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The effect of the cobalt-content on the magnetic properties of iron-based amorphous alloys
Journal of Magnetism and Magnetic Materials 477 (2019) 214–219
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Research articles
The effect of the cobalt-content on the magnetic properties of iron-based amorphous alloys
T
⁎
Marcin Nabiałek, Bartłomiej Jeż , Katarzyna Błoch, Paweł Pietrusiewicz, Joanna Gondro Institute of Physics, Faculty of Production Engineering and Materials Technology, Czestochowa University of Technology, 19 Armii Krajowej, 42-200 Częstochowa, Poland
A R T I C LE I N FO
A B S T R A C T
Keywords: Bulk amorphous materials Rapid-solidification Coercive field Curie temperature
This paper describes the results of studies carried out on bulk amorphous materials with the chemical compositions: Fe61+xCo10−xY8W1B20 (where x = 0, 1 or 2). The alloy samples were produced in the form of plates with the following dimensions: 10 mm × 5 mm × 0.5 mm. Massive amorphous glass structures were produced by two methods: injection and suction casting. On the basis of the recorded X-ray diffraction patterns, only the disordered phase was identified within the volume of each alloy. The only exception was the alloy with the chemical composition: Fe62Co9Y8W1B20, produced by the injection method, in the volume of which an (insignificant) trace quantity of the Fe5Y phase was registered. The presence of this phase explains the lower value of the coercive field compared to the other alloys. The investigations confirmed the effect of the cobalt content on the coercive field value, and the Curie temperature, of the amorphous alloys. However, the cobalt content was not found to have a significant influence on the saturation of magnetisation. Also, the method of producing the investigated amorphous materials does not seem to have a significant effect on the magnetic properties of the resulting alloys.
1. Introduction Engineering materials are subject to rapid development, influenced by the increasing specialisation of materials that are applied to specific tasks. These types of materials are known as: ‘functional materials’. One of the families of functional materials is that of the so-called ‘soft magnetic materials’. Such materials are characterized by a low coercive field value (of less than 1000 A/m) and a high saturation magnetisation [1–7]. Representative examples of the soft magnetic materials are, amongst others, the amorphous and nanocrystalline alloys based on iron [8–11]. The first amorphous materials were produced in the form of thin layers of ‘tapes’. The latter production method involves the rapid cooling of the liquid alloy on a rotating thermally conductive cylinder. On contact with the surface of the copper cylinder, the alloy solidifies in the form of a strip with a thickness of a few tens of micrometres. The cooling rate obtained when casting alloys by this method can reach 106 K/s [12,13]. This cooling rate ensures that an amorphous structure is obtained for many different chemical compositions. Tape alloys have very limited applications due to their dimensions. The search for new methods of producing amorphous materials led to the creation of a new family of materials: massive amorphous materials. A great contribution to the development of this group of materials was made by A. Inoue. ⁎
Due to a significant increase in the minimum dimensions of alloys obtainable whilst maintaining good magnetic properties, the application possibilities of amorphous materials have been significantly extended [14–21]. Currently, there are many methods for producing amorphous materials; the achieved cooling rates differ. The methods facilitating the production of massive amorphous alloys are of particular interest, examples of which are the methods known as: ‘suction casting’ and ‘injection casting’ [22,23]. This paper compares the structural and magnetic properties of massive iron-based amorphous materials, produced by two production methods that are characterized by similar cooling rates: the injection method and the suction method. During the production of the high-melt alloys, the same moulds were used to cast the alloy and the same level of vacuum was created in the working chambers. In both cases, the production process was carried out under a protective atmosphere of argon; the same level of negative pressure prevailing in the working chambers was used. The aim of the work was to ascertain the effect of cobalt content and the method of production on the probability of creating an amorphous structure for the proposed chemical compositions – using identical parameters for both methods. Further, to examine the impact of the production method on the magnetic properties of the resulting alloys produced.
Corresponding author. E-mail address: [email protected] (B. Jeż).
https://doi.org/10.1016/j.jmmm.2019.01.073 Received 5 December 2018; Received in revised form 15 January 2019; Accepted 18 January 2019 Available online 22 January 2019 0304-8853/ © 2019 Published by Elsevier B.V.
