A Study of Structure and Magnetic Properties of Low Purity Fe-Co-Based Metallic Glasses

This paper is related to the evaluation of the possibility of using ferroalloys for the production of conventional (CMGs) and bulk metallic glasses (BMGs) as well as determining their magnetic properties. The structure and magnetic properties of Fe-Co-based CMGs and BMGs prepared from ferroalloys and pure elements, were studied. The CMGs and BMGs were in the form of ribbons and rods, respectively. The thickness of the ribbons were 0.07, 0.12, and 0.27 mm and the diameters of the rods were 1.5 and 2.5 mm. The investigations of the structure of the test specimens were carried out using the X-ray diffraction (XRD) method and electron microscopy methods (HRTEM—high-resolution transmission electron microscope, SEM—scanning electron microscope). The relationship between the structure and magnetic properties of the Fe36.00Co36.00B19.00Si5Nb4 and Fe35.75Co35.75B18.90Si5Nb4Cu0.6 CMGs and BMGs was determined. The possibility of using new materials, i.e., CMGs and BMGs, prepared on the basis of ferroalloys, lies in the scope of the presently conducted research and allows us to obtain the utility properties, while avoiding high costs associated with the purchase of raw materials.


Introduction
The Fe-Co-based conventional (CMGs) and bulk metallic glasses (BMGs) constitute potential research materials for institutions and industry because of their excellent GFA-glass forming ability, thermal stability, superior magnetic properties (high saturation magnetization B s , low coercivity H c ) and good strength [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. So far, these alloys have been prepared mainly from high purity elements (greater than 99.5 wt %). These metallic glasses can achieve a larger critical diameter than low purity metallic glasses (e.g., based on the ferroalloys). However, amorphous alloys, prepared from high purity metals and metalloids are very expensive because of raw materials' purchase costs. Recently, the production of Fe-based metallic glasses (MGs) has been significantly increasing to improve energy saving needed for conservation of the environment, an urgent subject of the present times [17]. Because of the low core losses attributable to the good magnetic softness, energy saving is feasible. However, the thickness of the typical Fe-Si-B amorphous alloy ribbon produced by a melt-spinning method, in commercial production for many kinds of highly efficient transformers, is about 25 µm, which is much thinner than the other ordinary crystalline soft magnetic alloy sheets [1,18]. An Fe 84.2 Si 5.8 crystalline alloy (silicon steel) sheet produced by cold rolling has some advantages, like unlimited thickness, high saturation magnetization (B s~2 T), and very low material cost. However, the silicon steel has a great disadvantage of inferior magnetic softness fundamentally caused by a large magneto-crystalline anisotropy and its production is long and expensive.
Fe-and Co-based MGs have the advantage over silicon steel, because their core loss is about 85% lower. This would therefore reduce the cost of operating electrical appliances. When comparing the application prospects indicate the need for further development of the research on metallic glass production technologies, while fulfilling the criteria for the minimization of the energy and material consumption. The unrelenting pressure on cost reduction makes the economic factor crucial in the selection of engineering materials. It is reflected in striving for the reduction of material costs, for example by replacing pure elements with charged materials in the form of lower purity metals and alloys, such as ferroalloys, in the production of CMGs and BMGs.
However, for these alloys there are difficulties in achieving a fully amorphous structure due to the presence of impurities. Impurities may form crystal nuclei and cause problems with obtaining the eutectic composition. Thus, solving these problems becomes an issue of interesting and innovative scientific works. Available in the literature [1,3,8,9,13,14,17,25], the results of investigations of the structure and properties of metallic glasses are mainly based on alloys produced from pure elements. Although the metallic glasses have been manufactured and investigated for many years, the current knowledge about the structure and properties of MGs prepared on the basis of industrial materials, such as ferroalloys, remains limited [33][34][35][36]. Of the available ferroalloys, Fe-B is mainly used for manufacturing metallic glasses [33][34][35][36]. In the literature, there are still no data on the structure and properties of the metallic glasses made by combining a few ferroalloys (Fe-B, Fe-Si, Fe-Nb), which determine their industrial application [37]. Another issue that has not been presented in a comprehensive way so far is the analysis of structural components in the structure of partially amorphous Fe-Co-B-Si-Nb metallic glasses. A crucial importance for their application is to establish a relationship between the presence of crystalline boride phases and the maintenance of the magnetic properties of Fe-based BMGs.

