and HAADF-STEM images of Alnico8 alloy treated at 830 � C for 3 and 10 min under different Hext (ac) TEM images of the alloy treated at 830 � C for 3 min under 0 T, 0.7 T and 10 T respectively; (d), (e), (f) TEM images of the alloy treated at 830 � C for 10 min under 0 T, 0.7 T and 10 T respectively [the inserted figure in (e) and (f) was perpendicular to the Hext]; the scale bar is 100 nm. 2
X.Y. Sun et al.
Intermetallics 119 (2020) 106691
Fig. 3. HAADF STEM images and the line scanning results of Alnico8 alloy treated at 830 � C for 10 min under different Hext (a, b) 0 T; (c, d) 0.7 T, ?Hext; (d, e) 10 T, ?Hext; Scale bar is 20 nm.
external magnetic field would make the atoms diffuse faster. Therefore, the percentage of peak area for the (100) and (200) diffraction peaks would be increased for the alloy treated under an external magnetic field. The difference between two phase composition was also larger for the alloy treated under an external magnetic field, which could lead to the (200) diffraction peak shifted.
Fig. 2 shows the evolution of modulated structure depending on external magnetic field intensity and heat-treatment duration. The observed crystal directions were all the same as the orientation of <001>, and the corresponding selected area electron diffraction (SAED) patterns was shown as the inserted figure in Fig. 2(a). Compared to the morphology of the decomposed particles for the Alnico8 alloy treated at 830 � C for 10 min, it was not clear for that of the Alnico8 alloy treated for 3 min no matter how much about the external magnetic field intensities. The contrast of the TEM image was mainly contributed by the composition difference. It was due to the difference of the compositioin between two phases increased as the time extended, so the interface between two
3.2. Spinodal structure Alnico8 alloy was treated at temperature of 830 � C for 3 and 10 min under different external magnetic field intensities (0 T, 0.7 T, 10 T). 3
X.Y. Sun et al.
Intermetallics 119 (2020) 106691
areas correspond to the average Z smaller. The HAADF STEM image indicates that the positions at which heavy metal atoms richly gathered were the bright regions, such as Ni and Cu. Therefore, the decomposed particles (α1 phase) were shown as the dark regions, while the bright regions were the matrixes. Which was obviously different from that of the finally aged alloys, the HAADF STEM image of α1 phase was the bright regions in Alnico 5–7, Alnico 8 and Alnico 9 [7]. It also illustrated that the composition diffusion greatly occurred during the annealing processing after the TMT stage. When the Alnico8 alloy treated without magnetic field, the HAADF STEM image was shown as Fig. 3(a), the line scanning results of the elements distribution was shown as Fig. 3(b). The Fe was mainly distributed in the α1 phase, but the Co was not obviously different in the two phases. While the Ni, Al, Cu and Ti were concentrated in the α2 phase. As the alloy treated under an external magnetic field, the element distribution was changed a little. The difference of Fe content between the two phases was increased with a Hext. The Ni content in the α2 phase was also increased under an external magnetic field. There were some oscillations for the Co distribution between two phases under the 10 T magnetic field (shown as Fig. 3 f), compared to the results under the 0.7 T magnetic field (shown as Fig. 3 d) or zero magnetic field (shown as Fig. 3 b). The composition of the alloy in the different locations were shown as Table 1 (the point analysis results), which was corresponding to the results of line scanning. The Fe was mainly distributed in the α1 phase. The Co content in the α1 phase was lower than that in the α2 phase, and the Co content in the α1 phase of the alloy treated without magnetic field was higher than that of the alloy treated with a magnetic field.
Table 1 The compositions of different locations (locations marked as Fig.2) in Alnico8 alloy treated at 830 � C for 10 min under different Hext conditions (wt%). Position 0T 0.7T 10T
1 2 3 1 2 3 1 2 3
Fe
Co
Al
Ni
Ti
Cu
53.17 36.17 23.41 59.95 40.35 19.73 58.71 38.5 26.76
34.66 41.27 41.25 31.3 31.91 40.73 32.08 39.46 41.78
1.89 4.72 9.13 2.32 7.98 11.68 1.6 3.43 5.02
5.89 8.41 11.87 1.11 8.47 9.44 2.85 12 13.6
2.77 6.76 9.88 0.45 3.35 11.95 1.81 4.54 9.03
1.59 2.64 4.41 4.84 7.94 6.44 2.04 2.04 3.81
phases became clearer as the time kept for longer. The average diameter of the Fe–Co rich phase particles (α1 phase) decreased as increasing external magnetic field intensities. It was about 29 nm, 26 nm and 20 nm treated at 830 � C for 10 min under 0 T, 0.7 T and 10 T, respectively. When the intensity of external magnetic field was 0.7 T, the α1 phase particles were anisotropic, they were mainly round shape perpendicular to the Hext, as shown in the inserted figure of Fig. 2e. It illustrated that the shape of α1 phase particles was mainly rodshape, but the aspect ratio of α1 phase particles was less than 2.0. When the intensity of external magnetic field increased to 10 T, the decom posed particles were obviously connected parallel to the direction of the external magnetic field, and as well many particles perpendicular to the Hext were also connected together to become laminae structure (shown in Fig. 2 f). It indicated that the connection between α1 phase particles in the alloy treated under high magnetic field might be much more shapes, besides rod-shaped, and Π-shaped, H-shaped and Ш-shaped geometries. The diameter of α1 phase particles was the smallest, but its aspect ratio (about 4.2) was obviously the largest. The volume fraction of the α1 phase particles was also the largest among three conditions of the external magnetic field. The reason is that the magnetic field makes the atoms diffuse faster parallel to the direction of the external magnetic field than that perpendicular to the direction of the external magnetic field [18]. In a word, the external magnetic field mainly affected the diffusion of the atoms and increased the anisotropy of α1 phase particles in the Alnico8 alloy.
