Optimum Soft Magnetic Properties of the FeSiBNbCu Alloy Achieved by Heat Treatment and Tailoring B / Si Ratio

: To increase the saturation magnetization (M s ) of commercially available soft magnetic Finemet alloys to the level comparable to that of Si-steel and Fe-based nanocrystalline alloys such as Nanoperm, Nanomet, the B or Si content in combination with annealing heat treatment was tailored. The ribbons of Fe 95 − x Si 1 B x Nb 3 Cu 1 ( x = 11, 12, 13) and Fe 87 − x Si x B 9 Nb 3 Cu 1 ( x = 6, 8, 10) were prepared by melt-spinning and annealed at di ﬀ erent temperatures to develop nanocrystalline microstructure optimizing the soft magnetic properties. The magnetic properties of the as-spun and annealed ribbons were measured using a vibrating sample magnetometer and AC B-H loop tracer to acquire M s of above 1.4 T in all as-spun ribbons. Among the alloys, Fe 84 Si 1 B 11 Nb 3 Cu 1 annealed at 545 ◦ C showed the highest M s of 2 T, which exceeds that of the conventional Finemet and other Fe-based nanocrystalline alloys.


Introduction
Due to their superior soft magnetic properties, magnetic materials such as Si-steel have been widely used to produce actuators, sensors, transformer cores, or electric motors [1,2]. Recently, attention has been paid to low core loss (P cv ) due to the increasing demand on energy efficiency [3]. Fe-based soft magnetic or nanocrystalline alloys could be a strong candidate for the emerging engineering technologies thanks to their potential for low P cv and sufficient mechanical properties [4][5][6]. However, the saturation magnetization (M s ) of these materials is relatively lower than that of Si-steels [4]. Therefore, improving the soft magnetic properties of the alloys to keep P cv low and, at the same time, to increase M s to the level exceeding that of Si-steel is required for the development of next-generation soft magnetic materials.
Finemet alloys are based on Fe-Si-B-Nb-Cu, which are derived from the conventional Fe-Si-B system with a minor addition of Cu and Nb [7]. High soft magnetic properties in these alloys are originated from the precipitation of nanocrystalline α-Fe dispersed in an amorphous matrix [8][9][10][11]. Although the overall magnetic properties of Finemet were considered innovatory when it was first produced by Yoshizawa et.al in 1988 [10], currently, there are some competing nanocrystalline alloys that have remarkable soft magnetic properties including Nanoperm, Hitperm, or Nanomet. These alloys show very high saturation magnetization flux density (B s ) above 1.5 T higher than that of Finemet which is 1.23 T [12][13][14][15]. However, the Finemet-based alloys are still attractive as soft magnetic materials because of the potential for further improvement including excellent magnetic permeability (µ r~1 0 3 at the frequency of 1 kHz), low coercivity (H c ), as well as low P cv (~300 kW/m 3 ), which make the alloys suitable for the use in electric power applications [16].
There are numerous reports that the substitution of elements in the Finemet alloy leads to the improvement of its magnetic properties. In particular, the Finemet-based alloys with a higher Fe content or with B replacing part of Si of a Fe-Si-B-Cu alloy system resulted in an increase of their soft magnetic properties although the latter case induced higher H c [7,17]. Therefore, effect of B and Si content variation in connection with the Fe content on the soft magnetic properties would be of interest in the further optimization of the Finemet-based alloy system. In this study, we focused on increasing the value of saturation magnetization M s while maintaining low P cv by varying the B or Si contents. We modified the atomic concentration of B or Si in the Finemet-based alloy system in exchange with Fe content. In addition, in order to further optimize soft magnetic properties, annealing treatment at various temperatures (T a ) was applied.

