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Effect of hydrogen on the internal friction of pure polycrystalline iron at low temperatures
Materials Science and Engineering, 50 (1981) 263 - 270
263
Effect of Hydrogen on the Internal Friction of Pure PolycrystaUine Iron at Low Temperatures HIROAKI WADA and KOSHIRO SAKAMOTO Department of Applied Physics, Faculty of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo 113 (Japan) (Received August 11, 1980;in revised form March 17, 1981)
SUMMARY
The behaviour o f hydrogen which had been introduced into high purity polycrystalline iron by thermal quenching was studied by measuring the internal friction and the change in the elastic modulus. The internal friction and the change in the modulus were determined in the temperature range 4 . 2 - 370 K using a three-component composite oscillator m e t h o d at resonant frequencies near 100 kHz at a strain amplitude o f about 10 -7 in a longitudinal magnetic field o f 6.4 × 104 A m - l . A new internal friction peak and a pronounced change in elastic modulus were observed at 14 K. The profile o f the internal friction versus temperature curve is asymmetric; the internal friction increases markedly on the low temperature side o f the 14 K peak with increasing temperature, decreases gradually on the high temperature side o f the peak and reaches a m i n i m u m at 33 K. The origin o f the 14 K peak is quite different from that o f relaxation peaks such as the hydrogen S n o e k peak and the hydrogen cold-worked peak. It was f o u n d that the drastic p h e n o m e n a at 14 K were caused by the phase transition between solid and liquid molecular hydrogen in defects such as voids and microcracks which had f o r m e d along grain boundaries owing to the evolution o f hydrogen gas at high pressures immediately after thermal charging. The internal friction in the temperature range 14 - 33 K can be interpreted in terms o f the viscosity o f the liquid hydrogen in the defects.
1. INTRODUCTION The nature of the hydrogen introduced into iron and steel has been investigated extensively, because it causes serious embrittle0025-5416/81/0000-0000/$02.50
m e n t of these materials. The behaviour of hydrogen in iron has been shown to be strongly affected by interactions between hydrogen and other lattice defects. Therefore basic studies on high purity iron should be made to obtain more accurate information on the behaviour of hydrogen in iron. In recent years, the internal friction [1 - 6 ] , diffusion [7, 8], the magnetic after-effect [2, 9 - 12], the electrical resistivity [ 1 3 - 15] and the specific heat [16] have been investigated to clarify the behaviour of hydrogen in high purity iron. It is n o t easy to determine the intrinsic nature of solute hydrogen in iron because the solubility of hydrogen is very small [17] and the " j u m p i n g " rate of hydrogen is still fairly high [18] even at room temperature. Hydrogen charging frequently causes permanent damage (e.g. voids, microcracks and blisters) to the iron specimen. It is very difficult to introduce a large a m o u n t of supersaturated hydrogen into the iron lattice without inducing such defects. Since internal friction is sensitive to the structure of materials, a study of internal friction is indicative of the behaviour of hydrogen in iron. Two types of internal friction peaks have been reported for iron and steel containing hydrogen. One peak has been reported to be caused by the stress-induced diffusion of hydrogen in solution [19 - 22] ; this is called " t h e hydrogen Snoek peak". The other peak is due to the hydrogen-dislocation interaction; this is called " t h e hydrogen coldworked peak" [1 - 6, 22 - 26]. The hydrogen Snoek peak has been reported in the temperature range 48 - 120 K and the hydrogen coldworked peak in the temperature range 140 220 K depending on the frequency of the measurement. In this paper we report a new internal friction peak which is quite different from the © Elsevier Sequoia/Printed in The Netherlands
264
two peaks mentioned above. The new peak is related to molecular hydrogen segregated in defects (such as voids and microcracks) produced by the hydrogen which had been introduced into a specimen.
2. EXPERIMENTAL PROCEDURE
The starting material for the present investigation was high purity polycrystalline iron (Marz grade) purchased from Materials Research Corporation. The specimen was 3 mm in diameter and 25.5 mm long. The specimens were treated with dry hydrogen at 1770 K for 0.5 h in order to remove interstitial impurities such as carbon, nitrogen and oxygen [ 2 7 ] . After the purification treatment, the total impurity content was less than 20 wt. ppm and in particular the total interstitial impurity content was a b o u t 1 wt.ppm. The internal friction and the change in the elastic modulus were measured in the temperature range 4.2 - 370 K by a three-component composite oscillator m e t h o d [28] at resonant frequencies near 100 kHz at a strain amplitude of a b o u t 10 -7. To suppress the internal friction of magnetic origin, a longitudinal magnetic field of 6.4 × 104 A m -z was applied using a solenoid. The temperature of the specimen was controlled by a non-inductive furnace surrounding the specimen; the low pressure helium gas in the furnace served as a heat exchanger. To maintain highly supersaturated concentrations of hydrogen in the specimens below room temperature, hydrogen was introduced by holding the specimens in a pure hydrogen atmosphere of 0.1 MPa in the temperature range 8 4 0 - 1770 K for 0.5 h and then quenching the specimens in water at 273 K. Several specimens were analysed for total hydrogen content by the vacuum fusion technique. The total hydrogen concentrations were in the range 50 - 500 at.ppm.
