HYDROGEN IN STAINLESS STEEL AND Fe-Ni ALLOYS

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HYDROGEN IN STAINLESS STEEL AND Fe-Ni

ALLOYS

F. Fujita, T. Sohmura

To cite this version:

JOURNAL DE PHYSIQUE Colloque C6, supple'ment au no 12, Tome 37, De'cembre 1976, page C6-379

HYDROGEN IN STAINLESS STEEL AND

Fe-Ni ALLOYS

F. E. FUJITA and T. SOHMURA

Faculty of Engineering Science, Osaka University, Toyonaka, Osaka Pref., Japan

Rbsumb.

-

La repartition de l'hydrogkne et les interactions avec les atomes de fer dans l'acier inoxydable et les alliages Fe-Ni, ont kt6 BtudiQs par spectromktrie Mossbauer du 57Fe. Le spectre paramagnktique de I'acier inoxydable hydrogknk et analyse en deux composantes. L'une provient des atomes de fer qui ne sont pas affectes par l'hydrogkne, l'autre de ceux qui.sont profondement affect& par l'hydrogkne. Le grand deplacement isomerique de cette dernikre composante ne peut &tre expliqu6 par la dilatation du reseau mais plut6t par I'accroissement du nombre des Bectrons 3d du fer du fait des interactions avec l'hydrogkne. Le sextuplet ferromagnetique des Fe-Ni hydro- gknks comporte aussi deux contributions, l'une venant de la phase y' dans laquelle les atomes ont peu d'interaction avec l'hydrogkne du fait de la distribution homogkne, et l'autre provenant d'une phase hybride de type p, dans laquelle les atomes defer interagissent obligatoirement avec les atomes voisins d'hydrogkne ; leur moment magnktique est r6duit de 15

%.

I1 apparait que la distribution de l'hydrogkne dans les alliages de fer n'est pas uniforme et qu'il existe des interactions importantes entre fer et hydrogkne.

Abstract.

-

By means of 57Fe Mossbauer effect, distribution of hydrogen and its interaction

with iron in stainless steel and Fe-Ni alloys are studied. Paramagnetic single line spectrum of hydrogenated stainless steel is analyzed into two components. One arises from the iron atoms totally unaffected by hydrogen and the other from those largely affected by hydrogen. A large positive isomer shift of the latter can not be explained by the lattice expansion but by the increase in the number of 3d electrons of iron due to the interaction with hydrogen. Ferromagnetic six line spectra of hydrogenated Fe-Ni alloys also consist of two parts, one being from the y' phase, in which iron atoms have little interaction with hydrogen because of its inhomogeneous distribution, and the other from the hydride like Pphase, in which the iron atoms inevitably interact with nearby hydrogen atoms and their magnetic moment is reduced by 15%. This experiment shows non- uniform distribution of infused hydrogen in iron alloys and, at the same time, remarkable effects of the electronic interaction between hydrogen and iron. Disagreements in the past experiments are well interpreted from the above results and point of view.

1. Introduction.

-

In recent years, studies of

hydrogen in metals and alloys, such as Fe-Bd 11-31, Ni [4], Fe-Ni-Cr [5], and Ta [6], by means of Moss- bauer effect have been carried out to a considerable extent. Positive isomer shift change by hydrogen absorption was observed in all investigations. However, the interpretations of this effect of hydrogen are not always in agreements with each other. Roughly speak- ing, two alternative interpretations exist, that is, a) the

d-band holes of metals are partly filled by 1s electrons from solute hydrogen, or b) lattice expansion caused by the interstitially dissolved hydrogen atoms reduces the density of conduction electrons as a whole.

On the other hand, behavior of hydrogen in metals and alloys is rather complicated, differs from metal to metal, and makes the straightforward understanding of interaction between hydrogen and constituent metals difficult. For instance, the solubility of hydrogen in pure a-iron under the thermal equilibrium is so small that the Mossbauer spectroscopy of Fe-H system is not available [4]. Alloying with another metal element, which dissolves hydrogen to a large extent, to make the measurement possible is not always in favor to the precise analysis of the Mossbauer data,

because it produces nonuniform distribution of hydrogen in the alloy due to different affinities with different constituents, and induces local changes in bonding states, local lattice distortions, and lattice transformations, all of which must be properly taken into account in the spectral analysis.

In the present investigation, by means of the

57Fe

Mossbauer effect the distribution of solute hydrogen atoms and their interactions with iron atoms in stainless steel and Fe-Ni binary alloys are studied. The austenitic

stainless steels containing 8.6

%

Ni-20.3

%

Cr,

12.6% Ni-18.8

%

Cr, and 18.4% Ni-26.5

%

Cr

respectively and ferromagnetic Fe-45

%

Ni and

Fe-75% Ni alloys are employed. They are first [heat treated for homogenization and chemically and electro- lytically polished to obtain the specimen thickness of about 30 ym. Hydrogen is introduced cathodically in 1 N H2S04 solution. Immediately after the hydrogena- tion, each specimen is cooled by liquid nitrogen to prevent hydrogen desorption and then quickly transfer- ed into a cryostat to measure the Mossbauer spectrum at that or lower temperatures.