Materials and Methods
Multicomponent ingots with the compositions of Fe 95−x Si 1 B x Nb 3 Cu 1 (x = 11, 12, 13) and Fe 87−x Si x B 9 Nb 3 Cu 1 (x = 6, 8, 10) were prepared by arc melting under Ti-gettered argon atmosphere and remelted at least four times for homogeneity. Amorphous ribbons were produced by using melt-spinner in an argon atmosphere with a wheel speed of 56.3 m/s. The width and thickness of ribbons were 2 mm and 20-30 µm, respectively. As-spun ribbons were annealed in a vertical furnace at various temperatures for 60 min under an argon atmosphere. The cooling rate that can be achieved in this method was of order 0.67 • C/s. The structural properties of as-spun ribbons were identified by X-ray diffraction (XRD) with Cu-Kα radiation (D8 Advance, Bruker, Germany). The M s was measured with a vibrating sample magnetometer (VSM) at room temperature under the in-plane applied magnetic field ranging from −10,000 to 10,000 Oe. Additionally, the density of the specimens was determined using a helium pycnometer (AccuPyc II, Micromeritics). The µ r and P cv were investigated by using an AC B-H loop tracer. The values of µ r were measured under the maximum applied filed (H m ) of 800 A/m, and the values of P cv were measured under a frequency (f) of 100 kHz and the H m of 0.1 A/m. For annealing, a temperature above the crystallization temperature (T x ) of the as-spun amorphous ribbons was chosen for each specimen. T x was measured by differential scanning calorimetry (DSC) at a heating rate of 0.34 • C/s.

Results and Discussion
The atomic structure of as-spun Fe 95−x Si 1 B x Nb 3 Cu 1 (x = 11, 12, 13) and Fe 87−x Si x B 9 Nb 3 Cu 1 (x = 6, 8, 10) ribbons was determined using XRD where Cu-Kα radiation was used. Figure 1 shows the XRD patterns for the as-spun ribbons with variation of Si and B content. The patterns of ribbons with different B contents (upper 3 patterns in Figure 1) consist of broad halos without any sharp diffraction peaks corresponding to crystalline phases, indicating that the structure is fully amorphous for the alloys considered here. However, the as-spun ribbons of Fe 81 Si 6 B 9 Nb 3 Cu 1 and Fe 79 Si 8 B 9 Nb 3 Cu 1 (4th and 5th patterns from the top of the Figure 1, respectively) have obvious crystalline peaks in the 2θ range of 40-50 • . This suggests that the ribbons are partially crystallized. The alloys Fe 95−x Si 1 B x Nb 3 Cu 1 (x = 11, 12, 13) and Fe 77 Si 10 B 9 Nb 3 Cu 1 (x = 10, y = 9) maintain amorphous phase in as-spun state suggesting their good glass-forming ability (GFA).
The hysteresis loops for Fe 95−x Si 1 B x Nb 3 Cu 1 (x = 11, 12, 13) and Fe 87−x Si x B 9 Nb 3 Cu 1 (x = 6, 8, 10) of the as-spun ribbons are shown in Figure 2. All specimens exhibit high soft magnetic properties with a high M s about of 1.4-1.5 T. The measured M s for all as-spun ribbons are summarized in Table 1.   As can be seen in Table 1, with decrease in the Si or B content, the values of Ms increase mainly because of the accompanying increase in Fe content [18].
For the optimum conditions of formation, the dependence of the onset crystallization on temperature and Cu content is noteworthy [19]. It was determined by DSC. The DSC curves, which   As can be seen in Table 1, with decrease in the Si or B content, the values of Ms increase mainly because of the accompanying increase in Fe content [18].
For the optimum conditions of formation, the dependence of the onset crystallization on temperature and Cu content is noteworthy [19]. It was determined by DSC. The DSC curves, which  As can be seen in Table 1, with decrease in the Si or B content, the values of M s increase mainly because of the accompanying increase in Fe content [18].
For the optimum conditions of formation, the dependence of the onset crystallization on temperature and Cu content is noteworthy [19]. It was determined by DSC. The DSC curves, which indicate the crystallization behavior of the amorphous ribbons with different B or Si content, are shown in Figure 3a,b and compared with Finemet as a reference (placed on top of both Figure 3a,b with composition of Fe 73.5 Si 13.5 B 9 Nb 3 Cu 1 ). The exothermic peaks are observed for all ribbons, which suggest the crystallization reaction of the as-spun alloys. The T x values are increased from 394 to 422 • C and from 425 to 475 • C along with an increase in the B content from 11 to 13 at.% and the Si content from 6 to 10 at.%, respectively. For the Fe-based amorphous alloys that are mostly intended for the soft magnetic applications, the first crystalline phase precipitating at the lowest temperature is likely to be α-iron, which dominantly contributes to the high magnetization. Therefore, for commercial nanocrystalline alloys, the suitable T a is usually in the range between the end of the first crystallization and the start of the second crystallization to effectively control the volume fraction, precipitate size, and distribution of the α-iron [20]. Based on the acquired T x values, the annealing temperatures (T a ) were determined. For obtaining the nanocrystalline state, each as-spun ribbon was annealed under argon atmosphere at various T a covering a wide range. Four different annealing temperatures for each alloy were applied: the lowest T a is near the onset temperature T x with three more annealing conditions of higher temperature [19]. The values of T a and T x are listed in Table 2.