Journal of Magnetism and Magnetic Materials 477 (2019) 214–219
M. Nabiałek et al.
2. Materials and methods
reaching the proper consistency of the alloy, the suction valve was released. The molten alloy was then drawn into the mould. The moulds used for the rapid cooling of these alloys were made of copper due to the thermal conductivity of this material. The copper moulds were water-cooled. The batch solidified in the mould at a relatively low cooling rate (of the order of 103 K/s), which, however, allowed the formation of an amorphous structure. The production of the hightemperature melt was carried out under an inert gas atmosphere after the chamber has been evacuated. In the presented solution, it was possible to use a mould that allowed the simultaneous execution of several castings. The second method used to produce the investigated massive amorphous materials was the injection casting method. Rapid cooling of the alloy was carried out using a copper mould placed in a copper block that was cooled by a continuous stream of water. The ingot was placed in a quartz capillary, which in turn was mounted to the working chamber of the device. After sealing the chamber, the air was pumped out. The small size of the chamber allowed a satisfactory level of vacuum to be obtained relatively quickly. The working chamber was subjected to a purging process with pure argon – in the same way as in the production of a polycrystalline ingot. After the vacuum had been restored, argon was introduced into the chamber in the atmosphere where the melting process was carried out. The ingot located in the quartz capillary was melted by means of eddy current heating. After obtaining the appropriate consistency of the alloy, a valve was opened to allow the liquid alloy to be injected into the water-cooled copper mould. The method achieved a cooling rate of 103 K/s, which was a sufficient cooling rate for iron- and boron-based alloys. The alloy was obtained in the form of rectangular plates of various dimensions; in the described case: 5 mm wide, 10 mm long and 0.5 mm thick. The structure of the obtained material was examined using a Bruker Advanced 8 X-ray diffractometer. Diffraction measurements were made for the particulate material. The diffractometer was equipped with a CuKα lamp and a semiconductor counter. The structure was tested for an angle of 2-Θ from 30° to 100°, irradiating the sample for 7 s per measuring step, at a resolution of 0.02°. The magnetic properties of the rapidly-cooled alloy were tested on a Faraday magnetic balance and a vibration magnetometer. The magnetic balance allowed measurements at a temperature of up to 850 K. The obtained results made it possible to assess the thermal stability of the magnetic properties and to determine the Curie temperature of the alloys. Measurements using the Lake Shore vibration magnetometer were carried out in an external magnetic field of up to 1.7 T. The obtained results were used to determine the saturation magnetisation and the
On the basis of available specialist literature and the Author’s own research, the chemical composition of the alloy was developed. The proposed composition: Fe61+xCo10−xY8W1B20 (where x = 0, 1 or 2) complies with the criteria established by A. Inoue. The batch material was weighed with an accuracy of 0.001 g. The elements were characterized by a high degree of purity of greater than 99.99%. Five gram batches were weighed to produce each polycrystalline ingot. The ingots were made in an arc furnace. The process of melting the charge was undertaken under a high vacuum, achieved by evacuating the working chamber using a pumping system. The vacuum in the working chamber attained a pressure value of 5 × 10−5 mbar; this provided a significant reduction in the presence of oxygen during the melting of the ingot. In order to improve the purity of the atmosphere before the melting process of the ingot, the chamber was flushed with argon. After the high vacuum was restored, argon was pumped into the working chamber–into the atmosphere where the ingot was melted. The batch was placed on a water-cooled copper plate. The disc also has space for a titanium ingot – its re-melting before the alloy ingot was produced improved the purity of the chamber due to the absorption of residual oxygen. Melting of the charge took place using a plasma arc, fired inside the working chamber by means of two electrodes. The arc temperature was regulated by changing the current flowing through the electrode. The first melting of the charge took place at a slower rate, with less current, so as to avoid scattering the alloy components inside the working chamber. Subsequent re-melting was carried out at a higher temperature after the ingot was first produced. Use of an appropriately high temperature and manoeuvring of the ingot ensured high homogeneity of the chemical composition of the ingot. Usually, a satisfactory level of homogeneity of the ingot was obtained after it had been re-melted five times – each time following the smelting of pure titanium. Ingots prepared in this way were subjected to eradication of external impurities and then were crushed into smaller pieces. The polycrystalline ingots thus obtained were used for the production of high-temperature alloys using the suction casting method and the injection casting method. The alloy was obtained in the form of rectangular tiles with the following dimensions: 10 mm × 5 mm × 0.5 mm. The suction method allows the production of an amorphous alloy with dimensions significantly exceeding the thicknesses obtained for amorphous tapes. A diagram of the suction method is shown in Fig. 1. The polycrystalline ingot was melted using an electric arc. Before melting the ingot, in order to get rid of the remaining oxygen and impurities from the working chamber, pure titanium was melted. After
Fig. 1. Schematic diagrams of the apparatus for the production of massive amorphous materials by the suction casting method: a) top view, b) side view. 215
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Fig. 2. XRD diffractograms for the alloy in powder form, made from plates of thickness 0.5 mm: a) and d) Fe61Co10Y8W1B20, b) and e) Fe62Co9Y8W1B20, c) and f) Fe63Co8Y8W1B20.
coercivity field of the investigated high-temperature alloys.
amorphous structure. A very important parameter, often decisive for the applicability of magnetic materials and in particular for the construction of cores, is the Curie temperature – which determines the working temperature range of a ferromagnetic material. Taking advantage of the fact that the manufactured ferromagnetic materials met the Heisenberg assumptions, a critical determinant β, equal to 0.36 was used to calculate the Curie temperature. All of the manufactured alloys were found to be characterized by high thermomagnetic stability. This is indicated by the similar waveforms of magnetic saturation polarization, measured in the direction of increasing and decreasing temperature. The curves obtained for the various chemical compositions are similar to each other. Within the tested temperature range, the X-ray diffraction phase of the crystalline phase – identified in the image – cannot be revealed because the Curie temperature of this phase is outside the measurement range. The Curie temperature of the alloys was determined, based on the critical factor β equal to 0.36. Fig. 4 contains (μ0Ms)^(1/β) curves for the investigated samples. The Curie temperatures of the alloys