Effect of heat treatment on the microstructure and soft magnetic properties of nanocrystalline Fe_80.5−xCo_xCu_1.5Si₄B₁₄ alloys (x = 0, 2, 4, 10)

Abstract

The improvement of soft magnetic properties at elevated temperature was realized by doping Co into the Nb-free Fe_80.5Cu_1.5Si₄B₁₄ alloy. The Curie temperature of amorphous alloy T_C increases to 400 °C when 10 at.% Fe is substituted by Co. For the nanocrystalline Fe_70.5Co₁₀Cu_1.5Si₄B₁₄ alloy, a considerable initial permeability μ_i can be maintained above 450 °C, which is much higher than the T_C of amorphous alloy. It is found that the cyclic annealing is effective to improve the saturation flux density B_S, which is attributed to the enhanced crystallization volume fraction. Especially for the nanocrystalline Fe_78.5Co₂Cu_1.5Si₄B₁₄ alloy, the B_S is increased to 1.83 T. Furthermore, the cyclic annealing reduces the grain size, giving rise to the decrease of coercivity H_C and the increase of μ_i.

1 Introduction

FeCuSiB nanocrystalline alloys have been extensively studied for their ability to realize both high magnetic flux density B_S and low iron loss P simultaneously [1,2,3,4,5]. Their B_S is more than 20% larger than that of conventional Fe-based nanocrystalline alloys [6], and P is about the half of Si steels [7]. The excellent soft magnetic properties of the FeCuSiB nanocrystalline alloy are attributed to the high-density α-Fe nanocrystals as well as the high Fe content.

There has been an increasing need for the identification of new materials with superior soft magnetic properties for high-temperature applications, while the high-temperature magnetic softness of Nb-free FeCuSiB alloys was seldom dealt with in the past literatures because of the difficulty to realize for this alloy family. Generally, Fe-based nanocrystalline alloy cannot be used at elevated temperature due to the relatively low Curie temperature of the residual amorphous matrix [8]. The partial substitution of Fe by Co in Nb-containing FeCuSiB alloy has confirmed the improvement of magnetic response at high temperatures with respect to the Co-free alloys [9], which is ascribed to the reinforced magnetic exchange coupling between the crystals [10]. However, superfluous Co doping into Nb-containing FeCuSiB would damage the magnetic softness because of larger magnetostriction [11]. It has been confirmed that the substitution of 10 at.% Co for Fe in Fe_74.5Nb₂Si_17.5B₅Cu₁ alloy can lower the onset primary crystallization temperature and enhance crystallized interval temperature [12]. Thus, the substitution of Fe with appropriate amount of Co in Nb-free FeCuSiB alloy should be a possible approach to realize the improvement of high-temperature magnetic softness. Additionally, the uniform and fine nanostructure is the key to obtain excellent soft magnetic properties for nanocrystalline alloys. The high Fe content in FeCuSiB alloy usually leads to the formation of pre-existing α-Fe nuclei in amorphous precursors [13] and these nuclei will grow in size rapidly during the annealing process, which results in a non-uniform precipitation of α-Fe grains [14]. It has been reported that the cyclic annealing is effective to refine and uniform α-Fe grains, the B_S of FeBCu nanocrystalline alloy was increased and the H_C was decreased simultaneously when the alloy was subjected to cyclic annealing [15].

In this work, Fe_80.5−xCo_xCu_1.5Si₄B₁₄ (x = 0, 2, 4, 10) amorphous alloys were fabricated on the purpose of improvement of magnetic softness at elevated temperature. The alloys were subjected to cyclic annealing to achieve the refined nanostructure and thus the excellent soft magnetic properties.