Metals 2020, 10, x FOR PEER REVIEW 4 of 9 indicate the crystallization behavior of the amorphous ribbons with different B or Si content, are shown in Figure 3a,b and compared with Finemet as a reference (placed on top of both Figure 3a,b with composition of Fe73.5Si13.5B9Nb3Cu1). The exothermic peaks are observed for all ribbons, which suggest the crystallization reaction of the as-spun alloys. The Tx values are increased from 394 to 422 °C and from 425 to 475 °C along with an increase in the B content from 11 to 13 at.% and the Si content from 6 to 10 at.%, respectively. For the Fe-based amorphous alloys that are mostly intended for the soft magnetic applications, the first crystalline phase precipitating at the lowest temperature is likely to be α-iron, which dominantly contributes to the high magnetization. Therefore, for commercial nanocrystalline alloys, the suitable Ta is usually in the range between the end of the first crystallization and the start of the second crystallization to effectively control the volume fraction, precipitate size, and distribution of the α-iron [20]. Based on the acquired Tx values, the annealing temperatures (Ta) were determined. For obtaining the nanocrystalline state, each as-spun ribbon was annealed under argon atmosphere at various Ta covering a wide range. Four different annealing temperatures for each alloy were applied: the lowest Ta is near the onset temperature Tx with three more annealing conditions of higher temperature [19]. The values of Ta and Tx are listed in Table 2.  After cooling down to room temperature, the annealed ribbons were measured again by VSM. Figure 4 depicts the hysteresis loops for the Fe84Si1B11Nb3Cu1 alloy, for their as-spun state and after annealing treatment at 395, 445, 495 and 545 °C for 60 min. The Fe84Si1B11Nb3Cu1 alloy has been selected since this alloy shows the highest values of Ms among the annealed alloys with different element atomic ratio. In addition, Figure 5a,b shows the variation of Ms and Hc of the Fe84Si1B11Nb3Cu1 along with Ta. The graphs show the trend that the tendencies of Ms and Hc are opposite. As can be seen in Figure 5, there is a correlation between Ms and Ta. Ms considerably increased with an increase of Ta. Furthermore, Ms decreased at Ta = 445 °C, then reached the maximum value of 2.06 T at 545 °C. After cooling down to room temperature, the annealed ribbons were measured again by VSM. Figure 4 depicts the hysteresis loops for the Fe 84 Si 1 B 11 Nb 3 Cu 1 alloy, for their as-spun state and after annealing treatment at 395, 445, 495 and 545 • C for 60 min. The Fe 84 Si 1 B 11 Nb 3 Cu 1 alloy has been selected since this alloy shows the highest values of M s among the annealed alloys with different element atomic ratio. In addition, Figure 5a,b shows the variation of M s and H c of the Fe 84 Si 1 B 11 Nb 3 Cu 1 along with T a . The graphs show the trend that the tendencies of M s and H c are opposite. As can be seen in Figure 5, there is a correlation between M s and T a . M s considerably increased with an increase of T a . Furthermore, M s decreased at T a = 445 • C, then reached the maximum value of 2.06 T at 545 • C. Based on the DSC patterns in Figure 3, we propose that structural reordering, eventually leading to crystallization, begins at annealing temperatures of around 394 • C. The alloy becomes magnetically harder after annealing in the temperature range between 395 and 445 • C compared with either its relaxed amorphous state or the nanostructured state after crystallization [21].
