Change in soft magnetic properties of Fe-based metallic glasses during hydrogen absorption and desorption

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Change in soft magnetic properties of Fe-based metallic glasses during hydrogen absorption and desorption

L. Novák

Department of Physics, Technical University of Kosice, Kosice, Park Komenského 2, Slovakia A. Lovas^a^兲

Department of Vehicle Manufacturing and Repairing, Faculty of Transportation Engineering, Budapest University of Technology and Economics, Bertalan L. u. 2., H-1111 Budapest, Hungary

L. F. Kiss

Research Institute for Solid State Physics and Optics, Hungarian Academy of Sciences, P.O. Box 49, H-1525 Budapest, Hungary

共Received 11 February 2005; accepted 13 June 2005; published online 18 August 2005兲

The stress level can be altered in soft magnetic amorphous alloys by hydrogen absorption. The resulting changes in the soft magnetic parameters are reversible or irreversible, depending on the chemical composition. Some of these effects are demonstrated in Fe–B, Fe–W–B, and Fe–V–B glassy ribbons, in which various magnetic parameters are measured mainly during hydrogen desorption. The rate of hydrogen desorption is also monitored by measuring the pressure change in a hermetically closed bomb. The observed phenomena are interpreted on the basis of induced stresses and chemical interactions between the solute metal and hydrogen. © 2005 American Institute of Physics.关DOI: 10.1063/1.1994939兴

INTRODUCTION

The characteristics of hydrogen absorption共temperature and pressure dependences of absorption or desorption in metals and alloys兲are determined by the sign of the enthalpy related to the process. The process is exothermic for typical hydride-forming systems共e.g., Zr, Ti, V, and the rare earths兲, and endothermic for metals and alloys, which do not form stable hydrides around room temperature.¹Besides, when the H concentration exceeds a critical value, the hydrogen uptake is connected with a substantial structural rearrangement as well² 共hydride-forming alloys兲. Both the chemical short- range order and the lattice symmetry are changed in this case 共first-order transformation with compositional change兲. On the other hand, no structural rearrangement can be detected in the case of an endothermic solution, solely a lattice expan- sion. The interstitial site occupancies of H atoms are typical in both types of metal-hydrogen systems.

In hydrogen-absorbing amorphous alloys, such as Zr–Ni, the site-energy distribution for hydrogen occupying tetrahe- dral sites are determined mainly by the chemical composition.^3,4 The solubility of H is significant in these glasses 共0.5–0.8 H/M, depending on the ambient pressure and on the temperature of saturation兲. In addition, a chemical reordering in the amorphous state can be induced by dissolved hydrogen. The dimension of these reordered regions can extend over a few nanometers in alloys containing elements with positive and negative heats of solution with hy- drogen共microphase separation in the amorphous state兲.⁵

In contrast, the solubility of hydrogen is negligible in Fe-based metallic glasses, as a consequence of the dominant chemical character of the metallic host共endothermic heat of

solution for H兲. In spite of this, the resulting effects on the soft magnetic properties are considerable, resulting from the local stresses induced by the H incorporation 共magnetome- chanical coupling兲.^6,7It means that the soft magnetic proper- ties共being sensitive to the magnitude of the stress兲are influenced by the H absorption. On the other hand, the absorption or desorption process can be monitored by measuring the coercive force共H_c兲, the initial permeability共␮i兲, or the magnetoelastic anisotropy, though the sensitivity of these measurements is highly influenced by the magnitude of the magnetostriction constant, so they are not applicable for low- magnetostriction alloys.

In this paper, the time dependence of selected magnetic properties during dehydrogenation will be presented for Fe₈₅B₁₅, Fe₇₉W₅B₁₆, and Fe₈₀V₅B₁₅ glassy alloys. The ribbon compositions were selected on the basis of different af- finity of hydrogen to the applied alloying elements. All of the measurements were carried out at room temperature.

Hydrogenation was performed electronically in 1N H₂SO₄ solution containing a few CS₂. In these experiments hydrogenation was carried out using 5.0 mA/ cm² for 2 h.

Dehydrogenation was followed in air at room temperature.⁸

EXPERIMENTAL RESULTS

The amorphous alloys were prepared by planar flow casting 共PFC兲 method. The glassy state of the as-quenched samples was confirmed by x-ray diffraction. The binary Fe₈₅B₁₅ribbon was used as a reference material. Due to the negligible interaction with H共positive heat of solution for H in Fe兲, most of the change in magnetic properties are revers- ible共H-induced changes disappear within 20 h subsequent to the hydrogenation兲. The appropriate sample properties in the as-quenched state are shown in Table I. A current density of

a兲Electronic mail: lovas@kgtt.bme.hu

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5 mA/ cm²was applied⁸with 120 min saturation time in every case. The dehydrogenation process was controlled continuously by measuring various physical properties every 30 or 60 min.