3. EXPERIMENTAL RESULTS
3.1. Quenching temperature and hydrogen concentration Figure 1 shows the relation between the reciprocal of the quenching temperature and the logarithm of the hydrogen concentration
Temperature IK) 2000 ]5(]0 1000 8~
~I000 i
._~
c_~ I00
i
•
i i
liar. I
~
~ ron
To
,Ii o 5 10 15 Reciprocal of Ouenchin9 Temperature 10oo0 (K-I) To
Fig. 1. The relation between the reciprocal of the quenching temperature and the logarithm of hydrogen concentrations introduced into pure polycrystalline iron by thermal charging: e, present data; o, ref. 17.
measured by the vacuum fusion technique. The results of the present experiment are indicated by full circles, and the data obtained by Martin [17] b y open circles. Using an Arrhenius plot of Sieverts' law [ 29 ] C = Co PZl 2 e x p ( - - R ~ )
where C is the solubility of hydrogen in iron, Co a constant, P the pressure of hydrogen gas, Q the heat of solution of hydrogen in iron, R the gas constant and T the temperature, we found Q to be 27.6 kJ mo1-1. This value of the heat of solution is in good agreement with other data on pure iron (27.2 kJ mo1-1) [30, 31].
3.2. The internal friction and the change in the frequency A sharp internal friction peak and a marked change in the resonant frequency were observed at 14 K in the specimen held in a pure hydrogen atmosphere at 1140 K and quenched in water at 273 K. The change in the resonant frequency shown in Fig. 2 is the difference between the resonant frequency at 4.2 K and that at each temperature. Lattice defects {e.g. point defects and dislocations) may be produced in the specimen by a thermal stress during quenching. To determine whether or not the drastic phenomena at 14 K were due to these defects, the speci-
265
1000
g 2000
3C
g 2E
I
3000
x rc~
~-000
io
16o
I~O
26o
2~o
3~o
Temperature(K)
Fig. 2. Effect of hydrogen introduced into pure polycrystalline iron by thermal quenching from 1140 K on the internal friction and the change in the resonant frequency: o, as annealed ; e, in vacuum at 1440 K, water quenched to 273 K; A, in hydrogen at 1140 K, water quenched to 273 K.
IODO
30
2000 c.>
g 20
3000 '~
lO
4000
x 'c::~
~'o
i~o
I~O
2~o
2~o
3bo
3~o
Temperature { K )
Fig. 3. Effect of hydrogen introduced into pure polycrystalline iron by thermal quenching from 1770 K on the internal friction and the change in the resonant frequency: A, as annealed; $, in hydrogen at 1770 K, water quenched to 273 K, heating run from 4.2 to 360 K; o, in hydrogen at 1770 K, water quenched to 273 K, cooling run from 360 to 4.2 K.
mens were held in vacuum at 1140 K and q u en ch ed in water at 273 K. The internal friction and the change in the resonant freq u e n c y o f th e v a c u u m - q u e n c h e d specimens are s o mewh at larger than those o f t he annealed specimens over the whole t e m p e r a t u r e range from 4.2 to 370 K, but neither t he internal friction peak nor the marked change in
the resonant f r e q u e n c y was observed near 14 K, as shown by the full circles in Fig. 2. T h e r e f o r e the p h e n o m e n a at 14 K are related to hydrogen. Hereafter these p h e n o m e n a , i.e. the internal friction peak and the change in modulus, are referred to as the 14 K anomalies. As shown in Fig. 3, the internal friction of t he specimen quenched from a h y d r o g e n
266
atmosphere at 1770 K shows a peak at 14 K and a minimum at 33 K, increases gradually with increasing temperature and has a broad peak centred near 300 K. A cooling run was made to confirm the stability of the internal friction. The broad internal friction peak near 300 K disappeared, and the internal friction and the change in the resonant frequency decreased over the whole temperature range. The 14 K anomalies, however, were reproduced. Consequently, the 14 K anomalies are caused by the hydrogen introduced into the specimen and the broad peak near 300 K is apparently due to the decay of point defects produced by quenching. Only the internal friction and the change in the resonant frequency in the temperature range 4.2 - 33 K will be discussed in detail. To examine the effect of hydrogen c o n t e n t on the 14 K anomalies, the specimen was held in a pure hydrogen atmosphere at various temperatures and quenched in water at 273 K. The internal friction values and the changes in the resonant frequency at various temperatures are shown in Fig. 4. Both the peak height and the pronounced change in the resonant frequency at 14 K increased with increasing hydrogen concentration, as shown in Fig. 5. These phenomena can scarcely be seen in the specimen quenched from temperatures lower than 840 K. The internal friction increased markedly on the low temperature side
20
~_o~o_.o.oo___o____o__._o___o11000 ~,
15
2000i~ Q=
1C
I
10 15 20 25 30 35 Temperature (K) Fig. 4. Effect of the quenching temperature on the internal friction and the change in the resonant frequency of pure polycrystalline iron (all samples were held in pure hydrogen gas at various temperatures and water quenched to 273 K): o, 1770 K; A, 1140 K;D, 940 K ; e , 840 K.
z500
lOOO~ "E
To
m
i 1500 0
n
I
,00
200
H;0
Hydrogen Concentration { at.ppm)
Fig. 5. Effect of the hydrogen concentration on the peak height of the internal friction and the change in the resonant frequency of pure polycrystalline iron at 14K.
of the 14 K peak, decreased gradually on the high temperature side with increasing temperature and reached a minimum at 33 K. The change in the resonant frequency occurred abruptly at 14 K; the change in the frequency in the temperature range 14 - 33 K was small and gradual.