that were produced by the injection method are a few Kelvin lower than for the samples made by the suction method. Table 1 presents the determined Curie temperature values for the amorphous alloys produced by both methods. As expected, a reduction in the cobalt content of the alloy reduces the Curie temperature. This relationship is visible both for the injection casting and suction casting methods. An interesting phenomenon is the differences in the Curie temperature values for the same chemical compositions of alloys produced by various methods. The suction cast alloys show a slightly higher Curie temperature in each case studied. Fig. 5 presents static magnetic hysteresis loops that were obtained for samples of the tested alloys. Analysing the course of the static magnetic hysteresis loops, it can be said that they are similar to each other, especially for the alloys: Fe61Co10Y8W1B20 and Fe62Co9Y8W1B20. In the case of the Fe63Co8Y8W1B20 alloy, produced using the two different methods, it can be concluded that both of the static magnetic hysteresis loops have the so-called ‘wasp-like’ shape [25]. The visible widening of the static magnetic hysteresis loop in the ‘approach to ferromagnetic saturation’ area is related to a change of the magnetic structure and the formation of a hard magnetic phase. The presence of this phase in the initial stage of its growth (i.e. with the residual volume of the sample tested) does not make a significant contribution to the value of the coercive field. However, despite this, the Fe63Co8Y8W1B20 alloy is still classified within the group of soft magnetic alloys [26]. Compared to the other investigated alloys, the value of the coercive field for the Fe63Co8Y8W1B20 alloy increased by several times. A slight change in the proportion of iron and cobalt within the amorphous alloy
3. Results Fig. 2 presents the measured X-ray diffractograms for the investigated rapidly-cooled alloys in powder form. The recorded X-ray diffraction images were found to be similar for the investigated injection and suction cast alloy samples. However, in the case of the injection cast Fe62Co9Y8W1B20 alloy sample, diffraction reflections, associated with the existence of longrange ordering between atoms within the volume of the tested alloy, were visible near the 2-theta angles of 44 and 50°. Based on analysis using the COD database (via Match! software) it was shown that these reflections originated from the crystalline phase of Fe5Y. Until recently, this phase was considered hypothetical. The Fe5Y phase is formed by the disintegration of the high temperature Fe17Y2 phase. This decay occurs at a temperature of 1173 K; the second product of decomposition is the α Fe phase [24]. Although the conducted tests do not confirm the existence of Fe5Y in the produced samples, due to the resolution of the X-ray apparatus, its residual presence in the volume of the Fe62Co9Y8W1B20 alloy cannot be ruled out. On the remaining diffraction patterns (Fig. 2a, c, d, e, f) only a single, wide, fuzzy diffraction maximum is observed – typical for materials in which there is no long-range ordering between atoms. This maximum occurs within the range of 2-theta angle of: 35–55°, which may suggest that crystallographic systems in which the main component is iron will be preferred in the atomic systems that form. The structure of the investigated alloys was also examined by an indirect method, using a Faraday magnetic balance. This test gave the opportunity for analysis of the structural quality within the range of up to 850 K. Additionally, when analysing the measurements of magnetic polarization curves, as a function of temperature, the intensity of the transition from the ferromagnetic state to the paramagnetic state can be determined. Fig. 3 presents reduced magnetic saturation polarization curves, as a function of temperature, within a homogeneous magnetic field with an intensity of 0.7 T. These curves were measured in two temperature directions (room temperature – 850 K and 850 K – room temperature). Heating of the bifilarwound coil of the resistance furnace was tapped with a measuring card at 10 K/s, whereas cooling of the system was carried out by gravity. All curves (μ0Ms)^(1/β) were found to be similar in both the heating and cooling processes. However, it should be noted that, during the heating of the samples, the material was partially relaxed, which affects the small change in the return curve (μ0Ms)^(1/β). Based on analysis of the curves shown in Fig. 3, it can be concluded that, in the tested samples up to 850 K, there is one magnetic phase coming from the 216
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Fig. 3. Magnetic saturation polarization as a function of temperature: a) and d) Fe61Co10Y8W1B20, b) and e) Fe62Co9Y8W1B20, c) and f) Fe63Co8Y8W1B20.
is of great importance to the value of the coercive field. In the case of these injection cast or suction cast alloys, an increase in the iron content – at the expense of the cobalt content-yields a concomitant increase in the value of the coercive field. The exception to this rule was an alloy sample Fe62Co9Y8W1B20 which was made by pressing, for which the coercive field value of 33 A/m was determined. At this point, reference should be made to Fig. 2b in which narrow peaks from crystalline
phases are visible, which affect the lowering of the value of the coercive field [27]. According to the literature, the improvement of magnetic properties in amorphous alloys can be obtained by partial nanocrystallization, as a result of thermal processing [28]. Another method of nanocrystallization can be one-step nanocrystallization-designed during the very production of samples [29]. In the case of fine nanocrystalline grains smaller than 50 nm in iron-based alloys, the flow of domain walls
Fig. 4. Curie temperatures for the investigated alloys: a) and d) Fe61Co10Y8W1B20, b) and e) Fe62Co9Y8W1B20, c) and f) Fe63Co8Y8W1B20. 217
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Table 1 Curie temperature for the manufactured alloys. Method Alloy
Injection [9]
Table 2 Hc, Ms and Keff parameters, determined for the alloys with chemical composition: Fe61+xCo10−XY8W1B20 (where: x = 0, 1 or 2), produced by the injection or suction casting methods.
Suction
Curie temperature [K] Fe61Co10Y8W1B20 Fe62Co9Y8W1B20 Fe63Co8Y8W1B20
561 549 541
568 557 545
within the sample volume occurs without blocking them, which significantly reduces the losses from the hysteresis loop. This means that, for such an alloy, the surface of the hysteresis loop at the beginning of the M-H system will be smaller, which is consistent with the obtained result (Fig. 5b). In contrast, the magnetization of all alloys obtained,
Alloy
Method
Saturation magnetization [T]
Coercive field [A/m]
Keff [kJ/ m3]
Fe61Co10Y8W1B20
injection suction
1.11 1.14
71 61
78.7 77.6
Fe62Co9Y8W1B20
injection suction
1.09 1.21
33 159
75.9 75.7
Fe63Co8Y8W1B20
injection suction
1.10 1.13
241 223
74.1 73.3
Fig. 5. Static magnetic hysteresis loops for the alloy produced by the injection method: a) Fe61Co10Y8W1B20, b) Fe62Co9Y8W1B20, c) Fe63Co8Y8W1B20 and suction method: d) Fe61Co10Y8W1B20 e) Fe62Co9Y8W1B20, f) Fe63Co8Y8W1B20. 218
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regardless of the method of manufacture, is relatively high and amounts to over 1 T. The results from analysis of static magnetic hysteresis loops and primary magnetisation curves, i.e. Hc, Ms and Keff, are presented in Table 2.