3.4. M€ ossbauer spectra €ssbauer spectrometry results of the Alnico8 alloy treated at 830 � C Mo €ssbauer for 10 min under different Hext were shown as Fig. 4. All the Mo spectrums gotten under different Hext were magnetic six-line spectrums, and there was no paramagnetic peak in the spectrums. It indicated that the two phases were both ferromagnetic. The width between the 1st and 6th peak under 0 T was slightly larger than that of 0.7 T or 10 T. It illustrated that the local structure of the alloy was different, the amount of Fe and Co atoms surrounding nearest neighbor the Fe atom in the alloy treated without magnetic field were more than that with an external magnetic field. The average hyperfine field of Alnico8 alloy treated under an external magnetic field of 0.7 T or 10 T was decreased a little, compared to that without magnetic field, shown as Table 2. The obvious difference for the P(H) distribution curve was that the peak of P(H) in high field was shifted from 355.0 kOe (without Hext) to 335.0 kOe (with a 0.7 T or 10 T Hext), shown as Fig. 4 (b). When the
3.3. Compositions of spinodal phases The HAADF STEM images and the line scanning results of Alnico8 alloy at 830 � C for 10 min under different external magnetic field in tensities were shown as Fig. 3. The image (contrast) intensity of a HAADF STEM pattern is proportional to Z2 (Z is the atomic number); thus the bright areas correspond to the average Z larger, while the dark
Fig. 4. M€ ossbauer spectrometry results of Alnico8 alloy treated at 830 � C for 10 min under different Hext. (a) M€ ossbauer spectrums; (b) Hyperfine field distribution. 4
X.Y. Sun et al.
Intermetallics 119 (2020) 106691
properties changed with the external magnetic field intensities, espe cially for the Hc. The holding time mainly affected the difference of composition between two phases under zero magnetic field. While the external magnetic field intensities affected not just the difference of composition between two phases, but also the anisotropy of the α1 phase. The covercivitiy of the alloy was decided not only by the differ ence of composition between two phases. While the shape, spacing, size, volume fraction, arrangements, and branching types of the magnetic α1 phase in the Ni–Al rich matrix all affected the coercivity, but aside from the packing fraction, the most important features were the shapes of the α1 phase ends or tips and interactions between them [20–22]. The shapes of the α1 phase in the alloy treated under high magnetic field were not a single rod-shape, much more Π-shaped, H-shaped, Ш-shaped geometries. Though the shapes of α1 phase differently contributed to the coercivity of the alloy, the shapes of Π-shaped, H-shaped, Ш-shaped geometries were not benefited to the coercivity of alloy [20]. As the results measured by HAADF STEM, the difference of composition be tween two phases under a 10T magnetic field was more benifited to the covercivities than that under a 0.7 T magnetic field, according to the Stoner-Wohlfarth mechanism [21]. Although the difference of compo sition between two phases in the alloy treated without magnetic field, by contrast, was good to the covercivities, the α1 phase particles were isotropic, which was not benifited to the covercivities. In a way, the
Table 2 Average hyperfine field of Alnico8 alloy treated under different external magnetic field conditions. Hext
0T
0.7 T
10 T
(kOe)
301.3
292.0
296.1
intensity of hyperfine field was about 330.0 kOe, the Fe atom was mainly surrounded by the Fe atoms. While the intensity of hyperfine field was over 330.0 kOe, there would be some Co atoms located nearby the Fe atoms. If the area of the P(H) distribution in high field was more, there were more Co atoms surrounding nearby the Fe atoms. These re sults were corresponding to the composition of the spinodal phases shown in Fig. 3 and Table .1. The Co in the α1 phase of the alloy treated without magnetic field was higher than that of the alloy treated with a magnetic field, shown in Table .1. 3.5. Magnetic properties of the alloy The evalution of magnetic properties for the Alnico8 alloy isother mally treated at 830 � C under different external magnetic field was shown as Fig. 5 it was obviously that the magnetic properties become better with the time kept longer, which was similar to the magnetic
Fig. 5. The evalution of magnetic properties in Alnico8 alloy isothermally treated at 830 � C under different external magnetic fields. (a) Br; (b) Hc; (c) (BH)max; (d) Ms. 5
X.Y. Sun et al.
Intermetallics 119 (2020) 106691
covercivities of the alloy treated under an external magnetic field was mainly contributed by the anisotropy of the α1 phase(besides the aspect ratio of the α1 phase) and the composition difference between two phases. In other word, the magnetic properties of the alloy isothermally treated at 830 � C were obviously improved by the high magnetic field, especially for the coercivity. When the alloy treated at 830 � C under a 10T magnetic field, Br, Hc and (BH)max were about 1.09T, 396Oe and 9.5 kJ/m3 respectively. The Ms of the alloy treated at 830 � C under the different magnetic field was slightly changed, the Ms was the best for the samples treated at 830 � C for different time under 10T. while the isothermal time was 3 min or 5 min, the Ms was increased a little with the intensity of magnetic field increasing, shown as Fig. 5 d. When the isothermal time was extended to 10 min, the Ms was almost the same under the different magnetic field. The change might be due to the magnetic field accelerating the diffusion of atoms [18]. The magnetic field also enhanced the interaction of atoms, especially for the Fe and Co atoms. When the isothermal time was prolonged further, the composition of two phases would reach to the equilibrium. The change of Ms was indicated the composition changing of two phases in the alloy treated under the different magnetic field.
