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Crystallization progress and soft magnetic properties of FeGaBNbCu alloys
Accepted Manuscript Crystallization progress and soft magnetic properties of FeGaBNbCu alloys Qianke Zhu, Zhe Chen, Shuling Zhang, Qiushu Li, Yong Jiang, Peixuan Wu, Kewei Zhang PII: DOI: Reference:
S0304-8853(18)33029-4 https://doi.org/10.1016/j.jmmm.2018.11.105 MAGMA 64664
To appear in:
Journal of Magnetism and Magnetic Materials
Received Date: Revised Date: Accepted Date:
19 September 2018 18 November 2018 21 November 2018
Please cite this article as: Q. Zhu, Z. Chen, S. Zhang, Q. Li, Y. Jiang, P. Wu, K. Zhang, Crystallization progress and soft magnetic properties of FeGaBNbCu alloys, Journal of Magnetism and Magnetic Materials (2018), doi: https:// doi.org/10.1016/j.jmmm.2018.11.105
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Crystallization progress and soft magnetic properties of FeGaBNbCu alloys Qianke Zhu1, Zhe Chen1, Shuling Zhang2, Qiushu Li1, Yong Jiang1, Peixuan Wu3, Kewei Zhang1* 1 School of Materials Science and Engineering, Taiyuan University of Science and Technology, Taiyuan, China 2 School of Material of Science and Engineering, Beifang University of Nationalities, Yinchuan, China 3 Guangdong Provincial Key Laboratory of Micro-nano Manufacturing Technology and Equipment, Guangdong University of Technology, Guangzhou, China Corresponding author: Kewei Zhang School of Materials Science and Engineering Taiyuan University of Science and Technology 030024 Taiyuan, China Email: [email protected] Abstract Fe81-xGaxB15Nb3Cu1 (x=0, 3, 6, 9, 12 at. %) ribbons were fabricated by using melt-spun method and their crystallization kinetics, magnetic properties and microstructure were investigated. We found that the small quenched-in Fe-Ga clusters were formed in the as-spun ribbons with Ga addition. Besides, proper Ga addition could suppress the precipitation of Fe2B phase. During annealing, addition of Ga could provide Fe-Ga clusters to be nucleation sites for the α-Fe(Ga) phase, and could
inhibit the initial grain growth. With the increase of annealing temperature above 475 ℃, the crystallization was controlled by both Fe-Ga and Cu clusters, which effectively refined the grain size by facilitating the nucleation and increased the crystallization volume fraction. Consequently, in the ribbon with 6 at% Ga, the grains turned out to be the minimum in the mean size of 8 nm, and the optimal soft magnetic properties with coercivity at 6.8 A/m and saturation magnetization at 167 emu/g were obtained at annealing temperature of 490 ℃ for 3 min. Keywords: annealing, coercivity, nanocrystalline, Ga, nucleation sites, amorphous 1. Introduction Finemet alloys have attracted great attention due to their excellent comprehensive magnetic properties [1-4]. However, fabrication of Finemet alloys with higher saturation magnetization Ms and lower coercivity Hc is still a future development trend. According to the random anisotropy model, the soft magnetic properties of Finemet alloys can be improved by minimizing the size of grains in uniform and dense distribution in the amorphous matrix[5]. For example, addition of small insoluble atoms (e.g. Cu, Au) can facilitate the formation of heterogeneous nucleation sites for the primary crystallization while addition of large atoms (e.g. Nb, Hf, Zr) can prevent the diffusion of atoms and restrain the growth of primary crystallization[6-8]. Previous studies have suggested that the enhancement of nucleation rate and reduction of growth rate could confine grain size within about 10 nm [1,9] (less than ferromagnetic exchange length Lex) and effectively average out the magnetocrystalline anisotropy K1 and thus Hc [10]. Moreover, low saturation
magnetostriction λs can also contribute to the soft magnetic properties. In Finemet alloys, λs can be close to 0 with proper Si addition, which significantly improves the initial permeability. In all, many works [11-15] have been done for further improving the magnetic properties of the Finemet alloys by alloying different kinds of elements and achieved good magnetic properties, such as Fe73.5Si16.1B6.4Nb2.9Cu1.1 alloy [13] with Ms of 138 emu/g, Fe73.5Si7.5Ge6Nb3B9Cu1 alloy [14] with Ms of 138 emu/g. It should be noticed that high Si content can not be avoidable in this alloys for magnetic property improvement. In such case, quite amount of Fe is replaced with Si which significantly reduces the Ms although the α-Fe(Si) phase has a great contribution to Hc owing to the interaction