The 14.4 keV y-ray radiation of 5 7 ~ e from 30 mCi j7Co source dissolved in metallic copper is measured by

C6-380 F. E. FUJITA AND T. SOHMURA using the transmission method and the constant

acceleration mode with the 1 024 channels multi- scaler analyzer. X raydiffr action measurements are carried out by using CoK, radiation a t room tempera- ture in rather short time to reduce the hydrogen desorption. The amount of absorbed hydrogen is determined by measuring the volume of collected desorbing gas in glycerine and triethyleneglycol mixture.

the above assumption is carried out. The calculation is to find the isomer shift distribution by the Fourier decomposition of the observed spectrum which is considered to consist of a number of Lorentzian curves 17, 81. A result of the isomer shift distribution curve D(IS) for 12.6 Ni-18.8 Cr alloy is shown together with the observed Mijssbauer spectrum in figure 2.

2. Results on stainless steel.

-

In the case of austenitic stainless steels, the paramagnetic single line spectrum at 77 K shows a considerable change by hydrogen absorption, as seen in figure 1, which is

L I

-1.0 -0.5 0 0.5 1.0

D o p p l e r v e l o c ~ t y I m m l s l

FIG. 2.

-

The spectrum the same as in figure 1 and its computer analyzed isomer shift distribution curve.

D o p p l e r v e l o c ~ t y I r n r n l s l

FIG. 1.

-

The Mossbauer spectrum at 77 K of 18.4% Ni- 26.5 % Cr stainless steel hydrogenated for 10 hr with the current density of 0.5 A/cm2, Po, and its analytical curves, PA and PB, where Pa corresponds to the original unhydrogenated spectrum.

taken from the 18.4 Ni-26.5 Cr alloy hydrogenated for 10 hr a t 20 OC with a current density of 0.5 A/cm2. The peak center shifts from the original position toward the positive velocity side by 0.08 mmls, the half width is broadened from 0.43 mm/s to 0.55 mm/s, and an asymmetric tail appears in the high energy side. All of these variations disappear and the original spectrum, denoted as PA hereafter, is recovered after desorption of hydrogen by aging at about 100 O C .

It is therefore obvious that these changes are only due to hydrogen. Similar changes are observed for the other two types of stainIess steel.

In analyzing the spectrum after hydrogenation, Po, first it is simply assumed that the spectrum consists of two components, one of which is the absorption from the iron atoms not affected by the infused hydrogen that may correspond to PA and the other from the affected iron atoms that we denote as PB. Decom- position of the spectrum is successfully done as shown by broken lines in figure 1. The fraction of the compo- nent PB increases linearly with the increasing nickel

content and the hydrogen charging time. A computer

analysis to decompose the spectrum P, without using

Undoubtedly, the peak

1

)

,

and DB appearing in the distribution curve correspond to PA and

P,

respecti- vely. The third additional peak D c at -t 0.25 mm/s in the figure always appears in the computer analysis and seems to arise from a newly formed hydride. Wertheim et al. [4] detected a positive isomer shift of 0.58 mm/s relative to the peak PA in our notation in nickel hydride, while the isomer shift, D,

-

DA

(or D c

-

PA) in the present experiment is about 0.47 mm/s. These two results are in agreement if the alloying elements, iron and chromium, of high concen- trations in the stainless steel are taken into account. As for the identification of PB (or D,), three possibili- ties seem to exist. They are : (1) a-phase, (2) &-phase, and (3) y-phase affected by hydrogen. Since the hydro- gen induced a-phase is ferromagnetic, its possibility is excluded. X-ray diffraction data show that the hydro- gen induced hexagonal &-phase in stainless steel does not disappear by aging except for 18.4 Ni-26.5 Cr alloy, while the Mossbauer peak P, easily disappear by the aging. Therefore, PB is not due to the E-phase. As a conclusion PB is attributed to the y-phase affected by hydrogen.

