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Resistivity recovery at low temperatures in high-purity iron charged with hydrogen
Materials Science and Engineering, 32 ( 1 9 7 8 ) 71 - 79
71
© Elsevier S e q u o i a S.A., L a u s a n n e -- P r i n t e d in t h e N e t h e r l a n d s
Resistivity Recovery at Low Temperatures in High-purity Iron Charged with Hydrogen
S. M O R I Y A
Graduate School o f Engineering, Tohoku University, Sendai (Japan) S. T A K A K I a n d H. K I M U R A
Research Institute for Iron, Steel and Other Metals, Tohoku University, Sendai (Japan) (Received in revised f o r m J u l y 6, 1 9 7 7 )
SUMMARY
High-purity iron specimens (RRRH = 1 800 - 5 000) were electrolytically charged with hydrogen so as to avoid any observable damage in the specimens. They were quenched into liquid helium and aged at below room temperature. The change in resistivity was measured to investigate the behavior of hydrogen. The resistivity recovers in two stages, a large recovery at 110 K and a small recovery at 170 K, in specimens without silicon contamination, but at 150 K and above 200 K (with a small stage around 100 K in less contaminated specimens) in specimens contaminated with silicon during dry hydrogen treatment. The two-stage (110 K and 170 K) recovery is considered to be the representative behavior of hydrogen in iron for this range of purity. A large part of the quenched hydrogen is likely to condense at sites (most likely dislocations) in the crystal in the 110 K stage. The activation energy for the 110 K stage is measured to be about 0.2 eV, which implies that the hydrogen diffusion around 100 K is affected by impurity trapping, even in this high-purity iron. The 170 K stage seems to be due to the release of hydrogen from dislocations and its escape to the surface.
in iron below room temperature [ 1] ; hydrogen was frozen into specimens by electrolytic charging and quenching into liquid helium, and the resistivity due to quenched-in hydrogen was found to recover at two stages below room temperature as long range migration of hydrogen occurred. This is a new technique for studying hydrogen diffusion below room temperature and is expected to be useful in investigating the interactions of hydrogen with impurities and defects, since the interactions are more readily observed at lower temperatures. Recently, Takaki and Kimura have succeeded in preparing very pure iron [2]. It has also been found that iron is contaminated by silicon during dry hydrogen treatment to remove interstitial impurities if iron is placed in a silica tube and heated by a furnace surrounding the tube [3]. Since the diffusion of hydrogen is strongly influenced by impurities [4], high-purity iron treated so as to avoid silicon contamination should be used to clarify the mechanism of the two stages reported in the previous paper. The purpose o f the present research is to investigate the low temperature behavior o f hydrogen in " p u r e " iron of various degrees of purity and perfection by the new technique, and to discuss the mechanism of the recovery stages.
1. I N T R O D U C T I O N 2. E X P E R I M E N T A L
Studies o f the diffusion of hydrogen in pure iron are important steps in clarifying the effects o f hydrogen in steels, e.g., hydrogen embrittlement. The present authors have reported preliminary results of the investigation o f the long range migration of hydrogen
(i) Specimens A high-purity iron was prepared from Johnson-Matthey iron by electron-beam, floating-zone-melting (10 or 12 passes) under ultra-high vacuum (about 10 -7 Pa) [2].
72 The pure iron was swaged and drawn at room temperature to wire 0.3 mm in diameter. Contamination during fabrication was avoided as much as possible by cleaning with trichloroethylene and acetone and by etching. The wire was then annealed in a quartz t u b e for 1 h at 1 200 K in dry hydrogen, 24 h in wet hydrogen and 20 h in dry hydrogen, both at 1 070 K. The dry hydrogen is purified by palladium foil permeation, and the wet hydrogen was produced by passing the dry hydrogen through distilled water at room temperature. After hydrogen treatment, the wire was heavily etched with a solution o f HF, H20 and H202 (3, 6 and 50 by volume, respectively) to a diameter o f 0.10 mm to remove the surface layer contaminated by silicon [3]. After these treatments, the grain structure o f the wire was b a m b o o t y p e with an average grain length of about 1 mm. The wire was cut into pieces about 40 mm long and potential leads of the same wire were spot-welded with a gage length of about 25 mm to make specimens for resistivity measurement. The original Johnson-Matthey iron was also used for comparison after exactly the same process of fabrication, hydrogen treatment and etching. The grain structure is again b a m b o o t y p e with an average grain length o f about 0.2 mm. The purity o f the specimens was estimated using the residual resistivity ratio, RRRH, where the resistivity at 4.2 K was measured with a magnetic field of about (2/n) × 10 s A/m. The RRRH'S of the zone-melted iron specimens were a b o u t 5 000 and that of the original Johnson-Matthey iron specimen was about 1 800.
(ii) Hydrogen charging and resistivity measurement Hydrogen was charged electrolytically in 0.1 N NaOH solution, with a small a m o u n t o f NaAsO2 added, with a current density between 10 and 30 mA cm -2. It has already been shown that after charging under these conditions the resistivity recovers to the precharged value by aging at below room temperature [1]. After charging, the specimen was removed from the bath, quickly plunged into liquid helium, and aged either isochronally or isothermally in a furnace placed in the cryostat with the temperature controlled to -+0.2 K. The resistivity was measured at 4.2 K with a magnetic field of
(2/n) × 105 A/m with an accuracy of 1 × 10 -zz ~2 cm.
