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Influence of the production method of Fe61Co10Y8W1B20 amorphous alloy on the resulting microstructure and hyperfine field distribution
Journal of Alloys and Compounds 628 (2015) 424–428
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Influence of the production method of Fe61Co10Y8W1B20 amorphous alloy on the resulting microstructure and hyperfine field distribution M. Nabiałek, P. Pietrusiewicz ⇑, K. Błoch Institute of Physics, Faculty of Production Engineering and Materials Technology, Czestochowa University of Technology, Poland
a r t i c l e
i n f o
Article history: Received 25 September 2014 Received in revised form 22 December 2014 Accepted 23 December 2014 Available online 6 January 2015 Keywords: Mössbauer spectroscopy X-ray diffraction Magnetic properties Amorphous materials Ferromagnetic materials
a b s t r a c t Mössbauer spectroscopy enables the determination of the influences of quenching rate and production method on the microstructure and magnetic properties of resulting ferromagnetic amorphous materials. On the basis of analysis of the hyperfine field distributions obtained from Mössbauer spectra, it has been ascertained that, in the volume of three samples manufactured by different production methods, differences exist in the local environments of the Fe atoms; further, that these differences contribute to observed variations in the magnetic properties. Analysis of the high-field magnetization curves has facilitated the calculation of the spin wave stiffness parameter. From this parameter, the range and strength of the exchange interactions could be determined. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Metallic glasses, called amorphous alloys, possess much better application properties in comparison with their crystalline counterparts, given the same chemical composition [1,2]. Depending on their chemical composition, amorphous materials can exhibit very good magnetic and mechanical properties [3]. Fe-based amorphous alloys possess high microhardness and corrosion resistance, in addition to very low hysteresis losses, virtually zero magnetostriction and very low values of coercivity [4]. Therefore, these materials are ideal for application in the electrotechnical industry, especially as a material for transformer cores. For more than fifty years now, there has been interest in these materials, from the time when Douwez and co-workers developed a novel technology for the production of metallic amorphous materials [2]. The production cycle of this technology relied on the constant, unidirectional casting of the molten alloy onto a rotating, copper wheel (the ‘melt-spinning’ method). Unfortunately, using this method, alloy samples could only be obtained in the form of a ribbon of thickness approximately 40 lm – which is a major limitation in respect of practical applications. However, due to the exceptional properties of the amorphous alloys, further attempts have been undertaken in order to produce materials with larger thicknesses. In 1989, Inoue and his co-workers from Tohoku University formulated three ⇑ Corresponding author. E-mail address: [email protected] (P. Pietrusiewicz). http://dx.doi.org/10.1016/j.jallcom.2014.12.136 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.
criteria, adherence to which gives the possibility of repeatable manufacturing of amorphous alloy samples of thicknesses greater than 100 lm [5]. This date is now perceived as the birth of a new group of functional materials: the bulk amorphous alloys [1]. A. Inoue postulated that such an alloy should consist of more than three components (1), the atomic radii of the main constituents of the alloy should differ by more than 12% (2) and there should be negative mixing heat (3). An increase in the number of atomic constituents influences the complexity of the atomic structure of the alloy, and larger differences between the atomic radii favour a higher atomic packing density. The negative-heat mixing increases the viscosity of the liquid alloy and hinders atomic migration. All three criteria decrease the distances between atoms in the alloy, thereby influencing the short-range interactions between them. The bulk amorphous alloys are mostly manufactured in the form of rods or plates, using two main production methods: suction and injection casting of the molten material, (in both cases) into a cooled, copper die [6,7]. The first group of bulk amorphous alloys consisted of alloys based on: palladium, platinium, zirconium and titanium [1]. The other group of bulk amorphous materials is that of the Fe-based alloys, which are characterized by very good mechanical and magnetic properties. The magnetic materials are divided into two groups: soft magnetic materials (for which the field of the hysteresis loop approaches zero) and hard magnetic materials (for which the field of the hysteresis loop approaches one). For applications in the electrotechnical industry, the most interesting group is the former – the amorphous materials with soft magnetic properties.