Magnetic properties, including coercivity 共Hc兲, initial permeability共␮i兲, saturation magnetization共Js兲, and magnetoelastic anisotropy energy 共K_␴兲, were determined from the hysteresis loop. The total demagnetizing factor 共D兲 was determined from the anhysteretic magnetization curve. All of the measurements were carried out on 100-mm-long strips at room temperature, using a computer-controlled Förster mag- netometer. The appropriate data, referring to the as-quenched state, are listed in Table I. In agreement with earlier results,⁹ Jsand the saturation magnetostriction␭sare decreased due to the host metal replacement with W and V.

The inner demagnetizing factorDican be derived from D by subtraction of the geometrical factor Dg. The demagnetizing field is not homogeneous in the case of the investigated samples 共100-mm-long ribbons兲. Therefore, only the measurable共total兲demagnetizing factor was used, assuming that the geometrical factor is not changed significantly during the hydrogenation-dehydrogenation cycle. In this sense, one can assume that the total measurable change inDresults from the inner demagnetizing factor, which can be influenced by the H absorption.

The details of the magnetization curves are shown in Fig. 1. In order to gain more detailed information from the hydrogen-induced changes, the curves are plotted only for the field range 0 – 400 A / m. The shape of the curves is similar after a complete cycle for the investigated glasses, except the saturation value, which is slightly lowered after desorption. This change is less pronounced for the investigated ternary glasses共see Fig. 1兲. Nevertheless, the virgin curves are flattened and the hysteresis loop is widened after hydrogena- tion共hydrogen absorption兲in each case. The saturation value of the magnetic polarization is reached at 8 kA/ m at room temperature for the investigated alloys. Following the small transient increase due to hydrogen saturation, a monotonic decrease ofJsto below the as-quenched value was observed in every case.共The possible origin can be the dissolution of a small amount of Fe host during electrochemical treatment, as shown in Fig. 2. However, this effect is near the experimental error in most cases.

A considerable increase in the anisotropy was observed in all samples during the H uptake 共see Fig. 3兲, which is in qualitative agreement with the results published in Ref. 6.

According to Fig. 4 theHcalso increases significantly in all samples during H absorption共45%, 130%, and 150% for the Fe–V–B, Fe–B, and Fe–W–B samples, respectively兲. As

a consequence of stress relaxation, the coercive force decreases again as the desorption proceeds, and slowly approaches a value which is slightly higher than that of the corresponding as-quenched state, in each case. Figure 4 shows that the time dependence of theHcdecrease is specific to the sample composition. After the rapid drop during the first 5-h period, a very slow further decrease was observed in Fe–B and Fe–W–B alloys. The rapid decrease is absent in Fe–V–B. It means that the hydrogen-induced stresses do not completely relax during the 20-h period of dehydrogenation 共as Fig. 4 shows, the difference between the as-quenched and the dehydrogenated value ofHcis still 25% even after keep- ing the sample for 20 h at room temperature兲.

The permeability 共␮i兲 changes inversely during the

TABLE I. Coercivity, magnetic anisotropy, saturation magnetization共mea- sured in the field of 8000 A / m兲, and initial permeability of the investigated ribbons measured in as-quenched state.

HC

关A/m兴

K_␴ 关Jm⁻³兴

J₈₀₀₀

关T兴 ␮i

关H/m兴

Fe₈₅B₁₅ 11.68 432.4 1.027 0.002 053

Fe₇₉W₅B₁₆ 9.60 374.2 0.494 0.001 242

Fe₈₀V₅B₁₅ 7.14 352.8 0.617 0.002 240

FIG. 1. The details of the hysteresis curves in as-quenched state and after hydrogenation.

FIG. 2. Time dependence of saturation polarization during dehydrogenation.

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hydrogenation-dehydrogenation cycle; ␮i decrease was detected in every case due to H absorption, as is evident if one compares Figs. 4 and 5. The relation between the anisotropy and coercive force for different degrees of H uptake or H desorption is illustrated in Fig. 6. A linear relationship between H_c and K_s was found during the 5 – 25-h period of desorption. This linear relationship is not fulfilled at the very beginning of H desorption and during the long holding times for the samples at room temperature. One can suppose that the variation of stress level in the samples can supply a rea- sonable explanation for the existence of this linear relationship between the two physical parameters.

In Fig. 7, the values of demagnetizing factor共D兲versus the desorption time are plotted. No detectable change of D was observed in the binary Fe–B glass. On the contrary, the changes are appreciable in the ternary Fe–W–B and Fe–V–B glasses. There is a rapid drop inDdue to the H absorption, and an increase again as the desorption proceeds.