between Fe and Si elements. Therefore, it is of great interest to explore other additives which can play the role of Si and enhance magnetic moment, giving a new Finemet-type nanocrystalline alloy with α-Fe(M) (where M is additive element) phase. Discovering proper additives would auspicate new soft-magnetic materials with high Ms and low Hc and meet the increasingly demanding in the soft-magnetic industry. Previous research works [16-17] show how Ga additions on Fe increases saturation magnetization, Ms. Therefore, by alloying Ga element and controlling annealing condition, the Finemet-type alloy with high Ms and low Hc can be obtained, in which the Si is substituted by Ga. In this work, Fe81-xGaxB15Nb3Cu1 (x=0, 3, 6, 9, 12 at. %) alloys have been fabricated and the influence of Ga content and annealing temperature on the magnetic
properties, thermal stability and microstructure of the alloys has been investigated. 2. Experimental procedure Fe81-xGaxB15Nb3Cu1 (x=0, 3, 6, 9, 12 at. %) ribbons were obtained using the melt spinning technique with the wheel speed of 29.9 m/s and then were annealed at different temperatures (from 430 to 540 ℃ ) under vacuum for 3 min. The as-spun and annealed ribbons were inspected by X-ray diffraction (XRD) (MiniFlex 600) using Cu Kα radiation (λ=1.5418 Å ) and the mean diameters for the α-Fe(Ga) crystallites in the annealed samples were estimated by line-broadening analysis. The thermal stability was determined by Differential scanning calorimetry (DSC) using a Mettler Toledo TGA/DSC3 calorimeter at heating rate of 5, 15, 25 and 35 ℃/min under N2 protection. Hysteresis loops, by which the Ms and Hc were obtained, were measured with vibrating sample magnetometer (VSM, Versalab) under an applied field of 40 kA/m. The microstructure of the annealed alloy was examined by transmission electronic microscopy (TEM, JEM 2100F). 3. Results In Fig. 1(a), the broad diffraction peaks of XRD patterns show that the ribbons with x=0 and 6 are amorphous. For the ribbons with x=3 and 12, it seems that there is small part of crystalline phase together with amorphous phase based on the relatively sharp diffraction peak. For the ribbon with x=9, obvious crystalline diffraction peaks are observed, which reveals the poor glass forming ability of the alloy. In addition, with the increase of Ga content, the broad diffraction peak near 44° shifts to lower 2θ, even for the amorphous ribbon with x=6, indicating that the small quenched-in
clusters with short-range order of Fe and Ga atoms are embedded in the amorphous phase, since the small Fe-Ga clusters with a similar structure of α-Fe(Ga) phase exhibit an expanding lattice parameter by comparison with pure α-Fe [18]. In other words, for the ribbon with x=3 and 12, the small diffraction peaks of XRD patterns indicate the relatively large size of Fe-Ga clusters that compared to the x=6 ribbon. Fig. 1(b), shows DSC curves of each alloy showing three exothermic peaks (Tx1, Tx2, Tx3) corresponding to the crystallization processes of α-Fe(Ga), Fe2B, and FeNbB phase [19], respectively. With the increase of Ga content, the first crystallization peak becomes broader which is due to inhibition of atom diffusion by Ga which increases the difficulty of initial growth of α-Fe(Ga) phase during primary crystallization [20], revealing that Ga could inhibit the diffusion of atoms. Moreover, Tx2 firstly increases significantly and then remains unchanged, which is ascribed to that the addition of Ga weakens the attractive bonding nature between Fe and B [21] since Ga is soluble to Fe and immiscible to B. As a result, the Fe element is prior involved in the precipitation of α-Fe(Ga) due to the attractive bonding nature between Fe and Ga, which rises the B content of residual amorphous phase. High B content could enhance the thermal stability of the residual amorphous phase and suppress the crystallization process. Thus, the Tx2 increases dramatically. However, with further increasing Ga content to 9 at% and 12 at%, more amorphous phase transforms into α-Fe(Ga) phase, leading to a higher ratio of B to Fe in residual amorphous phase, which results to the precipitation of Fe2B phase, without the necessity of long-range atomic rearrangements. That is why the Tx2 remains unchanged at x=9 and 12. The decrease in Tx3 with increasing Ga
content is attributed to the appropriate chemical composition of residual amorphous phase which is more easily crystallized due to the increase in Nb/Fe ratio since more Fe precipitates from amorphous phase.