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Results of the investigations into the Fe61Co10Y8W1B2 alloy, obtained by two different production methods, have been presented previously in [8]. These investigations allowed the influence of the production method to be connected with the resulting structure, microstructure and properties of the obtained alloy. The results, presented in [8], were found to be so interesting that a third sample, obtained by the suction casting method, has been added for comparison. This work presents the results of investigations for the following alloy: Fe61Co10Y8W1B20, obtained by three different production methods. 2. Methods and materials The samples used in the investigations were produced using high purity elements: Fe – 99.98%, Co – 9998%, Y – 99.98 and W – 99.999%. The element boron was added in the form of an alloy of known composition: Fe45.4B54.6. The samples were obtained by application of the following three different production methods: melt-spinning, suction-casting and injection-casting. Ribbon-shaped samples were obtained by unidirectional quenching of the molten alloy on a copper wheel which rotated at high speed; these samples have an average thickness of 35 lm and width of 4 mm. Bulk amorphous alloy samples were obtained through the process of radial cooling of the molten material in a copper die through either suction or injection therein; these samples are in the form of plates of approximate thickness 0.5 mm and length and width 15 mm and 10 mm, respectively. All of the alloy samples were subjected to microstructure investigations by means of X-ray diffractometry (using a BRUKER Advanced D8 X-ray diffractometer) and Mössbauer spectroscopy (using a POLON Mössbauer spectrometer). The Advanced D8 X-ray diffractometer was equipped with a cobalt lamp, and the samples were scanned in the 2H range from 20° to 100° with a measurement step of 0.02° and exposure time of 14 s per step. The Mössbauer spectrometer used the 57 Co radiation source, with an intensity of 50 mCi and half-life time of 270 days. The magnetic properties of the samples were determined from analysis of the static hysteresis loops and initial magnetization curves, obtained from measurements using a vibrating sample magnetometer (the ‘LakeShore 7301 VSM’).
Fig. 2. The transmission Mössbauer spectra for the investigated samples, obtained by the three different production methods: (a) melt spinning, (b) injection-casting and (c) suction-casting.
3. Results and discussion Fig. 1 shows X-ray patterns obtained for the investigated samples of the Fe61Co10Y8W1B20 alloy samples, produced using the three different production methods. The X-ray diffraction patterns, presented in Fig. 1, are characterized by a wide amorphous halo, with a maximum near the 2H angle equal to 42°. This shape of X-ray diffraction pattern confirms that, within the alloy volume, the interactions between the atoms are short- or medium-ranged. Fig. 2 shows the transmission Mössbauer spectra, recorded for the investigated samples obtained by the three different production methods. The Mössbauer investigations facilitated confirmation of the amorphicity of the samples, with short- and medium-range interactions between the atoms. The transmission Mössbauer spectra, presented in Fig. 2, consist of wide, asymmetrical, overlapping lines, which is typical for materials featuring amorphous structure [9]. The transmission Mössbauer spectra were analysed using NORMOS software. On the basis of this analysis, the hyperfine field distributions have been obtained, and these are shown in Fig. 3. The analysis shows the distribution of the hyperfine field, defining the distribution of atoms in the near-neighbourhoods of the Fe atoms. Table 1 presents data obtained from analysis of the
Fig. 1. The X-ray diffraction patterns for Fe61Co10Y8W1B20 alloy samples, obtained by: (a) melt spinning [8], (b) injection casting [8] and (c) suction casting.
Fig. 3. The hyperfine field distribution on the 57Fe nuclei derived from analysis of the transmission Mössbauer spectra (Fig. 2) for the investigated samples of Fe61Co10Y8W1B20 alloy, obtained by: (a) melt spinning, (b) injection casting and (c) suction casting.
Table 1 The average value of the induction of the hyperfine field on the 57Fe nuclei (Bef), dispersion of the distribution of the hyperfine fields on the 57Fe nuclei (Dam). Composition
Form
Bef (T)
Dam (T)
Fe61Co10Y8W1B20
Ribbon Plate (injection casting) Plate (suction casting)
19.19 (2) 20.16 (2) 20.19 (2)
5.004(3) 5.552(3) 5.059(3)
Mössbauer spectra, i.e. dispersion of the distribution of the hyperfine fields on the 57Fe nuclei, and the average value of the induction of the hyperfine field.
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When compared with the ribbon sample, the plate-shaped samples are characterized by higher atomic packing densities, and this is confirmed by higher values of the average induction of the hyperfine field. An increase in the value of Bhf may be connected with the higher value of the topological order of the structure. Material obtained in the form of plates is produced at a lower quenching speed, therefore, already during its production process, structural relaxations can occur. These structural relaxations lead to a more stable structure. In addition, for these samples, the dispersion of the distribution of the hyperfine field of the amorphous phase is larger than for the ribbon-shaped sample. This confirms the lower value of homogeneity of these materials, from the point of view of the local surroundings of the Fe atoms. This behaviour is also related with the cooling speed of the liquid alloy during its production process. The hyperfine field distributions for all of the investigated samples consist of two, clearly separated peaks; these include a wide peak with the maximum of its average hyperfine field at the magnetic field induction of 20–22 T, depending on the production method, or to be more precise, the quenching speed of the alloy during the production process. It has been noticed that, for samples obtained at a lower value of cooling speed (102–103 K/s), with the radial heat exchange into die, the values of maximal average induction of the hyperfine field were the same. This means that, in the case of the alloy, the quenching speed and the manner of heat exchange are the deciding factors in the formation of the high-field part of the distribution of the hyperfine field on the 57Fe. The low-field component, described by the first peak, possesses the maximal value of the average induction of the hyperfine field at the induction field of 8–11 T. It has been postulated that this component is related to the nearest neighbourhood of the Fe atoms and the presence of Y atoms in it [10]. Presence of the Y atoms in the first co-ordination zone of Fe should result in a significant decrease in the hyperfine field. It is well known that Mössbauer spectroscopy investigations provide the possibility to gain information about the nearest neighbourhood of Fe atoms; therefore, also changes in the spacial configuration of the elements, caused by re-distribution of the magnetic moments of the atomic pairs (Fe–Fe, Fe–Co, Co–Co) and the surrounding pairs of Fe–Y, which depend only on the production method of the alloy. Fig. 4 shows static hysteresis loops, measured for the investigated alloy, as obtained by three different production methods. Data obtained from analysis of the static hysteresis loops are presented in Table 2.
