Microstructure and Magnetic Properties of (Fe100-xCox)84.5Nb5B8.5P2 Alloys

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Microstructure and magnetic properties of (Fe100−xCox)84.5Nb5B8.5P2 alloys • S.N. Kanea, • S. Tripathib, c, • M. Coissonb, , , • E.S. Olivettib, • P. Tibertob, • F. Vinaib, • M. Bariccoc, • G. Fiorec, • Apolináriod, • C.T. Sousad, • J.P. Araujod, • L.K. Vargae Abstract

Partial substitution of Fe with Co in gradually devitrified Si-free Nanoperm-type alloys improves their soft magnetic properties, increases the Curie temperature of the amorphous matrix, and allows casting in air. In the present work we report the temperature dependence of structural and magnetic properties of

(Fe100−xCox)84.5Nb5B8.5P2 (x = 20, 40, 60) alloys using differential scanning calorimetry, magnetic, thermo-magnetic and X-ray diffraction measurements, to get complementary information on the microstructure, composition of the precipitated crystalline phase and their correlation with magnetic properties. Higher Co concentrations improve the magnetic softness of the alloy, whose properties can be further optimized by suitable thermal treatments. Magnetic data is discussed in terms of the precipitation of a Fe–Co crystalline phase consisting of different Co content.

________________________________________ 1. Introduction

Si-free Nanoperm-type alloys are widely studied because of their soft magnetic properties coupled with the possibility of casting in air [1] and [2]. The addition of Co increases the Curie temperature of both the amorphous phase and of the crystalline precipitates, thus allowing a better performance of the alloy at high temperatures [3] and [4]. The ability to tailor the soft magnetic properties of nanocrystalline Fe-based alloys is of utmost importance because they can achieve high saturation magnetization values, making them useful in power applications where low coercive field and high saturation are required [5] and [6]. In this paper three (Fe100−xCox)84.5Nb5B8.5P2 compositions (x = 20, 40, 60) with different Co content have been rapidly solidified and their structural and magnetic properties have been studied. Suitable thermal treatments aimed to optimize the material properties have been performed and their effects are investigated. The role of Co substitution is discussed in terms of improved soft magnetic properties of the alloy.

2. Experimental

Ribbons of nominal composition (Fe100−xCox)84.5Nb5B8.5P2 (x = 20, 40, 60 at.%) have been prepared by planar flow casting in air on a Cu wheel. The composition has been later evaluated through

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have been measured with a Bruker D8 Advance diffractometer in the ϑ–2ϑ configuration, equipped with a fast counting detector (Bruker LynxEye) and using Cu Kα radiation (λ = 0.154 nm). Differential Scanning Calorimetry (DSC) has been measured with a TA Instruments model 9210 with a heating rate of 20 °C/min. Hysteresis loops have been measured by means of an inductive technique at 50 Hz (maximum field: 1 kA/m) with compensated and calibrated pick-up coils. Magnetization as a function of temperature has been measured by means of a vibrating sample magnetometer (VSM, LakeShore Model 7410) equipped with an oven capable of reaching temperatures up to 1000 °C. The magnetization has been measured as a function of temperature, up to 800 °C, both upon heating and cooling, with an applied field of 1 kOe. The heating and cooling rate was approximately 3 K/min. Hysteresis loops have been measured by means of the same VSM at room temperature before and after the M vs. T ramps, with an applied field up to 10 kOe. Selected samples have been annealed in furnace in a protective Ar atmosphere at temperatures up to 500 °C for 1 h. 3. Results and discussion

As-quenched ribbons of the three compositions have been investigated through EDS analysis in order to determine their actual composition. For each alloy, Table 1 depicts the nominal composition in the first row, the nominal composition normalized as if no B were present (as EDS is not sensitive to light elements and B cannot be detected) in the second row, and the measured composition in the third row. For each alloy, the second and third rows should be compared. As Table 1 suggests, there is always a little excess of Nb and possibly P, at the expense of either Fe or Co. However, these variations do not distort the relative amount of Fe and Co in the three alloys.

Table 1.

Composition (at.%) of the three alloys. For each alloy, the first row reports the nominal composition, the second row reports the composition normalized without B, the third row reports the measured

composition.