The change in the macroscopic geometry of the ribbons 共due to the partial dissolution of metallic matrix兲 is negligible compared with the observed drastic change inD. The continuous increase in D during the desorption, the very slow approach to the value for the as-quenched state, is a consequence of the reconstruction of the original stress distribution on a microscopic scale.

In order to provide an independent evidence on the H absorption and desorption, the quantity of desorbed hydrogen was determined by monitoring the pressure change共aris-

ing from the H evolution from the electrochemically charged sample兲in a hermetically closed bomb. As an example, the desorption kinetic is plotted for the FeB and FeVB ribbons in Fig. 8. The quantity of the absorbed H in binary FeB is nearly two times higher than that for the FeVB. Neverthe- less, the H content in the samples approaches zero over the same time scale.

DISCUSSION

It is obvious that both hydriding and dehydriding are coupled with significant changes in the stress-sensitive soft magnetic properties of the investigated alloys. In qualitative agreement with Refs. 6 and 7, the changes are mostly reversible in all properties, with the exception of the saturation magnetization which slightly decreases during the hydrogenation-dehydrogenation cycle for Fe–B and Fe–

W–B, the presumed reason for which is the relatively low corrosion resistance of these alloys¹⁰ 共see Fig. 2兲. On the contrary, there is no detectable decrease in Fe–V–B glass as a consequence of the highest chemical stability of this glass.

A simple explanation for this phenomenon would thus be a loss in weight 共dissolution of metallic components in the acetic electrolyte兲 during the hydrogenation 共FeB and FeWB兲. In spite of the low saturation concentration共see Fig.

8兲, the dissolved hydrogen atoms do cause detectable local volume dilatation around the occupying interstice, causing a net increase in the average Fe–Fe atomic distances, hence

FIG. 4. Time dependence of coercivity during hydrogen desorption.

FIG. 5. Time dependence of the magnetic permeability during hydrogen desorption.

FIG. 6. Correlation between anisotropy and coercive force during dehydrogenation.

FIG. 3. The change of magnetoelastic anisotropy during hydrogen desorption.

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resulting in a transient increase in the net ferromagnetic coupling at the end of the saturation period. This effect does probably contribute to the small reversible increase of I_sat the end of the saturation period. The results indicate directly and indirectly the absence of irreversible hydrogen-induced structural changes in these alloys, which is in agreement with the basic thermodynamic fact that the solution enthalpy of H is endothermic and thus negligibly small in Fe. Accordingly, the chemical properties of these alloys are dominated by the main component 共host metal兲. The chemical interaction between the dissolved H atoms and the electronic structure of the host metal is weak, similar to the crystalline solid solu- tions, in which the pressure dependence of H solubility follows Sievert’s rule. The chemical potential of H in the ambient electrolyte, and so the driving force for H absorption, constant. As the rate of H diffusion in Fe is several orders higher than that for the self-diffusion of Fe atoms, one can suppose that the equilibrium H concentration is reached during the saturation period. When the H-saturated sample is placed into the ambient atmosphere, the dissolved H is re- leased from the sample and the introduced stresses will be relaxed.

The distribution of H atoms over the interstitial sites is not random, however. The probability of H incorporation is influenced mainly by the local chemical composition of the atomic environment. As it is generally accepted, the binary hypoeutectic Fe–B glasses are composed of two different types of clusters with characteristic chemical short-range order.^11,12In the compoundlike clusters the boron concentra-

tion is higher than the average composition of the alloy 共Fe3B-like chemical environment兲. The probability of H incorporation is negligible in these regions, as a consequence of high bonding strength between the Fe and boron atoms.

As the alloy composition is hypoeutectic, the off- stoichiometric Fe atoms are accumulated in the form of Fe- rich clusters with low boron content. The local symmetry of Fe-rich environments can be fcc 共compressed, ␥-like兲 and also bcc 共stressed, ␣-like, with higher local Fe moments兲, with fluctuating strengths of the magnetic coupling between the neighboring Fe atoms.^13,14The probability of H incorporation into the fcc-like environments is higher, resulting in local dilatation in the compressed regions. As a consequence of this, the interatomic distances and the local coupling increase, hence the polarization may also increase slightly. At the end of saturation共i.e., on removing the sample from the electrolyte兲, rapid H effusion starts from the specimen and, after 100 min desorption time,Js drops down below the as- quenched value.