169–175. [4] M.E. McHenry, M.A. Willard, D.E. Laughlin, Amorphous and nanocrystalline materials for applications as soft magnets, Prog. Mater. Sci. 44 (1999) 291–433. [5] F. Wang, A. Inoue, Y. Han, F.L. Kong, S.L. Zhu, E. Shalaan, F. Al-Marzouki, A. Obaid, Excellent soft magnetic Fe-Co-B-based amorphous alloys with extremely high saturation magnetization above 1.85 T and low coercivity below 3 A/m, J. Alloys Compd. 711 (2017) 132–142. [6] Y. Geng, Y. Wang, Z. Wang, J. Qiang, H. Wang, C. Dong, O. Tegus, Formation and structure-property correlation of new bulk Fe–B–Si–Hf metallic glasses, Mater. Des. 106 (2016) 69–73. [7] K. Gruszka, Analysis of the structural-defect influence on the magnetization process in and above the Rayleigh region, Mater. Tehnol. 50 (5) (2016) 707–718. [8] B. Jeż, M. Nabialek, P. Pietrusiewicz, K. Gruszka, K. Błoch, J. Gondro, J. Rzącki, M.M.A.B. Abdullah, A.V. Sandu, M. Szota, K. Jeż, A. Sałagacki, The structure and properties of rapid cooled iron based alloy, IOP Conf. Series: Mater. Sci. Eng. 209 (2017) 012023. [9] B. Jeż, Modification of Curie temperature in bulk amorphous alloys as a result of their annealing, Rev. Chim. 68 (8) (2017) 1903–1907. [10] R.K. Roy, A.K. Panda, A. Mitra, Effect of Co content on structure and magnetic behaviors of high induction Fe-based amorphous alloys, J. Magn. Magn. Mater. 418 (2016) 236–241. [11] J. Li, X. Wang, X. Liu, S. Zhao, K. Yao, Effect of fluxing treatment on the properties of Fe66Co15Mo1P7.5C5.5B2Si3 bulk metallic glass by water quenching, Phys. B: Phys. Condensed Matter 528 (2018) 24–26. [12] W. Klement, R.H. Willens, P. Duwez, Non-crystalline structure in solidified goldsilicon alloys, Nature 187 (1960) 869–870. [13] H.S. Chen, C.E. Miller, A rapid quenching technique for the preparation of thin uniform films of amorphous solids, Rev. Scientific Instrum. 41 (1970) 1237. [14] K. Błoch, M. Nabiałek, The influence of heat treatment on irreversible structural relaxation in bulk amorphous Fe61Co10Ti3Y6B20 alloy, Acta. Phys. Pol. A 127 (2015) 442–444. [15] A. Inoue, A. Kato, T. Zhang, S.G. Kim, T. Masumoto, Mg-Cu-Y amorphous alloys with high mechanical strengths produced by a metallic mold casting method, Mater. Trans. JIM 32 (1991) 609–616. [16] A. Inoue, T. Zhang, T. Masumoto, Zr-Al-Ni amorphous alloys with high glass transition temperature and significant supercooled liquid region, Mater. Trans. JIM 31 (1990) 177–183. [17] K. Błoch, Magnetic properties of the suction-cast bulk amorphous alloy: (Fe0.61Co0.10Zr0.025Hf0.025Ti0.02W0.02B0.20)96Y4, J. Magn. Magn. Mater. 390 (2015) 118–122. [18] M. Nabiałek, Influence of the quenching rate on the structure and magnetic properties of the Fe-based amorphous alloy, Arch. Metall. Mater. 61 (2016) 439–444. [19] W. Li, Y.Z. Yang, J. Xu, Crystallization and soft magnetic properties of metalloidfree Fe89Hf7Al3Zr1 amorphous alloy, J. Non-Crystalline Solids 461 (2017) 93–97. [20] A. Wang, C. Zhao, A. He, H. Men, C. Chang, X. Wang, Composition design of high Bs Fe-based amorphous alloys with good amorphous-forming ability, J. Alloys Compd. 656 (2016) 729–734. [21] K. Brzózka, A. Ślawska-Waniewska, P. Nowicki, K. Jezuita, Hyperfine magnetic fields in FeZrB(Cu) alloys, Mater. Sci. Eng. A (1997) 654–658. [22] D.K. Li, H.F. Zhang, A.M. Wang, Z.W. Zhu, Z.Q. Hu, Effects of Sn addition on the glass-forming ability and mechanical properties of Ni-Nb-Zr bulk metallic glasses, Chin. Sci. Bull. 56 (36) (2011) 3926–3931. [23] J.F. Wang, S. Huang, S.F. Guo, Y.Y. Wei, F.S. Pan, Effects of cooling rate on microstructure, mechanical and corrosion properties of Mg-Zn-Cu Alloy, Trans. Nonferrous Metals Soc. China 23 (2013) 1930–1935. [24] K. Gruszka, M. Nabiałek, T. Noga, Ab initio study of structure, electronic and magnetic properties of YFe5 phase compound in the DFT formalism, Arch. Metall. Mater. 61 (1) (2016) 369–374. [25] K. Błoch, M. Nabiałek, S. Garus, Structure and magnetic properties of amorphous Fe60Co10Mo2WxY8B20-x (x = 0, 1) alloys, Acta Phys. Polonica A 130 (2016) 905–908. [26] H. Liebermann, Rapidly Solidified Alloys, Springer, New Jersey, 1993. [27] A. Suzuki, A. Makino, T. Inoue, Masumoto, Soft magnetic properties of nanocrystalline bcc Fe-Zr-B and Fe-M-B-Cu (M=transition metal) alloys with high saturation magnetization (invited), J. Appl. Phys. 70 (1991) 6232–6237. [28] T. Kulik, T. Horubała, H. Matyja, Flash annealing nanocrystallization of Fe-Si-B based glasses, Mater. Sci. Eng. A 157 (1992) 107–112. [29] T. Yanai, M. Yamasaki, M. Nakano, H. Fukunaga, Y. Yoshizawa, Investigation of development process soft creep-induced anisotropy in nanocrystalline Fe73.5Cu1Nb3Si15.5B7 ribbon for mass production, Soft Magn. Mater. 10 (2003) 737–741.