HYDROGEN IN STAINLESS STEEL AND Fe-Ni ALLOYS C6-381 between the isomer shift change and the volume change tion, and the isomer shift moves by

+

0.08

+

0.04 mm/s by compression, d(IS)/a(A V/ V) = 1.4 mm/s reported as an average. A careful computer fitting is done to find by Pipcorn et al. [9] on pure iron and nearly the same the internal magnetic field distribution function D ( H ) value by Rhiger et al. [lo] on stainless steel, the isomer by a similar method to obtain D(IS), and the spectrum

shift changes expected from the lattice expansion will be is decomposed into two components as are shown in

0.009 8 mm/s and 0.045 mm/s respectively which are figure 4. One arises from the hydrogen infused solid far smaller than the above observed values. Therefore,

we are apt to conclude that 1s electrons of hydrogen I n t e r n a l magnet~c field I kOe1

I I

- 6 -L - 2 0 2 1 6

D o p p l e r v e l o c i t y [rnmlsl

partially occupy the 3d holes of iron producing such 2 00 150 l o o 1 5 0 2 00

large positive isomer shift as observed. The isomer shift

2

originates in the density at nuclear position of s elec- trons, which are on the other hand screened by the inner shell 3d electrons. Consequently, the both effects

of 3d and 4s must be taken into account to clarify the

E

origin of the isomer shift change. According to Walker

et al. [ l l ] the isomer shift is evaluated by determining

2

the electronic configuration of iron. If the hydrogena- tion changes the electronic state of iron in stainless steel

2

from 3d7+' 4s1, where e arises from alloying, to either 3d7+'+" 4s' or 3d7+' 4 ~ ' - ~ , a positive change of isomer

₂

shift is expected. The former state represents the 3d hole filling model, while the latter is expected from

lattice expansion. Wertheim et [4] have interpreted FIG. 4. - The calculated internal magnetic field distribution the positive isomer shift of nickel hydride by suggesting curves for the spectrum in figure 3.

the configuration, 3d7.35 4s'. On the other hand, Phillips et al. [3] and Sech et al. [2] observed that the

saturation moment of ferromagnetic Fe-Pd alloys was solution phase, Y', and the other from the Fe-Ni h ~ d r i d e not reduced by hydrogenation, and concluded that like phase,

P.

An X-ray diffraction study shows that the 1s electrons of hydrogen do not fill the 3d holes. This two phases have the f. c. c. structure like the original y will be discussed later with regard to our result on the phase. At average hydrogen concentration of about

Fe-Ni alloys. 20

%.

y' phase has a lattice expansion of only with 1

%

but a large lattice distortion. On the other hand, Pphase 3. ~ ~on F ~ - N ~ ~ alloys and discussion. ~ l t ~

-

~h~ has a lattice expansion as large as 6

%

but little distor-

above interpretations of inhomogeneous distribution of tion. An X-ray data is in figure 5-

hydrogen and filling of 3d holes of iron by 1s electrons

FIG. 3. -The Mossbauer spectrum at 77 K of Fe-45% Ni

alloy hydrogenated for 15 hr. Hydrogen content is 24.4 at. %.

of 45% Ni alloy the ferromagnetic six line spectrum produces remarkable line broadening and shoulder on the inner side of each peak. The amount of absorbed hydrogen in this case is 24.4 at.

%.

The apparent inter- nal field is about 4

%

less than that before hydrogena- of hydrogen are confirmed by the study of ferro- magnetic Fe-45

%

Ni and Fe-75

%

Ni solid solution alloys. As figure 3 shows, by an intense hydrogenation

>

-8-

FIG. 5. -X-ray diffraction profiles of the original y(200) and the hydrogen induced y'(200) and /3(200) line. Charging time is

5 hr. CoK, radiation is used.

In accordance with the X-ray diffraction data, the y'

component in the Mossbauer spectrum exhibits a reduction in internal field less than 1

%,

very small isomer shift change, and no appreciable line broaden- ing as compared with the y phase, while the

B

phase shows the internal field reduction as large as 15%,

C6-382 F. E. FUJITA AND T. SOHMURA isomer shift change of

+

0.4 mm/s, and a line broaden-

ing by about 20

%.

It is worthy of note that the y'

phase has large local lattice distortions as seen in figure 5 presumably due to inhomogeneous distribu- tion of hydrogen but the Mossbauer spectrum shows very small change of iron environment in it. On the other hand, the sharp X-ray line of

/?

phase in the figure indicates a good ordering of atomic arrange- ments or rather homogeneous distribution of hydrogen, while the Mossbauer spectrum shows that the iron atoms are largely influenced by hydrogen.

