Effect of hydrogen on internal friction of some F.C.C. metals

Acta metall, mater. Vol. 38, No. 12, pp. 2573 2582, 1990 Printed in Great Britain. All rights reserved

0956-7151/90 $3.00 + 0.00 Copyright © 1990 Pergamon Press plc

EFFECT OF HYDROGEN ON INTERNAL FRICTION OF SOME F.C.C. METALS A. Z I E L I N S K I Merchant Marine Academy, Institute of Marine Engineering, 81-962 Gdynia, Poland (Received 14 August 1989; in revised form 26 April 1990)

Abstract--A review is given of the internal friction and ultrasonic attenuation investigations made so far on hydrogen charged austenitic stainless steels, high nickel alloys, nickel and copper. The experimental results show that only some forms of hydrogen in f.c.c, metals are of importance for modification of internal friction spectrum: atoms located in the interstices and their pairs, interstitials segregated at the dislocations, and hydrogen bound in hydride and molecular forms. These different hydrogen forms are concluded to affect some already existing anelastic phenomena or to result in an appearance of some new processes. R 6 s u m ~ n pr6sente une revue des &udes de frottement int6rieur et d'att6nuation ultrasonique effectu6es jusqu'fi pr6sent sur des aciers inoxydables aust6nitiques, sur des alliages riches en nickel, sur le nickel et sur le cuivre charg6s en hydrogdne. Les r6sultats exp&imentaux montrent qu'il n'y a que quelques formes de l'hydrogdne, dans les m6taux c.f.c., qui soient importantes pour modifier le spectre de frottement int6rieur; les atomes situ6s sur les interstices et leurs paires, les interstitiels qui ont s6gr6g6 sur dislocations, el I'hydrogdne li6 dans les hydrures et les formes mol6culaires. Ces formes d'hydrog6ne diff6rentes affectent quelques phenomdnes an61astiques d6jfi pr6sents, ou provoquent l'apparition de quelques nouveaux m6canismes.

Zusammenfassung--Die bisher durchgefiihrten Untersuchungen zur inneren Reibung und Ultraschallabsorption in Wasserstoff-beladenen austenitischen St/ihlen, Hochnickel-Legierungen, Nickel und Kupfer werden zusammengestellt. Die experimentellen Ergebnisse zeigen, dab nur einige Formen des Wasserstoffs for die Ver/inderungen im Spektrum der inneren Reibung von k.f.z. Metallen wichtig sind: Atome im Zwischengitter und deren Paare, an Versetzungen segregierte Zwischengitteratome und Wasserstoff, gebunden in Hydriden oder molekularen Formen, Es wird gefolgert, dab diese verschiedenen Formen des Wasserstoffs einige schon bestehende anelastischen Erscheinungen beeinflussen oder zum Auftreten einiger neuer Prozesse ffihren.

INTRODUCTION A presence of hydrogen in a variety of metallic materials results in degradation of their mechanical properties and in some microstructural changes: decrease in strength and plasticity, hydrogen induced delayed cracking, appearance of bubbles, fissures and crevices, etc. Although the hydrogen embrittlement is the most serious and best documented for ferritic and martensitic high strength steels and alloys, the recent years have brought an increasing number of research works on the hydrogen due failure of many f.c.c. metals and engineering materials, and on general hydrogen behaviour in f.c.c, lattice. Hydrogen atoms exist in metals as interstitial screened protons. Neutronographic studies have shown that hydrogen atoms locate in f.c.c, metals in octahedral interstices, at least in nickel [1] and palladium [2]. The presence of hydrogen in f.c.c. metals results in a local lattice distortion of about 4%, much less than that of about 1 3 o found for b.c.c, metals [3]. The hydrogen solubility in f.c.c, metals is remarkably higher than that in b.c.c, structures, with the solubility enthalpy estimated at 0.13-0.17 eV

in nickel [4-6], 0.37-0.60 eV in copper [4, 7, 8] and about 0,61eV in austenitic stainless steels [9, 10], and the solubility constant Ks of 10-4-10 -6 (at.H/at.Me)Pa-1/2. The hydrogen concentration may reach quite large values, e.g. 10-4-10 -3 at.H/at.Me in nickel and steels as the averages [11, 12] and even 0 . 5 - 1 a t . H / a t . M e within a thin surface layer in austenitic steels [13, 14]. The diffusion of hydrogen in f.c.c, metals is relatively slow. The activation enthalpy H o of hydrogen lattice diffusion has been estimated at 0.41-0.42 eV for nickel [4, 5, 15-20], 0.54-0.68eV for austenitic steels [10,21-23] and 0.40-0.42eV for copper [4, 7, 8, 16, 24] at D Oabout 10 6 m2/s for all metals. At r o o m temperature the diffusivity coefficient has been determined at 6-7 x 10-14m2/s in annealed nickel [25-27] and 2-8 x 10-16m2/s in steels [9, 14, 28, 29]. Plastic deformations lowers the diffusivity in Ni [30] and Cu [7]. The hydrogen atoms in nickel have been suggested to move always along the (111) direction, alternately through the octa- and tetrahedral interstices [3]. There is an evidence of hydrogen trapping by some point defects. The hydrogen-vacancy interaction enthalpy has been reported as of 0.05-0.30 eV