Fig. 1 (a) XRD patterns and (b) DSC curves of as-spun Fe81-xGaxB15Nb3Cu1 (x=0, 3, 6, 9, 12 at. %) ribbons The endothermic peak is corresponding to the melting of ribbons. It is interesting that the melting point of main phase in ribbons increases with the addition of Ga. In such case, the poor glass forming ability is obtained due to the decrease of reduced glass transition temperature Trg (Trg =Tg/TL) (where Tg is glass transition temperature and TL is liquidus temperature of main phase) since Tg keeps almost unchanged [22]. That is why the crystalline phases is appeared in x=3 and 12 ribbons. It is worth noting that with Ga content increasing to 6 at. %, the shallow broad endothermic peak becomes deep and sharp, meaning that the composition of the alloy moves close to the eutectic point. The eutectic composition is beneficial for glass forming ability, thus leading to the amorphous microstructure of x=6 ribbon. For ribbon with x=9, the extreme broad endothermic peak indicates that there is a wide solid-liquid coexistence zone, which worsens the glass forming ability that is the reason for the crystalline
structure for the as-spun ribbon. The crystallization activation energy E1 and E2 for the first and second exothermic peak of Fe81-xGaxB15Nb3Cu1 (x=0, 3, 6, 12 at. %) ribbons are calculated by Kissinger equation [23] and the results are shown in Table 1. Clearly, with the increase of Ga content, the value of E1 increases firstly, then decreases at x=6, and finally increases again at x=12. The value of E2 increases firstly and then decreases at x=12. The results further confirm that proper addition of Ga could enhance the thermal stability of Fe2B phase. Table 1 Values of E1 and E2 of the as-spun Fe81-xGaxB15Nb3Cu1 (x=0, 3, 6, 12 at. %) ribbons. x
E1/ kJ·mol-1
E2/ kJ·mol-1
0
227
375
3
234
426
6
226
488
12
254
396
Fig. 2 XRD patterns (a, b, c) and grain sizes (d) of Fe81-xGaxB15Nb3Cu1 (x=0, 3, 6 at. %) ribbons as a function of annealing temperature. Considering the poor glass forming ability for the ribbon with x=9 and the low value of Trg for the ribbon with x=12, we further study the ribbons with x=0, 3, 6 under annealing conditions. As illustrated in Fig. 2(a), the XRD pattern shows that the ribbon with x=0 is in the process of structural relaxation until the annealing temperature rises to 475 ℃. With rising annealing temperature, the x=0 ribbon begins crystallization and the grain size increases as shown in Fig. 2(d). On the other hand, the ribbons with x=3 and 6 are crystallized after annealed at 430 ℃, which indicates that Ga addition could promote the nucleation of α-Fe(Ga) phase due to the preexisting of Fe-Ga clusters. The grain size of the ribbon with x=3 firstly increases and then decreases when the annealing temperature rises to 490 ℃, which could be due to the increase of nucleation ratio. For the ribbon with x=6, the grain size decreases to the minimum of 8 nm and then increases. To well explain the crystallization process of the annealed ribbons in this paper, the corresponding microstructure schematics are illustrated in Fig. 3. The temperature dependence of crystallization process can be divided into two stages, below and above
475 ℃. For the ribbon with x=0, the crystallization mechanism is controlled by Cu clusters which can serve as heterogeneous nucleation sites for the primary crystallization[24-25]. When the annealing temperature is higher than 475 ℃, the ‘Cu clusters mechanism’ starts to take effect. However, the ribbons with x=3 and 6 are crystallized at annealing temperature below 475 ℃ (430 ℃), which could be controlled by the Fe-Ga clusters. It can be explained that, the small quenched-in FeGa clusters that embedded in the ribbons with x=3 and 6 can also serve as heterogeneous nucleation sites, together with the bonding nature between Fe and Ga elements, making the formation of primary crystallization easier than the ribbon with x=0. When the annealing temperature exceed 475 ℃, both the Fe-Ga and Cu clusters controlled crystallization occur in the ribbons with x=3 and 6, which enhances the nucleation of α-Fe(Ga) phase. That is why the grain sizes of the ribbons with x=3 and 6 decrease dramatically at 490 ℃, as shown in Fig. 2(d). The relatively coarse grain of the ribbon with x=3 at the annealing temperature below 475 ℃ is probably because that the low Fe-Ga clusters content leads to a low nucleation rate until the ‘Cu clusters mechanism’ works. Similar behaviors have been found by other authors [26] on magnetic wires.
Fig. 3 Crystallization process and the corresponding microstructure schematics of the ribbons with x=0, 3, and 6 at different annealing stages, below and above 475 ℃. In Fig. 4(a), the Ms of as-spun ribbons increases significantly when Ga content increases from 0 to 3 at. % and then remains unchanged with further increase of x. This could be due to that the formation of Fe-Ga clusters with addition of Ga enhances the magnetic moment of the as-spun ribbons to some extent. When the annealing temperature increases but below 475 ℃, the Ms for x=0 ribbon decreases slightly with the rise of annealing temperature, but for the ribbons with x=3 and 6, the Ms increase dramatically, due to that the ribbon with x=0 is in the process of structural relaxation while the ribbons with x=3 and 6 begin crystallization. When the annealing temperature higher than 475 ℃, the Ms for x=0 increases because of the crystallization.