National Basic Research Program of China (2012CB934102). X.Y. Sun was supported by Lab Foundation of National Key Laboratory for Pre cision Hot Processing of Metals. We wish to thank G. Q. Chen at Dalian University of Technology for his kind permission to use thermomagnetic treatment equipment. M.L. Liu at Jilin University is €ssbauer analysis. acknowledged for her help and useful discussion on Mo Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.intermet.2019.106691. References [1] T. Mishima, Nickel-aluminum steel for permanent magnets, Stahl u. Eisen. 53 (1931) 79. [2] M. Ibrahim, L. Masisi, P. Pillay, Design of variable-flux permanent-magnet machines using alnico magnets, IEEE Trans. Ind. Appl. 51 (2015) 4482–4491. [3] Q. Xing, M.K. Miller, L. Zhou, , et al.Constantinides, M.J. Kramer, Design of variable-flux permanent-magnet machines using alnico magnets, IEEE Trans. Magn. 49 (2013) 3314–3317. [4] F. Mohseni, A. Baghizadeh, A.A.C.S. Lourenco, M.J. Pereira, et al., Interdiffusion processes in high-coercivity RF-sputtered alnico thin films on Si substrates, JOM 69 (2017) 1427–1431. [5] A. Palasyuk, E. Blomberg, R. Prozorov, et al., Advances in characterization of nonrare-earth permanent magnets: exploring commercial alnico grades 5-7 and 9, JOM 65 (2013) 862–869. [6] L. Zhou, W. Tang, L.Q. Ke, et al., Microstructural and magnetic property evolution with different heat-treatment conditions in an alnico alloy, Acta Mater. 133 (2017) 73–80. [7] L. Zhou, M.K. Miller, P. Lu, et al., Architecture and magnetism of alnico, Acta Mater. 74 (2014) 224–233. [8] F. Zhu, L.V. Alvensleben, P. Haasen, A study of Alnico magnets by atom probe, Scr. Mater. 18 (1984) 337–342. [9] M. Fan, Y. Liu, R. Jha, et al., On the formation and evolution of Cu-Ni-rich bridges of alnico alloys with thermomagnetic treatment, IEEE Trans. Magn. 52 (2016) 2101710. [10] Z. Ahmad, Z.W. Liu, A. ul Haq Synthesis, Magnetic and microstructural properties of Alnico magnets with additives, J. Magn. Magn. Mater. 428 (2017) 125–131. [11] Y.L. Sun, J.T. Zhao, Z. Liu, et al., The phase and microstructure analysis of Alnico magnets with high coercivity, J. Magn. Magn. Mater. 379 (2015) 58–62. [12] S.M. Zhu, J.T. Zhao, W.X. Xia, et al., Magnetic structure and coercivity mechanism of AlNiCo magnets studied by electron holography, J. Alloy. Comp. 720 (2017) 401–407. [13] M. Fan, Y. Liu, R. Jha, et al., On the evolution of Cu-Ni-rich bridges of Alnico alloys with tempering, J. Magn. Magn. Mater. 420 (2016) 296–302. [14] H.M. Dillon, Effects of Heat Treatment and Processing Modifcations on Microstructure in Alnico 8H Permanent Magnet Alloys for High Temperature Applications, M.S. thesis, Iowa State Univ., Ames, IA, USA, 2014. [15] O.A. Ushakova, E.H. Dinislamova, M.V. Gorshenkov, et al., Structure and magnetic properties of Fe-Cr-Co nanocrystalline alloys for permanent magnets, J. Alloy. Comp. 586 (2014) 291–293. [16] I.E. Anderson, A.G. Kassen, E.M.H. White, et al., Development of controlled solidstate alignment for alnico permanent magnets in near-final shape, AIP Adv. 7 (2017), 056209. [17] K. Lowe, M. Durrschnabel, L. Molina-Luna, et al., Microstructure and magnetic properties of melt-spun Alnico-5 alloys, J. Magn. Magn. Mater. 407 (2016) 230–234. [18] X.Y. Sun, C.L. Chen, L. Yang, et al., Experimental study on modulated structure in Alnico alloys under high magnetic field and comparison with phase-field simulation, J. Magn. Magn. Mater. 348 (2013) 27–32. [19] X.Y. Sun, L. Zhen, C.Y. Xu, et al., M€ ossbauer spectrometry study of early stage spinodal decomposition in Fe-Cr-Co alloy under high magnetic field, Mater. Lett. 63 (2009) 64–65. [20] L.Q. Ke, R. Skomski, T.D. Hoffmann, et al., Simulation of alnico coercivity, Appl. Phys. Lett. 111 (2017), 022403. [21] S. Chikazumi, Physics of Magnetism, Wiley, New York, 1964. [22] M.C. Nguyen, X. Zhao, C.Z. Wang, K.M. Ho, Cluster expansion modeling and Monte Carlo simulation of alnico 5-7 permanent magnets, J. Appl. Phys. 117 (2015), 093905.