2573

2574

ZIELINSKI: EFFECT OF HYDROGEN ON INTERNAL FRICTION

for Ni [3], 0.20 eV for Pd and 0.50 eV for A1 [3, 3 I]. A weak interaction of that kind has been also suggested for hydrogen in Cu [32-34]. The appearance of some Me-H complexes (Me = Cu, Fe, Cr, Mn, Pd) in Ni alloys has been concluded from the magnetic relaxation studies [35]. The trapping of hydrogen by Si atoms in dopped nickel has been deduced from the electrical resistivity measurements [6]. The well-documented is an interaction of hydrogen and dislocations, e.g. for copper from the positron [36, 37] and internal friction [38, 39] studies, and for nickel from the positron testing [40,41], investigations of hydrogen effect on mechanical properties [15, 42-45], permeation studies of stressed specimens [46] and internal friction tests [47-50]. The binding enthalpy of hydrogen at dislocations in nickel has been estimated at 0.08 eV [15] and 0.14eV [44], at elastic interaction rather small [15] as compared to the electrostatic contribution [30]. The interaction of hydrogen and dislocations in austenitic stainless steels and alloys has been intensively studied, and the binding enthalpy reported as of 0.10-1.0 eV, with the most reasonable values at 0.17-0.19 eV [9]. Investigations of internal friction have also confirmed the dislocation pinning of dislocations by hydrogen atoms [51-53]. Some other structural defects may trap substantial amounts of hydrogen in f.c.c, metals: crack tips in steels [10, 54-56] and nickel with binding enthalpy 0.32eV [57, 58], stacking faults [59], interfaces [9], inclusions [60] and grain boundaries [46, 57, 60-62]. At higher hydrogen concentrations in some f.c.c. metals the hydride phases may appear: in nickel charged with gaseous or electrolytic hydrogen [23, 63, 64], Fe-Ni alloys [63, 65], austenitic stainless steels and high nickel alloys [66-74]. The phase transitions and decomposition of hydrides may lead to the hydrogen surface cracking and destruction of metal as observed sometimes at moderate charging conditions [10, 13, 14, 51,75-77], even at cathodic current density as low as 40 A/m 2 and in absence of hydrogen recombination poison [78]. In some other f.c.c, metals in which hydride phases are thought not to appear the hydrogen entry may result in formation of microbubbles, e.g. in copper [79-81]. Among different experimental techniques a study of internal friction is one to give very valuable informations characterizing hydrogen behaviour in metals including: hydrogen~lislocation binding enthalpy, distribution of hydrogen between lattice and some crystal defects, charging conditions resulted in irreversible change in microstructure, creation of hydride phases, etc. So far developed internal friction models indicate that hydrogen entry may cause the appearance of new or modify already existing phenomena in f.c.c, metals: Snoek and Zener relaxation due to a stress-induced reorientation of pairs of point defects, Bordoni relaxation associated with intrinsic properties of dislocations, dislocation resonance and hysteresis (frequency- and amplitude-

dependent internal friction), Snoek-K6ster and Hasiguti relaxations resulted from interaction of dislocations and point defects, precipitation peaks, magnetomechanical internal friction, scattering on inhomogeneities, and some phenomena caused by hydride phases and microbubbles created by gaseous hydrogen. Some undesired secondary effects observed during charging, difficult interpretation of experimental results, and limited industrial application may explain why the research works on the hydrogen effects in f.c.c, metals made by internal friction technique are still fewer and sometimes controversial. In presented paper the new and so far earlier results of the investigations of the above subject are reviewed in order to describe the hydrogen behaviour and its contribution to the internal friction in some f.c.c. metals: austenitic stainless steels, high nickel alloys, nickel and cooper. RESULTS AND DISCUSSION

Internal friction of hydrogen charged austenitic stainless steels An austenitic 18Cr-14Ni steel was investigated in annealed and 10% cold worked state. The samples, wires 0.5 mm in diameter and 100mm long, were charged in a pure sulphuric acid at cathodic current density ik of 10, 20 and 40 A/m 2 for 1000 h or in the acid with an addition of 3mg/l of As203, as a hydrogen recombination poison, at ik of 300 A/m 2 for 50 h. Internal friction and eigenfrequency were measured by the inverted torsion pendulum at frequency f of 0.7 Hz in a temperature range between 140 and 300 K (for other details see [78, 82]). The charging of annealed steel at 20 A/m 2 results in a broad flat peak at 230 K accompanied by a modulus effect (Fig. 1). The charging at 300A/m 2 causes an appearance of much higher peak at similar temperature. Some microscopic examinations have shown in the latter test numerous cracks up to 2 #m in depth. A high single internal friction peak has been found in a number of earlier investigations made by

(fl 25

1.08 1.06

I' o

15 1.04

1D2 0~ 1.0(? 100 140 18() 220 260 300 TEMPERATURE T( K } Fig. 1. Internal friction and eigenfrequency of annealed 18Cr-14Ni steel charged with hydrogen at 20A/m2 for 1000h (0) and at 300A/m2 for 50h (O).