3.3. Effect of aging As described in Section 3.1, the hydrogen c o n t e n t detected in the specimen was nearly equal to the solubility of hydrogen in iron for thermal equilibrium at the quenching temperature. Then the specimen was supersaturated with hydrogen after quenching. The diffusion coefficient of hydrogen in iron at 373 K is about 9 X 1 0 - 9 m 2 s-1 [18] ; therefore almost all the dissolved hydrogen in supersaturated solid solution can escape from the iron lattice (an iron rod specimen 3 mm in diameter was used) within 600 s at 373 K. Figure 6 shows the effect o f aging on the internal friction peak height and on the change in the resonant frequency at 14 K. The 14 K anomalies were observed for the specimen aged in vacuum for 1 h at 373 K. The resonant frequency below 14 K steadily decreased with aging, but that above 14 K did not. The resonant frequency of the specimen aged in vacuum for 2 h at 473 K showed no anomalies at 14 K. These observations suggest that the 14 K anomalies are not caused by dissolved hydrogen in a solid solution but by, hydrogen in other states. Figure 7 shows some voids and microcracks along the grain boundaries in the specimen charged with hydrogen. These defects in-
ot r;
Temperature (K)
Fig. 6. Effect of aging on the internal friction and the resonant frequency of pure polycrystalline iron charged with hydrogen by water quenching from 1770 to 273 K: 0, heating run from 4.2 to 300 K; 0, cooling run from 300 to 4.2 K; 0, aged at 300 K for 24h;A,agedat373Kforlh;O,agedat473Kfor2h.
creased in size and number as the quenching temperature increased. The hydrogen existing in excess in the specimen should have segregated as molecular hydrogen gas in such defects. According to the discussion in Section 3.2 about the asymmetric shape of the internal friction uersus temperature curve and the drastic change in the resonant frequency, it is clear that the mechanism associated with the phenomena is quite different from any relaxation mechanism such as the hydrogen Snoek peak and the hydrogen cold-worked peak.
4. DISCUSSION
It was found in Section 3.3 that almost all the hydrogen introduced by quenching was contained as molecular hydrogen gas in voids and microcracks produced along grain boundaries. According to Sieverts’ law, the pressure of the hydrogen gas in these defects should be very high, so that the equation of state of the hydrogen gas deviates from that of an ideal gas. The following approximate equation of state for non-ideal gaseous hydrogen gives the
Fig. 7. Longitudinal cross section of pure polycrystalline iron charged with hydrogen by thermal quenching from various temperatures: (a) quenched from 840 K; (b) quenched from 1140 K; (c) quenched from 1770 K. Voids and microcracks were formed along grain boundaries owing to the evolution of molecular hydrogen gas at high pressures.
amount [32,33] X=
of the gas contained
in these defects
:
34SPV T(1+0.006P)
(1)
where X is the hydrogen content in voids and microcracks (in cubic centimetres per 100 g of iron), P the pressure (in megapascals), V the porosity (the relative volume of these de-
268
fects in volume per cent} and T the temperature (in kelvins). The size and number of voids and microcracks were measured microscopically a large number of times. The calculated porosities are in the ranges 0.15% - 0.5%, 0.04% - 0.12% and 0.01%- 0.04% for the specimens containing 500 at.ppm H, 150 at.ppm H and 50 at.ppm H respectively. Substituting these values into eqn. (1), we can estimate the pressures in the voids and microcracks at 273 K; they are in the ranges 17 - 76 MPa, 22 - 90 MPa and 22 150 MPa in the specimens with 500 at.ppm H, 150 at.ppm H and 50 at.ppm H respectively. 10(3
Sol.
,,'
',~i~
ported by the observation of the specific heat due to phase transition between the solid and liquid hydrogen in the defects in the specimen charged with hydrogen by quenching [ 1 6 ] . The electrical resistivity usually exhibits a discontinuous change on a phase transition of the matrix [36, 37]. The electrical resistivity of iron specimens (1.2 mm in diameter and 600 mm long) charged with hydrogen by thermal quenching were measured in the present study. The electrical resistivity of the specimens, however, did not show any discontinuous change at 14 K. This result indicates that the hydrogen associated with the 14 K anomalies is not hydrogen atoms in the lattice b u t hydrogen molecules precipitated in voids and microcracks. 30
,~-
,, ,,' ,'~ ,~
<~.~ ,~,~--
I 2C
~o.i
Gas
0.0 01
1E
/
0.01
§
Triple point
10
15 20 25 Temperature (K)
30
35
40
Fig. 8. The phase diagram of molecular hydrogen: . . . . , lines of c o n s t a n t density (in grams per cubic centimetre). (After Jacob and Erk [ 34 ] .)
Figure 8 is the phase diagram of hydrogen [ 3 4 ] . The peak temperature (14 K) and the minimum temperature (33 K) of the internal friction correspond to the triple point and the critical point in the phase diagram respectively. From the volume of voids and microcracks and the pressure, the density of hydrogen in the present specimens is calculated to range from about 0.014 to 0.071 g c m -3. Hydrogen in the present specimens exists as a liquid in the voids and microcracks between 14 K and 33 K for most cases. Below 14 K, it is solid. Generally, a phase transition causes marked changes in the internal friction and the elastic modulus [35 - 3 7 ] . Therefore the above considerations lead to the conclusion that the 14 K anomalies are induced by the liquid-solid transition of molecular hydrogen in the voids and microcracks. This conclusion is also sup-
10
20 Temperature (K)
30
40
Fig. 9. The viscosity coefficient for compressed parah y d r o g e n ; that of normal hydrogen is o n l y up to 5% greater t h a n that of parahydrogen at the same temperature. (After Diller [38 ] .)