4. Conclusions The magnetic properties of Alnico8 alloy isothermally treated at 830 C were obviously improved by a high magnetic field of 10 T, which increased the anisotropy of the α1 phase of the alloy. The two phases in the alloy treated at 830 � C were ferromagnetic no matter how much about the external magnetic field. The composition of the two phases was affected by the external magnetic field, the Fe was mainly distrib uted in the α1 phase. The Co in the α1 phase was lower than that in the α2 phase, and the Co in the α1 phase of the alloy treated without magnetic field was higher than that of the alloy treated with a magnetic field. Which leaded the average hyperfine field of alloy treated with external magnetic field slightly deceased, compared to that of the alloy treated without magnetic field. When the alloy treated at 830 � C under a 10T magnetic field, the magnetic properties of the alloy were the best. Br, Hc and (BH)max were about 1.09T, 396Oe and 9.5 kJ/m3 respectively. �
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement X.Y. Sun: Writing - original draft, Conceptualization. C.L. Chen: Investigation. M.Y. Ma: Investigation. L. Yang: Investigation. L.X. Lv: Investigation. S. Atroshenko: Writing - original draft. W.Z. Shao: Visualization. L. Zhen: Writing - review & editing, Supervision, Writing - original draft. Acknowledgements This work was financially supported by National Key R&D Program of China (No. 2018YFB2003900), and National Natural Science Foun dation of China (No. 50901024), International/Regional Cooperation and Exchange Program of NSFC (NSFC-RFBR, No. 51211120186), and
6
Contents lists available at ScienceDirect
Intermetallics journal homepage: http://www.elsevier.com/locate/intermet
Structure and magnetic properties of Alnico8 alloy thermo-magnetically treated under a 10 T magnetic field X.Y. Sun a, b, C.L. Chen a, M.Y. Ma a, L. Yang a, L.X. Lv a, S. Atroshenko c, W.Z. Shao a, b, L. Zhen a, * a
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, 150001, China National Key Laboratory for Precision Hot Processing of Metals, Harbin Institute of Technology, Harbin, 150001, China c School of Mathematics and Mechanics, Saint-Petersburg State University, Saint-Petersburg, 199034, Russia b
A R T I C L E I N F O
A B S T R A C T
Keywords: Alnico8 alloy High magnetic field Structure Magnetic properties
The structure and compositions of spinodal phases in Alnico8 alloy isothermally treated at 830 � C under external magnetic field (Hext) up to 10 T were investigated by transmission electron microscopy (TEM). Under ultra-high magnetic field (10 T), the spinodal structure of Alnico8 alloy presents interesting diversity for the shapes of α1 phase particles, namely, Π-shape, H-shape and Ш-shape apart from regular rod-shape. Composition analysis shows that the Fe was mainly distributed in the α1 phase. The Co content in the α1 phase was lower than that in the α2 phase as evidenced by the point analysis results, and Co content inα1 phase of the alloy treated without magnetic field was higher than that of the alloy treated with a magnetic field. In contrast, the Ni, Al, Cu and Ti were concentrated in the α2 phase. The average hyperfine field of the alloy treated without magnetic field was about 301.3 kOe. While the alloy treated under magnetic field, it was slightly decreased, which was about 292.0 and 296.1 kOe under 0.7 and 10 T, respectively. Magnetic properties of the alloy treated under a 10 T magnetic field were the best, which were about 1.09T, 396Oe and 9.5 kJ/m3 for Br, Hc and (BH)max respectively. Especially for the coercivity, which was mainly contributed by the anisotropy of the α1 phase induced by the high magnetic field.
1. Introduction Alnico alloy was firstly discovered in 1931, and can be used in some high-technological fields, especially on some extreme environment such as space field and nuclear industry due to their excellent comprehensive performance (such as high Curie temperature and good thermal stabil ity) [1–3]. Recently, Alnico alloys were further studied and developed again due to these rare-earth-free magnets (the high cost of rare-earth magnets) [4–7]. The spinodal structure of the alloys has been system atically characterized by the advanced analytical facilities, such as atom probe tomography (APT) [7,8], high-resolution transmission electron microscope (HRTEM), Lorentz transmission electron microscope (LTEM) [7], high angle annular dark-field scanning transmission elec tron microscopy (HAADF-STEM), etc [9–13], aiming to understand how the chemistry and processing affect its magnetic properties. Spinodal decomposition will occur during the thermomagnetic treatment (TMT) stage, and the external magnetic field will significantly affect the mag netic properties and spinodal structure of these kinds of alloys. The external magnetic field was used to enhance the magnetic properties for
these magnetic materials, such as Alnico alloys and Fe–Cr–Co alloys [14–16], because the anisotropy of magnetic α1 phase particles in these alloys was improved during the TMT stage under the condition of the external magnetic field. Many researches have been focused on the improvement of micro alloying and processing of the alloy to increase the magnetic properties [6,10,15]. Compared with the rare-earth magnets (such as NdFeB and SmCo magnets), the coercivity of the Alnico alloys is much low. Usually, the coercivity is less than 2000 Oe, which leads its maxim magnetic energy less than 80 kJ/m3 [17]. In previous research, the Hext. intensity applied during the TMT processing was usually less than 3T [6,9]. In this work, the high magnetic field up to 10T was applied during the isothermal TMT processing. The aim is to find whether the high mag netic field could obviously improve its magnetic properties, and to analysis how the high magnetic field affects the spinodal structure and magnetic properties of the alloy during isothermal TMT processing. Alnico8 alloy was isothermally treated at 830 � C (In order to get good magnetic properties, the Alnico8 alloy was usually TMT around 830 � C, and then multi-step aged [14]) under different Hext. intensities,
* Corresponding author. E-mail address: [email protected] (L. Zhen). https://doi.org/10.1016/j.intermet.2019.106691 Received 16 September 2019; Received in revised form 28 December 2019; Accepted 31 December 2019 Available online 23 January 2020 0966-9795/© 2020 Elsevier Ltd. All rights reserved.