For the ribbon with x=3, the changes of Ms present a tendency of ‘decrease-increase’ which could result from the variation of crystallization volume fraction. In detail, when the annealing temperature is higher than 475 ℃, both the ‘Fe-Ga clusters mechanism’ and ‘Cu clusters mechanism’ increase the nucleation rate. Thus, the competitive process between nanocrystallites may diminish the crystallization volume fraction, decreasing the Ms of ribbons that annealed at 475 ℃ and 490 ℃. Then, with the rise of annealing temperature, the Ms increases again due to the grain growth which increases the crystallization volume fraction. The Ms of the x=6 ribbon keeps increasing with the rise of annealing temperature. Finally, the Ms increase to 176 and 169 emu/g for the ribbons with x=3 and 6, respectively, but only to 154 emu/g for the the x=0 ribbon which has a high content of Fe. The result can be attributed to that the ribbons with x=3 and 6 have a higher volume fraction of nanocrystalline phase compared to the ribbon without Ga content at the same annealing condition. The enhancement of magnetic moment of Fe atom by adding Ga element may also have a contribution to the high value of Ms. In Fig. 4(b), the variation of Hc for x=0, 3 and 6 with annealing temperature shows a “V” curve tendency but the corresponding annealing temperature for the minimum Hc is different. According to Hc∝D6 [3], the change in Hc is mainly attributed to the variation of grain size D with increasing annealing temperature. For the ribbon with x=0, when the annealing temperature lower than 475 ℃, the Hc decreases due to the release of internal stress. When it comes to 475 ℃, the ribbon begins crystallization. The two-phase structure with fine grain size and amorphous matrix decreases the Hc to the minimum. With rising
annealing temperature, the grain size of the x=0 ribbon increases, which deteriorates the Hc. For the ribbon with x=3, the relatively high value of Hc is owing to the coarse grains when the annealing temperature lower than 475 ℃. With the increase of annealing temperature, even though the grain size decreases, the relatively large size of pre-existing Fe-Ga clusters in x=3 ribbon induces grain size inhomogeneous, which deteriorates the soft magnetic properties, as shown in Fig. 3. That is why the Hc of the ribbon with x=3 increases dramatically with rising annealing temperature. For the ribbon with x=6, the release of internal stress and the minimal grain size decrease the Hc to the minimum at 490 ℃, where both the ‘Fe-Ga clusters mechanism’ and ‘Cu clusters mechanism’ refine the grain size by facilitating the nucleation. The Nb addition also makes important contribution to grain refining by leading to solubility limitations and kinetic slowdown of crystal growth process. The Ga element also could inhibit the growth process of α-Fe(Ga) phase as discussed in DSC curves. Finally, the minimum Hc of 6.8 A/m is obtained in the x=6 ribbon after annealed at 490 ℃, together with the Ms of 167 emu/g.
Fig. 4 Saturation magnetization (a) and coercivity (b) of ribbons as a function of annealing temperature.
In Fig. 5(a), α-Fe(Ga) nanocystals embedded in amorphous matrix are identified from the selected area diffraction pattern. Besides, the average grain size of about 8 nm is confirmed. In Fig. 5 (b), the interplanar spacing d of (110) of α-Fe(Ga) is detected which is larger than that of pure α-Fe (0.2027 nm) due to the lattice distortion. The variation of d indicates that Ga atoms randomly substitute for Fe atoms in α-Fe(Ga) lattice like A2 structure [18].
Fig. 5 selected area diffraction pattern (a) and high-resolution transmission electron microscope images (b) of the Fe75Ga6B15Nb3Cu1 ribbon annealed at 490 ℃. 4. Conclusion Fe81-xGaxB15Nb3Cu1 (x=0, 3, 6, 9, 12 at. %) ribbons have been fabricated and the effect of Ga content and annealing temperature on the crystallization kinetics, structures and magnetic properties has been investigated. Several points are concluded as follows: (1) Optimal Ga addition could suppress the precipitation of Fe2B phase but the glass forming ability of the alloys is deteriorated with the increase of Ga content. (2) After annealed, the ribbons with Ga addition precipitate α-Fe(Ga) phase with Ga atoms randomly substituting for Fe atoms. (3) With Ga addition, the quenched-in Fe-Ga clusters could serve as heterogeneous
nucleation sites for the primary crystallization during annealing. When the annealing temperature increases to 470 ℃, the nucleation is contributed by both Fe-Ga and Cu clusters. (4) The optimal soft magnetic properties with saturation magnetization of 167 emu/g and coercivity of 6.8 A/m are obtained in Fe75Ga6B15Nb3Cu1 ribbon with the mean grain size of 8 nm after annealed at 490 ℃ for 3 min. Acknowledgements This study is supported by the Program for the Top Young Academic Leaders of Higher Learning Institutions of Shanxi (2014), “131” Leading Talents Project of Higher Education Institutions in Shanxi (2015), Overseas Students Science and Technology Activities Project Merit Funding (2016), the Fund for Shanxi Key Subjects Construction and Guangdong Provincial Natural Science Foundation (No. 2018A030313246). References [1] Yoshizawa Y, Oguma S, Yamauchi K. New Fe-based soft magnetic alloys composed of ultrafine grain structure[J]. Journal of Applied Physics, 1988, 64(10):6044-6046. [2] Moya J A. Improving soft magnetic properties in FINEMET-like alloys. A study[J]. Journal of Alloys & Compounds, 2015, 622:635-639. [3] Herzer G. Grain size dependence of coercivity and permeability in nanocrystalline ferromagnents[J]. IEEE Transactions on Magnetics, 1990, 26:1397– 402.
