Improved soft magnetic properties and thermal stability for Fe 82·65 Cu 1·35 Si 2 B 14 alloys with Nb,Zr,Ta or Co addition

Introduction

Fe–Si–B alloys have been used for soft magnets for many years since they exhibit good soft magnetic properties.1^–6 Recently, Fe–Cu–B and Fe–Cu–Si–B nanocrystalline alloys with high saturation magnetisation M_s of >1·8 T have been developed.7^–10 Ohta and Yoshizawa7,8 reported that nanocrystalline Fe_82·65Cu_1·35Si₂B₁₄ alloy with defined microstructure showed good soft magnetic properties. However, the coercivities H_c of Fe–Cu–B and Fe–Cu–Si–B alloys are relatively high due to the relatively large grain size in these alloys. Therefore, by reducing the grain size, further improvement of soft magnetic properties can be expected. In order to reduce the grain size, increasing the nucleation sites and suppressing the grain growth are necessary. It has been reported that IVB to VIB elements, such as Nb, Zr, Ta, V and Hf, can effectively inhabit the grain growth during the crystallisation process and form a nanosized phase. Based on this idea, Fe–Cu–Nb–Si–B nanocrystalline alloys with excellent soft magnetic properties have been developed, and a rather small grain size was achieved.11^–15 The other problems with this alloy are the narrow temperature range (ΔT_x) between the first and second crystallisation temperatures (T_x1 and T_x2) and the low Curie temperature (T_c) of the amorphous phase. Here, the first and second crystallisations correspond to the crystallisations of α-Fe and Fe–B phase (Fe₂B or Fe₃B). Generally, α-Fe are magnetically soft with high saturation magnetisation, but Fe–B phases are relatively hard. To achieve good soft magnetic properties, heat treatment at a temperature between T_x1 and T_x2 is required. A small ΔT_x will lead to a narrow temperature region for heat treatment, which is not favourable for real applications. Moreover, the low T_c of the amorphous phase is not beneficial to thermal stability. In the present work, therefore, additions of Nb, Zr and Ta and substitution of Co for Fe are employed to refine the grain structure and increase both ΔT_x and T_c.

Experimental

Alloy ingots were prepared by arc melting the mixtures of pure metals, ferroboron and polycrystalline silicon with purities higher than 99·9 in an argon atmosphere. Small pieces of ingots were placed into a quartz crucible and melt spun in argon atmosphere with a wheel speed of 50 m s⁻¹, argon injection pressure of 0·05 MPa and melt temperature of 1200°C. Ribbons with width of ∼1 mm and thickness of ∼30 μm were obtained. For heat treatment, selected ribbons were sealed in vacuum quartz tubes and annealed at various temperatures for 1 h. The structure of the as spun and annealed samples was examined by X-ray diffraction (XRD) with Cu K_α radiation. Thermal analysis was carried out by differential scanning calorimeter (DSC) in an argon atmosphere at a heating rate of 10 K min⁻¹. The saturation magnetisation M_s was measured at room temperature by vibrating sample magnetometer with a maximum applied magnetic field of 50 kOe. The coercivity was obtained with a B–H loop tracer using open circuit measurement.

Results and discussion

The XRD patterns for as spun Fe_82·65Cu_1·35Si₂B₁₄ and Fe_81·65Cu_1·35Si₂B₁₄M₁ (M = Nb, Ta and Zr) alloys are shown in Fig. 1. All alloys show almost a fully amorphous structure. The DSC curves for the above four alloys are shown in Fig. 2. Each curve shows two peaks. The first and second peaks correspond to the crystallisation of α-Fe and Fe₂B phases respectively. The onset temperatures T_x1 and T_x2, peak temperatures T_p1 and T_p2 and ΔT_x = T_x1−T_x2 for all alloys are provided in Table 1. The temperature range between the first and second crystallisation temperatures (ΔT_x) is only 91°C for the Fe_82·65Cu_1·35Si₂B₁₄ alloy. The addition of Ta, Nb or Zr significantly increased the ΔT_x of the starting alloy. In particular, 1Zr increased ΔT_x from 91 to 155·2°C (Fig. 2).