Materials
The Fe 36 . Ferroalloys belong to the alloys with less than 50 wt % content of iron and with one or more other elements, e.g., boron, silicon. These contain besides the main elements, like iron and boron, other constitutes such as aluminium (0.2-1.5 wt %), silicon (0.3-3.0 wt %), carbon (0.1-0.5 wt %), sulphur (0.015-0.1 wt %), and phosphorus (0.02-0.1 wt %). They are used as raw materials for steel manufacturing. The ferroalloys applied in this work were obtained from industrial large-scale production. The content of boron, niobium, and silicon in the cast alloy were adjusted by adding the Fe-B, Fe-Nb, and Fe-Si alloys, respectively, which are much cheaper than pure elements. Pure elements, such as Fe (99.99 wt %), Co (99.99 wt %), and Cu (99.99 wt %) were added to achieve the chemical composition of the alloys. The ingots were re-melted a few times in a furnace with purified argon atmosphere to ensure their homogeneity, and the Fe 36 Ribbons with the thicknesses of 0.07, 0.12, and 0.27 mm and width of 2.3 mm were produced by the single copper roller melt spinning method. The master alloy was melted in a quartz crucible using an induction coil and pushed thereafter on a copper wheel by applying an ejection pressure of about 0.02 MPa. Bulk samples in rod form with the diameters of 1.5 and 2.5 mm were prepared by the pressure die casting method into a copper mold.

Methods
The microstructure of the samples (ribbons and rods) was examined by the X-ray diffraction (XRD) method and high-resolution transmission electron microscopy (HRTEM, FEI, Eindhoven, The Netherlands). XRD measurements were performed at ambient temperature using a X-Pert PRO MP diffractometer (PANalytical, Almelo, The Netherlands) with Co-Kα radiation (λ = 0.17888 nm), a tube voltage of 30 kV, and a current of 10 mA, in Bragg-Brentano geometry. Phase qualitative analysis was performed using classical X-ray diffraction. X-ray Diffraction (XRD) can be used to determine phase composition (commonly called phase ID) for mixtures of crystalline and nanocrystalline metals, foils, coatings, films, etc. [38]. The result of the analysis consists of an identified phase list with experimentally observed X-ray patterns and known diffraction patterns from various sources. The sources and the notation describing the quality of data are maintained by the ICDD (International Centre for Diffraction Data) [39]. The peaks were compared to probable crystalline species using the software X'Pert Highscore Plus 3.0e correlated with PAN-ICSD data base (2013-1, PANalytical, Almelo, The Netherlands) The structures of ribbons and rods were also examined by a high-resolution transmission electron microscope (HRTEM) S/TEM TITAN 80-300 FEI. Structures were observed on the small lamellas. The lamella-formed samples of 20 µm × 8 µm were cut out with a gallium-focused ion beam from a specific cross-sectional area of the test specimen and were then thinned with this ion beam to the thickness of approximately 50-70 nm using a FEI FIB Quanta 3D200 machine (FEI, Eindhoven, The Netherlands) [40]. The lamella-preparation process was observed in situ using a scanning electron microscope.
The morphology of fracture surfaces of the samples after decohesion was examined by means of a scanning electron microscope (SEM) SUPRA 35, with a voltage of 20 kV (Carl Zeiss, Jena, Germany). Analysis of the chemical composition of the samples was performed using an EDAX energy-dispersive X-ray spectroscope (EDS), coupled with the SEM. The boron content in the alloy composition was not determined.
A vibrating sample magnetometer (VSM), (LakeShore, Euclid, OH, USA), was used to measure the magnetic properties of the samples. High field magnetization curves were measured using magnetic induction up to 2 T. The magnetizing field was parallel to the sample length to minimize the demagnetization effect. The magnetization curves were analyzed using the least squares method. The initial magnetic permeability µ i of the ribbon samples was measured by using the Maxwell-Wien bridge (Agilent HP, Tokyo, Japan). The applied magnetic field had a value of 0.5 A/m and a frequency of about 1 kHz. The magnetic after effects (∆µ/µ) were determined by measuring changes of magnetic permeability of the examined alloys as a function of time after demagnetization, where ∆µ is the difference in magnetic permeability measured at t 1 = 30 s and t 2 = 1800 s after demagnetization [41,42].