As shown in Fig. 9, the viscosity coefficient of compressed liquid parahydrogen decreases with increasing temperature and with decreasing pressure [ 3 8 ] . The viscosity coefficient of normal hydrogen is only up to 5% greater than that of parahydrogen at the same temperature [38]. The temperature dependence of the internal friction in the temperature range 14 - 33 K shown in Figs. 4 and 6 is very similar to that of the viscosity coefficient of liquid hydrogen. Thus the viscosity of the liquid hydrogen in the voids and microcracks may contribute to the energy dispersion. The internal friction of the vacuum-quenched specimen increases gradually with temperature from 4.2 K, as shown in Fig. 2. The magnitude
269 o f t h e i n t e r n a l f r i c t i o n n e a r 33 K is n e a r l y equal to that of the specimen charged with hydrogen. The internal friction due to the v i s c o s i t y o f liquid h y d r o g e n a r o u n d 33 K m a y be c o n s i d e r a b l y smaller t h a n t h a t d u e t o t h e d e f e c t s i n d u c e d b y q u e n c h i n g . T h u s t h e internal f r i c t i o n has a m i n i m u m at a r o u n d 33 K. Since a l m o s t all t h e h y d r o g e n in t h e voids a n d m i c r o c r a c k s solidifies b e l o w 14 K, t h e s e d e f e c t s are s t u c k b y t h e solid h y d r o g e n w h i c h acts as a paste. T h u s t h e average elastic m o d u lus o f t h e s p e c i m e n increases d r a s t i c a l l y ; t h e c h a n g e in t h e r e s o n a n t f r e q u e n c y decreases m a r k e d l y . This i n t e r p r e t a t i o n is s u p p o r t e d b y t h e b e h a v i o u r o f t h e r e s o n a n t f r e q u e n c y at a r o u n d 14 K in Fig. 6. T h e r e s o n a n t f r e q u e n c y did n o t s h o w a n y d i s c o n t i n u o u s increase n e a r 14 K w i t h d e c r e a s i n g t e m p e r a t u r e in t h e s p e c i m e n aged in v a c u u m at 4 7 3 K f o r 2 h. T h a t is, a l m o s t all t h e h y d r o g e n c o n t a i n e d in t h e voids a n d m i c r o c r a c k s was e x t r a c t e d b y t h e aging t r e a t m e n t in v a c u u m ; t h e n t h e s e d e f e c t s c o u l d n o t be f i x e d b y t h e h y d r o g e n even b e l o w t h e triple p o i n t . T h e r e f o r e t h e elastic m o d u l u s o f t h e s p e c i m e n did n o t increase.
5. CONCLUSIONS T h e results o f t h e p r e s e n t i n v e s t i g a t i o n o n pure polycrystalline iron charged with hydrogen are s u m m a r i z e d as follows. (1} An internal f r i c t i o n p e a k a n d a m a r k e d change in t h e elastic m o d u l u s w e r e o b s e r v e d at 14 K. T h e i n t e r n a l f r i c t i o n in t h e t e m p e r a t u r e range 1 4 - 3 3 K a n d t h e p r o n o u n c e d c h a n g e in t h e elastic m o d u l u s at 14 K c a n b e i n t e r p r e t e d as d u e t o h y d r o g e n m o l e c u l e s in voids a n d m i c r o c r a c k s . T h e voids a n d m i c r o cracks w e r e f o r m e d a l o n g grain b o u n d a r i e s in t h e s p e c i m e n s c h a r g e d w i t h h y d r o g e n b y therm a l q u e n c h i n g . T h e s e d e f e c t s are filled w i t h h y d r o g e n gas a t a p r e s s u r e in t h e range 20 - 1 5 0 MPa at 273 K d e p e n d i n g o n t h e a m o u n t o f h y d r o g e n in a n d t h e p o r o s i t y o f t h e s p e c i m e n . {2) T h e drastic changes in t h e elastic m o d u l u s a n d t h e i n t e r n a l f r i c t i o n a t 14 K are caused b y t h e l i q u i d - s o l i d t r a n s i t i o n o f mole c u l a r h y d r o g e n in t h e s e d e f e c t s . T h e internal f r i c t i o n in t h e t e m p e r a t u r e r a n g e 14 - 33 K is i n d u c e d b y t h e viscosity o f t h e liquid h y d r o gen.
ACKNOWLEDGMENTS T h e a u t h o r s w o u l d like t o t h a n k Mr. T. Igarashi a n d Mr. M. S h i m a d a f o r c a r r y i n g o u t some of the experiments.
REFERENCES 1 K. Takita and K. Sakamoto, Scr. Metall., I0 (1976) 399. 2 J. F. Dufresne, A. Seeger, P. Groh and P. Moser, Phys. Status Solidi A, 36 (1976) 579. 3 E. Lunarska, A. Zielinski and M. Smialowski, Acta Metall., 25 (1977) 551. 4 P. Moser, J. F. Dufresne and I. G. Ritchie, Proc. 6th Int. Conf. on Internal Friction and Ultrasonic Attenuation in Solids, University of Tokyo Press,
Tokyo, 1977, p. 473. 5 M. Shimada and K. Sakamoto, Proc. 2nd JIM Int. Syrup. on Hydrogen in Metals, Minakami, 1979,
Japanese Institute of Metals, Tokyo, 1979, p. 569. 6 I. G. Ritchie, J. F. Dufresne and P. Moser, Phys. Status Solidi A, 52 (1979) 331. 7 W. Dresler, M. G. Frohberg and H. G. Feller, Z. Metallkd., 63 (1972) 94. 8 Th. Heumann and E. Domke, Bet. Bunsenges. Phys. Chem., 76 (1972) 825. 9 J. J. Au and H. K. Birnbaum,Scr. Metall., 7 (1973) 579. 10 H. Steeb and H. KrSnmuller, Phys. Status Solidi A, 16 (1973) K175. 11 H. KrSnmuller, H. Steeb and N. KSnig, Nuovo Cimento B, 33 (1956) 205. 12 J. J. Au and H. K. Birnbaum, Acta Metall., 26 (1978) 1105. 13 S. Moriya, S. Takaki and H. Kimura, Scr. Metall., 8 (1974) 937. 14 S. Moriya, S. Takaki and K. Kimura, Mater. Sci. Eng., 32 (1978) 71. 15 K. Yamakawa, M. Tada and F. E. Fujita, Scr. Metall., 9 (1975) 1. 16 H. Wadaand K. Sakamoto,Scr. Metall., 13 (1979) 573. 17 E. Martin,Arch. Eisenhiittenwes., 3 (1929 - 1930) 407. 18 S. Asano, K. Hara, Y. Nakai and N. Ohtani, J. Jpn. Inst. Met., 38 (1974) 626. 19 L. C. Weiner and M. Gensanmer, Acta Metall., 5 (1957) 692. 20 A. E. Lord, Jr., Acta Metall., 15 (1967) 1241. 21 W. R. Heller, Acta Metall., 9 (1961) 600. 22 R. Gibala, Trans. Metall. Soc. AIME, 239 (1967) 1574. 23 G. M. Sturges and A. P. Miodownik, Acta Metall., 17 (1969) 1197. 24 A. P. Miodownik and B. S. Achar, Proc. 1st Int. Congr. on Hydrogen in Metals, Chatenay Malabry, 1972, Editions Sciences et Industrie, Paris, 1972,
p. 84. 25 Y. Kikuta, K. Sugimoto, S. Ochiai and K. Iwata, Trans. Iron Steel Inst. Jpn., 15 (1975)87.