4. Conclusions On the basis of the obtained results, it can be concluded that ironbased alloys with a significant content of cobalt and boron are characterized by a good glass-transition ability. In addition, the published research results have led to the following conclusions: 1. It is possible to produce, in a single-stage, an amorphous alloy with a small volume of crystalline phase featuring nanometric grain dimensions; 2. The alloys produced are characterized by good thermomagnetic stability as indicated by the similar shapes of the magnetic saturation polarization curves (as a function of temperature) of both the heating and cooling processes of samples; 3. A change in the cobalt content within the investigated amorphous alloys results in measureable effects on the Curie temperature and the value of the coercive field, while its effect on saturation of magnetisation is insignificant; 4. Amorphous alloys produced by the suction casting method are characterized by Curie temperatures that are higher by a few Kelvin, compared to the same alloys produced by injection casting; 5. The method of producing the investigated amorphous materials has been found not to exert a significant influence on their structure and properties. 6. For samples obtained regardless of the production method, effective anisotropy decreases with increasing iron content in the alloy. The resolution of the X-ray apparatus, used in this study, does not allow the detection of a small number of crystalline phases within the volume of an alloy; therefore the possibility of the presence of a small quantity of grains or nuclei of the Fe5Y crystal phase in the remaining alloys cannot be excluded. Indirectly, the presence of this phase can be explained by a decrease in the coercivity field, which is clearly visible in the case of a melt produced by pressing (Figs. 2b and 5b). It is possible to design a heat treatment regime for the amorphous alloy, in a manner allowing the Fe5Y crystal phase grains to grow to desired dimensions. Conducting heat treatment at different temperatures and heating durations gives the opportunity to obtain the lowest coercive field value for the tested chemical compositions whilst maintaining their good thermomagnetic stability and high saturation of magnetisation. References [1] G. Herzer, Modern soft magnets: amorphous and nanocrystalline materials, Acta Mater. 61 (2013) 718–734. [2] G. Herzer, Soft magnetic nanocrystalline materials, Scripta Metall. Mater. 33 (1995) 1741–1756. [3] Y. Han, C.T. Chang, S.L. Zhu, A. Inoue, D.V. Louzguine-Luzgin, E. Shalaan, F. AlMarzouki, Fe-based soft magnetic amorphous alloys with high saturation magnetization above 1.5 T and high corrosion resistance, Intermetallics 54 (2014)
219
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Research articles
The effect of the cobalt-content on the magnetic properties of iron-based amorphous alloys
T
⁎
Marcin Nabiałek, Bartłomiej Jeż , Katarzyna Błoch, Paweł Pietrusiewicz, Joanna Gondro Institute of Physics, Faculty of Production Engineering and Materials Technology, Czestochowa University of Technology, 19 Armii Krajowej, 42-200 Częstochowa, Poland
A R T I C LE I N FO
A B S T R A C T
Keywords: Bulk amorphous materials Rapid-solidification Coercive field Curie temperature
This paper describes the results of studies carried out on bulk amorphous materials with the chemical compositions: Fe61+xCo10−xY8W1B20 (where x = 0, 1 or 2). The alloy samples were produced in the form of plates with the following dimensions: 10 mm × 5 mm × 0.5 mm. Massive amorphous glass structures were produced by two methods: injection and suction casting. On the basis of the recorded X-ray diffraction patterns, only the disordered phase was identified within the volume of each alloy. The only exception was the alloy with the chemical composition: Fe62Co9Y8W1B20, produced by the injection method, in the volume of which an (insignificant) trace quantity of the Fe5Y phase was registered. The presence of this phase explains the lower value of the coercive field compared to the other alloys. The investigations confirmed the effect of the cobalt content on the coercive field value, and the Curie temperature, of the amorphous alloys. However, the cobalt content was not found to have a significant influence on the saturation of magnetisation. Also, the method of producing the investigated amorphous materials does not seem to have a significant effect on the magnetic properties of the resulting alloys.