Results and Discussion
The results of the investigations of the structure of ribbons with different thicknesses produced from Fe 36.00 Co 36.00 B 19.00 Si 5 Nb 4 and Fe 35.75 Co 35.75 B 18.90 Si 5 Nb 4 Cu 0,6 alloys carried out by the X-ray phase analysis method have shown that the ribbons are in the amorphous state (Figure 1a,b). The structure of the rods with the diameter of 1.5 mm is mainly amorphous (Figure 2a,b). In Figure 2a,b only one pattern respectively has a broad-angle peak. This is evidenced by numerous structural images taken by the latest techniques of high-resolution transmission electron microscopy (HRTEM) (Figures 3a,b and 4a,b) and scanning electron microscopy (SEM) (Figures 5 and 6).   (Figures 3a and 4a), we can observe the positions with randomly distributed dark and bright contrasts. From the HRTEM images we may conclude that local short-range order (SRO) regions have occurred during the cooling of the Fe-Co-based metallic glass. The selected area electron diffraction (SAED) pattern consisted only of halo rings (Figures 3b and 4b). The diffuse diffraction halos may be taken as characteristic of the amorphous state. However, this characteristic alone is not sufficient to describe the atomic arrangements within the solid. Although the precise description of atomic structures for metallic glasses is still open even now.

Results and Discussion
The results of the investigations of the structure of ribbons with different thicknesses produced from Fe36.00Co36.00B19.00Si5Nb4 and Fe35.75Co35.75B18.90Si5Nb4Cu0,6 alloys carried out by the X-ray phase analysis method have shown that the ribbons are in the amorphous state (Figure 1a,b). The structure of the rods with the diameter of 1.5 mm is mainly amorphous (Figure 2a,b). In Figure 2a,b only one pattern respectively has a broad-angle peak. This is evidenced by numerous structural images taken by the latest techniques of high-resolution transmission electron microscopy (HRTEM) (Figures 3a,b and 4a,b) and scanning electron microscopy (SEM) (Figures 5 and 6).   Vein pattern morphology was observed in the investigated ribbons, typical of amorphous alloys [43]. The morphology is changing from a smooth fracture inside with a few vein networks in the surface freely solidified (shining surface) (Figures 5a and 6a). Macroscopically metallic glasses behave in a brittle manner. Microanalysis of the chemical composition in the EDS plots, examined by SEM, is shown in Figures 5b and 6b (Figures 3b and 4b). The diffuse diffraction halos may be taken as characteristic of the amorphous state. However, this characteristic alone is not sufficient to             However, rods with the diameter of 2.5 mm are partially crystallized (in Figure 2a,b only one pattern). The results of the X-ray diffraction method using the Fe 36 (Figure 2a,b). In the XRD patterns, a few of the Bragg's peaks are superimposed on the diffused diffraction maxima, which means that the rods contain crystalline phases. The crystalline phases are identified as (Fe, Co)α, (Fe, Co, Si) 3 B, and Nb 5 Si 3 [39]. The results of the HRTEM confirm the existence of the amorphous (Figure 7a-c) and crystalline phases (Figures 7a,d,e, 8a,b, 9a,b and  10a,b) in the structure of the rods.
For alloy rods containing copper, the saturation magnetic polarization Js was from 1.03 to 1.18 T (Figure 14b), which is similar to that of the base Fe36.00Co36.00B19.00Si5Nb4 alloy-Js was from 0.97 to 1.17 T (Figure 14a). It has been reported that the addition of copper into Fe-Co-B-Si-Nb alloys improves the soft magnetic properties of the alloy, by decreasing the coercive force and increasing the relative initial magnetic permeability-μi. This is probably connected with the impurity content. Jia reported that the Cu addition reduces the oxygen content of the alloy [2].    The alloy rods prepared on the basis of pure metals and non-metals have a lower coercive force [45][46][47] compared to the alloy rods prepared on the basis of ferroalloys, which is related to the reduced amount of impurities present in the alloy [2]. The use of high-purity charge materials makes it possible to obtain an increased critical casting thickness/diameter of BMGs [47,48]. The critical diameter of BMGs is the maximal dimension of the sample having an amorphous structure. The highest coercive force is observed in the rods with the diameter of 2.5 mm prepared on the basis of ferroalloys that can be ascribed to the appearance of boride phases in the structure of the alloy.
