Figure 1 shows the XRD patterns of the as-melt spun ribbons. It can be seen that the curves for x = 3, 5, 7 all exhibit only one broad peak at 45°, indicating that each has a typical FeNi based amorphous structure. For the ribbon with x = 1, the XRD curve exhibits a sharp peak together with a broad peak, which suggests partial crystallization in the form of (Fe,Ni)₂₃B₆ with the Cr₂₃C₆ prototype structure. The Nb is a large radius atom which can enhance the atomic density of the whole alloy, and could result in the enhancement of the stability of the undercooled liquid and suppress the atomic rearrangement for the process of crystallization reaction. It clearly demonstrates that the GFA of the alloys is improved with the addition of Nb. Table 1 shows the actual atomic percentage of the alloy, determined by ICP. The result suggests that the examined composition of the ribbon is close to the nominal composition.

Fig. 1.

	Figure Option ViewDownloadNew Window
	Fig. 1. XRD patterns of (Fe40Ni40B19Cu1)100Nbx (x = 1, 3, 5, 7) as-melt-spun ribbons.

Table 1.

Table 1.

Table 1. Actual atomic percentage of (Fe40Ni40B19Cu1)100−xNbx (x = 1, 3, 5, 7) as-melt spun ribbons. . Nominal compositions Actual atomic percentage of each element/at.% Fe Ni B Cu Nb (Fe40Ni40B19Cu1)99Nb1 38.49 42.24 17.62 0.99 0.63 (Fe40Ni40B19Cu1)97Nb3 39.12 40.08 17.15 0.99 2.6 (Fe40Ni40B19Cu1)95Nb5 37.28 39.88 17.05 1.02 4.7 (Fe40Ni40B19Cu1)93Nb7 37.31 38.59 16.56 1.00 6.5

Table 1. Actual atomic percentage of (Fe40Ni40B19Cu1)100−xNbx (x = 1, 3, 5, 7) as-melt spun ribbons. .

Figure 2 shows the DSC curves of the as-melt spun ribbons. Two obvious exothermic peaks corresponding to two different crystallization processes can be seen in all the DSC curves. To further determine the crystallization behavior, we select (Fe₄₀Ni₄₀B₁₉Cu₁)₉₇Nb₃ ribbons for being pre-annealed at 653 K, 713 K, and 753 K. The XRD results in Fig. 3 show that the (Fe,Ni)₂₃B₆ precipitates prior to γ-(Fe, Ni) phase. Therefore, we can determine that the first exothermic peak of DSC, , the sharp peak on the broad peak of (Fe₄₀Ni₄₀B₁₉Cu₁)₉₉Nb₁ XRD pattern, is believed to result from the formation of (Fe, Ni)₂₃B₆ phase, while the second peak is believed to be due to the formation of γ (Fe, Ni) phase. The first crystallization temperature ( and the second crystallization temperature ( obtained from Fig. 2 are listed in Table 2. Clearly, with the increase of Nb content, increases from 696 K to 754 K and increases from 712 K to 829 K. As a result, the corresponding increases from 16 K to 75 K, indicating that with the increase of Nb content, the thermal stability of the (Fe₄₀Ni₄₀B₁₉Cu₁)_100-−xNb_x ribbons is gradually improved. The effect of Nb element on the onset crystallization temperature of the secondary crystalline phase is stronger than that of the first crystalline phase, which results in the gradual increase of . Meanwhile, with the increase of Nb content, the exothermic peak width of the secondary crystalline phase increases gradually, indicating that the precipitation process of the γ (Fe, Ni) phase is sensitive to Nb. This may be due to the enhancement of L-S percolating network structure with the increase of Nb, leading to a higher degree of dense and randomly packed atomic configuration.^[20,21] Therefore, the diffusion process of atoms becomes more difficult and further affects the crystallization process.

Fig. 2.

	Figure Option ViewDownloadNew Window
	Fig. 2. DSC curves of (Fe40Ni40B19Cu1)100−xNbx (x = 1,3,5,7) as-melt spun ribbons.

Fig. 3.

	Figure Option ViewDownloadNew Window
	Fig. 3. XRD patterns of as-melt-spun (Fe40Ni40B19Cu1)97Nb3 ribbons annealed at different temperatures.

Table 2.

Table 2.

Table 2. Thermal stabilities of (Fe40Ni40B19Cu1)100-−xNbx (x = 1, 3, 5, 7) as-melt spun ribbons . Thermal stabilities Ribbons /K /K /K (Fe40Ni40B19Cu1)99Nb1 696 712 16 (Fe40Ni40B19Cu1)97Nb3 713 760 47 (Fe40Ni40B19Cu1)95Nb5 736 801 65 (Fe40Ni40B19Cu1)93Nb7 754 829 75

Table 2. Thermal stabilities of (Fe40Ni40B19Cu1)100-−xNbx (x = 1, 3, 5, 7) as-melt spun ribbons .