270 26 A. Zielinski, E. Lunarski and M. Smialowski, Acta Metall., 25 (1977) 551. 27 H. Wada and K. Sakamoto, J. Jpn. Inst. Met., 41 (1977) 981. 28 J. Marx, Rev. Sci. Instrum., 22 (1951) 503. 29 A. Sieverts, Z. Metallkd., 21 (1929) 37. 30 O. D. Gonzales, Trans. Metall. Soc. AIME, 245 (1969) 607. 31 W. Geller and T. H. Sun, Arch. Eisenhiittenwes., 21 (1950) 423. 32 G. Phragmen, JernkontoretsAnn., 128 (1944) 537.
33 F. de Kazinczy, J. Iron Steel Inst., London, 1 77 (1954) 85. 34 M. Jacob and S. Erk, Z. Gesamte K~lte-Ind., 35 (1928) 125. 35 S. Imoto and G. Mima, Technol. Rep. Osaka Univ., 6 {1956) 141. 36 W. KSster, Z. Metallkd., 32 (1940) 151, 156, 160. 37 M. Amano and Y. Sasaki, J. Jpn. Inst. Met., 38 (1974) 969. 38 D. E. Diller, J. Chem. Phys., 43 (1965) 2089.
263
Effect of Hydrogen on the Internal Friction of Pure PolycrystaUine Iron at Low Temperatures HIROAKI WADA and KOSHIRO SAKAMOTO Department of Applied Physics, Faculty of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo 113 (Japan) (Received August 11, 1980;in revised form March 17, 1981)
SUMMARY
The behaviour o f hydrogen which had been introduced into high purity polycrystalline iron by thermal quenching was studied by measuring the internal friction and the change in the elastic modulus. The internal friction and the change in the modulus were determined in the temperature range 4 . 2 - 370 K using a three-component composite oscillator m e t h o d at resonant frequencies near 100 kHz at a strain amplitude o f about 10 -7 in a longitudinal magnetic field o f 6.4 × 104 A m - l . A new internal friction peak and a pronounced change in elastic modulus were observed at 14 K. The profile o f the internal friction versus temperature curve is asymmetric; the internal friction increases markedly on the low temperature side o f the 14 K peak with increasing temperature, decreases gradually on the high temperature side o f the peak and reaches a m i n i m u m at 33 K. The origin o f the 14 K peak is quite different from that o f relaxation peaks such as the hydrogen S n o e k peak and the hydrogen cold-worked peak. It was f o u n d that the drastic p h e n o m e n a at 14 K were caused by the phase transition between solid and liquid molecular hydrogen in defects such as voids and microcracks which had f o r m e d along grain boundaries owing to the evolution o f hydrogen gas at high pressures immediately after thermal charging. The internal friction in the temperature range 14 - 33 K can be interpreted in terms o f the viscosity o f the liquid hydrogen in the defects.
1. INTRODUCTION The nature of the hydrogen introduced into iron and steel has been investigated extensively, because it causes serious embrittle0025-5416/81/0000-0000/$02.50
m e n t of these materials. The behaviour of hydrogen in iron has been shown to be strongly affected by interactions between hydrogen and other lattice defects. Therefore basic studies on high purity iron should be made to obtain more accurate information on the behaviour of hydrogen in iron. In recent years, the internal friction [1 - 6 ] , diffusion [7, 8], the magnetic after-effect [2, 9 - 12], the electrical resistivity [ 1 3 - 15] and the specific heat [16] have been investigated to clarify the behaviour of hydrogen in high purity iron. It is n o t easy to determine the intrinsic nature of solute hydrogen in iron because the solubility of hydrogen is very small [17] and the " j u m p i n g " rate of hydrogen is still fairly high [18] even at room temperature. Hydrogen charging frequently causes permanent damage (e.g. voids, microcracks and blisters) to the iron specimen. It is very difficult to introduce a large a m o u n t of supersaturated hydrogen into the iron lattice without inducing such defects. Since internal friction is sensitive to the structure of materials, a study of internal friction is indicative of the behaviour of hydrogen in iron. Two types of internal friction peaks have been reported for iron and steel containing hydrogen. One peak has been reported to be caused by the stress-induced diffusion of hydrogen in solution [19 - 22] ; this is called " t h e hydrogen Snoek peak". The other peak is due to the hydrogen-dislocation interaction; this is called " t h e hydrogen coldworked peak" [1 - 6, 22 - 26]. The hydrogen Snoek peak has been reported in the temperature range 48 - 120 K and the hydrogen coldworked peak in the temperature range 140 220 K depending on the frequency of the measurement. In this paper we report a new internal friction peak which is quite different from the © Elsevier Sequoia/Printed in The Netherlands
264
two peaks mentioned above. The new peak is related to molecular hydrogen segregated in defects (such as voids and microcracks) produced by the hydrogen which had been introduced into a specimen.