ZIELINSKI:

EFFECT OF HYDROGEN ON INTERNAL FRICTION

resonant techniques in kHz frequency region for: austenitic stainless steels charged at 2000A/m 2 [11, 83] or 500A/m: [84-88], Fe-Ni [89,90] and F e ~ r - M n f.c.c, alloys [91]. The measurements of a peak shift with frequency gave a peak enthalpy H of 0.50-0.52 eV [11,83-88], close to the activation enthalpy of hydrogen lattice diffusion in an austenite. Therefore the peak was concluded to originate from stress-induced reorientation of pairs of point defects described by the Snoek model. This hypothesis is also supported by an independence of the peak temperature on hydrogen content (Fig. 1) and on chemical composition of steel [85], in agreement with the above model. The relaxation strength, on the other hand, should decrease with increasing temperature, as really observed [85]. There is no, however, agreement as to what atoms constitute the relaxing pair. Some investigators have suggested the reorientation of hydrogen atom pair [85 88], others relaxation of H-Ni pair [83]. In favour of the first hypothesis is a discovery of the Snoek peak even in Ni-free F e - C r - M n alloys [91]. In the above described tests the peak appears at 230K for 0.7 Hz frequency. From the Arrhenius relation of relaxation time one might find at the above values of H the frequency factor f0 as of 9.5 x 10Jr'-2.7 x 101 S -1, close to the determined by Asano [85] value 2.4 x 101~ s 1. Therefore the peaks found in these low frequency tests may be also attributed to the Snoek relaxation of hydrogen atoms. The Snoek relaxation strength is very sensitive to the chemical composition of steel. For example, in tests made by Asano [85] at similar frequency and charging conditions the Snoek peak height for the most resistant to the hydrogen damage 25Cr-20Ni steel was about 1.6 x 10 3Np, for less resistant 17Cr-I 1Ni steel about 1 x 10 3Np, and for the most susceptible to hydrogen embrittlement 19Cr-9Ni steel the peak was on b about 6 x 10 4Np. As the hydrogen solubility is very similar for all these steels, the observed difference may be attributed mainly to the appearance of surface hydrogen microcracks which absorb substantial amounts of hydrogen that can no more contribute to the Snoek peak. More brittle steel, more hydrogen trapped inside cracks and blisters, less interstitial hydrogen atoms and pairs, and lower Snoek peak. The irreversible embrittlement may occur even at moderate charging conditions and for relatively stable 18Cr-14Ni steel charged at 50 A/m 2 in presence of 3 mg/1 of As203 [92, 93] and at 40 A/m 2in pure acid [78. 82], and even in 25Cr-20Ni steel charged at 60 A/m 2 in solution with no As203 [94]. In all studies so far the crack incubation time was not very long and comprised between 100 and 3000 min [95]. Therefore the hydrogen damage may have appeared for all here reported investigations in steels charged in presence of a great amount 250 mg/1 As203 and at relatively high current density 500 A / m 2

2575

20 15

~

s 0

, 160

200

2/.0

TEMPERATURE

280 TiK)

Fig. 2. Internal friction of 10% cold worked lgCr 14Ni steel H-free (©) and charged with hydrogen at 10 (A), 20 (A), and 40 A/m2 (0) for 1000h. The measurements made on cold worked steel show very complex internal friction spectrum (Fig. 2). Microscopic examinations have revealed after charging at 40 A/m 2 microcracks deep to 30/~m. The detailed discussion of the effects observed in Hcharged cold worked steels is, unfortunately, very difficult because of a great number of processes which are caused or modified by hydrogen entry. They include: modification of the carbon and nitrogen deformation peaks that according to earlier papers [96-98] may be associated with interstitial-dislocation interaction, and which appear here at 205 K and 250 K; formation of hydrogen atom pairs resulting in possible Snoek-type relaxation peak; trapping of hydrogen at dislocations, associated with a SnoekK6ster peak; plastic deformation (after charging at 40 A/m 2) that causes an increase in dislocation background. The double peak observed at 20 A/m 2 seems to consist of two peaks, very likely the hydrogen Snoek-type peak at lower and hydrogen SnoekKfster peak at higher temperature. As a result of charging at 40 A/m 2 the microcracks are formed which absorb some part of hydrogen, and following plastic deformation changes equilibrium between hydrogen in lattice and at dislocations. Both processes decrease the amount of interstitial hydrogen so that the Snoek peak due to hydrogen atom pairs may even disappear. The peak observed in this test may then be originated mainly from the Snoek-K6ster relaxation of hydrogen atoms bound to the dislocations. In support of this statement is an observed here decrease in the peak height and peak temperature, in comparison to those after charging at 20 A/m 2, as expected from the Snoek-K6ster relaxation model for steel more deformed and containing less hydrogen.