Fe Co Nb B P

Nominal (x = 60) 33.8 50.7 5.0 8.5 2.0 Nominal (no B) 36.9 55.4 5.5 n.a. 2.2

Measured 36.9 52.5 7.7 n.a. 2.9

Measured 53.8 36.4 7.8 n.a. 2.0

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Fig. 1 reports XRD data on all as-quenched and annealed samples, for the three studied alloys. In all cases, a crystalline phase is already present in the as-quenched ribbons, superimposed to the amorphous phase. Upon annealing at temperatures up to 500 °C, no additional crystalline phases develop, as the peaks remain at the same 2ϑ position. However, the crystalline phase grows at the expense of the amorphous one. The coherent domain scattering regions are mostly of α-Fe (with a possible presence of Co in small amounts) in the alloy with x = 20, and of a Fe–Co alloy in the samples with x = 40 and 60. Additionally, upon increasing the Co content the peaks become broader, indicating that the particles become progressively smaller.

Fig. 1.

X-ray diffractograms on as-quenched and annealed samples of the three compositions.

Fig. 2 depicts the DSC curve of the sample with the lowest Fe content (x = 60), measured with a heating rate of 20 °C/min. The onset of crystallization appears at Tx = 416 °C. A second crystallization process takes place only above 500 °C. A similar behavior is observed also for the other two compositions. As all the studied samples are either as-quenched or annealed at Ta ≤ 500 °C, only one crystalline phase is expected. This is in agreement with the X-ray diffractograms discussed in Fig. 1.

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Fig. 2.

Differential scanning calorimetry on the alloy with x = 60. The heating rate is 20 °C/min and the onset of crystallization is at Tx = 416 °C.

The onset of crystallization can be followed also by studying the thermo-magnetic properties of these alloys. As-quenched samples have been characterized by a VSM by measuring their magnetization at 1 kOe from room temperature up to 800 °C and then back to room temperature. The resulting curves are shown in Fig. 3 for the three alloys. Similar features characterize the three compositions. Starting at room

temperature, the magnetization decreases on increasing temperature, as the Curie temperature of the amorphous phase is approached. The sample with highest Fe content (x = 20) has the lowest TC of the amorphous matrix; on increasing the Co content, TC increases as expected. This fact is clearly evidenced by the slower reduction of the magnetization with temperature in samples with more Co. All the three samples seem to be characterized by a microstructure which is only partly amorphous. In fact, when the

temperature approaches 400 °C, the samples have not yet reached their Curie temperature (samples with x = 40 and 60 are far away from it), which might suggest that a crystalline phase is already present in the as-quenched ribbons. This is in agreement with the results reported by XRD and discussed in Fig. 1.

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Fig. 3.

Magnetization vs. temperature measured with a VSM on the three studied alloys. The applied field is 1 kOe. Red curves are measured on heating with a rate of ≅3 K/min. Blue curves are measured at the same rate but on cooling. (For interpretation of the references to color in the figure caption, the reader is referred to the web version of the article. The arrows may be used for determining the temperature sweep direction in the black-and-white figure.)

For all the three alloys, the M vs. T curves mark the onset of crystallization at a temperature of

approximately 380 °C. This temperature is slightly lower than the one suggested by DSC measurements (see Fig. 2); this difference can be accounted for by the different heating rates, VSM measurements being performed much more slowly at approximately 3 K/min. The M vs. T curves confirm that just one crystalline phase develops at temperatures below 500 °C. Indeed, a second phase precipitates at a temperature slightly above 600 °C. This second phase is characterized by a lower magnetization at 1 kOe, which could be either due to a lower saturation magnetization, a much higher coercive field, or a Curie temperature close to, or lower than the second crystallization temperature. All three possibilities are consistent with the

precipitation of borides [6], [7], [8] and [9]. At the maximum heating temperature (800 °C) all samples are still ferromagnetic, indicating that the precipitates are composed by a Fe–Co alloy, whose Curie temperature is above 800 °C and reaches ≅900 °C already for very low Co concentrations [10].