Fig. 4. The M–H relationship for the investigated alloy, Fe61Co10Y8W1B20, obtained by different production methods: (a) melt spinning [8], (b) injection casting [8] and (c) suction casting.
Table 2 Data from analysis of the static hysteresis loops: l0M – magnetization, Hc – coercivity. Alloy composition
Shape/method
l0M (T)
Hc (A/m)
Ref.
Fe61Co10Y8W1B20
Ribbon/melt spinning Plate/injection casting Plate/suction casting
1.50(2) 1.19(2) 1.14(2)
61(3) 24(3) 37(3)
[8] [8]
All of the static hysteresis loops, presented in Fig. 4, are typical for soft magnetic materials. A significantly lower coercivity value was found for each of the samples obtained by suction- and injection casting methods. It is believed that the decrease in the coercivity values for these samples is connected to the longer time for migration of magnetic atoms within the alloy volume during the solidification process. This is in agreement with the results of the Mössbauer spectroscopy investigations; i.e. the higher values of Bhf obtained for these samples as shown in Table 1. Fig. 5 shows the initial magnetization curves for the investigated samples, obtained by the three different production methods. The area called the ‘approach to ferromagnetic saturation’ is indicated by an oval shape in this figure. The initial magnetization curves in the vicinity of the area called the approach to ferromagnetic saturation (the so-called Ewing knee) were analysed according to the Kronmuller theorem [11]. This theorem allows for indirect investigation of structural stresses, their description, and their influence on the high-field magnetization process. In the amorphous alloys, two types of structural defects could be distinguished: free volumes and quasidislocational dipoles. The ‘free volumes’ play a similar role to that of point defects in crystalline materials; conversely, the ‘quasidislocational dipoles’ are equivalent to linear defects in crystalline materials. Magnetization in a strong magnetic field could be described by the following equation:
"
l0 MðHÞ ¼ l0 Ms 1
a1=2 ðl0 HÞ1=2
a1 ðl0 HÞ1
a2 ðl0 HÞ2
# þ bðl0 HÞ1=2 ; ð1Þ
where the first three factors are connected to the presence of the structural defects. The last factor describes the effect of the spin waves (thermally-induced by the magnetic field) on the magnetization process in the region where the Holstein–Primakoff paraprocess occurs [12,13]. Analysis of the M–H curves according to Eq. (1) allowed for the calculation of the parameters of the law of the approach to the
Fig. 5. The magnetization curves for the investigated alloy Fe61Co10Y8W1B20, obtained by different production methods: (a) melt spinning, (b) injection casting and (c) suction casting.
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Table 3 Data obtained from analysis of the magnetization as a function of the magnetic field, as powers of ½, 1, 2 and ½. Dspf – spin wave stiffness parameter, Aex – exchange constant, lh – exchange length, Ndip – density of quasidislocational dipoles. Form/method
a1 (102 T1)
a2 (102 T2)
b (102 T1/2)
Dspf (102 meV nm2)
Aex (1012 J m1)
lh (nm)
Ndip (1016 m2)
Ref.
Ribbon/melt spinning Plate/injection casting Plate/suction casting
0.104(7) – –
– 0.07(3) 0.11(2)
3.43(1) 4.51(1) 4.06(1)
62.92(3) 52.42(4) 56.23(2)
2.95(2) 2.00(2) 2.06(2)
6.08(2) 3.67(2) 3.32(2)
2.70(2) – –
[8] [8]
Fig. 6. Free volume model and the quasidislocation dipole formation model, as a result of a conglomeration of free volumes [14].
Fig. 7. (a) Magnetization distribution around a point defect and (b) magnetization distribution around a quasi-dislocalized dipoles [15,16].
ferromagnetic saturation. On the basis of the obtained results for the investigated samples, it was found that the number of point defects was insignificant (lack of a1/2). For the ribbon-shaped sample of alloy, a good fit was obtained for the second law of the approach to the ferromagnetic saturation. For the plate-shaped samples, made by suction- and injection-casting, the high-field magnetization process was described by the third law of the approach to the ferromagnetic saturation. Table 3 features data assembled from analysis of the initial magnetization curves, according to the Kronmuller theorem. In the case of the investigated samples, the close relationship with the second and third laws of the ferromagnetic saturation indicates that, in the volume of the alloy, the local structural stresses are connected with two-dimensional quasidislocational dipoles. The internal stresses in the material, existing within the micro-regions, are related to the presence of the structural defects and could be graphically presented [14,15] as in Fig. 6.