Surface ultrasonic wave attenuation in hydrogen charged f.c.c, metals The surface wave attenuation was investigated in polycrystalline nickel, austenitic stainless steel 18Cr14Ni, two high nickel alloys Hastelloy C-276 and Carpenter 20Cb3, and high purity iron undergoing different thermomechanical treatment. The charging

2576

ZIELINSKI: EFFECT OF HYDROGEN ON INTERNAL FRICTION

was carried out in sulphuric acid with 3 mg/l of AszO 3 at 50 or 200A/m 2 for 1.5-1000h. Special surface wave transducers were fixed to the outer surfaces of the strip sample bent into a 90 ° vee. The samples were in part immersed into the electrolyte so that the measurement of ultrasonic attenuation was made both during and after charging. The attenuation of generated non-dispersive surface wave was measured by the standard pulse echo technique in a 3-7 MHz frequency range. The logarithmic decrement of internal friction was calculated as 3 --0.115~/f, where ct the surface wave attenuation in dB/#s and f the frequency in MHz. Because of very high damping the single echo technique was applied and only relative changes in attenuation were measured (for other details see [53, 93]). The investigations made during hydrogen egress have shown the decreasing attenuation with increasing cathodic current and charging time up to 90 min for all materials. During long term charging the attenuation grows again after some time (Fig. 3) for both current densities. The change in attenuation during hydrogen desorption becomes very quick at the beginning and then the attenuation increases much slower up to some steady value (Fig. 4). Intensive studies of this effect on C-276 alloy and ~t-Fe [71, 99] have shown that among a number of possible processes which may occur during hydrogen ingress and egress only a few are of importance for the observed effects: segregation of hydrogen at dislocations, changes within a double surface layer and on the surface, and surface hydrogen cracking. The possibility of hydrogen trapping was later verified by a specially developed model that has involved the Granato-Lficke model of dislocation resonance damped by interstitials segregated at dislocations, and the Cottrell-Bilby kinetic law for the diffusion of interstitials from the dislocations. The final equations were well fit by experimental relations of change in dislocation attenuation on the hydrogen desorption time [53] except the very beginning. Therefore the hydrogen atoms which segregate at the dislocations may be assumed to decrease the dislocation background and surface wave attenuation. Interruption of cathodic polarization results in a gradual

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hydrogen effusion out of the specimen accompanied by hydrogen unlocking of dislocations and increase in dislocation attenuation. The segregation of hydrogen atoms at dislocations within a thin surface layer cannot explain a very fast increase in attenuation at the beginning of desorption [100]. In order to determine the origin of this effect the attenuation was measured for 18Cr-14Ni steel during and after its charging at 50 A/m 2 for 1.5 h in electrolytes of different composition [101,102]. As illustrated in Fig. 5, the instantaneous change in attenuation appears only in presence of sulphate ions and increased with their concentration. However, postulated earlier [99, 100] the redistribution of sulphate ions within a double surface layer cannot be a direct cause of the effect which has not been observed for the samples covered with a passive film. The most reliable explanation seems associated with different properties of surface layers which determine the hydrogen surface concentration, hydrogen permeation and surface wave propagation conditions. When the passive film is present on the surface,

,~ -2

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DESORPTION TIME td(mfn) Fig. 4. Change in surface wave attenuation of 2OCb3 alloy during hydrogen desorption; arrows indicate the attenuation level at the beginning of desorption for different cathodic current density (in A/m2).

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CHARGING TIME tk{rnin) Fig. 3. Change in surface wave attenuation for various annealed materials during long term cathodic charging at 50 A/m2.

DESORPTION

TIME

td ( m i n )

Fig. 5. Change in surface wave attenuation in 316L steel during hydrogen resorption after cathodic polarization in various NaOH-based electrolytes; arrows indicate the beginning of desorption.