Figure 4 shows the plots of saturation magnetization (M_s) versus coercivity (H_c) of the as-melt spun ribbons with different Nb content. The specific values of M_s and H_c are summarized in Fig. 5. The lowest H_c of 2 A/m for x = 3 is obtained. Moreover, the M_s decreases monotonically from to with Nb content increasing from x = 1 to x = 7. It is well known that the saturation magnetization (M_s) reflects the quantity of total magnetic moment. According to the energy band theory, increasing Nb content means that the concentration of Fe and Ni decrease, thereby leading the total atomic magnetic moment to decrease, and resulting in the decline of M_s. The H_c is known as a structure-sensitive quantity, and it is linked to the internal stress, the relaxation and the local structure. The H_c is relatively high with x = 1. The reason may be that the low volume fraction of crystalline phase produced pinning effect hinders the movement of magnetic domains. Table 3 lists the values of H_c and B_s (or M_s) for the (Fe₄₀Ni₄₀B₁₉Cu₁)₉₇Nb₃ alloy and some related alloys. Compared with other FeNi magnetic alloys, the as-melt spun ribbon with 3 at.% Nb exhibits excellent soft magnetic properties. It has a relatively low coercivity of 2 A/m and large saturation magnetization of . Therefore, considering the minimum H_c and relatively high M_s, the ribbon with x = 3 is annealed for further research.

Fig. 4.

	Figure Option ViewDownloadNew Window
	Fig. 4. Saturation magnetization (Ms) versus coercivity (Hc) of as-melt spun (Fe40Ni40B19Cu1)100-−xNbx (x = 1, 3, 5, 7) ribbons.

Fig. 5.

	Figure Option ViewDownloadNew Window
	Fig. 5. Saturation magnetization (Ms) and coercivity (Hc) versus Nb content for (Fe40Ni40B19Cu1)100-−xNbx (x = 1, 3, 5, 7) alloy.

Table 3.

Table 3.

Table 3. Values of Hc and Bs (or Is) of FeNi magnetic alloys. . Materials Hc/(A/m) Bs/T or References Fe40Ni38Mo4B18 31.4 .637 T [22] Fe40Ni40P14B6 – [23] (Fe0.39Ni0.39B0.16P0.06)97.5Nb2.5 1.0 0.73 T [24] Fe42Ni41.7 C7Si4.5B3.9P0.9 822.4 [25] (Fe40Ni40B19Cu1)97Nb3 2.0 this work

Table 3. Values of Hc and Bs (or Is) of FeNi magnetic alloys. .

Figure 6 shows the XRD patterns of the (Fe₄₀Ni₄₀B₁₉Cu₁)₉₇Nb₃ ribbons annealed at 809 K for different times. It is found that the primary phase is (Fe,Ni)₂₃B₆ which precipitates in a very short annealing time of 10 s, followed by the occurrence of FCC γ (Fe, Ni) phase at 0.5 min and then further growing with annealing time increasing. Firstly, the atomic radius of Fe is similar to that of Ni, so that there is a competitive relationship between the formation of γ (Fe, Ni) phase and that of α-Fe phase in the annealing process. On the other hand, the thermal stability of α-Fe phase at high temperature is lower than that of γ (Fe, Ni) phase, so that γ (Fe, Ni) phase prefers to precipitate from an amorphous matrix.^[26,27] In contrast, the enthalpy as well as the stability of (Fe,Ni)₂₃B₆ phase increases with high Ni content increasing. The short annealing time for the formation of (Fe,Ni)₂₃B₆ phase is probably attributed to its instantaneous crystallization process without substantial rearrangement of constituent atoms over longer distance.^[28] On the other hand, due to the non-solubility of B in γ-(Fe, Ni) phase, the residual amorphous phase surrounding γ (Fe, Ni) is enriched with B increasing and forms a barrier inhibiting γ (Fe, Ni) phase for further growth, leading to a longer crystallization time than the (Fe, Ni)₂₃B₆ phase.^[14,29]

Fig. 6.

	Figure Option ViewDownloadNew Window
	Fig. 6. XRD patterns of as-melt-spun (Fe40Ni40B19Cu1)97Nb3 ribbons annealed at 809 K for different times.

The values of crystallization activation energy E₁ ((Fe,Ni)₂₃B₆) and E₂ (γ-(Fe, Ni)) for the first 2 exothermic peaks of (Fe₄₀Ni₄₀B₁₉Cu₁)₉₇Nb₃ ribbons are calculated by both Kissinger equation and Ozava equation and the results are listed in Table 4. Clearly, the value of E₂ is larger than that of E₁, which further confirms the time required for the precipitation of γ-(Fe, Ni) phase is longer than that of the (Fe,Ni)₂₃B₆ phase.

Table 4.

Table 4.

Table 4. Crystallization activation energy of as-melt-spun (Fe40Ni40B19Cu1)97Nb3 ribbons. . Method Kissinger 259.67 437.65 Ozava 258.38 428.39

Table 4. Crystallization activation energy of as-melt-spun (Fe40Ni40B19Cu1)97Nb3 ribbons. .