In the amorphous alloy, the domain walls are free to move due to the low magnetocrystalline anisotropy [26], resulting in a low Hc. The lower coercivity could be obtained upon annealing well below Tx in order to reduce the stress field in the as-cast amorphous matrix or structural relaxation phenomena. A relaxed amorphous phase in the material produced at lower rates of cooling is characterized by the presence of the local atomic order and magnetic structure (different than the nano-structure). Cu atom clusters are present in the amorphous structure of the Fe-Co-Si-Nb-B alloy containing Cu. The Cu atoms aggregate to form small clusters and the size of the clusters is approximately a few nm. [51,52]. Improvement of the soft magnetic properties of the alloy is caused by structural relaxation. However, when there are impurities in the alloy, they can pin the domain The existence of the crystalline phases Fe 23 B 6 and Fe 3 B and Nb-rich phase have been reported in the Fe 77−x Co x Nb 7 B 15 Cu 1 (x = 0, 2, 4, 8 at %) alloy [43]. The metastable iron boride Fe 23 B 6 phase was confirmed by the XRD (X-Ray Diffraction) and D (Differential Scanning Calorimetry) results achieved for the Fe-Co-Si-B-Nb alloy prepared with the high purity metals [16,44]. The metastable Fe 3 B phase is favored in the alloy with the replacement of Fe by Co [44]. Comparing with the DSC measured for the other [(Fe 0.5 Co 0.5 ) 0.75 B 0.20 Si 0.05 ] 96 Nb 4 alloy, the middle of the second crystallization peak and some small additional peaks can be seen and they could be identified as (Fe,Co) 3 [46]. These differences in the crystallization behavior are associated with the different atomic arrangements in the two types of amorphous alloys. This hypothesis was confirmed by synchrotron studies [45,46]. The analysis of structural components in the structure of partially crystallized amorphous Fe-Co-B-Si-Nb rods with the diameter of 2.5 mm carried out on the basis of the results of the X-ray examinations (Figure 2a,b) and transmission electron microscopy (TEM) (Figures 7-10) allowed us to establish the relationship between the presence of crystalline boride phases and the maintenance of magnetic properties. These results have significant informative and application importance.
The ability to obtain the amorphous state for the rods with the diameter of 2.5 mm is highly affected by the purity of the charged materials that the test specimens are made from. If pure metals and non-metals are used, the rods with the diameter of 2.5 mm are also obtained in the amorphous state [47,48].
The fracture morphology using the Fe 36 (Figures 11 and 12). These fracture surfaces of the rods with the diameters of 1.5 and 2.5 mm are typical for the relaxation of metallic glasses [49,50]. Smooth fracture morphology with areas of vein and chevron pattern morphology have been observed in the region between the surface and the core of the rod and in the region of direct contact of the liquid alloy in a copper mold. A smooth pattern is usually observed at the fracture surface of brittle MGs [43]. Thermal methods, such as differential thermal analysis (DTA) (NETZSCH, Selb, Germany) and differential scanning calorimetry (DSC) (NETZSCH, Selb, Germany), allowed the characteristic temperatures of the examined alloys to be determined, i.e., crystallization onset temperature (Tx1), eutectic temperature (Te), melting point (Tm), and glass forming ability (GFA) indicators (Tg-glass transition temperature; Tl-liquidus temperature; ΔTx = Tx1 − Tg-range of supercooled liquid; Trg = Tg/Tl-reduced glass transition temperature) [42,55]. The chemical composition of the examined alloys was similar to the eutectic one, which assures their good glass forming ability [42,55]. differential scanning calorimetry (DSC) (NETZSCH, Selb, Germany), allowed the characteristic temperatures of the examined alloys to be determined, i.e., crystallization onset temperature (Tx1), eutectic temperature (Te), melting point (Tm), and glass forming ability (GFA) indicators (Tg-glass transition temperature; Tl-liquidus temperature; ΔTx = Tx1 − Tg-range of supercooled liquid; Trg = Tg/Tl-reduced glass transition temperature) [42,55]. The chemical composition of the examined alloys was similar to the eutectic