The internal stresses of the structure, in the form of the quasidislocational dipoles and point defects, have a direct influence on the distribution of the magnetization within these defects [15,16] Fig. 7. In higher magnetic fields, the magnetization process of the investigated samples was related to the dumping of spin waves induced by the magnetic field. Fig. 8 presents the linear fits of l0 M S ¼ f ððl0 HÞ1=2 Þ, showing the Holstein–Primakoff paraprocess. The spin wave stiffness parameter values (Dspf) were calculated using Eq. (2).
b ¼ 3:54g l0 lB
1 4pDspf
3=2
kTðg lB Þ1=2 ;
ð2Þ
where: k – Boltzmann constant, lB – Bohr magneton, g – gyromagnetic coefficient. According to the postulation by Kaul [17] and Corb [18], the number of atoms in the nearest neighbourhood of a magnetic atom changes with relaxation of the amorphous state. A decrease in the
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field magnetization, as a function of the magnetic field induction in the region of the Holstein–Primakoff paraprocess. The value of the spin wave stiffness parameter (Dspf) for this ribbon-shaped sample is the highest, which according to [19] indicates shorter distances between the adjacent magnetic atoms, than in the case of the samples obtained in the form of plates. This is believed to be connected to the creation of the short-range chemical order. References
Fig. 8. The Holstein–Primakoff paraprocess obtained for the investigated alloy of Fe61Co10Y8W1B20, obtained by different production methods: (a) melt spinning, (b) injection casting and (c) suction casting.
value of the spin wave stiffness parameter is associated with an increase in the atoms in the nearest neighbourhood of the central magnetic atom. Data presented in Table 3 show that the most homogeneous structure was obtained using the unidirectional melt spinning method. 4. Conclusions The investigated methods for quenching the Fe61Co10Y8W1B20 alloy from the liquid state to the solid state facilitated the manufacture of materials with amorphous structure. The atomic structure of amorphous materials usually is characterized by a function of the distribution of the atomic pairs therein. The physical properties of the amorphous alloys are defined by the microstructure, spanning ranges longer than the nearest or second nearest atomic neighbours. Related influences include: fluctuations in density, composition gradient, and creation of the chemical and structural ordering of the medium range. Structural inhomogeneities of the amorphous alloys, for example areas with higher or lower values of density, are created during the solidification process of the amorphous alloy; this results in the existence of elastic stresses in the material matrix. On the basis of the obtained Mössbauer spectroscopy results, the highest value of the topological order (in terms of the possible configurations of the nearest neighbours in the vicinity of the central atom of Fe), was found for the sample obtained by the melt spinning method. This was confirmed by measurements of the high
[1] A. Inoue, Stabilization of metallic supercooled liquid and bulk amorphous alloys, Acta Mater. 48 (2000) 279–306. [2] P. Duwez, R.H. Willens, Rapid quenching of liquid alloys, T. Metall. Soc. Aime 227 (1963) 362–365. [3] 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. [4] M. Nabiałek, K. Sobczyk, J. Gondro, M. Gwoz´dzik, M. Szota, Surface and bulk properties of thick amorphous iron alloys, Opt. Appl. 39 (2009) 781–785. [5] A. Inoue, T. Zhang, T. Masumoto, Al–La–Ni amorphous alloys with a wide supercooled liquid region, Mater. Trans. JIM 30 (1989) 965–972. [6] M. Dos´piał, M. Nabiałek, M. Szota, P. Pietrusiewicz, K. Gruszka, K. Błoch, Modeling the hysteresis loop in hard magnetic materials using T(x) model, Acta Phys. Pol. A 126 (2014) 170. [7] P. Pietrusiewicz, M. Nabiałek, M. Dos´piał, K. Gruszka, K. Błoch, J. Gondro, P. Bragiel, M. Szota, Z. Stradomski, Influence of production method on the magnetic parameters and structure of Fe61Co10Y8Nb1B20 amorphous alloys in the as-quenched state, J. Alloys Comp. 615 (2014) S67–S70. [8] M. Nabiałek, P. Pietrusiewicz, M. Dos´piał, M. Szota, J. Gondro, K. Gruszka, A. Dobrzan´ska-Danikiewicz, S. Walters, A. Bukowska, Influence of the cooling speed on the soft magnetic and mechanical properties of Fe61Co10Y8W1B20 amorphous alloy, J. Alloys Comp. 615 (2014) S56–S60. [9] K. Błoch, Structure and magnetic properties of bulk amorphous (Fe0.61Co0.10Zr0.025Hf0.025Ti0.02W0.02B0.20)94Y6 alloy, Arch. Mater. Sci. Eng. 64 (2013) 97–102. [10] P. Gupta, A. Gupta, A. Shukla, Tapas