X.Y. Sun et al.
Intermetallics 119 (2020) 106691
830 � C for different durations (up to 10 min) were adopted for the alloy without and with an external magnetic field (0.7 and 10 T), and the direction of magnetic field applied was parallel to the length of the sample. The details of heat treatment were shown in our previous work [18]. Phase identification of the alloy treated under different conditions were carried out using X-ray diffraction. Conventional 2θ-ω Bragg diffraction data were collected on a PANalyticalX’pert Pro MPD diffractometer. TEM thin foils were jet-polished and then observed on a FEI G2 F30 trans mission electron microscope at 300 kV. The composi tions of decomposed phases in Alnico8 alloy treated under different conditions were analyzed by HAADF STEM. The hyperfine structure of Alnico8 alloy treated under different Hext. intensities was also obtained €ssbauer spectrometry. Detailed experimental processing was by Mo described in detail in a previous study [19]. The magnetic properties of the samples were measured on NIM-200 magnetic properties test system at room temperature. 3. Results and discussion 3.1. XRD patterns Fig. 1. XRD patterns with 2θ ω scans for the Alnico8 alloy treated under different conditions (solid-solution state, isothermally treated at 830 � C for 10 min under 0T, 0.7T, 10T, respectively).
Fig. 1 shows 2θ-ω scan results of the Alnico8 alloy treated under different conditions. There are mainly three peaks for the alloy between 20� and 90� , the bcc (100), (110), and (200) diffraction peak, respec tively. After the alloy was solid solution treated at 1250 � C for 40 min, peaks were indexed to bcc α phase, and the percentage of peak area for the (110) diffraction peak was about 25.8%. While the alloy was isothermally treated at 830 � C for 10 min under different Hext. in tensities, the percentage of peak area for the (110) diffraction peak was decreased as the Hext. intensities increasing. The percentage of peak area for the (110) diffraction peak was about 10.5%, 3.6%, 3.4%, as the alloy treated under 0T, 0.7T, 10T, respectively. At same time, the (200) diffraction peak was slightly changed for the alloy treated under different conditions. The (200) diffraction peak was at 64.1 � 0.1� for the alloy with solid-solution state. When the alloy treated under 0T, 0.7T, 10T, the (200) diffraction peak was at 64.4 � 0.1� , 64.7 � 0.1� , 64.7 � 0.1� , respectively. When the alloy was isothermally treated at 830 � C, the α phase was spinodally decomposed into α1 phase and α2 phase, and the α1 phase would grow along the orientation of <001>. If there was an external magnetic field during spinodal decomposition, the
including zero magnetic field, low magnetic field (0.7 T) and high magnetic field (10 T). The morphology and compositions of spinodal structure were observed and measured by TEM and HAADF-STEM. The €ssbauer hyperfine structure of Alnico8 alloy was also investigated by Mo spectrometry. The relationship among the structure, composition and magnetic properties of the Alnico8 alloy was discussed in this work. 2. Experimental The compositions and local structure of decomposed phases in Alnico8 alloy (Fe-34.2Co-14.6Ni–7Al-6.1Ti-2.7Cu, wt%) treated under different Hext conditions were systemically studied. The Alnico8 alloy was isotropic crystal. Flake-like samples of 12 � 10 � 4 mm3 cut from the bulk were solid solution treated at 1250 � C for 40 min, and then quenched into iced brine. The same parameters of isothermal ageing at
Fig. 2. Bright-field TEM images and HAADF-STEM images of Alnico8 alloy treated at 830 � C for 3 and 10 min under different Hext (ac) TEM images of the alloy treated at 830 � C for 3 min under 0 T, 0.7 T and 10 T respectively; (d), (e), (f) TEM images of the alloy treated at 830 � C for 10 min under 0 T, 0.7 T and 10 T respectively [the inserted figure in (e) and (f) was perpendicular to the Hext]; the scale bar is 100 nm. 2
X.Y. Sun et al.
Intermetallics 119 (2020) 106691
Fig. 3. HAADF STEM images and the line scanning results of Alnico8 alloy treated at 830 � C for 10 min under different Hext (a, b) 0 T; (c, d) 0.7 T, ?Hext; (d, e) 10 T, ?Hext; Scale bar is 20 nm.
external magnetic field would make the atoms diffuse faster. Therefore, the percentage of peak area for the (100) and (200) diffraction peaks would be increased for the alloy treated under an external magnetic field. The difference between two phase composition was also larger for the alloy treated under an external magnetic field, which could lead to the (200) diffraction peak shifted.