2 Experimental procedure

Multicomponent alloy ingots with nominal compositions of Fe_80.5−xCo_xCu_1.5Si₄B₁₄ (x = 0, 2, 4, 10) were synthesized by melting the mixtures of raw materials Fe (99.9 mass%), Si (99.99 mass%), Fe-18 mass% B, Cu (99.9%) and Co (99.9%) at an argon atmosphere using high frequency induction melting technology. Amorphous ribbons with about 1 mm width and 30 μm thickness were fabricated by single-roller melt spinning method under argon atmosphere onto a copper wheel with circumferential speed of 4000 r/min. These ribbons were wound into toroidal core samples of 16 mm inner diameter and 18 mm outer diameter. To obtain a two-phase composite structure, the melt-spun ribbons and cores were annealed at 410 °C for 10 min with different thermal cyclic numbers (N = 1 and 5) under an Ar flow, where N = 1 represents an isothermal annealing treatment without cycle. N = 5 represents the cyclic annealing, per heat preservation stage at 410 °C was kept for 2 min and subsequently cooled to room temperature, such a process was repeated 5 times continuously. The total holding time at 410 °C of the alloy ribbon and cores is 10 min. The heating and cooling rates in the program of cyclic annealing are 40 °C/min and 10 °C/min, respectively. The thermal characteristics of the melt-spun ribbons were evaluated by differential scanning calorimetry (DSC) at a constant heating rate of 40 °C/min. The microstructure of amorphous and annealed samples was identified by X-ray diffractometry (XRD) with Cu-Kα radiation. The B_S and H_C of the annealed samples were measured by a DC B-H loop tracer under a maximum applied field of 1000 A/m. The permeability was in situ measured in a furnace with Ar atmosphere protection by using an HP4294A impedance analyzer at H = 0.4A/m and f = 10 kHz, and the heating rate was 10 °C/min.

3 Results and discussion

Figure 1 shows the XRD patterns of melt-spun Fe_80.5−xCo_xCu_1.5Si₄B₁₄ (x = 0, 2, 4, 10) ribbons. The sample with x = 2 shows a small crystallization peak at 2θ = 44.5°, which corresponds to the (110)-refection of α-FeCo(Si) crystalline phase. For the other samples, only a typical broad scattering peak is observed, indicating the fully amorphous structure of alloys. It has been reported that the primary crystals in the as-quenched state are necessary for the excellent soft magnetic properties in the present alloy system [1]. Therefore, the superior magnetic softness could be expected in the 2at.% Co-containing FeCuSiB alloy.

Fig. 1

XRD patterns of melt-spun Fe_80.5−xCo_xCu_1.5Si₄B₁₄ (x = 0, 2, 4, 10) alloys

The temperature dependence of initial permeability (μ_i ~ T curve) was measured for the Curie temperatures of as-quenched Fe_80.5−xCo_xCu_1.5Si₄B₁₄ (x = 0, 2, 4, 10) alloys, as shown in Fig. 2. A characteristic sharp Hopkinson peak is observable on all μ_i ~ T curves at the Curie temperature of amorphous alloys, T_C, which is due to a faster decrease of magnetic anisotropy than that of saturation magnetization. The T_C of alloys was extracted and displayed in the inset of Fig. 2. T_C increases from 365 °C for the Co-free alloy (x = 0) up to 400 °C for the Co-containing alloy (x = 10), indicating that the partial substitution Co for Fe is in favor of enhancing the T_C. The effect of Co on the Curie temperature for Nb-free FeCuSiB alloy is similar to that for Nb-containing FeCuSiB alloy [10].

Fig. 2

μ_i ~ T curves of as-quenched Fe_80.5−xCo_xCu_1.5Si₄B₁₄ (x = 0, 2, 4, 10) alloys. The inset shows the evolution of Curie temperature T_C with the Co content

The thermal performance of the as-quenched ribbons was investigated by DSC, as shown in Fig. 3. Two exothermic peaks for each alloy could be observed, suggesting the crystallization of all these ribbons occurs in two stages. The onset temperature of primary crystallization T_X1 and the second crystallization T_X2 marked with arrows are originated from the precipitation of α-Fe(Co)Si soft magnetic phase and Fe(Co)B compounds, respectively [16]. With the increasing of Co content, T_X1 increases from 370 to 405 °C corresponding to the samples with x = 0 and 10, respectively. The evolution of T_X2 with Co content is similar to that of T_X1, increasing from 520 to 535 °C. The amorphous precursor would transform into a single nanocrystalline structure without any precipitation of other compounds in the temperature interval between T_X1 and T_X2, the heat treatment temperature was chosen from the DSC results.

Fig. 3

DSC curves of melt-spun Fe_80.5−xCo_xCu_1.5Si₄B₁₄ (x = 0, 2, 4, 10) alloys

Figure 4a and b shows the XRD patterns of Fe_80.5−xCo_xCu_1.5Si₄B₁₄ ribbons annealed at 410 °C with and without cycle, respectively. As presented patterns of all the annealed samples show the characteristic diffraction peaks corresponded to α-Fe(Co)Si crystalline precipitates. It is indicated that all the annealed samples partially crystallize after annealing and their microstructure consists of the residual amorphous phase and the nanocrystalline phase. The volume fraction of crystalline phase V_cry can be estimated from the XRD spectra according to the following equation [17]

$$ V_{{cry}} = {\raise0.7ex\hbox{${I_{{cry}} }$} \!\mathord{\left/ {\vphantom {{I_{{cry}} } {\left( {I_{{cry}} + I_{{am}} } \right)}}}\right.\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{${\left( {I_{{cry}} + I_{{am}} } \right)}$}} $$

(1)

where I_cry and I_am are the integral intensities of diffraction peaks of crystalline phase and amorphous phase, respectively. These integral intensities are determined from peak areas.