Ganguli, A.K. Sinha, G. Principi, A. Maddalena, Structural evolution and the kinetics of Cu clustering in the amorphous phase of Fe–Cu–Nb–Si–B alloy, J. Appl. Phys. 110 (2011) 033537. [11] M. Vazquez, W. Fernengel, H. Kronmüller, Approach to magnetic saturation in rapidly quenched amorphous alloys, Phys. Status Solidi (a) 115 (1989) 547– 553. [12] T. Holstein, H. Primakoff, Field dependence of the intrinsic domain magnetization of a ferromagnet, Phys. Rev. 58 (1940) 1098–1113. [13] T. Holstein, H. Primakoff, Magnetization near saturation in polycrystalline ferromagnet, Phys. Rev. 59 (1941) 388–394. [14] M.G. Nabialek, P. Pietrusiewicz, M.J. Dospial, M. Szota, K. Błoch, K. Gruszka, K. Oz´ga, S. Garus, Effect of manufacturing method on the magnetic properties and formation of structural defects in Fe61Co10Y8Zr1B20 amorphous alloy, J. Alloys Comp. 615 (2014) S51–S55. [15] H. Kronmuller, Micromagnetism and microstructure of amorphous alloys, J. Appl. Phys. 52 (1981) 1859–1864. [16] H. Kronmüller, M. Fähnle, Micromagnetism and the Microstructure of Ferromagnetic Solids, Cambridge University Press, 2003. [17] N. Kaul, Magnetic properties of amorphous (Fe, Ni)80B20, (Fe, Ni)80B19Si1, and (Fe, Ni)80P14B6 alloy, IEEE Trans. Magn. 17 (1981) 1208–1215. [18] B.W. Corb, R.C. O’Handley, N.J. Grant, Chemical bonding, magnetic moments, and local symmetry in transition-metal—metalloid alloys, Phys. Rev. B 27 (1983) 636. [19] N. Lenge, H. Kronmüller, Low temperature magnetization of sputtered amorphous Fe–Ni–B films, Phys. Status Solidi (a) 95 (1986) 621–633.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Influence of the production method of Fe61Co10Y8W1B20 amorphous alloy on the resulting microstructure and hyperfine field distribution M. Nabiałek, P. Pietrusiewicz ⇑, K. Błoch Institute of Physics, Faculty of Production Engineering and Materials Technology, Czestochowa University of Technology, Poland
a r t i c l e
i n f o
Article history: Received 25 September 2014 Received in revised form 22 December 2014 Accepted 23 December 2014 Available online 6 January 2015 Keywords: Mössbauer spectroscopy X-ray diffraction Magnetic properties Amorphous materials Ferromagnetic materials
a b s t r a c t Mössbauer spectroscopy enables the determination of the influences of quenching rate and production method on the microstructure and magnetic properties of resulting ferromagnetic amorphous materials. On the basis of analysis of the hyperfine field distributions obtained from Mössbauer spectra, it has been ascertained that, in the volume of three samples manufactured by different production methods, differences exist in the local environments of the Fe atoms; further, that these differences contribute to observed variations in the magnetic properties. Analysis of the high-field magnetization curves has facilitated the calculation of the spin wave stiffness parameter. From this parameter, the range and strength of the exchange interactions could be determined. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Metallic glasses, called amorphous alloys, possess much better application properties in comparison with their crystalline counterparts, given the same chemical composition [1,2]. Depending on their chemical composition, amorphous materials can exhibit very good magnetic and mechanical properties [3]. Fe-based amorphous alloys possess high microhardness and corrosion resistance, in addition to very low hysteresis losses, virtually zero magnetostriction and very low values of coercivity [4]. Therefore, these materials are ideal for application in the electrotechnical industry, especially as a material for transformer cores. For more than fifty years now, there has been interest in these materials, from the time when Douwez and co-workers developed a novel technology for the production of metallic amorphous materials [2]. The production cycle of this technology relied on the constant, unidirectional casting of the molten alloy onto a rotating, copper wheel (the ‘melt-spinning’ method). Unfortunately, using this method, alloy samples could only be obtained in the form of a ribbon of thickness approximately 40 lm – which is a major limitation in respect of practical applications. However, due to the exceptional properties of the amorphous alloys, further attempts have been undertaken in order to produce materials with larger thicknesses. In 1989, Inoue and his co-workers from Tohoku University formulated three ⇑ Corresponding author. E-mail address: [email protected] (P. Pietrusiewicz). http://dx.doi.org/10.1016/j.jallcom.2014.12.136 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.