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2002, 47(6):559-619. [21]Inoue A, Shinohara Y, Gook J S. Thermal and magnetic properties of bulk FeBased glassy alloys prepared by copper mold casting[J]. Materials Transactions Jim, 1995, 36(12):1427-1433. [22]Lu Z P, Tan H, Ng S C, et al. The correlation between reduced glass transition temperature and glass forming ability of bulk metallic glasses[J]. Scripta Materialia, 2000, 42(7):667-673. [23]Altlizar P, Valenzuela R. Avrami and Kissinger theories for crystallization of metallic amorphous alloys[J]. Materials Letters. 1991, 11(3-4):101-104. [24]Hono K, Ping D, Ohnuma M, Onodera H. Cu clustering and Si partitioning in the early crystallization stage of an Fe73.5Si13.5B9Nb3Cu1 amorphous alloy[J]. Acta Mater 1999, 47:997-1006. [25]Vázquez M, Marin P, Davies H A, Olofinjana A O. Magnetic hardening of FeSiBCuNb ribbons and wires during the first stage of crystallization to a nanophase structure[J]. Applied Physics Letters, 1994, 64(23):3184-3186. [26]Marin P, Vazquez M, Olofinjana A O, et al. Influence of the as-cast state on the crystallization process and magnetic properties for FeSiBCuNb wires[J]. IEEE Transactions on Magnetics, 1994, 30(6):4794-4796. Highlights (1) The Fe81-xGaxB15Nb3Cu1 (x=0, 3, 6, 9, 12 at. %) ribbons were fabricated and their crystallization kinetics, magnetic properties and microstructure were investigated. (2) Proper Ga addition could suppress the precipitation of Fe2B phase. (3) During annealing, addition of Ga could provide Fe-Ga clusters to be nucleation sites for the α-Fe(Ga) phase, and could inhibit the initial grain growth. (4) The crystallization was controlled by both Fe-Ga and Cu clusters at proper
annealing temperature, which effectively refined the grain size. [27]
[28]
S0304-8853(18)33029-4 https://doi.org/10.1016/j.jmmm.2018.11.105 MAGMA 64664
To appear in:
Journal of Magnetism and Magnetic Materials
Received Date: Revised Date: Accepted Date:
19 September 2018 18 November 2018 21 November 2018
Please cite this article as: Q. Zhu, Z. Chen, S. Zhang, Q. Li, Y. Jiang, P. Wu, K. Zhang, Crystallization progress and soft magnetic properties of FeGaBNbCu alloys, Journal of Magnetism and Magnetic Materials (2018), doi: https:// doi.org/10.1016/j.jmmm.2018.11.105
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Crystallization progress and soft magnetic properties of FeGaBNbCu alloys Qianke Zhu1, Zhe Chen1, Shuling Zhang2, Qiushu Li1, Yong Jiang1, Peixuan Wu3, Kewei Zhang1* 1 School of Materials Science and Engineering, Taiyuan University of Science and Technology, Taiyuan, China 2 School of Material of Science and Engineering, Beifang University of Nationalities, Yinchuan, China 3 Guangdong Provincial Key Laboratory of Micro-nano Manufacturing Technology and Equipment, Guangdong University of Technology, Guangzhou, China Corresponding author: Kewei Zhang School of Materials Science and Engineering Taiyuan University of Science and Technology 030024 Taiyuan, China Email: [email protected] Abstract Fe81-xGaxB15Nb3Cu1 (x=0, 3, 6, 9, 12 at. %) ribbons were fabricated by using melt-spun method and their crystallization kinetics, magnetic properties and microstructure were investigated. We found that the small quenched-in Fe-Ga clusters were formed in the as-spun ribbons with Ga addition. Besides, proper Ga addition could suppress the precipitation of Fe2B phase. During annealing, addition of Ga could provide Fe-Ga clusters to be nucleation sites for the α-Fe(Ga) phase, and could
inhibit the initial grain growth. With the increase of annealing temperature above 475 ℃, the crystallization was controlled by both Fe-Ga and Cu clusters, which effectively refined the grain size by facilitating the nucleation and increased the crystallization volume fraction. Consequently, in the ribbon with 6 at% Ga, the grains turned out to be the minimum in the mean size of 8 nm, and the optimal soft magnetic properties with coercivity at 6.8 A/m and saturation magnetization at 167 emu/g were obtained at annealing temperature of 490 ℃ for 3 min. Keywords: annealing, coercivity, nanocrystalline, Ga, nucleation sites, amorphous 1. Introduction Finemet alloys have attracted great attention due to their excellent comprehensive magnetic properties [1-4]. However, fabrication of Finemet alloys with higher saturation magnetization Ms and lower coercivity Hc is still a future development trend. According to the random anisotropy model, the soft magnetic properties of Finemet alloys can be improved by minimizing the size of grains in uniform and dense distribution in the amorphous matrix[5]. For example, addition of small insoluble atoms (e.g. Cu, Au) can facilitate the formation of heterogeneous nucleation sites for the primary crystallization while addition of large atoms (e.g. Nb, Hf, Zr) can prevent the diffusion of atoms and restrain the growth of primary crystallization[6-8]. Previous studies have suggested that the enhancement of nucleation rate and reduction of growth rate could confine grain size within about 10 nm [1,9] (less than ferromagnetic exchange length Lex) and effectively average out the magnetocrystalline anisotropy K1 and thus Hc [10]. Moreover, low saturation