ZIELINSKI: EFFECT OF HYDROGEN ON INTERNAL FRICTION hydrogen atoms are used to dissolve the film and at the beginning no permanent surface hydrogen adsorption occurs. As observed in ellipsometric study the presence of sulphates in an electrolyte affects the composition and structure of surface layer: in NaOH solution an amorphous hydrated layer, and in NaOH + Na2SO4 solution much thinner crystalline layer is formed. The most likely, only on the latter crystalline layer, of properties similar to those of metal, the hydrogen atoms may be permanently adsorbed and change properties of metal-electrolyte interface, surface hydrogen concentration, wave propagation conditions and attenuation. When the hydrogen charging is prolongated, the attenuation increases after an initial decrease (cf. Fig. 5). The time to attain the minimum in attenuation is dependent on kind of material, its thermomechanical treatment and current density. Generally, the minima appear at shorter time for b.c.c, structure (ct-Fe) and higher current density. The appearance of minima was accompanied by other minima in electrochemical potential and by surface damage as blisters, cracks and crevices along the grain boundaries, twin boundaries and slip bands [93]. All these effects are undoubtfully associated with high internal stresses introduced by hydrogen that causes a local plastic deformation and further propagation of surface cracks during long term charging. Due to deformation multiplication of dislocations increases the dislocation background as already observed [99], and the appearance of microcracks increases the scattering. The change in electrochemical potential is resulted from creation of anodic surfaces of cracks. Longitudinal ultrasonic wave attenuation in hydrogen charged nickel The ultrasonic attenuation of 30 MHz longitudinal wave in annealed high purity nickel single crystal was measured by the standard pulse echo technique after charging with hydrogen in pure sulphuric acid at current density i~ of 500 A/m z and temperature 363 K for 1000 h. The small sample, diameter 6 m m and height 10 mm, was put between the poles of a small magnet and placed together inside a He 4 cryostat. The attenuation measurements were made on the sample cooled and heated between 20 and 350K, with an external magnetic field between 0 and 0.65 T applied along the [001] hard magnetization direction in nickel. The attenuation relations on both temperature and magnetic field were investigated (for other details see [50]). The temperature dependence of non-magnetic part of internal friction 6, (at saturating magnetic induction 0.65 T) is shown in Fig. 6. For uncharged nickel the broad relaxation peak appears at higher temperature. Such single peak has already been found in H-free high purity nickel, its parameters estimated at )co of 1 x 101° s -1 and H of 0.18 eV, and the peak

2577

12 10 8 '

6

4 2

0

50 100 150 200 250 300 TEMPERATURE T(K)

Fig. 6. Temperature dependence of total internal friction (O), non-magnetic internal friction (A) and magnetic contribution to the internal friction (V1), before (open symbols) and after (solid symbols) hydrogen charging. recognized as resulted from the Bordoni relaxation [48]. At applied here frequency the Bordoni peak should appear at 365 K. Therefore the increase in internal friction observed in Fig. 6 at upper region of temperature is assumed to be a left side of the Bordoni peak. After hydrogen charging the dislocations are pinned by diffusing hydrogen atoms and the Bordoni relaxation strength should quickly decrease as it is believed to be observed here (Fig. 6). At present high hydrogen concentration in this test, estimated at 0.01-0.05 at.H/at.Ni as the averages, a substantial amount of hydrogen atom pairs may be expected. For the Snoek relaxation associated with hydrogen diffusion in nickel the value of activation enthalpy H is 0.41-0.42eV, the f0 value of 4 x 1013s J [39], and the Snoek peak should be seen at about 340 K. The distinct increase in internal friction of hydrogen charged nickel may then be concluded to be the beginning of the already reported [103] hydrogen Snoek peak. Tanaka has also found in deformed nickel another peak attributed to the interaction of hydrogen and dislocations [47-49]. This peak might be expected at here applied frequency and reported values f0 as of 3 x 1014S ! and H of 0.67 eV at temperature 490 K, well beyond studied temperature range and too high for the trapping of hydrogen atoms at dislocations, even in heavily cold worked nickel. In Fig. 6 the temperature dependence of magnetomechanical internal friction 6m for H-free and H-charged fully demagnetized nickel is shown. In the 320-240 K temperature range (measurements made during cooling) hydrogen causes a small decrease in the 6m value, and next down to 150 K a very small increase. Below 150K the magnetic contribution to the internal friction becomes negligible. Similar