1. Introduction Engineering materials are subject to rapid development, influenced by the increasing specialisation of materials that are applied to specific tasks. These types of materials are known as: ‘functional materials’. One of the families of functional materials is that of the so-called ‘soft magnetic materials’. Such materials are characterized by a low coercive field value (of less than 1000 A/m) and a high saturation magnetisation [1–7]. Representative examples of the soft magnetic materials are, amongst others, the amorphous and nanocrystalline alloys based on iron [8–11]. The first amorphous materials were produced in the form of thin layers of ‘tapes’. The latter production method involves the rapid cooling of the liquid alloy on a rotating thermally conductive cylinder. On contact with the surface of the copper cylinder, the alloy solidifies in the form of a strip with a thickness of a few tens of micrometres. The cooling rate obtained when casting alloys by this method can reach 106 K/s [12,13]. This cooling rate ensures that an amorphous structure is obtained for many different chemical compositions. Tape alloys have very limited applications due to their dimensions. The search for new methods of producing amorphous materials led to the creation of a new family of materials: massive amorphous materials. A great contribution to the development of this group of materials was made by A. Inoue. ⁎
Due to a significant increase in the minimum dimensions of alloys obtainable whilst maintaining good magnetic properties, the application possibilities of amorphous materials have been significantly extended [14–21]. Currently, there are many methods for producing amorphous materials; the achieved cooling rates differ. The methods facilitating the production of massive amorphous alloys are of particular interest, examples of which are the methods known as: ‘suction casting’ and ‘injection casting’ [22,23]. This paper compares the structural and magnetic properties of massive iron-based amorphous materials, produced by two production methods that are characterized by similar cooling rates: the injection method and the suction method. During the production of the high-melt alloys, the same moulds were used to cast the alloy and the same level of vacuum was created in the working chambers. In both cases, the production process was carried out under a protective atmosphere of argon; the same level of negative pressure prevailing in the working chambers was used. The aim of the work was to ascertain the effect of cobalt content and the method of production on the probability of creating an amorphous structure for the proposed chemical compositions – using identical parameters for both methods. Further, to examine the impact of the production method on the magnetic properties of the resulting alloys produced.
Corresponding author. E-mail address: [email protected] (B. Jeż).
https://doi.org/10.1016/j.jmmm.2019.01.073 Received 5 December 2018; Received in revised form 15 January 2019; Accepted 18 January 2019 Available online 22 January 2019 0304-8853/ © 2019 Published by Elsevier B.V.
Journal of Magnetism and Magnetic Materials 477 (2019) 214–219
M. Nabiałek et al.
2. Materials and methods
reaching the proper consistency of the alloy, the suction valve was released. The molten alloy was then drawn into the mould. The moulds used for the rapid cooling of these alloys were made of copper due to the thermal conductivity of this material. The copper moulds were water-cooled. The batch solidified in the mould at a relatively low cooling rate (of the order of 103 K/s), which, however, allowed the formation of an amorphous structure. The production of the hightemperature melt was carried out under an inert gas atmosphere after the chamber has been evacuated. In the presented solution, it was possible to use a mould that allowed the simultaneous execution of several castings. The second method used to produce the investigated massive amorphous materials was the injection casting method. Rapid cooling of the alloy was carried out using a copper mould placed in a copper block that was cooled by a continuous stream of water. The ingot was placed in a quartz capillary, which in turn was mounted to the working chamber of the device. After sealing the chamber, the air was pumped out. The small size of the chamber allowed a satisfactory level of vacuum to be obtained relatively quickly. The working chamber was subjected to a purging process with pure argon – in the same way as in the production of a polycrystalline ingot. After the vacuum had been restored, argon was introduced into the chamber in the atmosphere where the melting process was carried out. The ingot located in the quartz capillary was melted by means of eddy current heating. After obtaining the appropriate consistency of the alloy, a valve was opened to allow the liquid alloy to be injected into the water-cooled copper mould. The method achieved a cooling rate of 103 K/s, which was a sufficient cooling rate for iron- and boron-based alloys. The alloy was obtained in the form of rectangular plates of various dimensions; in the described case: 5 mm wide, 10 mm long and 0.5 mm thick. The structure of the obtained material was examined using a Bruker Advanced 8 X-ray diffractometer. Diffraction measurements were made for the particulate material. The diffractometer was equipped with a CuKα lamp and a semiconductor counter. The structure was tested for an angle of 2-Θ from 30° to 100°, irradiating the sample for 7 s per measuring step, at a resolution of 0.02°. The magnetic properties of the rapidly-cooled alloy were tested on a Faraday magnetic balance and a vibration magnetometer. The magnetic balance allowed measurements at a temperature of up to 850 K. The obtained results made it possible to assess the thermal stability of the magnetic properties and to determine the Curie temperature of the alloys. Measurements using the Lake Shore vibration magnetometer were carried out in an external magnetic field of up to 1.7 T. The obtained results were used to determine the saturation magnetisation and the
On the basis of available specialist literature and the Author’s own research, the chemical composition of the alloy was developed. The proposed composition: Fe61+xCo10−xY8W1B20 (where x = 0, 1 or 2) complies with the criteria established by A. Inoue. The batch material was weighed with an accuracy of 0.001 g. The elements were characterized by a high degree of purity of greater than 99.99%. Five gram batches were weighed to produce each polycrystalline ingot. The ingots were made in an arc furnace. The process of melting the charge was undertaken under a high vacuum, achieved by evacuating the working chamber using a pumping system. The vacuum in the working chamber attained a pressure value of 5 × 10−5 mbar; this provided a significant reduction in the presence of oxygen during the melting of the ingot. In order to improve the purity of the atmosphere before the melting process of the ingot, the chamber was flushed with argon. After the high vacuum was restored, argon was pumped into the working chamber–into the atmosphere where the ingot was melted. The batch was placed on a water-cooled copper plate. The disc also has space for a titanium ingot – its re-melting before the alloy ingot was produced improved the purity of the chamber due to the absorption of residual oxygen. Melting of the charge took place using a plasma arc, fired inside the working chamber by means of two electrodes. The arc temperature was regulated by changing the current flowing through the electrode. The first melting of the charge took place at a slower rate, with less current, so as to avoid scattering the alloy components inside the working chamber. Subsequent re-melting was carried out at a higher temperature after the ingot was first produced. Use of an appropriately high temperature and manoeuvring of the ingot ensured high homogeneity of the chemical composition of the ingot. Usually, a satisfactory level of homogeneity of the ingot was obtained after it had been re-melted five times – each time following the smelting of pure titanium. Ingots prepared in this way were subjected to eradication of external impurities and then were crushed into smaller pieces. The polycrystalline ingots thus obtained were used for the production of high-temperature alloys using the suction casting method and the injection casting method. The alloy was obtained in the form of rectangular tiles with the following dimensions: 10 mm × 5 mm × 0.5 mm. The suction method allows the production of an amorphous alloy with dimensions significantly exceeding the thicknesses obtained for amorphous tapes. A diagram of the suction method is shown in Fig. 1. The polycrystalline ingot was melted using an electric arc. Before melting the ingot, in order to get rid of the remaining oxygen and impurities from the working chamber, pure titanium was melted. After
Fig. 1. Schematic diagrams of the apparatus for the production of massive amorphous materials by the suction casting method: a) top view, b) side view. 215
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Fig. 2. XRD diffractograms for the alloy in powder form, made from plates of thickness 0.5 mm: a) and d) Fe61Co10Y8W1B20, b) and e) Fe62Co9Y8W1B20, c) and f) Fe63Co8Y8W1B20.