magnetostriction λs can also contribute to the soft magnetic properties. In Finemet alloys, λs can be close to 0 with proper Si addition, which significantly improves the initial permeability. In all, many works [11-15] have been done for further improving the magnetic properties of the Finemet alloys by alloying different kinds of elements and achieved good magnetic properties, such as Fe73.5Si16.1B6.4Nb2.9Cu1.1 alloy [13] with Ms of 138 emu/g, Fe73.5Si7.5Ge6Nb3B9Cu1 alloy [14] with Ms of 138 emu/g. It should be noticed that high Si content can not be avoidable in this alloys for magnetic property improvement. In such case, quite amount of Fe is replaced with Si which significantly reduces the Ms although the α-Fe(Si) phase has a great contribution to Hc owing to the interaction between Fe and Si elements. Therefore, it is of great interest to explore other additives which can play the role of Si and enhance magnetic moment, giving a new Finemet-type nanocrystalline alloy with α-Fe(M) (where M is additive element) phase. Discovering proper additives would auspicate new soft-magnetic materials with high Ms and low Hc and meet the increasingly demanding in the soft-magnetic industry. Previous research works [16-17] show how Ga additions on Fe increases saturation magnetization, Ms. Therefore, by alloying Ga element and controlling annealing condition, the Finemet-type alloy with high Ms and low Hc can be obtained, in which the Si is substituted by Ga. In this work, Fe81-xGaxB15Nb3Cu1 (x=0, 3, 6, 9, 12 at. %) alloys have been fabricated and the influence of Ga content and annealing temperature on the magnetic
properties, thermal stability and microstructure of the alloys has been investigated. 2. Experimental procedure Fe81-xGaxB15Nb3Cu1 (x=0, 3, 6, 9, 12 at. %) ribbons were obtained using the melt spinning technique with the wheel speed of 29.9 m/s and then were annealed at different temperatures (from 430 to 540 ℃ ) under vacuum for 3 min. The as-spun and annealed ribbons were inspected by X-ray diffraction (XRD) (MiniFlex 600) using Cu Kα radiation (λ=1.5418 Å ) and the mean diameters for the α-Fe(Ga) crystallites in the annealed samples were estimated by line-broadening analysis. The thermal stability was determined by Differential scanning calorimetry (DSC) using a Mettler Toledo TGA/DSC3 calorimeter at heating rate of 5, 15, 25 and 35 ℃/min under N2 protection. Hysteresis loops, by which the Ms and Hc were obtained, were measured with vibrating sample magnetometer (VSM, Versalab) under an applied field of 40 kA/m. The microstructure of the annealed alloy was examined by transmission electronic microscopy (TEM, JEM 2100F). 3. Results In Fig. 1(a), the broad diffraction peaks of XRD patterns show that the ribbons with x=0 and 6 are amorphous. For the ribbons with x=3 and 12, it seems that there is small part of crystalline phase together with amorphous phase based on the relatively sharp diffraction peak. For the ribbon with x=9, obvious crystalline diffraction peaks are observed, which reveals the poor glass forming ability of the alloy. In addition, with the increase of Ga content, the broad diffraction peak near 44° shifts to lower 2θ, even for the amorphous ribbon with x=6, indicating that the small quenched-in
clusters with short-range order of Fe and Ga atoms are embedded in the amorphous phase, since the small Fe-Ga clusters with a similar structure of α-Fe(Ga) phase exhibit an expanding lattice parameter by comparison with pure α-Fe [18]. In other words, for the ribbon with x=3 and 12, the small diffraction peaks of XRD patterns indicate the relatively large size of Fe-Ga clusters that compared to the x=6 ribbon. Fig. 1(b), shows DSC curves of each alloy showing three exothermic peaks (Tx1, Tx2, Tx3) corresponding to the crystallization processes of α-Fe(Ga), Fe2B, and FeNbB phase [19], respectively. With the increase of Ga content, the first crystallization peak becomes broader which is due to inhibition of atom diffusion by Ga which increases the difficulty of initial growth of α-Fe(Ga) phase during primary crystallization [20], revealing that Ga could inhibit the diffusion of atoms. Moreover, Tx2 firstly increases significantly and then remains unchanged, which is ascribed to that the addition of Ga weakens the attractive bonding nature between Fe and B [21] since Ga is soluble to Fe and immiscible to B. As a result, the Fe element is prior involved in the precipitation of α-Fe(Ga) due to the attractive bonding nature between Fe and Ga, which rises the B content of residual amorphous phase. High B content could enhance the thermal stability of the residual amorphous phase and suppress the crystallization process. Thus, the Tx2 increases dramatically. However, with further increasing Ga content to 9 at% and 12 at%, more amorphous phase transforms into α-Fe(Ga) phase, leading to a higher ratio of B to Fe in residual amorphous phase, which results to the precipitation of Fe2B phase, without the necessity of long-range atomic rearrangements. That is why the Tx2 remains unchanged at x=9 and 12. The decrease in Tx3 with increasing Ga
content is attributed to the appropriate chemical composition of residual amorphous phase which is more easily crystallized due to the increase in Nb/Fe ratio since more Fe precipitates from amorphous phase.