one, which assures their good glass forming ability [42,55]. In addition to purely utilitarian features, the investigations conducted on the magnetic properties have also provided results that are useful for analysis of the structure of the amorphous state. Magnetic methods, such as initial permeability, coercive force, and magnetic relaxation intensity, depend on the state of stress in the material and the concentration of microvoids. The results of investigations obtained with VSM vibrating magnetometer and presented in the form of hysteresis loop allow the examined alloys to be classified as soft magnetic materials. The obtained values for relative initial permeability μi and coercive force correlate with each other for ribbons and rods with the diameter of 1.5 mm.  Table 1. The results of the magnetic properties (the saturation magnetic polarization-J s and coercivity-H c ) obtained with a vibrating sample magnetometer (VSM) show the hysteresis loops, shown in Figures 13 and 14.  In addition to purely utilitarian features, the investigations conducted on the magnetic properties have also provided results that are useful for analysis of the structure of the amorphous state. Magnetic methods, such as initial permeability, coercive force, and magnetic relaxation intensity, depend on the state of stress in the material and the concentration of microvoids. The results of investigations obtained with VSM vibrating magnetometer and presented in the form of hysteresis loop allow the examined alloys to be classified as soft magnetic materials. The obtained values for relative initial permeability μi and coercive force correlate with each other for ribbons and rods with the diameter of 1.5 mm.    (Figure 13a,b). A similar decrease of the coercive force H c was observed using the Fe 36 (Figure 13a,b) and 0.27 mm (Figure 13a,b).
The investigated ribbons of alloys retain their ferromagnetic properties at room temperature, showing low coercive force H c and high saturation magnetic polarization J s (Table 1, Figure 13a (Table 1).
For alloy rods containing copper, the saturation magnetic polarization J s was from 1.03 to 1.18 T (Figure 14b), which is similar to that of the base Fe 36.00 Co 36.00 B 19.00 Si 5 Nb 4 alloy-J s was from 0.97 to 1.17 T (Figure 14a). It has been reported that the addition of copper into Fe-Co-B-Si-Nb alloys improves the soft magnetic properties of the alloy, by decreasing the coercive force and increasing the relative initial magnetic permeability-µ i . This is probably connected with the impurity content. Jia reported that the Cu addition reduces the oxygen content of the alloy [2].
The alloy rods prepared on the basis of pure metals and non-metals have a lower coercive force [45][46][47] compared to the alloy rods prepared on the basis of ferroalloys, which is related to the reduced amount of impurities present in the alloy [2]. The use of high-purity charge materials makes it possible to obtain an increased critical casting thickness/diameter of BMGs [47,48]. The critical diameter of BMGs is the maximal dimension of the sample having an amorphous structure. The highest coercive force is observed in the rods with the diameter of 2.5 mm prepared on the basis of ferroalloys that can be ascribed to the appearance of boride phases in the structure of the alloy.
In the amorphous alloy, the domain walls are free to move due to the low magnetocrystalline anisotropy [26], resulting in a low H c . The lower coercivity could be obtained upon annealing well below T x in order to reduce the stress field in the as-cast amorphous matrix or structural relaxation phenomena. A relaxed amorphous phase in the material produced at lower rates of cooling is characterized by the presence of the local atomic order and magnetic structure (different than the nano-structure). Cu atom clusters are present in the amorphous structure of the Fe-Co-Si-Nb-B alloy containing Cu. The Cu atoms aggregate to form small clusters and the size of the clusters is approximately a few nm. [51,52]. Improvement of the soft magnetic properties of the alloy is caused by structural relaxation. However, when there are impurities in the alloy, they can pin the domain walls [26]. This might result in a high coercive force-H c [53,54].