Metals 2020, 10, x FOR PEER REVIEW 5 of 9 Based on the DSC patterns in Figure 3, we propose that structural reordering, eventually leading to crystallization, begins at annealing temperatures of around 394 °C. The alloy becomes magnetically harder after annealing in the temperature range between 395 and 445 °C compared with either its relaxed amorphous state or the nanostructured state after crystallization [21].  For Fe84Si1B11Nb3Cu1, Ta = 545 °C, the Ms is 2.06 T, which is the highest value of all the considered alloys. Moreover, the lowest Hc measured at 1 kHz is 114.52 A/m when Ta = 545 °C at which the highest Ms is exhibited. The highest Hc measured at a 1 kHz is 301.88 A/m at Ta = 445 °C. This change in magnetic behavior with Ta containing both Cu and Nb can be interpreted based on the report that observed similar cases [21]. The latter magnetic hardening appears to be a consequence of the appearance of Cu-enriched clusters, which form even before the Tx. Simultaneously with Cu precipitation, α-Fe grains start to nucleate. Both the Cu-enriched clusters and the α-FeSi nanocrystals would act as pinning centers for the domain wall displacements, thereby increasing Hc [22]. Through the annealing treatment, the residual stress was removed, and it resulted in the structural relaxation. Therefore, the Hc were considerably reduced due to the reduced free volume and the nucleated clusters [22,23]. As a result, the Fe84Si1B11Nb3Cu1 represented the excellent soft magnetic properties such as high Ms and low Hc at Ta = 545 °C. The detailed magnetic properties such as Ms and Hc of the Fe84Si1B11Nb3Cu1 ribbons are summarized in Table 3. The annealed Fe84Si1B11Nb3Cu1 ribbons exhibit a Based on the DSC patterns in Figure 3, we propose that structural reordering, eventually leading to crystallization, begins at annealing temperatures of around 394 °C. The alloy becomes magnetically harder after annealing in the temperature range between 395 and 445 °C compared with either its relaxed amorphous state or the nanostructured state after crystallization [21].  For Fe84Si1B11Nb3Cu1, Ta = 545 °C, the Ms is 2.06 T, which is the highest value of all the considered alloys. Moreover, the lowest Hc measured at 1 kHz is 114.52 A/m when Ta = 545 °C at which the highest Ms is exhibited. The highest Hc measured at a 1 kHz is 301.88 A/m at Ta = 445 °C. This change in magnetic behavior with Ta containing both Cu and Nb can be interpreted based on the report that observed similar cases [21]. The latter magnetic hardening appears to be a consequence of the appearance of Cu-enriched clusters, which form even before the Tx. Simultaneously with Cu precipitation, α-Fe grains start to nucleate. Both the Cu-enriched clusters and the α-FeSi nanocrystals would act as pinning centers for the domain wall displacements, thereby increasing Hc [22]. Through the annealing treatment, the residual stress was removed, and it resulted in the structural relaxation. Therefore, the Hc were considerably reduced due to the reduced free volume and the nucleated clusters [22,23]. As a result, the Fe84Si1B11Nb3Cu1 represented the excellent soft magnetic properties such as high Ms and low Hc at Ta = 545 °C. The detailed magnetic properties such as Ms and Hc of the Fe84Si1B11Nb3Cu1 ribbons are summarized in Table 3. The annealed Fe84Si1B11Nb3Cu1 ribbons exhibit a For Fe 84 Si 1 B 11 Nb 3 Cu 1 , T a = 545 • C, the M s is 2.06 T, which is the highest value of all the considered alloys. Moreover, the lowest H c measured at 1 kHz is 114.52 A/m when T a = 545 • C at which the highest M s is exhibited. The highest H c measured at a 1 kHz is 301.88 A/m at T a = 445 • C. This change in magnetic behavior with T a containing both Cu and Nb can be interpreted based on the report that observed similar cases [21]. The latter magnetic hardening appears to be a consequence of the appearance of Cu-enriched clusters, which form even before the T x . Simultaneously with Cu precipitation, α-Fe grains start to nucleate. Both the Cu-enriched clusters and the α-FeSi nanocrystals would act as pinning centers for the domain wall displacements, thereby increasing H c [22]. Through the annealing treatment, the residual stress was removed, and it resulted in the structural relaxation. Therefore, the H c were considerably reduced due to the reduced free volume and the nucleated clusters [22,23]. As a result, the Fe 84 Si 1 B 11 Nb 3 Cu 1 represented the excellent soft magnetic properties such as high M s and low H c at T a = 545 • C. The detailed magnetic properties such as M s and H c of the Fe 84 Si 1 B 11 Nb 3 Cu 1 ribbons are summarized in Table 3. The annealed Fe 84 Si 1 B 11 Nb 3 Cu 1 ribbons exhibit a very high M s > 200 emu/g (1.8 T) at all T a . These values are considered very high, which are higher than that of Finemet or Fe-based nanocrystalline alloys of~1.5 T [7,[13][14][15]. The H c values of the annealed Fe 84 Si 1 B 11 Nb 3 Cu 1 significantly decreased from 272.96 to 114.52 at 1 kHz, from 329.01 to 157.18 at 10 kHz, and from 357.94 to 184.83 at 20 kHz along with increasing T a .