Fig. 2 shows the evolution of modulated structure depending on external magnetic field intensity and heat-treatment duration. The observed crystal directions were all the same as the orientation of <001>, and the corresponding selected area electron diffraction (SAED) patterns was shown as the inserted figure in Fig. 2(a). Compared to the morphology of the decomposed particles for the Alnico8 alloy treated at 830 � C for 10 min, it was not clear for that of the Alnico8 alloy treated for 3 min no matter how much about the external magnetic field intensities. The contrast of the TEM image was mainly contributed by the composition difference. It was due to the difference of the compositioin between two phases increased as the time extended, so the interface between two
3.2. Spinodal structure Alnico8 alloy was treated at temperature of 830 � C for 3 and 10 min under different external magnetic field intensities (0 T, 0.7 T, 10 T). 3
X.Y. Sun et al.
Intermetallics 119 (2020) 106691
areas correspond to the average Z smaller. The HAADF STEM image indicates that the positions at which heavy metal atoms richly gathered were the bright regions, such as Ni and Cu. Therefore, the decomposed particles (α1 phase) were shown as the dark regions, while the bright regions were the matrixes. Which was obviously different from that of the finally aged alloys, the HAADF STEM image of α1 phase was the bright regions in Alnico 5–7, Alnico 8 and Alnico 9 [7]. It also illustrated that the composition diffusion greatly occurred during the annealing processing after the TMT stage. When the Alnico8 alloy treated without magnetic field, the HAADF STEM image was shown as Fig. 3(a), the line scanning results of the elements distribution was shown as Fig. 3(b). The Fe was mainly distributed in the α1 phase, but the Co was not obviously different in the two phases. While the Ni, Al, Cu and Ti were concentrated in the α2 phase. As the alloy treated under an external magnetic field, the element distribution was changed a little. The difference of Fe content between the two phases was increased with a Hext. The Ni content in the α2 phase was also increased under an external magnetic field. There were some oscillations for the Co distribution between two phases under the 10 T magnetic field (shown as Fig. 3 f), compared to the results under the 0.7 T magnetic field (shown as Fig. 3 d) or zero magnetic field (shown as Fig. 3 b). The composition of the alloy in the different locations were shown as Table 1 (the point analysis results), which was corresponding to the results of line scanning. The Fe was mainly distributed in the α1 phase. The Co content in the α1 phase was lower than that in the α2 phase, and the Co content in the α1 phase of the alloy treated without magnetic field was higher than that of the alloy treated with a magnetic field.
Table 1 The compositions of different locations (locations marked as Fig.2) in Alnico8 alloy treated at 830 � C for 10 min under different Hext conditions (wt%). Position 0T 0.7T 10T
1 2 3 1 2 3 1 2 3
Fe
Co
Al
Ni
Ti
Cu
53.17 36.17 23.41 59.95 40.35 19.73 58.71 38.5 26.76
34.66 41.27 41.25 31.3 31.91 40.73 32.08 39.46 41.78
1.89 4.72 9.13 2.32 7.98 11.68 1.6 3.43 5.02
5.89 8.41 11.87 1.11 8.47 9.44 2.85 12 13.6
2.77 6.76 9.88 0.45 3.35 11.95 1.81 4.54 9.03
1.59 2.64 4.41 4.84 7.94 6.44 2.04 2.04 3.81
phases became clearer as the time kept for longer. The average diameter of the Fe–Co rich phase particles (α1 phase) decreased as increasing external magnetic field intensities. It was about 29 nm, 26 nm and 20 nm treated at 830 � C for 10 min under 0 T, 0.7 T and 10 T, respectively. When the intensity of external magnetic field was 0.7 T, the α1 phase particles were anisotropic, they were mainly round shape perpendicular to the Hext, as shown in the inserted figure of Fig. 2e. It illustrated that the shape of α1 phase particles was mainly rodshape, but the aspect ratio of α1 phase particles was less than 2.0. When the intensity of external magnetic field increased to 10 T, the decom posed particles were obviously connected parallel to the direction of the external magnetic field, and as well many particles perpendicular to the Hext were also connected together to become laminae structure (shown in Fig. 2 f). It indicated that the connection between α1 phase particles in the alloy treated under high magnetic field might be much more shapes, besides rod-shaped, and Π-shaped, H-shaped and Ш-shaped geometries. The diameter of α1 phase particles was the smallest, but its aspect ratio (about 4.2) was obviously the largest. The volume fraction of the α1 phase particles was also the largest among three conditions of the external magnetic field. The reason is that the magnetic field makes the atoms diffuse faster parallel to the direction of the external magnetic field than that perpendicular to the direction of the external magnetic field [18]. In a word, the external magnetic field mainly affected the diffusion of the atoms and increased the anisotropy of α1 phase particles in the Alnico8 alloy.
3.4. M€ ossbauer spectra €ssbauer spectrometry results of the Alnico8 alloy treated at 830 � C Mo €ssbauer for 10 min under different Hext were shown as Fig. 4. All the Mo spectrums gotten under different Hext were magnetic six-line spectrums, and there was no paramagnetic peak in the spectrums. It indicated that the two phases were both ferromagnetic. The width between the 1st and 6th peak under 0 T was slightly larger than that of 0.7 T or 10 T. It illustrated that the local structure of the alloy was different, the amount of Fe and Co atoms surrounding nearest neighbor the Fe atom in the alloy treated without magnetic field were more than that with an external magnetic field. The average hyperfine field
3.3. Compositions of spinodal phases The HAADF STEM images and the line scanning results of Alnico8 alloy at 830 � C for 10 min under different external magnetic field in tensities were shown as Fig. 3. The image (contrast) intensity of a HAADF STEM pattern is proportional to Z2 (Z is the atomic number); thus the bright areas correspond to the average Z larger, while the dark
Fig. 4. M€ ossbauer spectrometry results of Alnico8 alloy treated at 830 � C for 10 min under different Hext. (a) M€ ossbauer spectrums; (b) Hyperfine field distribution. 4
X.Y. Sun et al.