Fig. 1 (a) XRD patterns and (b) DSC curves of as-spun Fe81-xGaxB15Nb3Cu1 (x=0, 3, 6, 9, 12 at. %) ribbons The endothermic peak is corresponding to the melting of ribbons. It is interesting that the melting point of main phase in ribbons increases with the addition of Ga. In such case, the poor glass forming ability is obtained due to the decrease of reduced glass transition temperature Trg (Trg =Tg/TL) (where Tg is glass transition temperature and TL is liquidus temperature of main phase) since Tg keeps almost unchanged [22]. That is why the crystalline phases is appeared in x=3 and 12 ribbons. It is worth noting that with Ga content increasing to 6 at. %, the shallow broad endothermic peak becomes deep and sharp, meaning that the composition of the alloy moves close to the eutectic point. The eutectic composition is beneficial for glass forming ability, thus leading to the amorphous microstructure of x=6 ribbon. For ribbon with x=9, the extreme broad endothermic peak indicates that there is a wide solid-liquid coexistence zone, which worsens the glass forming ability that is the reason for the crystalline
structure for the as-spun ribbon. The crystallization activation energy E1 and E2 for the first and second exothermic peak of Fe81-xGaxB15Nb3Cu1 (x=0, 3, 6, 12 at. %) ribbons are calculated by Kissinger equation [23] and the results are shown in Table 1. Clearly, with the increase of Ga content, the value of E1 increases firstly, then decreases at x=6, and finally increases again at x=12. The value of E2 increases firstly and then decreases at x=12. The results further confirm that proper addition of Ga could enhance the thermal stability of Fe2B phase. Table 1 Values of E1 and E2 of the as-spun Fe81-xGaxB15Nb3Cu1 (x=0, 3, 6, 12 at. %) ribbons. x
E1/ kJ·mol-1
E2/ kJ·mol-1
0
227
375
3
234
426
6
226
488
12
254
396
Fig. 2 XRD patterns (a, b, c) and grain sizes (d) of Fe81-xGaxB15Nb3Cu1 (x=0, 3, 6 at. %) ribbons as a function of annealing temperature. Considering the poor glass forming ability for the ribbon with x=9 and the low value of Trg for the ribbon with x=12, we further study the ribbons with x=0, 3, 6 under annealing conditions. As illustrated in Fig. 2(a), the XRD pattern shows that the ribbon with x=0 is in the process of structural relaxation until the annealing temperature rises to 475 ℃. With rising annealing temperature, the x=0 ribbon begins crystallization and the grain size increases as shown in Fig. 2(d). On the other hand, the ribbons with x=3 and 6 are crystallized after annealed at 430 ℃, which indicates that Ga addition could promote the nucleation of α-Fe(Ga) phase due to the preexisting of Fe-Ga clusters. The grain size of the ribbon with x=3 firstly increases and then decreases when the annealing temperature rises to 490 ℃, which could be due to the increase of nucleation ratio. For the ribbon with x=6, the grain size decreases to the minimum of 8 nm and then increases. To well explain the crystallization process of the annealed ribbons in this paper, the corresponding microstructure schematics are illustrated in Fig. 3. The temperature dependence of crystallization process can be divided into two stages, below and above
475 ℃. For the ribbon with x=0, the crystallization mechanism is controlled by Cu clusters which can serve as heterogeneous nucleation sites for the primary crystallization[24-25]. When the annealing temperature is higher than 475 ℃, the ‘Cu clusters mechanism’ starts to take effect. However, the ribbons with x=3 and 6 are crystallized at annealing temperature below 475 ℃ (430 ℃), which could be controlled by the Fe-Ga clusters. It can be explained that, the small quenched-in FeGa clusters that embedded in the ribbons with x=3 and 6 can also serve as heterogeneous nucleation sites, together with the bonding nature between Fe and Ga elements, making the formation of primary crystallization easier than the ribbon with x=0. When the annealing temperature exceed 475 ℃, both the Fe-Ga and Cu clusters controlled crystallization occur in the ribbons with x=3 and 6, which enhances the nucleation of α-Fe(Ga) phase. That is why the grain sizes of the ribbons with x=3 and 6 decrease dramatically at 490 ℃, as shown in Fig. 2(d). The relatively coarse grain of the ribbon with x=3 at the annealing temperature below 475 ℃ is probably because that the low Fe-Ga clusters content leads to a low nucleation rate until the ‘Cu clusters mechanism’ works. Similar behaviors have been found by other authors [26] on magnetic wires.
Fig. 3 Crystallization process and the corresponding microstructure schematics of the ribbons with x=0, 3, and 6 at different annealing stages, below and above 475 ℃. In Fig. 4(a), the Ms of as-spun ribbons increases significantly when Ga content increases from 0 to 3 at. % and then remains unchanged with further increase of x. This could be due to that the formation of Fe-Ga clusters with addition of Ga enhances the magnetic moment of the as-spun ribbons to some extent. When the annealing temperature increases but below 475 ℃, the Ms for x=0 ribbon decreases slightly with the rise of annealing temperature, but for the ribbons with x=3 and 6, the Ms increase dramatically, due to that the ribbon with x=0 is in the process of structural relaxation while the ribbons with x=3 and 6 begin crystallization. When the annealing temperature higher than 475 ℃, the Ms for x=0 increases because of the crystallization.