2578

ZIELINSKI: EFFECT OF HYDROGEN ON INTERNAL FRICTION

effects were observed for nickel partially magnetized and tested at different temperature [50]. The decrease in magnetic contribution to the internal friction after hydrogen charging may be caused by a few processes. The high internal stresses associated with high hydrogen lattice concentrations decrease magnetomechanical damping, according to the Becker-Drring model [104]. Another possible important process is a magnetic hardening by hydrogen atoms, their clusters and/or hydride inclusions that may damp the oscillations of dislocations near and inside the domain walls. Finally, the hydride phases are non- or weak-magnetic and, if present even within a thin surface layer, may also be responsible for the decreasing internal friction and saturation induction. A very small increase in magnetomechanical damping below 150K cannot be, unfortunately, explained at the moment. Longitudinal ultrasonic wave attention in hydrogen charged copper Small samples, diameter 6 mm and height 10 mm, were cut from a high purity copper single crystal and then annealed at 573 K either in gaseous hydrogen of different pressure or in vacuum. Applied charging time, 16 or 24h, was sufficient to obtain a uniform hydrogen concentration across the sample. The attenuation of the longitudinal wave was measured by the standard pulse echo technique in the 10-250 MHz frequency range and temperature 293 K. Found, after subtracting the non-dislocation damping, the relations of dislocation internal friction on frequency, 5d( f ) , were analysed in terms of the Granato-Liicke model. In particular, the dislocation density and mean dislocation loop length were calculated after determining the resonance peak height and position. The 6 d ( f ) values measured after annealing were compared only for those of samples which had a similar dislocation background before annealing ( for other details see [39]). As shown in Fig. 7, the ~ d ( f ) values are lower for copper charged with 0.01 MPa gaseous hydrogen than those of the samples annealed in vacuum, for both 16 and 24 h of charging time. Similar decrease in dislocation background has already been observed for copper charged in an electrolyte at 200 A/m 2 for 40 h [38]. The exact calculations have shown that at the same annealing time the dislocation density does not depend on whether the specimen is annealed in hydrogen or in vacuum, within the limits of experimental error. However, the length of dislocation segment between pinning point is shorter and then the number of pinning points is higher for samples charged with hydrogen. This difference may be attributed to the additional pinning of dislocations by a certain amount of hydrogen atoms. Estimation of the hydrogen concentration at the dislocations and of the mean hydrogen content makes it possible to calculate the binding enthalpy of hydrogen and

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5

20

50

100

200

FREQUENCY f(MNz)

Fig. 7. Frequency characteristics of dislocation background for pairs of copper specimens annealed in vacuum (open symbols) or 0.01 MPa gaseous hydrogen (solid symbols) for 16h (O) or 24h (A).

dislocations that has been found as of 0.18 and 0.20 eV, in 16 and 24 h charging tests, respectively [39]. Opposite results were obtained for copper charged with 1 MPa or 2 MPa gaseous hydrogen (Fig. 8). In both tests the decrement of internal friction in hydrogen containing samples was higher than in H-free samples. The treatment of results in terms of the Granato-Liicke model has indicated that this growth can be associated with increasing dislocation background and dislocation density rather than with increasing non-dislocation scattering. These effects may be explained by a presence of large amounts of hydrogen at dislocations that may result in formation of hydrogen clusters and microbubbles. High hydrogen pressure in microbubbles causes a local plastic deformation and generation of fresh dislocations as already observed [79, 80, 105]. The microbubbles, here below 0.02#m, are too small to be serious scatters of applied 30 MHz ultrasonic wave.

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FREQUENCY f (MHz)

Fig. 8. Frequency charactersitics of dislocation background for pairs of copper specimens annealed in vacuum (open symbols) or gaseous hydrogen (solid symbols) of 1 MPa (A) or 2 MPa (O) pressure.

ZIELINSKI: EFFECT OF HYDROGEN ON INTERNAL FRICTION ORIGINS OF HYDROGEN DUE MODIFICATION OF INTERNAL FRICTION IN F.C.C. METALS

Presented results show that only some of possible forms of hydrogen in f.c.c, metals are able to remarkably change the internal friction: (a) interstitial hydrogen atoms located in interstices and their pairs; (b) atoms trapped at the dislocations; (c) hydrogen forms which may cause the irreversible change in microstructure, i.e. hydrides and molecular hydrogen in lattice discontinuities.

(1) Snoek relaxation of hydrogen atom pairs; (2)eigenfrequency, resonant frequency and wave velocity; (3) magnetomechanical damping. Introduction of point defect such as hydrogen atom causes a local lattice distortion. Interaction of hydrogen atom with stress field may be described by elastic dipole model. The elastic dipole consists of two hydrogen atoms in the closest octahedral interstices. Periodically changing stress causes a reorientation of atom pair well explained by the Snoek relaxation model and characterized, as any relaxation process, by the activation enthalpy H, frequency factor f0 [or % = (2rcf0)-l], and the peak height A. The relaxation process is equivalent to the jumping of one or both hydrogen atoms consisting the elastic dipole into the next interstice so that it represents an elementary act of a lattice diffusion. The good accordance of activation enthalpy of the relaxation peak with that of the lattice diffusion has always been observed. There is yet a certain doubt as regards f0 value that according to the theoretical predictions [106] should be described by the following expression )co = 6~Do/Znl~

(1)

where D O the pre-exponential factor in the diffusion equation, ld the mean path of gravity center of H - H pair at jumping of one of the atoms, and • the constant determined by a crystal structure and kind of diffusing atom. The ~t parameter may be taken as of 3/2, the value proposed for C - C pair in f.c.c. structure [106]. The ld parameter may be calculated as equal ax/~/4, where a the lattice parameter. For the above values fo = 36Do~ha z

value two rows of magnitude higher from measured by Asano [85] and calculated here, and four rows of magnitude lower than that of Peterson [83]. The origin of this remarkable discrepancy is difficult to determine. The Snoek peak height cannot still be simply calculated from the theory. An approximate expression indicates its dependence on an absolute temperature T, hydrogen lattice concentration Co and activation enthalpy of pair formation Hp Ap oc T -1 c~ exp(Ho/kT ).