Intermetallics 119 (2020) 106691
properties changed with the external magnetic field intensities, espe cially for the Hc. The holding time mainly affected the difference of composition between two phases under zero magnetic field. While the external magnetic field intensities affected not just the difference of composition between two phases, but also the anisotropy of the α1 phase. The covercivitiy of the alloy was decided not only by the differ ence of composition between two phases. While the shape, spacing, size, volume fraction, arrangements, and branching types of the magnetic α1 phase in the Ni–Al rich matrix all affected the coercivity, but aside from the packing fraction, the most important features were the shapes of the α1 phase ends or tips and interactions between them [20–22]. The shapes of the α1 phase in the alloy treated under high magnetic field were not a single rod-shape, much more Π-shaped, H-shaped, Ш-shaped geometries. Though the shapes of α1 phase differently contributed to the coercivity of the alloy, the shapes of Π-shaped, H-shaped, Ш-shaped geometries were not benefited to the coercivity of alloy [20]. As the results measured by HAADF STEM, the difference of composition be tween two phases under a 10T magnetic field was more benifited to the covercivities than that under a 0.7 T magnetic field, according to the Stoner-Wohlfarth mechanism [21]. Although the difference of compo sition between two phases in the alloy treated without magnetic field, by contrast, was good to the covercivities, the α1 phase particles were isotropic, which was not benifited to the covercivities. In a way, the
Table 2 Average hyperfine field
0T
0.7 T
10 T
301.3
292.0
296.1
intensity of hyperfine field was about 330.0 kOe, the Fe atom was mainly surrounded by the Fe atoms. While the intensity of hyperfine field was over 330.0 kOe, there would be some Co atoms located nearby the Fe atoms. If the area of the P(H) distribution in high field was more, there were more Co atoms surrounding nearby the Fe atoms. These re sults were corresponding to the composition of the spinodal phases shown in Fig. 3 and Table .1. The Co in the α1 phase of the alloy treated without magnetic field was higher than that of the alloy treated with a magnetic field, shown in Table .1. 3.5. Magnetic properties of the alloy The evalution of magnetic properties for the Alnico8 alloy isother mally treated at 830 � C under different external magnetic field was shown as Fig. 5 it was obviously that the magnetic properties become better with the time kept longer, which was similar to the magnetic
Fig. 5. The evalution of magnetic properties in Alnico8 alloy isothermally treated at 830 � C under different external magnetic fields. (a) Br; (b) Hc; (c) (BH)max; (d) Ms. 5
X.Y. Sun et al.
Intermetallics 119 (2020) 106691
covercivities of the alloy treated under an external magnetic field was mainly contributed by the anisotropy of the α1 phase(besides the aspect ratio of the α1 phase) and the composition difference between two phases. In other word, the magnetic properties of the alloy isothermally treated at 830 � C were obviously improved by the high magnetic field, especially for the coercivity. When the alloy treated at 830 � C under a 10T magnetic field, Br, Hc and (BH)max were about 1.09T, 396Oe and 9.5 kJ/m3 respectively. The Ms of the alloy treated at 830 � C under the different magnetic field was slightly changed, the Ms was the best for the samples treated at 830 � C for different time under 10T. while the isothermal time was 3 min or 5 min, the Ms was increased a little with the intensity of magnetic field increasing, shown as Fig. 5 d. When the isothermal time was extended to 10 min, the Ms was almost the same under the different magnetic field. The change might be due to the magnetic field accelerating the diffusion of atoms [18]. The magnetic field also enhanced the interaction of atoms, especially for the Fe and Co atoms. When the isothermal time was prolonged further, the composition of two phases would reach to the equilibrium. The change of Ms was indicated the composition changing of two phases in the alloy treated under the different magnetic field.