criteria, adherence to which gives the possibility of repeatable manufacturing of amorphous alloy samples of thicknesses greater than 100 lm [5]. This date is now perceived as the birth of a new group of functional materials: the bulk amorphous alloys [1]. A. Inoue postulated that such an alloy should consist of more than three components (1), the atomic radii of the main constituents of the alloy should differ by more than 12% (2) and there should be negative mixing heat (3). An increase in the number of atomic constituents influences the complexity of the atomic structure of the alloy, and larger differences between the atomic radii favour a higher atomic packing density. The negative-heat mixing increases the viscosity of the liquid alloy and hinders atomic migration. All three criteria decrease the distances between atoms in the alloy, thereby influencing the short-range interactions between them. The bulk amorphous alloys are mostly manufactured in the form of rods or plates, using two main production methods: suction and injection casting of the molten material, (in both cases) into a cooled, copper die [6,7]. The first group of bulk amorphous alloys consisted of alloys based on: palladium, platinium, zirconium and titanium [1]. The other group of bulk amorphous materials is that of the Fe-based alloys, which are characterized by very good mechanical and magnetic properties. The magnetic materials are divided into two groups: soft magnetic materials (for which the field of the hysteresis loop approaches zero) and hard magnetic materials (for which the field of the hysteresis loop approaches one). For applications in the electrotechnical industry, the most interesting group is the former – the amorphous materials with soft magnetic properties.
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Results of the investigations into the Fe61Co10Y8W1B2 alloy, obtained by two different production methods, have been presented previously in [8]. These investigations allowed the influence of the production method to be connected with the resulting structure, microstructure and properties of the obtained alloy. The results, presented in [8], were found to be so interesting that a third sample, obtained by the suction casting method, has been added for comparison. This work presents the results of investigations for the following alloy: Fe61Co10Y8W1B20, obtained by three different production methods. 2. Methods and materials The samples used in the investigations were produced using high purity elements: Fe – 99.98%, Co – 9998%, Y – 99.98 and W – 99.999%. The element boron was added in the form of an alloy of known composition: Fe45.4B54.6. The samples were obtained by application of the following three different production methods: melt-spinning, suction-casting and injection-casting. Ribbon-shaped samples were obtained by unidirectional quenching of the molten alloy on a copper wheel which rotated at high speed; these samples have an average thickness of 35 lm and width of 4 mm. Bulk amorphous alloy samples were obtained through the process of radial cooling of the molten material in a copper die through either suction or injection therein; these samples are in the form of plates of approximate thickness 0.5 mm and length and width 15 mm and 10 mm, respectively. All of the alloy samples were subjected to microstructure investigations by means of X-ray diffractometry (using a BRUKER Advanced D8 X-ray diffractometer) and Mössbauer spectroscopy (using a POLON Mössbauer spectrometer). The Advanced D8 X-ray diffractometer was equipped with a cobalt lamp, and the samples were scanned in the 2H range from 20° to 100° with a measurement step of 0.02° and exposure time of 14 s per step. The Mössbauer spectrometer used the 57 Co radiation source, with an intensity of 50 mCi and half-life time of 270 days. The magnetic properties of the samples were determined from analysis of the static hysteresis loops and initial magnetization curves, obtained from measurements using a vibrating sample magnetometer (the ‘LakeShore 7301 VSM’).
Fig. 2. The transmission Mössbauer spectra for the investigated samples, obtained by the three different production methods: (a) melt spinning, (b) injection-casting and (c) suction-casting.
3. Results and discussion Fig. 1 shows X-ray patterns obtained for the investigated samples of the Fe61Co10Y8W1B20 alloy samples, produced using the three different production methods. The X-ray diffraction patterns, presented in Fig. 1, are characterized by a wide amorphous halo, with a maximum near the 2H angle equal to 42°. This shape of X-ray diffraction pattern confirms that, within the alloy volume, the interactions between the atoms are short- or medium-ranged. Fig. 2 shows the transmission Mössbauer spectra, recorded for the investigated samples obtained by the three different production methods. The Mössbauer investigations facilitated confirmation of the amorphicity of the samples, with short- and medium-range interactions between the atoms. The transmission Mössbauer spectra, presented in Fig. 2, consist of wide, asymmetrical, overlapping lines, which is typical for materials featuring amorphous structure [9]. The transmission Mössbauer spectra were analysed using NORMOS software. On the basis of this analysis, the hyperfine field distributions have been obtained, and these are shown in Fig. 3. The analysis shows the distribution of the hyperfine field, defining the distribution of atoms in the near-neighbourhoods of the Fe atoms. Table 1 presents data obtained from analysis of the
Fig. 1. The X-ray diffraction patterns for Fe61Co10Y8W1B20 alloy samples, obtained by: (a) melt spinning [8], (b) injection casting [8] and (c) suction casting.
Fig. 3. The hyperfine field distribution on the 57Fe nuclei derived from analysis of the transmission Mössbauer spectra (Fig. 2) for the investigated samples of Fe61Co10Y8W1B20 alloy, obtained by: (a) melt spinning, (b) injection casting and (c) suction casting.