2. EXPERIMENTAL PROCEDURE
The starting material for the present investigation was high purity polycrystalline iron (Marz grade) purchased from Materials Research Corporation. The specimen was 3 mm in diameter and 25.5 mm long. The specimens were treated with dry hydrogen at 1770 K for 0.5 h in order to remove interstitial impurities such as carbon, nitrogen and oxygen [ 2 7 ] . After the purification treatment, the total impurity content was less than 20 wt. ppm and in particular the total interstitial impurity content was a b o u t 1 wt.ppm. The internal friction and the change in the elastic modulus were measured in the temperature range 4.2 - 370 K by a three-component composite oscillator m e t h o d [28] at resonant frequencies near 100 kHz at a strain amplitude of a b o u t 10 -7. To suppress the internal friction of magnetic origin, a longitudinal magnetic field of 6.4 × 104 A m -z was applied using a solenoid. The temperature of the specimen was controlled by a non-inductive furnace surrounding the specimen; the low pressure helium gas in the furnace served as a heat exchanger. To maintain highly supersaturated concentrations of hydrogen in the specimens below room temperature, hydrogen was introduced by holding the specimens in a pure hydrogen atmosphere of 0.1 MPa in the temperature range 8 4 0 - 1770 K for 0.5 h and then quenching the specimens in water at 273 K. Several specimens were analysed for total hydrogen content by the vacuum fusion technique. The total hydrogen concentrations were in the range 50 - 500 at.ppm.
3. EXPERIMENTAL RESULTS
3.1. Quenching temperature and hydrogen concentration Figure 1 shows the relation between the reciprocal of the quenching temperature and the logarithm of the hydrogen concentration
Temperature IK) 2000 ]5(]0 1000 8~
~I000 i
._~
c_~ I00
i
•
i i
liar. I
~
~ ron
To
,Ii o 5 10 15 Reciprocal of Ouenchin9 Temperature 10oo0 (K-I) To
Fig. 1. The relation between the reciprocal of the quenching temperature and the logarithm of hydrogen concentrations introduced into pure polycrystalline iron by thermal charging: e, present data; o, ref. 17.
measured by the vacuum fusion technique. The results of the present experiment are indicated by full circles, and the data obtained by Martin [17] b y open circles. Using an Arrhenius plot of Sieverts' law [ 29 ] C = Co PZl 2 e x p ( - - R ~ )
where C is the solubility of hydrogen in iron, Co a constant, P the pressure of hydrogen gas, Q the heat of solution of hydrogen in iron, R the gas constant and T the temperature, we found Q to be 27.6 kJ mo1-1. This value of the heat of solution is in good agreement with other data on pure iron (27.2 kJ mo1-1) [30, 31].
3.2. The internal friction and the change in the frequency A sharp internal friction peak and a marked change in the resonant frequency were observed at 14 K in the specimen held in a pure hydrogen atmosphere at 1140 K and quenched in water at 273 K. The change in the resonant frequency shown in Fig. 2 is the difference between the resonant frequency at 4.2 K and that at each temperature. Lattice defects {e.g. point defects and dislocations) may be produced in the specimen by a thermal stress during quenching. To determine whether or not the drastic phenomena at 14 K were due to these defects, the speci-
265
1000
g 2000
3C
g 2E
I
3000
x rc~
~-000
io
16o
I~O
26o
2~o
3~o
Temperature(K)
Fig. 2. Effect of hydrogen introduced into pure polycrystalline iron by thermal quenching from 1140 K on the internal friction and the change in the resonant frequency: o, as annealed ; e, in vacuum at 1440 K, water quenched to 273 K; A, in hydrogen at 1140 K, water quenched to 273 K.
IODO
30
2000 c.>
g 20
3000 '~
lO
4000
x 'c::~
~'o
i~o
I~O
2~o
2~o
3bo
3~o
Temperature { K )
Fig. 3. Effect of hydrogen introduced into pure polycrystalline iron by thermal quenching from 1770 K on the internal friction and the change in the resonant frequency: A, as annealed; $, in hydrogen at 1770 K, water quenched to 273 K, heating run from 4.2 to 360 K; o, in hydrogen at 1770 K, water quenched to 273 K, cooling run from 360 to 4.2 K.
mens were held in vacuum at 1140 K and q u en ch ed in water at 273 K. The internal friction and the change in the resonant freq u e n c y o f th e v a c u u m - q u e n c h e d specimens are s o mewh at larger than those o f t he annealed specimens over the whole t e m p e r a t u r e range from 4.2 to 370 K, but neither t he internal friction peak nor the marked change in
the resonant f r e q u e n c y was observed near 14 K, as shown by the full circles in Fig. 2. T h e r e f o r e the p h e n o m e n a at 14 K are related to hydrogen. Hereafter these p h e n o m e n a , i.e. the internal friction peak and the change in modulus, are referred to as the 14 K anomalies. As shown in Fig. 3, the internal friction of t he specimen quenched from a h y d r o g e n
266
atmosphere at 1770 K shows a peak at 14 K and a minimum at 33 K, increases gradually with increasing temperature and has a broad peak centred near 300 K. A cooling run was made to confirm the stability of the internal friction. The broad internal friction peak near 300 K disappeared, and the internal friction and the change in the resonant frequency decreased over the whole temperature range. The 14 K anomalies, however, were reproduced. Consequently, the 14 K anomalies are caused by the hydrogen introduced into the specimen and the broad peak near 300 K is apparently due to the decay of point defects produced by quenching. Only the internal friction and the change in the resonant frequency in the temperature range 4.2 - 33 K will be discussed in detail. To examine the effect of hydrogen c o n t e n t on the 14 K anomalies, the specimen was held in a pure hydrogen atmosphere at various temperatures and quenched in water at 273 K. The internal friction values and the changes in the resonant frequency at various temperatures are shown in Fig. 4. Both the peak height and the pronounced change in the resonant frequency at 14 K increased with increasing hydrogen concentration, as shown in Fig. 5. These phenomena can scarcely be seen in the specimen quenched from temperatures lower than 840 K. The internal friction increased markedly on the low temperature side
20
~_o~o_.o.oo___o____o__._o___o11000 ~,
15
2000i~ Q=
1C
I
10 15 20 25 30 35 Temperature (K) Fig. 4. Effect of the quenching temperature on the internal friction and the change in the resonant frequency of pure polycrystalline iron (all samples were held in pure hydrogen gas at various temperatures and water quenched to 273 K): o, 1770 K; A, 1140 K;D, 940 K ; e , 840 K.