National Basic Research Program of China (2012CB934102). X.Y. Sun was supported by Lab Foundation of National Key Laboratory for Pre cision Hot Processing of Metals. We wish to thank G. Q. Chen at Dalian University of Technology for his kind permission to use thermomagnetic treatment equipment. M.L. Liu at Jilin University is €ssbauer analysis. acknowledged for her help and useful discussion on Mo Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.intermet.2019.106691. References [1] T. Mishima, Nickel-aluminum steel for permanent magnets, Stahl u. Eisen. 53 (1931) 79. [2] M. Ibrahim, L. Masisi, P. Pillay, Design of variable-flux permanent-magnet machines using alnico magnets, IEEE Trans. Ind. Appl. 51 (2015) 4482–4491. [3] Q. Xing, M.K. Miller, L. Zhou, , et al.Constantinides, M.J. Kramer, Design of variable-flux permanent-magnet machines using alnico magnets, IEEE Trans. Magn. 49 (2013) 3314–3317. [4] F. Mohseni, A. Baghizadeh, A.A.C.S. Lourenco, M.J. Pereira, et al., Interdiffusion processes in high-coercivity RF-sputtered alnico thin films on Si substrates, JOM 69 (2017) 1427–1431. [5] A. Palasyuk, E. Blomberg, R. Prozorov, et al., Advances in characterization of nonrare-earth permanent magnets: exploring commercial alnico grades 5-7 and 9, JOM 65 (2013) 862–869. [6] L. Zhou, W. Tang, L.Q. Ke, et al., Microstructural and magnetic property evolution with different heat-treatment conditions in an alnico alloy, Acta Mater. 133 (2017) 73–80. [7] L. Zhou, M.K. Miller, P. Lu, et al., Architecture and magnetism of alnico, Acta Mater. 74 (2014) 224–233. [8] F. Zhu, L.V. Alvensleben, P. Haasen, A study of Alnico magnets by atom probe, Scr. Mater. 18 (1984) 337–342. [9] M. Fan, Y. Liu, R. Jha, et al., On the formation and evolution of Cu-Ni-rich bridges of alnico alloys with thermomagnetic treatment, IEEE Trans. Magn. 52 (2016) 2101710. [10] Z. Ahmad, Z.W. Liu, A. ul Haq Synthesis, Magnetic and microstructural properties of Alnico magnets with additives, J. Magn. Magn. Mater. 428 (2017) 125–131. [11] Y.L. Sun, J.T. Zhao, Z. Liu, et al., The phase and microstructure analysis of Alnico magnets with high coercivity, J. Magn. Magn. Mater. 379 (2015) 58–62. [12] S.M. Zhu, J.T. Zhao, W.X. Xia, et al., Magnetic structure and coercivity mechanism of AlNiCo magnets studied by electron holography, J. Alloy. Comp. 720 (2017) 401–407. [13] M. Fan, Y. Liu, R. Jha, et al., On the evolution of Cu-Ni-rich bridges of Alnico alloys with tempering, J. Magn. Magn. Mater. 420 (2016) 296–302. [14] H.M. Dillon, Effects of Heat Treatment and Processing Modifcations on Microstructure in Alnico 8H Permanent Magnet Alloys for High Temperature Applications, M.S. thesis, Iowa State Univ., Ames, IA, USA, 2014. [15] O.A. Ushakova, E.H. Dinislamova, M.V. Gorshenkov, et al., Structure and magnetic properties of Fe-Cr-Co nanocrystalline alloys for permanent magnets, J. Alloy. Comp. 586 (2014) 291–293. [16] I.E. Anderson, A.G. Kassen, E.M.H. White, et al., Development of controlled solidstate alignment for alnico permanent magnets in near-final shape, AIP Adv. 7 (2017), 056209. [17] K. Lowe, M. Durrschnabel, L. Molina-Luna, et al., Microstructure and magnetic properties of melt-spun Alnico-5 alloys, J. Magn. Magn. Mater. 407 (2016) 230–234. [18] X.Y. Sun, C.L. Chen, L. Yang, et al., Experimental study on modulated structure in Alnico alloys under high magnetic field and comparison with phase-field simulation, J. Magn. Magn. Mater. 348 (2013) 27–32. [19] X.Y. Sun, L. Zhen, C.Y. Xu, et al., M€ ossbauer spectrometry study of early stage spinodal decomposition in Fe-Cr-Co alloy under high magnetic field, Mater. Lett. 63 (2009) 64–65. [20] L.Q. Ke, R. Skomski, T.D. Hoffmann, et al., Simulation of alnico coercivity, Appl. Phys. Lett. 111 (2017), 022403. [21] S. Chikazumi, Physics of Magnetism, Wiley, New York, 1964. [22] M.C. Nguyen, X. Zhao, C.Z. Wang, K.M. Ho, Cluster expansion modeling and Monte Carlo simulation of alnico 5-7 permanent magnets, J. Appl. Phys. 117 (2015), 093905.
4. Conclusions The magnetic properties of Alnico8 alloy isothermally treated at 830 C were obviously improved by a high magnetic field of 10 T, which increased the anisotropy of the α1 phase of the alloy. The two phases in the alloy treated at 830 � C were ferromagnetic no matter how much about the external magnetic field. The composition of the two phases was affected by the external magnetic field, the Fe was mainly distrib uted in the α1 phase. The Co in the α1 phase was lower than that in the α2 phase, and the Co in the α1 phase of the alloy treated without magnetic field was higher than that of the alloy treated with a magnetic field. Which leaded the average hyperfine field of alloy treated with external magnetic field slightly deceased, compared to that of the alloy treated without magnetic field. When the alloy treated at 830 � C under a 10T magnetic field, the magnetic properties of the alloy were the best. Br, Hc and (BH)max were about 1.09T, 396Oe and 9.5 kJ/m3 respectively. �
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement X.Y. Sun: Writing - original draft, Conceptualization. C.L. Chen: Investigation. M.Y. Ma: Investigation. L. Yang: Investigation. L.X. Lv: Investigation. S. Atroshenko: Writing - original draft. W.Z. Shao: Visualization. L. Zhen: Writing - review & editing, Supervision, Writing - original draft. Acknowledgements This work was financially supported by National Key R&D Program of China (No. 2018YFB2003900), and National Natural Science Foun dation of China (No. 50901024), International/Regional Cooperation and Exchange Program of NSFC (NSFC-RFBR, No. 51211120186), and
6