Table 1 The average value of the induction of the hyperfine field on the 57Fe nuclei (Bef), dispersion of the distribution of the hyperfine fields on the 57Fe nuclei (Dam). Composition
Form
Bef (T)
Dam (T)
Fe61Co10Y8W1B20
Ribbon Plate (injection casting) Plate (suction casting)
19.19 (2) 20.16 (2) 20.19 (2)
5.004(3) 5.552(3) 5.059(3)
Mössbauer spectra, i.e. dispersion of the distribution of the hyperfine fields on the 57Fe nuclei, and the average value of the induction of the hyperfine field.
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When compared with the ribbon sample, the plate-shaped samples are characterized by higher atomic packing densities, and this is confirmed by higher values of the average induction of the hyperfine field. An increase in the value of Bhf may be connected with the higher value of the topological order of the structure. Material obtained in the form of plates is produced at a lower quenching speed, therefore, already during its production process, structural relaxations can occur. These structural relaxations lead to a more stable structure. In addition, for these samples, the dispersion of the distribution of the hyperfine field of the amorphous phase is larger than for the ribbon-shaped sample. This confirms the lower value of homogeneity of these materials, from the point of view of the local surroundings of the Fe atoms. This behaviour is also related with the cooling speed of the liquid alloy during its production process. The hyperfine field distributions for all of the investigated samples consist of two, clearly separated peaks; these include a wide peak with the maximum of its average hyperfine field at the magnetic field induction of 20–22 T, depending on the production method, or to be more precise, the quenching speed of the alloy during the production process. It has been noticed that, for samples obtained at a lower value of cooling speed (102–103 K/s), with the radial heat exchange into die, the values of maximal average induction of the hyperfine field were the same. This means that, in the case of the alloy, the quenching speed and the manner of heat exchange are the deciding factors in the formation of the high-field part of the distribution of the hyperfine field on the 57Fe. The low-field component, described by the first peak, possesses the maximal value of the average induction of the hyperfine field at the induction field of 8–11 T. It has been postulated that this component is related to the nearest neighbourhood of the Fe atoms and the presence of Y atoms in it [10]. Presence of the Y atoms in the first co-ordination zone of Fe should result in a significant decrease in the hyperfine field. It is well known that Mössbauer spectroscopy investigations provide the possibility to gain information about the nearest neighbourhood of Fe atoms; therefore, also changes in the spacial configuration of the elements, caused by re-distribution of the magnetic moments of the atomic pairs (Fe–Fe, Fe–Co, Co–Co) and the surrounding pairs of Fe–Y, which depend only on the production method of the alloy. Fig. 4 shows static hysteresis loops, measured for the investigated alloy, as obtained by three different production methods. Data obtained from analysis of the static hysteresis loops are presented in Table 2.
Fig. 4. The M–H relationship for the investigated alloy, Fe61Co10Y8W1B20, obtained by different production methods: (a) melt spinning [8], (b) injection casting [8] and (c) suction casting.
Table 2 Data from analysis of the static hysteresis loops: l0M – magnetization, Hc – coercivity. Alloy composition
Shape/method
l0M (T)
Hc (A/m)
Ref.
Fe61Co10Y8W1B20
Ribbon/melt spinning Plate/injection casting Plate/suction casting
1.50(2) 1.19(2) 1.14(2)
61(3) 24(3) 37(3)
[8] [8]
All of the static hysteresis loops, presented in Fig. 4, are typical for soft magnetic materials. A significantly lower coercivity value was found for each of the samples obtained by suction- and injection casting methods. It is believed that the decrease in the coercivity values for these samples is connected to the longer time for migration of magnetic atoms within the alloy volume during the solidification process. This is in agreement with the results of the Mössbauer spectroscopy investigations; i.e. the higher values of Bhf obtained for these samples as shown in Table 1. Fig. 5 shows the initial magnetization curves for the investigated samples, obtained by the three different production methods. The area called the ‘approach to ferromagnetic saturation’ is indicated by an oval shape in this figure. The initial magnetization curves in the vicinity of the area called the approach to ferromagnetic saturation (the so-called Ewing knee) were analysed according to the Kronmuller theorem [11]. This theorem allows for indirect investigation of structural stresses, their description, and their influence on the high-field magnetization process. In the amorphous alloys, two types of structural defects could be distinguished: free volumes and quasidislocational dipoles. The ‘free volumes’ play a similar role to that of point defects in crystalline materials; conversely, the ‘quasidislocational dipoles’ are equivalent to linear defects in crystalline materials. Magnetization in a strong magnetic field could be described by the following equation:
"
l0 MðHÞ ¼ l0 Ms 1
a1=2 ðl0 HÞ1=2
a1 ðl0 HÞ1
a2 ðl0 HÞ2
# þ bðl0 HÞ1=2 ; ð1Þ
where the first three factors are connected to the presence of the structural defects. The last factor describes the effect of the spin waves (thermally-induced by the magnetic field) on the magnetization process in the region where the Holstein–Primakoff paraprocess occurs [12,13]. Analysis of the M–H curves according to Eq. (1) allowed for the calculation of the parameters of the law of the approach to the
Fig. 5. The magnetization curves for the investigated alloy Fe61Co10Y8W1B20, obtained by different production methods: (a) melt spinning, (b) injection casting and (c) suction casting.