For the ribbon with x=3, the changes of Ms present a tendency of ‘decrease-increase’ which could result from the variation of crystallization volume fraction. In detail, when the annealing temperature is higher than 475 ℃, both the ‘Fe-Ga clusters mechanism’ and ‘Cu clusters mechanism’ increase the nucleation rate. Thus, the competitive process between nanocrystallites may diminish the crystallization volume fraction, decreasing the Ms of ribbons that annealed at 475 ℃ and 490 ℃. Then, with the rise of annealing temperature, the Ms increases again due to the grain growth which increases the crystallization volume fraction. The Ms of the x=6 ribbon keeps increasing with the rise of annealing temperature. Finally, the Ms increase to 176 and 169 emu/g for the ribbons with x=3 and 6, respectively, but only to 154 emu/g for the the x=0 ribbon which has a high content of Fe. The result can be attributed to that the ribbons with x=3 and 6 have a higher volume fraction of nanocrystalline phase compared to the ribbon without Ga content at the same annealing condition. The enhancement of magnetic moment of Fe atom by adding Ga element may also have a contribution to the high value of Ms. In Fig. 4(b), the variation of Hc for x=0, 3 and 6 with annealing temperature shows a “V” curve tendency but the corresponding annealing temperature for the minimum Hc is different. According to Hc∝D6 [3], the change in Hc is mainly attributed to the variation of grain size D with increasing annealing temperature. For the ribbon with x=0, when the annealing temperature lower than 475 ℃, the Hc decreases due to the release of internal stress. When it comes to 475 ℃, the ribbon begins crystallization. The two-phase structure with fine grain size and amorphous matrix decreases the Hc to the minimum. With rising
annealing temperature, the grain size of the x=0 ribbon increases, which deteriorates the Hc. For the ribbon with x=3, the relatively high value of Hc is owing to the coarse grains when the annealing temperature lower than 475 ℃. With the increase of annealing temperature, even though the grain size decreases, the relatively large size of pre-existing Fe-Ga clusters in x=3 ribbon induces grain size inhomogeneous, which deteriorates the soft magnetic properties, as shown in Fig. 3. That is why the Hc of the ribbon with x=3 increases dramatically with rising annealing temperature. For the ribbon with x=6, the release of internal stress and the minimal grain size decrease the Hc to the minimum at 490 ℃, where both the ‘Fe-Ga clusters mechanism’ and ‘Cu clusters mechanism’ refine the grain size by facilitating the nucleation. The Nb addition also makes important contribution to grain refining by leading to solubility limitations and kinetic slowdown of crystal growth process. The Ga element also could inhibit the growth process of α-Fe(Ga) phase as discussed in DSC curves. Finally, the minimum Hc of 6.8 A/m is obtained in the x=6 ribbon after annealed at 490 ℃, together with the Ms of 167 emu/g.
Fig. 4 Saturation magnetization (a) and coercivity (b) of ribbons as a function of annealing temperature.
In Fig. 5(a), α-Fe(Ga) nanocystals embedded in amorphous matrix are identified from the selected area diffraction pattern. Besides, the average grain size of about 8 nm is confirmed. In Fig. 5 (b), the interplanar spacing d of (110) of α-Fe(Ga) is detected which is larger than that of pure α-Fe (0.2027 nm) due to the lattice distortion. The variation of d indicates that Ga atoms randomly substitute for Fe atoms in α-Fe(Ga) lattice like A2 structure [18].
Fig. 5 selected area diffraction pattern (a) and high-resolution transmission electron microscope images (b) of the Fe75Ga6B15Nb3Cu1 ribbon annealed at 490 ℃. 4. Conclusion Fe81-xGaxB15Nb3Cu1 (x=0, 3, 6, 9, 12 at. %) ribbons have been fabricated and the effect of Ga content and annealing temperature on the crystallization kinetics, structures and magnetic properties has been investigated. Several points are concluded as follows: (1) Optimal Ga addition could suppress the precipitation of Fe2B phase but the glass forming ability of the alloys is deteriorated with the increase of Ga content. (2) After annealed, the ribbons with Ga addition precipitate α-Fe(Ga) phase with Ga atoms randomly substituting for Fe atoms. (3) With Ga addition, the quenched-in Fe-Ga clusters could serve as heterogeneous
nucleation sites for the primary crystallization during annealing. When the annealing temperature increases to 470 ℃, the nucleation is contributed by both Fe-Ga and Cu clusters. (4) The optimal soft magnetic properties with saturation magnetization of 167 emu/g and coercivity of 6.8 A/m are obtained in Fe75Ga6B15Nb3Cu1 ribbon with the mean grain size of 8 nm after annealed at 490 ℃ for 3 min. Acknowledgements This study is supported by the Program for the Top Young Academic Leaders of Higher Learning Institutions of Shanxi (2014), “131” Leading Talents Project of Higher Education Institutions in Shanxi (2015), Overseas Students Science and Technology Activities Project Merit Funding (2016), the Fund for Shanxi Key Subjects Construction and Guangdong Provincial Natural Science Foundation (No. 2018A030313246). References [1] Yoshizawa Y, Oguma S, Yamauchi K. New Fe-based soft magnetic alloys composed of ultrafine grain structure[J]. Journal of Applied Physics, 1988, 64(10):6044-6046. [2] Moya J A. Improving soft magnetic properties in FINEMET-like alloys. A study[J]. Journal of Alloys & Compounds, 2015, 622:635-639. [3] Herzer G. Grain size dependence of coercivity and permeability in nanocrystalline ferromagnents[J]. IEEE Transactions on Magnetics, 1990, 26:1397– 402.
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annealing temperature, which effectively refined the grain size. [27]
[28]