In order to further investigate the effect of grain size on soft magnetic properties, the average grain size of (Fe, Ni)₂₃B₆ phase and γ (Fe, Ni) phase for the (Fe₄₀Ni₄₀B₁₉Cu₁)₉₇Nb₃ ribbons are estimated by using Scherrerʼs equation and the results are shown in Fig. 7. It can be seen that the grain size of (Fe, Ni)₂₃B₆ increases from 33 nm to 38 nm with annealing time increasing from 10 s to 0.5 min and then remains almost unchanged with further annealing while the grain size for γ (Fe, Ni) keeps increasing from 10 nm to 80 nm as annealing time rises, indicating the precipitation of γ (Fe, Ni) can restrain the growth of (Fe, Ni)₂₃B_6, which is confirmed by the corresponding peak intensity in the XRD patterns in Fig. 6.

Fig. 7.

	Figure Option ViewDownloadNew Window
	Fig. 7. Dependence of grain size of (Fe, Ni)23B6 and γ-(Fe, Ni) on annealing time in (Fe40Ni40B19Cu1)97Nb3 ribbon at 809 K.

Figure 8 shows the changes of H_c and H_s with annealing time for the (Fe₄₀Ni₄₀B₁₉Cu₁)₉₇Nb₃ ribbon. It is found that as the annealing time increases, H_c exhibits an obvious upward trend and eventually reaches to ∼42.2 A/m. As indicated in the results of Hernando model, soft magnetic properties depend strongly on the nature of the nanocrystal + amorphous+nanocrystal coupling. The increase of H_c after annealing the ribbon for 1 min is attributed to weak intergranular exchange due to the fact that amorphous phases are converted into (Fe, Ni)₂₃B₆ and γ-(Fe, Ni), while the internal stress has not yet been completely released. When the annealing time increases to 15 min, the internal stress σ disappears, which leads to a slight decrease in H_c.^[30,31] However, the gradual growth of (Fe, Ni)₂₃B₆ and γ-(Fe, Ni) grains cause H_c to rise, thus deteriorating the soft magnetic properties. Compared with other FeNi-based alloys,^[32–34] despite lacking α-Fe phase due to the low stability of α -Fe phase in high Ni content FeNi-based alloys, the ribbon still exhibits a relatively low coercivity due to the absence of hard magnetic phases Fe₂B and Fe₃B. Moreover, M_s is low in the early stage of annealing, which is due to the low volume fraction of γ-(Fe, Ni) phase. However, with further annealing, both (Fe, Ni)₂₃B₆ phase and γ-(Fe, Ni) phase are precipitated from the amorphous phase, leading to the increase of M_s and then remaining nearly constant at after being annealed for 0.5 min. As M_s is an insensitive parameter, its value is independent of size, distribution and morphology of grains, so M_s maintains a stable value.

Fig. 8.

	Figure Option ViewDownloadNew Window
	Fig. 8. Dependence of Hc and Ms on annealing time for (Fe40Ni40B19Cu1)97Nb3 ribbon at 809 K.

Figure 9 shows the high resolution transmission electron micrograph and selected area electron diffraction (SAED) pattern of the as-melt spun (Fe₄₀Ni₄₀B₁₉Cu₁)₉₇Nb₃ ribbon. Clearly, the homogeneous amorphous structure is consistent with the XRD pattern. The TEM image in Fig. 10(a) shows that the ribbon is nearly full of nano grains (∼40 nm on average) in contact with each other with a little amorphous phase, which is consistent with the XRD patterns as shown in Fig. 6. In such a case, the ferromagnetic exchange reaction is greatly enhanced which explains that the coercivity remains at a relative low value though there are no ferromagnetic nanocrystallites like α-Fe. Moreover, the three diffraction rings corresponding to the crystalline structures of (Fe, Ni)₂₃B₆ are indexed by planes (422) and (531), and γ-(Fe, Ni) phase is indexed by plane (111). In addition, the detailed analysis of the interplanar fringes of the nanograins confirms the fcc-FeNi structure with the (111) spacing of 0.208 nm in Fig. 10(b) and (Fe, Ni)₂₃B₆ with the (420) spacing of 0.235 nm in Fig. 10(c).

Fig. 9.

	Figure Option ViewDownloadNew Window
	Fig. 9. (a) TEM bright field image and (b) SAED pattern for as-melt-spun (Fe40Ni40B19Cu1)97Nb3 ribbon.

Fig. 10.

	Figure Option ViewDownloadNew Window
	Fig. 10. (a) TEM bright field images, SAED patterns, and grain size distribution for (Fe40Ni40B19Cu1)97Nb3 ribbon after being annealed at 809 K for 0.5 min. (b) and (c) HREM of (Fe40Ni40B19Cu1)97Nb3 ribbon after being annealed at 809 K for 0.5 min with marking different interplanar spacing values.