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Table 3 Data obtained from analysis of the magnetization as a function of the magnetic field, as powers of ½, 1, 2 and ½. Dspf – spin wave stiffness parameter, Aex – exchange constant, lh – exchange length, Ndip – density of quasidislocational dipoles. Form/method
a1 (102 T1)
a2 (102 T2)
b (102 T1/2)
Dspf (102 meV nm2)
Aex (1012 J m1)
lh (nm)
Ndip (1016 m2)
Ref.
Ribbon/melt spinning Plate/injection casting Plate/suction casting
0.104(7) – –
– 0.07(3) 0.11(2)
3.43(1) 4.51(1) 4.06(1)
62.92(3) 52.42(4) 56.23(2)
2.95(2) 2.00(2) 2.06(2)
6.08(2) 3.67(2) 3.32(2)
2.70(2) – –
[8] [8]
Fig. 6. Free volume model and the quasidislocation dipole formation model, as a result of a conglomeration of free volumes [14].
Fig. 7. (a) Magnetization distribution around a point defect and (b) magnetization distribution around a quasi-dislocalized dipoles [15,16].
ferromagnetic saturation. On the basis of the obtained results for the investigated samples, it was found that the number of point defects was insignificant (lack of a1/2). For the ribbon-shaped sample of alloy, a good fit was obtained for the second law of the approach to the ferromagnetic saturation. For the plate-shaped samples, made by suction- and injection-casting, the high-field magnetization process was described by the third law of the approach to the ferromagnetic saturation. Table 3 features data assembled from analysis of the initial magnetization curves, according to the Kronmuller theorem. In the case of the investigated samples, the close relationship with the second and third laws of the ferromagnetic saturation indicates that, in the volume of the alloy, the local structural stresses are connected with two-dimensional quasidislocational dipoles. The internal stresses in the material, existing within the micro-regions, are related to the presence of the structural defects and could be graphically presented [14,15] as in Fig. 6.
The internal stresses of the structure, in the form of the quasidislocational dipoles and point defects, have a direct influence on the distribution of the magnetization within these defects [15,16] Fig. 7. In higher magnetic fields, the magnetization process of the investigated samples was related to the dumping of spin waves induced by the magnetic field. Fig. 8 presents the linear fits of l0 M S ¼ f ððl0 HÞ1=2 Þ, showing the Holstein–Primakoff paraprocess. The spin wave stiffness parameter values (Dspf) were calculated using Eq. (2).
b ¼ 3:54g l0 lB
1 4pDspf
3=2
kTðg lB Þ1=2 ;
ð2Þ
where: k – Boltzmann constant, lB – Bohr magneton, g – gyromagnetic coefficient. According to the postulation by Kaul [17] and Corb [18], the number of atoms in the nearest neighbourhood of a magnetic atom changes with relaxation of the amorphous state. A decrease in the
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field magnetization, as a function of the magnetic field induction in the region of the Holstein–Primakoff paraprocess. The value of the spin wave stiffness parameter (Dspf) for this ribbon-shaped sample is the highest, which according to [19] indicates shorter distances between the adjacent magnetic atoms, than in the case of the samples obtained in the form of plates. This is believed to be connected to the creation of the short-range chemical order. References
Fig. 8. The Holstein–Primakoff paraprocess obtained for the investigated alloy of Fe61Co10Y8W1B20, obtained by different production methods: (a) melt spinning, (b) injection casting and (c) suction casting.
value of the spin wave stiffness parameter is associated with an increase in the atoms in the nearest neighbourhood of the central magnetic atom. Data presented in Table 3 show that the most homogeneous structure was obtained using the unidirectional melt spinning method. 4. Conclusions The investigated methods for quenching the Fe61Co10Y8W1B20 alloy from the liquid state to the solid state facilitated the manufacture of materials with amorphous structure. The atomic structure of amorphous materials usually is characterized by a function of the distribution of the atomic pairs therein. The physical properties of the amorphous alloys are defined by the microstructure, spanning ranges longer than the nearest or second nearest atomic neighbours. Related influences include: fluctuations in density, composition gradient, and creation of the chemical and structural ordering of the medium range. Structural inhomogeneities of the amorphous alloys, for example areas with higher or lower values of density, are created during the solidification process of the amorphous alloy; this results in the existence of elastic stresses in the material matrix. On the basis of the obtained Mössbauer spectroscopy results, the highest value of the topological order (in terms of the possible configurations of the nearest neighbours in the vicinity of the central atom of Fe), was found for the sample obtained by the melt spinning method. This was confirmed by measurements of the high
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