z500
lOOO~ "E
To
m
i 1500 0
n
I
,00
200
H;0
Hydrogen Concentration { at.ppm)
Fig. 5. Effect of the hydrogen concentration on the peak height of the internal friction and the change in the resonant frequency of pure polycrystalline iron at 14K.
of the 14 K peak, decreased gradually on the high temperature side with increasing temperature and reached a minimum at 33 K. The change in the resonant frequency occurred abruptly at 14 K; the change in the frequency in the temperature range 14 - 33 K was small and gradual.
3.3. Effect of aging As described in Section 3.1, the hydrogen c o n t e n t detected in the specimen was nearly equal to the solubility of hydrogen in iron for thermal equilibrium at the quenching temperature. Then the specimen was supersaturated with hydrogen after quenching. The diffusion coefficient of hydrogen in iron at 373 K is about 9 X 1 0 - 9 m 2 s-1 [18] ; therefore almost all the dissolved hydrogen in supersaturated solid solution can escape from the iron lattice (an iron rod specimen 3 mm in diameter was used) within 600 s at 373 K. Figure 6 shows the effect o f aging on the internal friction peak height and on the change in the resonant frequency at 14 K. The 14 K anomalies were observed for the specimen aged in vacuum for 1 h at 373 K. The resonant frequency below 14 K steadily decreased with aging, but that above 14 K did not. The resonant frequency of the specimen aged in vacuum for 2 h at 473 K showed no anomalies at 14 K. These observations suggest that the 14 K anomalies are not caused by dissolved hydrogen in a solid solution but by, hydrogen in other states. Figure 7 shows some voids and microcracks along the grain boundaries in the specimen charged with hydrogen. These defects in-
ot r;
Temperature (K)
Fig. 6. Effect of aging on the internal friction and the resonant frequency of pure polycrystalline iron charged with hydrogen by water quenching from 1770 to 273 K: 0, heating run from 4.2 to 300 K; 0, cooling run from 300 to 4.2 K; 0, aged at 300 K for 24h;A,agedat373Kforlh;O,agedat473Kfor2h.
creased in size and number as the quenching temperature increased. The hydrogen existing in excess in the specimen should have segregated as molecular hydrogen gas in such defects. According to the discussion in Section 3.2 about the asymmetric shape of the internal friction uersus temperature curve and the drastic change in the resonant frequency, it is clear that the mechanism associated with the phenomena is quite different from any relaxation mechanism such as the hydrogen Snoek peak and the hydrogen cold-worked peak.
4. DISCUSSION
It was found in Section 3.3 that almost all the hydrogen introduced by quenching was contained as molecular hydrogen gas in voids and microcracks produced along grain boundaries. According to Sieverts’ law, the pressure of the hydrogen gas in these defects should be very high, so that the equation of state of the hydrogen gas deviates from that of an ideal gas. The following approximate equation of state for non-ideal gaseous hydrogen gives the
Fig. 7. Longitudinal cross section of pure polycrystalline iron charged with hydrogen by thermal quenching from various temperatures: (a) quenched from 840 K; (b) quenched from 1140 K; (c) quenched from 1770 K. Voids and microcracks were formed along grain boundaries owing to the evolution of molecular hydrogen gas at high pressures.
amount [32,33] X=
of the gas contained
in these defects
:
34SPV T(1+0.006P)
(1)
where X is the hydrogen content in voids and microcracks (in cubic centimetres per 100 g of iron), P the pressure (in megapascals), V the porosity (the relative volume of these de-
268
fects in volume per cent} and T the temperature (in kelvins). The size and number of voids and microcracks were measured microscopically a large number of times. The calculated porosities are in the ranges 0.15% - 0.5%, 0.04% - 0.12% and 0.01%- 0.04% for the specimens containing 500 at.ppm H, 150 at.ppm H and 50 at.ppm H respectively. Substituting these values into eqn. (1), we can estimate the pressures in the voids and microcracks at 273 K; they are in the ranges 17 - 76 MPa, 22 - 90 MPa and 22 150 MPa in the specimens with 500 at.ppm H, 150 at.ppm H and 50 at.ppm H respectively. 10(3
Sol.
,,'
',~i~
ported by the observation of the specific heat due to phase transition between the solid and liquid hydrogen in the defects in the specimen charged with hydrogen by quenching [ 1 6 ] . The electrical resistivity usually exhibits a discontinuous change on a phase transition of the matrix [36, 37]. The electrical resistivity of iron specimens (1.2 mm in diameter and 600 mm long) charged with hydrogen by thermal quenching were measured in the present study. The electrical resistivity of the specimens, however, did not show any discontinuous change at 14 K. This result indicates that the hydrogen associated with the 14 K anomalies is not hydrogen atoms in the lattice b u t hydrogen molecules precipitated in voids and microcracks. 30
,~-
,, ,,' ,'~ ,~
<~.~ ,~,~--
I 2C
~o.i
Gas
0.0 01
1E
/
0.01
§
Triple point
10
15 20 25 Temperature (K)
30
35
40
Fig. 8. The phase diagram of molecular hydrogen: . . . . , lines of c o n s t a n t density (in grams per cubic centimetre). (After Jacob and Erk [ 34 ] .)