In Figure 6, the annealing temperature dependence of µ r and P cv of the Fe 84 Si 1 B 11 Nb 3 Cu 1 alloy are shown. The graph shows a trend that the tendencies of H c and P cv are similar. In addition, the values of µ r increase with increasing T a . The smallest value of P cv~4 06 mW/cm 3 and the highest value of µ r~1 340 are present in Fe 84 Si 1 B 11 Nb 3 Cu 1 annealed at 545 • C at the f of 1 kHz. These values of µ r and P cv are superior soft magnetic properties compared to the conventional value of Finemet (µ r~1 0 3 , P cv~3 00 kW/m 3 at the f of 1 kHz) [14]. Annealing at a higher temperature leads to an increase in the number density of nanocrystals. However, if the T a is as high as the second crystallization temperature, the overall properties such as µ r and P cv deteriorate due to the formation of other compounds, for instance, iron boride phases such as Fe 3 B (tetragonal structure) and Fe 23 B 6 (fcc structure) [24]. The µ r and P cv values of all specimens are compared in Table 4. Overall, the alloys show similar tendencies with Fe 84 Si 1 B 11 Nb 3 Cu 1 . The annealed Fe 79 Si 8 B 9 Nb 3 Cu 1 exhibits the highest µ r , and the annealed Fe 79 Si 10 B 9 Nb 3 Cu 1 exhibits the lowest P cv~1 00 mW/cm 3 at the f of 1 kHz. Although there is no obvious correlation between P cv and T a , the P cv values of the alloys with variation of Si ratio is relatively lower than that of the alloys with variation of B ratio. In Figure 6, the annealing temperature dependence of μr and Pcv of the Fe84Si1B11Nb3Cu1 alloy are shown. The graph shows a trend that the tendencies of Hc and Pcv are similar. In addition, the values of μr increase with increasing Ta. The smallest value of Pcv ~ 406 mW/cm 3 and the highest value of μr ~ 1340 are present in Fe84Si1B11Nb3Cu1 annealed at 545 °C at the f of 1 kHz. These values of μr and Pcv are superior soft magnetic properties compared to the conventional value of Finemet (μr ~ 10 3 , Pcv ~ 300 kW/m 3 at the f of 1 kHz) [14]. Annealing at a higher temperature leads to an increase in the number density of nanocrystals. However, if the Ta is as high as the second crystallization temperature, the overall properties such as μr and Pcv deteriorate due to the formation of other compounds, for instance, iron boride phases such as Fe3B (tetragonal structure) and Fe23B6 (fcc structure) [24]. The μr and Pcv values of all specimens are compared in Table 4. Overall, the alloys show similar tendencies with Fe84Si1B11Nb3Cu1. The annealed Fe79Si8B9Nb3Cu1 exhibits the highest μr, and the annealed Fe79Si10B9Nb3Cu1 exhibits the lowest Pcv ~100 mW/cm 3 at the f of 1 kHz. Although there is no obvious correlation between Pcv and Ta, the Pcv values of the alloys with variation of Si ratio is relatively lower than that of the alloys with variation of B ratio.

Conclusions
In this study, soft magnetic properties of Fe 95−x Si 1 B x Nb 3 Cu 1 (x = 11, 12, 13) and Fe 87−x Si x B 9 Nb 3 Cu 1 (x = 6, 8, 10) alloys were investigated by tailoring B or Si content ratio. All alloys were annealed at the various T a in order to achieve the precipitation of nanocrystalline α-Fe phase and to optimize the soft magnetic properties.
The X-ray diffraction patterns of the as-spun alloys with variation of Si and B content reveal an amorphous structure except for Fe 81 Si 6 B 9 Nb 3 Cu 1 and Fe 79 Si 8 B 9 Nb 3 Cu 1 , which are partially crystalline. As-spun Fe 95−x Si 1 B x Nb 3 Cu 1 (x = 11, 12, 13) alloys can maintain amorphous phase because of the high glass-forming ability of B.
In addition, the values of M s , H c , µ r and P cv were investigated. Annealed Fe 84 Si 1 B 11 Nb 3 Cu 1 exhibits excellent M s values above 1.8 T at the various annealing temperatures, T a and when the T a is 545 • C, the M s value reaches maximum of about 2 T, which is higher than that of Finemet or Fe-based nanocrystalline alloys of about 1.5 T with the low H c value less than 120 A/m. Moreover, the values of µ r and P cv are superior soft magnetic properties compared to the conventional value of Finemet.

Conflicts of Interest:
The authors declare no conflict of interest.