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Figure 1

X-ray diffraction patterns for as spun alloys

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Figure 2

Differential scanning calorimeter spectra for Fe_82·65Cu_1·35Si₂B₁₄ alloy with various elemental dopings

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Table 1

T_x, T_p and ΔT_x values for all samples

Composition	Tx1/°C	Tp1/°C	Tx2/°C	Tp2/°C	ΔTx/°C
Fe82·65Cu1·35Si2B14	402·2	420·1	493·2	498·9	91·0
Fe81·65Cu1·35Si2B14Nb1	393·9	408·7	533·3	539·3	139·4
Fe81·65Cu1·35Si2B14Zr1	397·6	411·2	552·8	557·9	155·2
Fe81·65Cu1·35Si2B14Ta1	388·8	403·1	529·1	536·2	140·3

Selected ribbons with various compositions were subjected to annealing at the temperatures varied between 400 and 550°C. At the annealing temperatures lower than T_x2, only α-Fe phase precipitated. The average grain sizes for α-Fe phases in all samples annealed at 400, 425 and 450°C were calculated by Scherrer equation, and the results are shown in Fig. 3. With increasing annealing temperature, the grain size increased. It is very clear that the mean grain sizes of Fe_81·65Cu_1·35Si₂B₁₄M₁ (M = Nb, Zr and Ta) alloys are smaller than that of Fe_82·65Cu_1·35Si₂B₁₄ alloy at each temperature. The reason can be attributed to the inhibition of the grain growth by Nb, Zr and Ta. In particular, Ta added alloy can maintain fine grain size below 35 nm until 450°C.

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Figure 3

Variation of grain size with annealing temperature for various temperatures

The M_s and H_c for the alloys before and after annealing are shown in Fig. 4a and b respectively. The values of M_s for Fe_81·65Cu_1·35Si₂B₁₄M₁ (M = Nb, Zr and Ta) alloys are lower than that of Fe_82·65Cu_1·35Si₂B₁₄ alloy due to the introduction of non-magnetic elements. Annealed alloys have higher M_s than as spun alloy, resulting from the crystallisation of ferromagnetic α-Fe phase. Low coercivities can be obtained in all experiment alloys. The coercivity is directly related to the magnetic anisotropy. According to Herzer's theory of effective magnetic anisotropy,16 the smaller the α-Fe grain size, the weaker the effective magnetic anisotropy that appeared in materials. Therefore, a small amount of Nb, Zr and Ta doping can benefit the soft magnetic properties through reducing the grain size of α-Fe. The Fe_82·65Cu_1·35Si₂B₁₄ alloy can maintain low coercivity while annealing between 350 and 425°C. With increasing annealing temperature to higher than 450°C, the coercivity increases rapidly due to the precipitation of Fe₂B phase, which has relatively high anisotropy. However, for the alloys doped with Nb, Zr or Ta, they can maintain the low coercivity in a wide temperature range. The reason is that the doped elements increase the value of ΔT_x, as shown early. The slow increase in coercivity with increasing temperature is likely due to the growth of α-Fe grains. At the annealing temperature of 500°C, precipitation of Fe₂B phase causes the rapid increase in coercivity in Fe_81·65Cu_1·35Si₂B₁₄Nb₁ and Fe_81·65Cu_1·35Si₂B₁₄Ta₁ alloys, but the alloy with Zr addition still maintains good soft magnetic properties. This is because the Fe_81·65Cu_1·35Si₂B₁₄Zr₁ alloy has high T_x2 and no Fe₂B phase precipitates at 500°C. The results indicate that the Fe_81·65Cu_1·35Si₂B₁₄Zr₁ alloy has good thermal stability for soft magnetic applications.

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Figure 4

Variation of M_s and H_c for alloys after annealing for 1 h

Co substituted (Fe_1−xCo_x)_82·65Cu_1·35Si₂B₁₄ (x = 0, 0·3, 0·5 and 0·7) alloys were also prepared. The XRD patterns in Fig. 5 also show an amorphous structure for all as spun alloys. The DSC curves are shown in Fig. 6. Two peaks corresponding to the two crystallisation temperatures can also be observed. With the increase in Co content, the crystallisation temperature of α-Fe (T_x1) decreased, and the crystallisation temperature of Fe₂B (T_x2) increased first and then decreased afterwards. Kolano-Burian et al. also found that the Co content can lower the first crystallisation temperature in research of effect of Co substitution for Fe in Finemet (FeCuNbSiB) alloys.17 Similar result was also found in Nanoperm (FeZrBCu) alloys.18