coercivity field of the investigated high-temperature alloys.
amorphous structure. A very important parameter, often decisive for the applicability of magnetic materials and in particular for the construction of cores, is the Curie temperature – which determines the working temperature range of a ferromagnetic material. Taking advantage of the fact that the manufactured ferromagnetic materials met the Heisenberg assumptions, a critical determinant β, equal to 0.36 was used to calculate the Curie temperature. All of the manufactured alloys were found to be characterized by high thermomagnetic stability. This is indicated by the similar waveforms of magnetic saturation polarization, measured in the direction of increasing and decreasing temperature. The curves obtained for the various chemical compositions are similar to each other. Within the tested temperature range, the X-ray diffraction phase of the crystalline phase – identified in the image – cannot be revealed because the Curie temperature of this phase is outside the measurement range. The Curie temperature of the alloys was determined, based on the critical factor β equal to 0.36. Fig. 4 contains (μ0Ms)^(1/β) curves for the investigated samples. The Curie temperatures of the alloys that were produced by the injection method are a few Kelvin lower than for the samples made by the suction method. Table 1 presents the determined Curie temperature values for the amorphous alloys produced by both methods. As expected, a reduction in the cobalt content of the alloy reduces the Curie temperature. This relationship is visible both for the injection casting and suction casting methods. An interesting phenomenon is the differences in the Curie temperature values for the same chemical compositions of alloys produced by various methods. The suction cast alloys show a slightly higher Curie temperature in each case studied. Fig. 5 presents static magnetic hysteresis loops that were obtained for samples of the tested alloys. Analysing the course of the static magnetic hysteresis loops, it can be said that they are similar to each other, especially for the alloys: Fe61Co10Y8W1B20 and Fe62Co9Y8W1B20. In the case of the Fe63Co8Y8W1B20 alloy, produced using the two different methods, it can be concluded that both of the static magnetic hysteresis loops have the so-called ‘wasp-like’ shape [25]. The visible widening of the static magnetic hysteresis loop in the ‘approach to ferromagnetic saturation’ area is related to a change of the magnetic structure and the formation of a hard magnetic phase. The presence of this phase in the initial stage of its growth (i.e. with the residual volume of the sample tested) does not make a significant contribution to the value of the coercive field. However, despite this, the Fe63Co8Y8W1B20 alloy is still classified within the group of soft magnetic alloys [26]. Compared to the other investigated alloys, the value of the coercive field for the Fe63Co8Y8W1B20 alloy increased by several times. A slight change in the proportion of iron and cobalt within the amorphous alloy
3. Results Fig. 2 presents the measured X-ray diffractograms for the investigated rapidly-cooled alloys in powder form. The recorded X-ray diffraction images were found to be similar for the investigated injection and suction cast alloy samples. However, in the case of the injection cast Fe62Co9Y8W1B20 alloy sample, diffraction reflections, associated with the existence of longrange ordering between atoms within the volume of the tested alloy, were visible near the 2-theta angles of 44 and 50°. Based on analysis using the COD database (via Match! software) it was shown that these reflections originated from the crystalline phase of Fe5Y. Until recently, this phase was considered hypothetical. The Fe5Y phase is formed by the disintegration of the high temperature Fe17Y2 phase. This decay occurs at a temperature of 1173 K; the second product of decomposition is the α Fe phase [24]. Although the conducted tests do not confirm the existence of Fe5Y in the produced samples, due to the resolution of the X-ray apparatus, its residual presence in the volume of the Fe62Co9Y8W1B20 alloy cannot be ruled out. On the remaining diffraction patterns (Fig. 2a, c, d, e, f) only a single, wide, fuzzy diffraction maximum is observed – typical for materials in which there is no long-range ordering between atoms. This maximum occurs within the range of 2-theta angle of: 35–55°, which may suggest that crystallographic systems in which the main component is iron will be preferred in the atomic systems that form. The structure of the investigated alloys was also examined by an indirect method, using a Faraday magnetic balance. This test gave the opportunity for analysis of the structural quality within the range of up to 850 K. Additionally, when analysing the measurements of magnetic polarization curves, as a function of temperature, the intensity of the transition from the ferromagnetic state to the paramagnetic state can be determined. Fig. 3 presents reduced magnetic saturation polarization curves, as a function of temperature, within a homogeneous magnetic field with an intensity of 0.7 T. These curves were measured in two temperature directions (room temperature – 850 K and 850 K – room temperature). Heating of the bifilarwound coil of the resistance furnace was tapped with a measuring card at 10 K/s, whereas cooling of the system was carried out by gravity. All curves (μ0Ms)^(1/β) were found to be similar in both the heating and cooling processes. However, it should be noted that, during the heating of the samples, the material was partially relaxed, which affects the small change in the return curve (μ0Ms)^(1/β). Based on analysis of the curves shown in Fig. 3, it can be concluded that, in the tested samples up to 850 K, there is one magnetic phase coming from the 216
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Fig. 3. Magnetic saturation polarization as a function of temperature: a) and d) Fe61Co10Y8W1B20, b) and e) Fe62Co9Y8W1B20, c) and f) Fe63Co8Y8W1B20.