Fig. 4

XRD patterns of the Fe_80.5−xCo_xCu_1.5Si₄B₁₄ (x = 0, 2, 4, 10) alloys annealed at 410 °C: a non-cyclic annealing; b cyclic annealing

The evolution of V_cry with Co content for the samples annealed at 410 °C with and without cycle are shown in Fig. 5a. With the increasing of Co content, the V_cry decreases from 60 to 40% for the samples with N = 1 and 67% to 49% for the samples with N = 5, respectively. In the condition of the same annealing temperature and time, the lower the crystallization temperature of alloy is, the stronger extent of crystallization of alloy becomes. Accordingly, the decreasing of V_cry is due to the raising of T_X1 with Co content (reflected in Fig. 3). The saturation flux density B_S of dual-phase nanocrystalline alloy is closely dependent on the V_cry, their relationship can be expressed as [18]

$$ B_{S} = B_{S}^{{cry}} V_{{cry}} + B_{S}^{{am}} (1 - V_{{cry}} ) $$

(2)

where $B_{S}^{{cry}}$ and $B_{S}^{{am}}$ is the saturation magnetic flux densities of the crystalline phase and the residual amorphous phase, respectively. The Co content dependence of B_S for the nanocrystalline Fe_80.5−xCo_xCu_1.5Si₄B₁₄ alloys (x = 0, 2, 4, 10) is observed in Fig. 5b. The Co content dependence of B_S shows a similar variation tendency for the samples annealed at 410 °C with and without cycle. For the samples with N = 1, the B_S increases from 1.51 to 1.80 T when 2at.% Co replaces Fe, which decreases subsequently with Co increasing. It is noteworthy that the $B_{S}^{{cry}}$ is larger than $B_{S}^{{am}}$ [19] and thus the higher V_cry obtained in the samples with N = 5 results in the increase of B_S. Especially for the 2 at.% Co-containing sample annealed with cycle, the B_S is about 1.83 T, which is the largest compared with that of the other samples studied here. However, the V_cry of 2 at.% Co-containing sample is smaller than that of Co-free sample, although the B_S of which is larger as shown in Fig. 5b. Except for the V_cry, the B_S of dual-phase nanocrystalline alloy is closely related to the $B_{S}^{{cry}}$ and/or the $B_{S}^{{am}}$. $B_{S}^{{cry}}$ of α-FeCo(Si) phase (~ 2.45 T) is higher than that of α-Fe(Si) phase (~ 2.18 T) [20]. Therefore, compared with the B_S of Co-free alloy, the relatively larger B_S of Co-containing alloy originated from the addition of Co content can be affected by the synergetic influence of V_cry and $B_{S}^{{cry}}$ for the Co-containing alloys.

Fig. 5

Change in V_cry (a) and B_S (b) of the Fe_80.5−xCo_xCu_1.5Si₄B₁₄ (x = 0, 2, 4, 10) alloys annealed at 410 °C as a function of the Co content (“N = 1” and “N = 5” denote the non-cyclic and cyclic annealing, respectively)

From the full width at half maximum (FWHM) of the (110) peak, the average grain size, D, can be estimated by means of the Scherrer formula from the XRD pattern. As shown in Fig. 6a, the D shows a slight increase with Co content for the N = 1 sample, which is about 21 ~ 23 nm. However, the D of N = 5 samples increases from 12 to 17 nm and then decrease to 12 nm with Co increasing. The D of the samples annealed with cycle is smaller than that of the ones annealed without cycle, indicating that the cyclic annealing effectively suppresses the coarsening of crystalline grain. From Fig. 6b, the coercivity, H_C, increases from 17.1 to 32.5 A/m corresponding to Co-free and 10 at.% Co-containing samples annealed without cycle. Similarly, the variation tendency of H_C is in accordance with that of D for the samples annealed with cycle, which increases from 11.2 to 18.9 A/m and then decreases to 13.4 A/m. H_C is directly related to the grain size D [9]

$$ H_{C} \approx P_{C} K_{1}^{4} D^{6} /J_{S} A^{3} $$

(3)

where P_C is dimensionless pre-factor of the order of unity, K₁ denotes the local magnetocrystalline anisotropy, J_S is the average saturation polarization and A is the exchange stiffness. It can be seen that H_C is proportional to D⁶, and thus the reduction of H_C for the samples annealed with cycle is attributed to the decrease of D.