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Figure 5

X-ray diffraction patterns for as spun (Fe_1−xCo_x)_82·65Cu_1·35Si₂B₁₄ alloys

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Figure 6

Differential scanning calorimeter curves for as spun (Fe_1−xCo_x)_82·65Cu_1·35Si₂B₁₄ alloys

The M∼T curves in the temperature range between room temperature and 700°C are shown in Fig. 7. For the Fe_82·65Cu_1·35Si₂B₁₄ alloy without dopant, as the temperature increases, the magnetisation (M) gradually decreases to nearly zero at 318°C, which indicated a ferromagnetism–paramagnetism transition of amorphous phase at Curie temperature (T_c) of ∼318°C. M increases afterwards, indicating the presence of ferromagnetic α-Fe phase due to its much higher M_s than the amorphous phase. The value of M reaches a maximum at 425°C, indicating the completion of the first crystallisation. At temperatures between 425 and 450°C, the decrease in M implies the dependence of M on temperature for α-Fe. At 450°C, a sharp increase in M results from the precipitation of another magnetic phase of Fe₂B, which is in good agreement with the DSC analysis in Fig. 6. M starts to decrease again over 475°C because of the temperature dependent saturation magnetisations for α-Fe and Fe₂B phases. For (Fe_1−xCo_x)_82·65Cu_1·35Si₂B₁₄ (x = 0·3, 0·5 and 0·7) alloys, the first drop of magnetisation is not obvious because the α-Fe(Co) phase precipitated before the temperature reaches T_c of the amorphous phase. Hence, the presence of Co can not only reduce T_x1 for (Fe_1−xCo_x)_82·65Cu_1·35Si₂B₁₄ (x = 0·3, 0·5 and 0·7) but also increase T_c of the amorphous phase. It can also be noticed from Fig. 7 that Co substitution has also increased the T_c of both α-Fe and Fe₂B phases.

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Figure 7

M∼T curves for (Fe_1−xCo_x)_82·65Cu_1·35Si₂B₁₄ alloys (x = 0, 0·3, 0·5 and 0·7)

The variations of M_s and H_c with temperature for (Fe_1−xCo_x)_82·65Cu_1·35Si₂B₁₄ alloys after annealing for 1 h are shown in Fig. 8. The M_s of the annealed alloys is higher than that of the as spun alloy due to the precipitation of α-Fe or α-Fe(Co) crystals. The M_s of crystalline phase is much higher than that of the amorphous phase. The alloys with 0 and 30 at-Co substitution for Fe after annealing at the temperatures between 400 and 500°C show high M_s values of over 190 emu g⁻¹. In contrast, the alloys with Co content x = 0·5 and 0·7 after annealing in the same temperature range have relatively low M_s of 178 and 130 emu g⁻¹ respectively. The alloy with x = 0·3 shows the highest M_s, which agrees well with the well known Slater–Pauling curve for Fe–Co binary alloy. Excess of Co concentration will reduce M_s. Furthermore, the alloy for x = 0·5 and 0·7 can maintain relatively low H_c even after annealing at 550°C (Fig. 8b). The reason can be attributed to the increased ΔT_x of the amorphous phase by Co substitution. The results indicated that Co substitution is helpful to improve the stability at high temperatures.

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Figure 8

Variation of M_s and H_c for (Fe_1−xCo_x)_82·65Cu_1·35Si₂B₁₄ alloys after annealing for 1 h

Conclusions

The nanocrystalline structure, thermal stability and magnetic properties of Fe_81·65Cu_1·35Si₂B₁₄M₁ (M = Nb, Zr and Ta) soft magnetic alloys obtained by the crystallisation of melt spun amorphous ribbon were investigated. The addition of 1·0 at.-Zr has increased ΔT_x from 90 to 155·2°C. Additionally, these elements can also reduce the grain size. The addition of Ta can keep the grain size below 35 nm until 450°C during annealing. The alloy with Zr addition has the best thermal stability, which has good soft magnetic properties even heat treated at 500°C. The effects of partial replacement of Fe with Co in (Fe_1−xCo_x)_82·65Cu_1·35Si₂B₁₄ alloys (x = 0, 0·3, 0·5 and 0·7) on the structure and magnetic properties have been investigated. It was found that increasing the Co content can reduce T_x1 and M_s but increase H_c and T_c of the amorphous phase. The alloy for x = 0·5 has the best thermal stability, which has low H_c even at 550°C. The results showed that these composition modified soft magnetic alloys have good prospect in magnetic core applications.