Figure 8 is the phase diagram of hydrogen [ 3 4 ] . The peak temperature (14 K) and the minimum temperature (33 K) of the internal friction correspond to the triple point and the critical point in the phase diagram respectively. From the volume of voids and microcracks and the pressure, the density of hydrogen in the present specimens is calculated to range from about 0.014 to 0.071 g c m -3. Hydrogen in the present specimens exists as a liquid in the voids and microcracks between 14 K and 33 K for most cases. Below 14 K, it is solid. Generally, a phase transition causes marked changes in the internal friction and the elastic modulus [35 - 3 7 ] . Therefore the above considerations lead to the conclusion that the 14 K anomalies are induced by the liquid-solid transition of molecular hydrogen in the voids and microcracks. This conclusion is also sup-
10
20 Temperature (K)
30
40
Fig. 9. The viscosity coefficient for compressed parah y d r o g e n ; that of normal hydrogen is o n l y up to 5% greater t h a n that of parahydrogen at the same temperature. (After Diller [38 ] .)
As shown in Fig. 9, the viscosity coefficient of compressed liquid parahydrogen decreases with increasing temperature and with decreasing pressure [ 3 8 ] . The viscosity coefficient of normal hydrogen is only up to 5% greater than that of parahydrogen at the same temperature [38]. The temperature dependence of the internal friction in the temperature range 14 - 33 K shown in Figs. 4 and 6 is very similar to that of the viscosity coefficient of liquid hydrogen. Thus the viscosity of the liquid hydrogen in the voids and microcracks may contribute to the energy dispersion. The internal friction of the vacuum-quenched specimen increases gradually with temperature from 4.2 K, as shown in Fig. 2. The magnitude
269 o f t h e i n t e r n a l f r i c t i o n n e a r 33 K is n e a r l y equal to that of the specimen charged with hydrogen. The internal friction due to the v i s c o s i t y o f liquid h y d r o g e n a r o u n d 33 K m a y be c o n s i d e r a b l y smaller t h a n t h a t d u e t o t h e d e f e c t s i n d u c e d b y q u e n c h i n g . T h u s t h e internal f r i c t i o n has a m i n i m u m at a r o u n d 33 K. Since a l m o s t all t h e h y d r o g e n in t h e voids a n d m i c r o c r a c k s solidifies b e l o w 14 K, t h e s e d e f e c t s are s t u c k b y t h e solid h y d r o g e n w h i c h acts as a paste. T h u s t h e average elastic m o d u lus o f t h e s p e c i m e n increases d r a s t i c a l l y ; t h e c h a n g e in t h e r e s o n a n t f r e q u e n c y decreases m a r k e d l y . This i n t e r p r e t a t i o n is s u p p o r t e d b y t h e b e h a v i o u r o f t h e r e s o n a n t f r e q u e n c y at a r o u n d 14 K in Fig. 6. T h e r e s o n a n t f r e q u e n c y did n o t s h o w a n y d i s c o n t i n u o u s increase n e a r 14 K w i t h d e c r e a s i n g t e m p e r a t u r e in t h e s p e c i m e n aged in v a c u u m at 4 7 3 K f o r 2 h. T h a t is, a l m o s t all t h e h y d r o g e n c o n t a i n e d in t h e voids a n d m i c r o c r a c k s was e x t r a c t e d b y t h e aging t r e a t m e n t in v a c u u m ; t h e n t h e s e d e f e c t s c o u l d n o t be f i x e d b y t h e h y d r o g e n even b e l o w t h e triple p o i n t . T h e r e f o r e t h e elastic m o d u l u s o f t h e s p e c i m e n did n o t increase.
5. CONCLUSIONS T h e results o f t h e p r e s e n t i n v e s t i g a t i o n o n pure polycrystalline iron charged with hydrogen are s u m m a r i z e d as follows. (1} An internal f r i c t i o n p e a k a n d a m a r k e d change in t h e elastic m o d u l u s w e r e o b s e r v e d at 14 K. T h e i n t e r n a l f r i c t i o n in t h e t e m p e r a t u r e range 1 4 - 3 3 K a n d t h e p r o n o u n c e d c h a n g e in t h e elastic m o d u l u s at 14 K c a n b e i n t e r p r e t e d as d u e t o h y d r o g e n m o l e c u l e s in voids a n d m i c r o c r a c k s . T h e voids a n d m i c r o cracks w e r e f o r m e d a l o n g grain b o u n d a r i e s in t h e s p e c i m e n s c h a r g e d w i t h h y d r o g e n b y therm a l q u e n c h i n g . T h e s e d e f e c t s are filled w i t h h y d r o g e n gas a t a p r e s s u r e in t h e range 20 - 1 5 0 MPa at 273 K d e p e n d i n g o n t h e a m o u n t o f h y d r o g e n in a n d t h e p o r o s i t y o f t h e s p e c i m e n . {2) T h e drastic changes in t h e elastic m o d u l u s a n d t h e i n t e r n a l f r i c t i o n a t 14 K are caused b y t h e l i q u i d - s o l i d t r a n s i t i o n o f mole c u l a r h y d r o g e n in t h e s e d e f e c t s . T h e internal f r i c t i o n in t h e t e m p e r a t u r e r a n g e 14 - 33 K is i n d u c e d b y t h e viscosity o f t h e liquid h y d r o gen.
ACKNOWLEDGMENTS T h e a u t h o r s w o u l d like t o t h a n k Mr. T. Igarashi a n d Mr. M. S h i m a d a f o r c a r r y i n g o u t some of the experiments.
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