Interstitial hydrogen atoms cause or affect the following internal friction processes and related test parameters:

(2)

For 310 austenitic stainless steel the Do value has been estimated at 5.15 × 10-7m2/s [10] and a = 3.58 × 10 ~°m. That gives f0 = 4.6 × 1013s-~, the

2579

(3)

Obtained results indicate that for both nickel and austenitic steels the relaxation strength of the Snoek peak should be about 10-3-10 -2 Np for 1 at.% of hydrogen atom pairs. For copper, in which the limit solubility of lattice hydrogen is likely no higher than 10-aat.H/at.Me, the relaxation strength will not surpass 10-5-10 -6 Np, and the Snoek peak will not be measurable. Hydrogen atoms located in interstices cause some lattice dilatational internal stresses and strains resulting in the change in the lattice parameters and elastic constants. The last effect may be observed in internal friction experiments as a change in ultrasonic wave propagation velocity in ultrasonic tests, change in resonant frequency in kHz tests and a change in eigenfrequency in tests made by torsion pendulum. Such effects, already observed for hydrogen charged high purity iron [107, 108], should also be noticeable in case of nickel and its alloys; the lattice strains caused by hydrogen are in f.c.c, metals three times lower than those in b.c.c, materials, but the hydrogen content in former metals up to a few rows of magnitude higher. The effect of hydrogen should be well observed in annealed nickel single crystal charged at higher temperature and mild conditions at which it seems possible to avoid the appearance of hydride phase. Dilatational stresses should also oppose the oscillation of domain walls and rotation of domains, and cause the decrease in magnetomechanical damping 6 m . Some damping models predict •m O~ O'i 2 where tr~ is the level of internal stresses. This effect should be seen especially in high frequency region in which the damping peak due to the microcurrents appears, and for high purity well annealed nickel having negligible internal stresses before charging. The effects cannot be expected at too high temperatures at which hydrogen related stresses decline, and at too low temperatures at which the magnetomechanical damping disappears. Hydrogen trapped at the dislocations seems to cause certain noticeable modification of internal friction spectrum, especially: (1) decrease in dislocation background; (2) disappearance of Bordoni peak; (3) appearance of Snoek-Kfster relaxation.

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ZIELINSKI: EFFECT OF HYDROGEN ON INTERNAL FRICTION

Variation of dislocation background is well described by the conventional Granato-Liicke model which predicts the decrease in dislocation damping following the segregation of interstitials at dislocations. The binding enthalpy of hydrogen and dislocations is rather low in f.c.c, metals. Despite that, even in copper for which the limit concentration of lattice hydrogen is probably low, it is still possible to observe the decrease in dislocation damping of slightly deformed metal in MHz region. The expected decrease in dislocation attenuation should be noticed in slightly deformed metals for high purity single crystals, having long dislocation loops, without grain boundaries and other impurity traps. The discussed effects might be difficult to find in austenitic steels and alloys. However, the high hydrogen concentration and sensitivity of dislocation resonance near the resonant frequency to the change of impurity concentration at dislocations result in the appearance of measurable effects in MHz region. This seems rather doubtful at low frequency experiments as a decrease in internal friction lies usually within the limits of experimental error. The Bordoni relaxation is determined by the length of dislocation segment, and a presence of interstitials pinning dislocations should also cause the disappearance of Bordoni peak. The physical picture seems clear: the appearance of additional stable pinning points leads to the decreasing kink width and area swept by the dislocation between its initial state (direct line) and position equivalent to the double kink. The effect of test frequency is complex: an application of higher frequency causes itself a decrease in the peak height but, on the other hand, it leads also to the unpinning of dislocations from hydrogen atoms and increase in the peak height. The interaction of mobile hydrogen atoms and dislocations will cause the appearance of the SnoekKrster relaxation. The description of observed effects may be made by a use of the Schoeck model that can be applied far from saturation state, or by the Seeger model. The results indicate that the Snoek-K6ster relaxation intensity increases slowly with an increasing hydrogen content and will likely be observed for slightly deformed metals, higher hydrogen contents, and low frequency. For nickel H = 0.67 eV and f 0 = 3 × 1014s-1 so that the hydrogen Snoek and Snoek-Krster peaks may be superpositioned in internal friction spectrum. Such behaviour seems typical of f.c.c, metals for which the activation enthalpy of lattice diffusion is usually much higher than the binding enthalpy of hydrogen at dislocations. For the latter peak the initial increase and then decrease in peak temperature may be expected at the ground of Seeger theory which anticipates such possibility in the near-saturation state.