Fig. 6

Change in D (a) and H_C (b) of the Fe_80.5−xCo_xCu_1.5Si₄B₁₄ (x = 0, 2, 4, 10) alloys annealed at 410 °C as a function of the Co content (“N = 1” and “N = 5” denote the non-cyclic and cyclic annealing, respectively)

Figure 7 presents the evolution of initial permeability with temperature (μ_i ~ T curve) for the nanocrystalline Fe_80.5−xCo_xCu_1.5Si₄B₁₄ alloys (x = 0, 2, 4, 10). The common feature of μ_i ~ T curves for the alloys is that μ_i maintains relatively higher value from room temperature to a certain temperature (depends on the composition of alloys) and then decreases with T increasing. It should be recognized that the μ_i value of studied alloys is significantly lower than that of Nb-containing FeCuSiB alloys [12]. It remains an open issue to enhance the initial permeability in Nb-free FeCuSiB alloys. It is an attention-worth feature that the temperature at which μ_i drops to zero increases with the Co content. The μ_i drops to zero at about 370 °C for the Co-free alloy, but the μ_i still keeps a relatively higher value above 450 °C for the 10 at.% Co-containing alloy. The soft magnetic behavior at elevated temperature is dependent on the magnetic exchange coupling between the crystalline grains through the intergranular phase. The grains will begin to decouple as the temperature exceeds T_C, which results from the thermal activation of exchange coupled moments in the ferromagnetic grains that are separated by significant amounts of paramagnetic intergranular amorphous phase [21]. The increase of T_C due to the doping of Co content is responsible for the improvement of magnetic softness at elevated temperature for the alloys described here.

Fig. 7

μ_i ~ T curves of the Fe_80.5−xCo_xCu_1.5Si₄B₁₄ (x = 0, 2, 4, 10) alloys annealed at 410 °C (“N = 1” and “N = 5” denote the non-cyclic and cyclic annealing, respectively)

Simultaneously, the magnitude of μ_i at room temperature is nearly doubled by the cyclic annealing for the samples (except for the sample with x = 2). As is reported, the grain size plays a dominant role in the evolution of the magnetic properties for the dual-phase nanocrystalline alloy, μ_i can be expressed as [10]

$$ \mu _{i} \approx P_{C} [(1 - V_{{cry}} )M_{S}^{{am}} + V_{{cry}} M_{S}^{{cry}} ]^{2} A/\mu _{0} V_{{cry}} K_{1}^{4} D^{6} $$

(4)

where μ₀ denotes the vacuum magnetic permeability, $M_{S}^{{cry}}$ and $M_{S}^{{am}}$ is the saturation magnetization of crystalline phase and residual amorphous phase, respectively. It is clear that μ_i is strongly related to the grain size D. From Fig. 6a, the D of samples annealed with cycle is significantly smaller than that of ones annealed without cycle, which is the reason for the increase of μ_i.

4 Conclusions

In this work, the effect of Co doping on the microstructure and soft magnetic properties was investigated for the Nb-free FeCuSiB alloys. The Curie temperature T_C increases with the Co content, which is enhanced to 400 °C for the amorphous Fe_70.5Co₁₀Cu_1.5Si₄B₁₄ alloy. The μ_i of Fe_70.5Co₁₀Cu_1.5Si₄B₁₄ alloy annealed at 410 °C exhibits relatively higher value above 450 °C, which is higher than the T_C of amorphous alloy. The effect of cyclic annealing was focused on the evolution of saturation flux density B_S and coercivity H_C together with room-temperature μ_i. The B_S of samples annealed with cycle is enhanced with respect to that of the ones annealed without cycle due to the increase of crystallization volume fraction. Especially for the Fe_78.5Co₂Cu_1.5Si₄B₁₄ alloy annealed at 410 °C with cycle, B_S is improved to 1.83 T. H_C of the samples is decreased significantly by cyclic annealing, which is due to the reduced D. For the same reason, the room-temperature μ_i of samples annealed with cycle is nearly twice as much as that of ones annealed without cycle.