Ribbons with lower thickness show higher magnetic permeability relaxation intensity after demagnetization (∆µ/µ) than those with higher thickness. The values of ∆µ/µ were 5, 3.6, and 3.0% using the Fe 36.00 Co 36.00 B 19.00 Si 5 Nb 4 alloy in the form of ribbons with the thicknesses of 0.07, 0.12, and 0.27 mm, respectively. In turn, higher values of ∆µ/µ = 11, 10, and 8% were obtained using the Fe 35.75 Co 35.75 B 18.90 Si 5 Nb 4 Cu 0.6 alloy in the form of ribbons with the thicknesss of 0.07, 0.12, and 0.27 mm, respectively ( Table 1). The magnetic permeability relaxation intensity after demagnetization is proportional to the concentration of microvoids (excess volume) in the magnetic metallic glasses [10,41,42]. The share of microvoids in the alloy depends on the conditions of the casting process.
Cu addition in the Fe-Co-Nb-Si-B alloy makes the magnetic permeability relaxation intensity increase after demagnetization (∆µ/µ) as a result of the presence of free volumes. Because copper has a lower melting temperature than iron and cobalt, it causes an increase of the free volume in the system during solidification. The copper diffusion produces a chemical inhomogeneity of the alloying elements, particularly Fe, which is induced by the Cu-cluster formation and which in turn causes an increase of the number of nucleation sites for the formation of the crystalline phase. The Cu atoms start to form a crystalline phase prior to the formation of the nanocrystalline phase [51,52].
Thermal methods, such as differential thermal analysis (DTA) (NETZSCH, Selb, Germany) and differential scanning calorimetry (DSC) (NETZSCH, Selb, Germany), allowed the characteristic temperatures of the examined alloys to be determined, i.e., crystallization onset temperature (T x1 ), eutectic temperature (T e ), melting point (T m ), and glass forming ability (GFA) indicators (T g -glass transition temperature; T l -liquidus temperature; ∆T x = T x1 − T g -range of supercooled liquid; T rg = T g /T l -reduced glass transition temperature) [42,55]. The chemical composition of the examined alloys was similar to the eutectic one, which assures their good glass forming ability [42,55].
In addition to purely utilitarian features, the investigations conducted on the magnetic properties have also provided results that are useful for analysis of the structure of the amorphous state. Magnetic methods, such as initial permeability, coercive force, and magnetic relaxation intensity, depend on the state of stress in the material and the concentration of microvoids. The results of investigations obtained with VSM vibrating magnetometer and presented in the form of hysteresis loop allow the examined alloys to be classified as soft magnetic materials. The obtained values for relative initial permeability µ i and coercive force correlate with each other for ribbons and rods with the diameter of 1.5 mm.

Conclusions
The results of this research provided the grounds for drawing the following conclusions:

•
Charge materials in the form of ferroalloys allow production of conventional and bulk metallic glasses with good soft magnetic properties, thus ensuring the reduction of manufacturing costs. For alloy rods containing copper, the saturation magnetic polarization J s was from 1.03 to 1.18 T, which is similar to that of the base Fe 36.00 Co 36.00 B 19.00 Si 5 Nb 4 alloy-J s was from 0.97 to 1.17 T. It is determined by the higher excess volume concentration for test specimens in the form of ribbons than for those in the form of rods, which is caused by the application of a higher cooling rate of ribbons.

•
The casting process parameters affect the share of microvoids in the alloy, and consequently the obtained magnetic properties. Ribbons with lower thickness show higher magnetic permeability relaxation intensity after demagnetization (∆µ/µ) than those with higher thickness. The magnetic permeability relaxation intensity after demagnetization is proportional to the concentration of microvoids (excess volume) in magnetic metallic glasses. Cu addition in the Fe-Co-Nb-Si-B alloy makes the magnetic permeability relaxation intensity increase after demagnetization (∆µ/µ) as a result of the presence of free volumes and Cu clusters in the amorphous structure of the alloy, even before the actual crystallization process has started. • Copper addition improves the soft magnetic properties of the alloy, by decreasing the coercive force and increasing the relative initial magnetic permeability-µ i . Lower coercive force is caused by structural relaxation, which allows the reduction in stress present in the amorphous matrix, or by the presence of Cu clusters in the alloys containing Cu. The presence of impurities in the alloy hinders the movement of domain walls, thus resulting in a high coercive force-H c .

•
The highest coercive force is observed in rods with the diameter of 2.5 mm. The deterioration in magnetic softness is connected with the appearance of boride phases in the structure of the alloy, which are characterized by strong magnetocrystalline anisotropy.