Hydrogen forms promoting irreversible change in microstructure affect also internal friction in f.c.c. metals. Such forms are: hydrogen in hydride phases

and molecular hydrogen in microbubbles and other discontinuities. These forms will be responsible for following changes in internal friction: (1) increase in dislocation damping; (2) scattering of elastic energy; (3) decrease in magnetomechanical damping. The hydrogen presence that results in brittle hydride phases during or after hydrogen charging of nickel, stainless steels and alloys causes the change in dislocation structure from planar to cell structure and increase in dislocation density. For copper similar effects may be expected but they are developed in a different way; the incubation and growth of microbubbles at the dislocations will result in a local overpassage of plastic limit and cold work associated with a generation of new dislocations. According to the Granato-Liicke model, these processes will be accompanied by a remarkable increase in internal friction due to the increase in dislocation damping. The decomposition of hydrides following decreasing hydrogen concentration near the hydride inclusions will cause the appearance of microcracks within a surface layer. Similar discontinuities will be microbubbles with molecular hydrogen e.g. in copper. The appearance of such obstacles will cause a scattering of elastic energy on discontinuities. The diapazon of effect will be different for different materials. The scattering seems greater for sharp discontinuities, with long dimensions and depth, as crevices and cracks in nickel alloys. The round bubbles, especially of small dimensions, seem to be no serious scatters. The increase in internal friction caused by increasing dislocation damping and scattering may be observed at lower frequency as well. This conclusion is supported by an increase in internal friction observed for strongly charged stainless steels. The presence of hydrides may be also a good reason for explaining the observed decrease in magnetomechanieal damping in nickel. The hydride phases are non-magnetic or weak-magnetic and their appearance must cause the decrease in magnetic properties. The segregation of hydrides on the dislocations may lead to the perturbation in the domain wall oscillation and to similar magnetic hardening as does a segregation of single hydrogen atoms at the dislocations. The expected modification of internal friction spectrum following hydrogen entry and its possible origins are here limited to a few f.c.c, metallic materials. There has not been so far important research work on the other metals except palladium-hydrogen system for which two relaxation processes, first originated from Zener-type stressinduced bulk lattice rearrangements of hydrogen interstitials and the second due to the hydrogendislocation interaction, have been reported [109, 110]. Future work could help to verify to what extent the considerations presented in this paper are representative for all f.c.c, metals.

ZIELINSKI:

EFFECT OF HYDROGEN ON INTERNAL FRICTION

CONCLUSIONS The presented results o f the internal friction studies o n austenitic stainless steels, high nickel alloys, nickel a n d copper enable to present the following conclusions: 1. Interstitial h y d r o g e n a t o m s in the lattice a n d their pairs, h y d r o g e n t r a p p e d at the dislocations, h y d r o g e n b o u n d in hydrides, a n d molecular hydrogen in m i c r o b u b b l e s affect mostly the internal friction o f a b o v e f.c.c, metals. 2. Interstitial h y d r o g e n a t o m s in the lattice a n d their pairs may: cause the Snoek relaxation o f H - H complexes: change the elastic constants, following a lattice distortion of h y d r o g e n atoms, a n d then change the ultrasonic wave velocity, r e s o n a n t frequency, a n d eigenfrequency; decrease the m a g n e t o m e c h a n i c a l d a m p i n g by i n t r o d u c i n g high internal stresses. 3. H y d r o g e n t r a p p e d at the dislocations may: decrease the dislocation b a c k g r o u n d ; cause the disappearance o f Bordoni relaxation; cause the a p p e a r a n c e of the Snoek K 6 s t e r relaxation. 4. H y d r o g e n b o u n d in hydride or molecular forms may: cause the increase in dislocation d a m p i n g by d e c o m p o s i t i o n of hydrides or d e v e l o p m e n t of microbubbles a n d local plastic deformation; increase the scattering of elastic energy following the a p p e a r a n c e o f h y d r o g e n due lattice discontinuities like crevices, cracks a n d bubbles; decrease the magnetoelastic d a m p i n g by f o r m a t i o n of n o n - m a g n e t i c hydride phases t h a t m a y decrease the m a g n e t i z a t i o n or cause the magnetic hardening. Acknowledgements--The author wishes to thank: N. F. Fiore, former Professor of the Notre Dame University; Professor Dr K. Liicke and Dr D. Lenz of the Technische Hochschule Aachen: Professor Dr M. Smialowski of the Ohio State University: and Professor Dr E. Lunarska of the Institute of Physical Chemistry in Warsaw, for their helpful comments, assistance with the measurements and encouragement. The financial assistance from the Alexander von Humboldt Foundation is gratefully acknowledged.

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