On cooling from 800 °C to room temperature, the M vs. T curves reported in Fig. 3 do not overlap with the ones acquired on heating. This fact marks the irreversibility of the transformations occurred during the heating process. For the samples with x = 40 and x = 60 a Curie temperature at approximately 400 °C is observed in the cooling curves, which can possibly be ascribed to a minority phase poor in Fe and Co. At room temperature, the magnetization of all three alloys is higher with respect to the as-quenched state, coherently with the precipitation of a crystalline phase. Besides, the obtained three magnetization values are very similar, indicating that more or less the same crystalline phase precipitates in the three samples.

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On all three alloys, hysteresis loops at fields up to 10 kOe have been measured before and after the M vs. T curves. From these loops, values of saturation magnetization and coercivity can be acquired, and are reported in Table 2.

Table 2.

Coercive field and magnetization measured at 10 kOe for samples of the three compositions before and after M vs. T curves.

HC (Oe)M10 kOe (emu/g) x = 20 before M vs. T <1 149.9 x = 20 after M vs. T 59.6 182.0 x = 40 before M vs. T <1 151.1 x = 40 after M vs. T 69.2 183.7 x = 60 before M vs. T ≅1 143.2 x = 60 after M vs. T 65.2 173.2

Coherently with the results discussed in Fig. 3, the loops after the heating process have a very large coercive field, which could be ascribed to the presence of borides among the precipitated crystalline phases. Indeed, the hysteresis loops after the heating process display a two-phase behavior, with a minority phase which is much softer, having a coercive field of the order of 1–2 Oe.

All the data discussed so far suggest that in order to optimize the soft magnetic properties of these alloys thermal treatments at an annealing temperature Ta ≤ 500 °C should be performed. Selected samples of all three compositions have then been annealed for 1 h a 400, 450 and 500 °C in Ar atmosphere, and their hysteresis loops have been measured with a loop tracer at 50 Hz, with a maximum applied field of 1 kA/m. The results are shown in Fig. 4.

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Fig. 4.

Hysteresis loops of as-quenched and furnace annealed ribbons of the three compositions, measured at 50 Hz with a maximum applied field equal to 1 kA/m.

The loops indicate that the sample with the highest Fe content (x = 20) has the highest coercive field. This is in agreement with the XRD data, which showed that for x = 20 the peaks were sharper, indicating the presence of a significant surface crystallization or of larger coherent domain scattering regions. Upon annealing, the coercive field of the x = 20 alloy increases even further, but the approach to saturation of the same alloy turns out to be very slow; this fact can be ascribed again both to a significant surface

crystallization or to an increase of the size of existing particles, associated to the precipitation of numerous nanocrystals from the amorphous matrix. This process is less evident for the other two compositions. Even though 1 kA/m is not sufficient for saturating the samples, the annealing process is also shown to largely increase the magnetization values, as a consequence of the precipitation of Fe–Co crystals.

The coercive field evolution as a function of the annealing temperature Ta is reported in Fig. 5 for the three compositions. For x = 20, the coercive field, which is already the highest in the as-quenched state, steadily increases with the annealing temperature, indicating that the precipitates continue to grow in size. For the intermediate composition (x = 40), the coercive field has a minimum at Ta = 450 °C. For x = 60, the minimum of the coercive field is reached at Ta = 400 °C. Upon further increasing the temperature, the coercive field increases, indicating that for both x = 40 and x = 60 compositions the precipitates grow in size and become magnetically harder.

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Fig. 5.

Room temperature coercive field as a function of the annealing temperature in ribbons of the three compositions.

4. Conclusions

Partial substitution of Fe with Co in gradually devitrified Si-free Nanoperm-type alloys has been

demonstrated to improve their soft magnetic properties, through a reduction of the coercive field without a significant decrease of the saturation magnetization. The soft magnetic properties can be optimized by means of thermal treatments in the 400–450 °C temperature range, where minimum coercivity is obtained. The studied alloys are characterized by a single Fe–Co crystalline phase at temperatures up to ≅600 °C, above which borides are formed that reduce the saturation magnetization and significantly increase the coercivity.

Acknowledgments

This work has been partially performed at NanoFacility Piemonte, INRIM, a laboratory supported by

Compagnia di San Paolo. S.N. Kane acknowledges gratefully for the excellent hospitality during June 2010 at IFIMUP, Departamento de Fisica, Universidade de Porto (Portugal), where part of the work was carried out. References

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