These results lead us to the following conclusions :

The infused hydrogen atoms first occupy the interstitial sites neighboring to nickel sites preferably because of the stronger affinity with nickel than iron, producing inhomogeneous distribution and local distortions. Most of the iron atoms will be therefore unaffected by hydrogen. This is what we find in the y' phase. By further hydrogenation, vacant interstitial sites in y' phase will be gradually occupied by hydrogen atoms to result in the more homogeneous and highly concentrat- ed state, and, thereby, the lattice will transform at a certain critical hydrogen concentration from the y'

phase to the

/?

phase. In the latter lattice structure hydrogen atoms must have an ordered arrangements with high concentration, and all iron atoms will inavoidably have strong direct interactions with surrounding liydrogen atoms. Although no precise theory on the interaction has appeared yet, it is likely that 1s-3d and other hybridizations take place between iron and nearby hydrogen to form a bonding level presumably below the top of 3d band. The positive isomer shift change is accounted for by this interaction since the number of inner core electrons screening the 4s electrons is increased. And, at the same time, this localized bonding will reduce the number of 3d unpair- ed electrons, and therefore reduce the magnetic moment of iron. Thus, the small internal field and large positive isomer shift in the

/3

phase is interpreted. The effect of lattice expansion on the positive isomer shift change in /?phase is also estimated, and it is found that only one third of the total change could be attribut- ed to the lattice expansion.

In order to verify the above change in 3d configura- tion, the Mossbauer spectroscopy of Fe-Ni alloys at low temperatures is done. The temperature dependence of the internal field of the y, y', and

/?

phase is shown in figure 6 . Curve a shows the relation between the reduced temperature, TIT,, and reduced internal field, H/H(O), for the y and y' phase, where Tc and H(0) are the Curie temperature and saturation field of y

respectively. The data of the y phase are determined by

[I] BEMSKI, G., DANON, J., DE GRAAF, A. M. and DA

SILVA, X. A., Phys. Lett. 18 (1965) 213.

[2] JECH, A. E. and ABELEDO, C . R., J. Phys. & Chem. Solids

28 (1967) 1371.

FIG. 6. -The temperature dependence of the internal field of y phase (46.9 % ; after Tomiyoshi et al. [12]), y' and p phase

(45 % Ni ; present experiment)

.

Tomiyoshi et al. [12]. No appreciable difference between y and y' is observed. Curve b, which is drawn so as to fit the present data, of the

/?

phase, clearly shows that the iron moment is reduced due to the interaction with solute hydrogen. Unlike the case of the Pd-Fe-H system studied by Jech and AbeIedo [2], to draw the curve c without reducing the saturation moment of iron is impossible in the present case. In summarizing, it is pointed out that in the hydro- genated iron alloys hydrogen atoms prefer the intersti- tial sites close to such atoms as nickel, palladium, etc., which have larger affinity with hydrogen than iron, producing an inhomogeneous distribution. Therefore, unless the hydrogen concentration becomes high enough to.make a uniform and ordered distribution or hydride like structure, iron atoms will scarcely interact with the infused hydrogen. In this case, the exchange interaction of iron atoms could be extensively changed by hydrogen but no change in iron moment will be expected. This explains the difference between the hydrogen-unaffected and affected spectral component in stainless steel and that between the y' phase and

/3

phase in Fe-Ni alloys. Strong local interactions between the iron atoms and surrounding hydrogen atoms in the affected and

/3

structure can reduce the magnetic moments of iron to a considerable extent and also give a large positive isomer shift change, as are actually observed. The Mossbauer studies on hydrogen in metals and alloys and their interpretations in the past seem to be required to reconsider by taking account of the above facts and point of view.

The authors wish to express their hearty thanks to Prof. K. Kamachi of Yamaguci University for supply- ing the alloys and for helpful suggestions. Thanks are aiso due to Dr. S. Nasu of Osaka University for advices on computer programing.

rences

[3] PHILLIPS, W. C. and KIMBALL, C. W., Phys. Rev. 165

(1968) 401.

[4] WERTHEIM, G. K. and BUCHANAN, D. N. E., J. Phys.

HYDROGEN IN STAINLESS STEEL AND Fe-Ni ALLOYS C6-383 [5] SOHMURA, T. and FUJITA, F. E., J. Japan Inst. Metals 39

(1975) 374.

[6] EIEIDEMANN, A., KAINDL, G., SALOMON, D. and WORT-

MANN, G., J. Physique Colloq. 35 (1974) C 6-515.

[7] GONSER, U., NASU, S., KEUNE, W. and WEISS, O., Solid

State Comrnun. 17 (1975) 233.

[8] WINDOW, B., J. Phys. E 4 (1971) 401.

[9] PIPCORN, D. N., EDGE. C. K., DEBRUNNER, P., DE PAS-

QUACI, G., DRICKAMER, H. G. and FRAUENFELDER, H.,

Phys. Rev. 135 (1964) A 1604.

[lo] RHIGER, D. H., INGALLS, R. and CHUN-MAI LIU, Solid

St. Commun. 18 (1976) 681.

[ l l ] WALKER, L. R., WERTHEIM, G. K. and JACCARINO, V.,

Phys. Rev. Lett. 6 (1961) 98.

[12] TOMIYOSHI, S., YAMAMOTO, H. and WATANABE, H., J.