An increase inK_␴, andHc, and a simultaneous decrease in␮i were observed in all investigated alloys during hydrogenation. These phenomena reveal a considerable stress build up as the H uptake proceeds. The stresses can relax rapidly in Fe–B and Fe–W–B during the initial period of desorption 共2 – 3 h兲, but is followed by a much slower process. The first abrupt decrease in H_c is the consequence of rapid H loss characteristic of these samples. In contrast, the abrupt decrease in H_c cannot be observed in the Fe–V–B ribbons. This fact hints to the possible significance of H trapping in this alloy, arising from the stronger interaction between the dissolved H and V atoms.共The enthalpy of solution for H is negative in the V metal, and positive in Fe and W兲.¹⁵ As the residence time of H in the interstitial sites neighboring the V atoms is prolonged, an increased stress level is maintained in the hydrogenated sample.

Opposite to the predominantly stress-sensitive properties treated above, the inner demagnetizing factor 共Di兲 is sensitive mostly to the presence of nonferromagnetic inclusions or to the dissolved nonmagnetic impurity atoms in the sample.

From Fig. 7 it is clear that the interaction between the dissolved hydrogen and metallic alloying elements is more pronounced. The influence of H is detectable in the ternary samples only. As the consequence of this interaction, the total demagnetizing factor decreases in Fe–W–B and Fe–V–B as the hydrogenation proceeds. On holding the samples at room temperature after the saturation共under ambient condi- tions兲,D increases again. Hence, the shape of theD–t共de- hydrogenation兲curves is a function of the sample composition. The values of D in the ternary alloys are appreciably lower compared with the as-quenched state, even after 10 h holding time.

The macroscopic change in the ribbon geometry due to the H dissolution can be neglected. Therefore, the observed very slow change in D during the desorption is of microscopic origin, arising from the reconstruction of the original stress distribution following the H depletion. One can suppose that the total demagnetizing factor D is composed of two parts:

FIG. 7. Time dependence of demagnetizing factor 共D兲 during hydrogen desorption.

FIG. 8. The time dependence of H content of as-quenched Fe₈₅B₁₅ and Fe₈₀V₅B₁₅samples after hydrogenation.

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D=Dg+Di,

whereDgandDiare the geometrical and inner demagnetizing factors, respectively. The magnitude of Dg 共geometric part兲 is determined by the ribbon geometry through Dg

=ap^b, where p= 0.5共wt/␲兲^1/2 共l, w, and t are the length, width, and the ribbon thickness, respectively兲. One should emphasize again that the inner demagnetizing factor origi- nates from the existence of nonferromagnetic or hydrogen- susceptible clusters共fcc-like, dense environment with low Fe moment兲. They give rise to magnetic poles behaving as nonmagnetic inclusions, which are continuously modified during the absorption-desorption process in the ternary glasses.

The very different character of time dependecies forDin various samples during the whole absorption-desorption cycle hints at the significance of small compositional modi- fications, which do modify the local chemical environments in the as-quenched samples; consequently, the susceptibility for the hydrogen site occupation in the adjacent interstitial sites will also be modified. Thus, the H absorption and the related response in the magnetic properties are influenced by the chemically predetermined site energy distribution. The absorption process causes a modification in the local stress distribution via local distortions. The modification of stress distribution in short-range-order scale acts directly onDi. On this basisDican be regarded as the inner sensor, being sensitive to stress accumulations or the change of chemical interactions in short-range-order scale. However, it is hardly possible to separate the various processes contributing to the change of Di, since it is a bulk quantity integrating all the effects over the whole sample volume.

SUMMARY

The results can be summarized as follows:

1. Though the solubility of H in the investigated amorphous alloys is negligible, the influence of dissolved H is considerable due to the increasing stress level, is obvious from the drastic changes in coercivity, initial permeability, and stress-induced anisotropy. The H-induced changes in the soft magnetic properties are mostly reversible. The relaxation of the H-induced stresses follows similar time dependence as the H release obtained from the direct measurements of H desorption.

2. The chemical interaction between the dissolved metallic additives and H is responsible for the partial irreversibil- ity of certain property changes. The strong interaction between the dissolved V and H in Fe–V–B results in trapping of H atoms, which decreases the rate of H diffusion in this alloy, causing a long-term stress accumu- lation.

3. The time scale of stress relaxation caused by the H solution is comparable with that H desorption determined in an independent measurement, indicating that reversible changes in the stress-sensitive properties are really coupled with the presence of dissolved H in the glassy host.

ACKNOWLEDGMENTS

This work has been supported by the Slovak Research Fund共VEGA兲through Grant No. 1/8128/01 and by the Hun- garian Scientific Research Fund共OTKA兲through Grant No.

T-046239. One of the authors共L. N.兲 greatly acknowledges Domus Hungarica for the one-month grant.

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