is of great importance to the value of the coercive field. In the case of these injection cast or suction cast alloys, an increase in the iron content – at the expense of the cobalt content-yields a concomitant increase in the value of the coercive field. The exception to this rule was an alloy sample Fe62Co9Y8W1B20 which was made by pressing, for which the coercive field value of 33 A/m was determined. At this point, reference should be made to Fig. 2b in which narrow peaks from crystalline
phases are visible, which affect the lowering of the value of the coercive field [27]. According to the literature, the improvement of magnetic properties in amorphous alloys can be obtained by partial nanocrystallization, as a result of thermal processing [28]. Another method of nanocrystallization can be one-step nanocrystallization-designed during the very production of samples [29]. In the case of fine nanocrystalline grains smaller than 50 nm in iron-based alloys, the flow of domain walls
Fig. 4. Curie temperatures for the investigated alloys: a) and d) Fe61Co10Y8W1B20, b) and e) Fe62Co9Y8W1B20, c) and f) Fe63Co8Y8W1B20. 217
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Table 1 Curie temperature for the manufactured alloys. Method Alloy
Injection [9]
Table 2 Hc, Ms and Keff parameters, determined for the alloys with chemical composition: Fe61+xCo10−XY8W1B20 (where: x = 0, 1 or 2), produced by the injection or suction casting methods.
Suction
Curie temperature [K] Fe61Co10Y8W1B20 Fe62Co9Y8W1B20 Fe63Co8Y8W1B20
561 549 541
568 557 545
within the sample volume occurs without blocking them, which significantly reduces the losses from the hysteresis loop. This means that, for such an alloy, the surface of the hysteresis loop at the beginning of the M-H system will be smaller, which is consistent with the obtained result (Fig. 5b). In contrast, the magnetization of all alloys obtained,
Alloy
Method
Saturation magnetization [T]
Coercive field [A/m]
Keff [kJ/ m3]
Fe61Co10Y8W1B20
injection suction
1.11 1.14
71 61
78.7 77.6
Fe62Co9Y8W1B20
injection suction
1.09 1.21
33 159
75.9 75.7
Fe63Co8Y8W1B20
injection suction
1.10 1.13
241 223
74.1 73.3
Fig. 5. Static magnetic hysteresis loops for the alloy produced by the injection method: a) Fe61Co10Y8W1B20, b) Fe62Co9Y8W1B20, c) Fe63Co8Y8W1B20 and suction method: d) Fe61Co10Y8W1B20 e) Fe62Co9Y8W1B20, f) Fe63Co8Y8W1B20. 218
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regardless of the method of manufacture, is relatively high and amounts to over 1 T. The results from analysis of static magnetic hysteresis loops and primary magnetisation curves, i.e. Hc, Ms and Keff, are presented in Table 2.
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4. Conclusions On the basis of the obtained results, it can be concluded that ironbased alloys with a significant content of cobalt and boron are characterized by a good glass-transition ability. In addition, the published research results have led to the following conclusions: 1. It is possible to produce, in a single-stage, an amorphous alloy with a small volume of crystalline phase featuring nanometric grain dimensions; 2. The alloys produced are characterized by good thermomagnetic stability as indicated by the similar shapes of the magnetic saturation polarization curves (as a function of temperature) of both the heating and cooling processes of samples; 3. A change in the cobalt content within the investigated amorphous alloys results in measureable effects on the Curie temperature and the value of the coercive field, while its effect on saturation of magnetisation is insignificant; 4. Amorphous alloys produced by the suction casting method are characterized by Curie temperatures that are higher by a few Kelvin, compared to the same alloys produced by injection casting; 5. The method of producing the investigated amorphous materials has been found not to exert a significant influence on their structure and properties. 6. For samples obtained regardless of the production method, effective anisotropy decreases with increasing iron content in the alloy. The resolution of the X-ray apparatus, used in this study, does not allow the detection of a small number of crystalline phases within the volume of an alloy; therefore the possibility of the presence of a small quantity of grains or nuclei of the Fe5Y crystal phase in the remaining alloys cannot be excluded. Indirectly, the presence of this phase can be explained by a decrease in the coercivity field, which is clearly visible in the case of a melt produced by pressing (Figs. 2b and 5b). It is possible to design a heat treatment regime for the amorphous alloy, in a manner allowing the Fe5Y crystal phase grains to grow to desired dimensions. Conducting heat treatment at different temperatures and heating durations gives the opportunity to obtain the lowest coercive field value for the tested chemical compositions whilst maintaining their good thermomagnetic stability and high saturation of magnetisation. References [1] G. Herzer, Modern soft magnets: amorphous and nanocrystalline materials, Acta Mater. 61 (2013) 718–734. [2] G. Herzer, Soft magnetic nanocrystalline materials, Scripta Metall. Mater. 33 (1995) 1741–1756. [3] Y. Han, C.T. Chang, S.L. Zhu, A. Inoue, D.V. Louzguine-Luzgin, E. Shalaan, F. AlMarzouki, Fe-based soft magnetic amorphous alloys with high saturation magnetization above 1.5 T and high corrosion resistance, Intermetallics 54 (2014)
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