(iii) Electron microscopy A wire of the zone-melted iron, 1 mm in diameter, was cold rolled to a thickness of a b o u t 160 pm. The width of the foil was a b o u t 1.7 mm. The foil specimens were treated in the same way as the resistivity specimens and finally chemically polished to about 100 pm thickness. After the desired treatment they were polished and examined with an electron microscope operated at 200 or 1 000 kV.
3. R E S U L T S
(i) Isochronal aging and recovery stages The isochronal aging program was 10 min aging at every 10 K from 50 K. Figure 1 shows the resistivity recovery curves in the zone-melted iron specimens together with their RRRH's. Their derivative curves are also shown in the lower part of the Figure. The resistivity recovers in two stages, one centering at about 110 K and the other at about 170 K. The recovery is complete at about 200 K. In these specimens, the charging, quenching and aging were repeated, and it was found that the recovery characteristics were the same for each run. In the fourth run, however, the recovery was not
7
RRR. 6
---o- 4850
-~ 5 u oc~ 4
--e- 4 8 8 0 4890 4910
b 2 I
,
5~o ,oo ,~o 260 2~o Aging Temp.(K)
Fig. 1. I s o c h r o n a l r e s i s t i v i t y r e c o v e r y c u r v e s a n d t h e i r d e r i v a t i v e s in h y d r o g e n c h a r g e d specimens.
73
5.0 o
As annealed
40
u. o i 0
s.o
E 3.0[ u
Temp.
2.o
o.. <~
%
I'0 I 0 o
I 50
I
~oo Aging
I I rso 200 Temp.(K)
[ "~. "9 I 250 500
Fig. 2. Isoehronal resistivity recovery curves in hydrogen-charged specimens. The specimens are of zonemelted iron, but the silicon-contaminated layer was not removed.
complete at 200 K and a small fraction of the quenched-in resistivity remained. Figure 2 shows the recovery in zone-melted specimens which were hydrogen-treated after being drawn to 0.10 mm diameter and not etched to remove the silicon contamination. There are three regions of recovery in an as-annealed specimen; a small recovery below 100 K, a distinct stage at 150 K, and a small and gradual recovery above 200 K. If the silicon contaminated specimen was deformed 1.5% in tension at room temperature before charging, the 150 K stage became larger and a small but clear stage appeared at 230 K. The recovery curves were obtained also on the original Johnson-Matthey iron specimens. The recovery occurs at two stages, of which the temperatures are the same as in the zonemelted iron. However, when the charging and aging were repeated the incomplete recovery at 200 K appeared in the second run, in contrast to the fourth run in the zone-melted iron specimens. It should be noted that the stage temperatures, especially that for the first stage, are independent of the specimen purity, ranging from RRRH of 1 800 to 5 000, provided that the specimens are heavily etched after hydrogen treatment to remove the silicon contaminated layer. The a m o u n t of quenched-in resistivity had no correlation with the charging time or the charging current density. It seems to depend, at least partly, on the quenching procedure; a part of the hydrogen seems to be lost from solution during the time involved in transferring a specimen from the charging
2:0 2.0
4.0
6.0
8.0
&pa( I0-'° D..cm)
Fig. 3. The resistivity remaining after the first stage, ApIi, against the total quenched-in resistivity, ApQ.
bath to the liquid helium. Figure 3 shows the relation between the resistivity remaining after the first stage, ~Pn, and the total quenched-in resistivity, ApQ. It should be noted that APH is almost independent of ApQ.
(ii) Isothermal aging and the activation energy for the recovery Since the a m o u n t of recovery at the second stage is too small for quantitative investigation, only the first stage was investigated further for kinetic analysis. Isothermal aging at 105 K was performed for specimens with various initial hydrogen concentrations, which were given by the quenched-in resistivity, ApQ, and varied from 3.4 X 10 -1° ~2 cm to 11.7 × 10 -1° ~2 cm. The specimen purities are about 5 000 in R R R n . Because of an experimental limitation, isothermal aging could not be completed. Hence, each isotherm is normalized with Apl corresponding to its Apq shown in Fig. 3*. Figure 4 shows examples taken from fifteen normalized isotherms for specimens 0.10 mm in diameter. It should be noted that while isochronal aging gives a peak temperature at 110 K with little scatter from specimen to specimen, the rate of isothermal recovery considerably depends on the specimens. The largest rate is about twice the smallest one. In the Figure, isotherms for specimens of 0.085 mm and 0.17 mm in diameter are also shown. The isothermal recovery is never represented by a first order reaction. Figure 5 shows chemical reaction rate analysis o f the isotherms at 100 and 105 K. The slope of the
* A p I for A p Q of 11.7 X 10 -1° ~ c m was estimated by extrapolating the curve in Fig. 3.
74 curves in the Figure gives the order of reaction, 7, in the following equation dn -
dt
(iii) Electron- and optical microscopy No difference was found in the electron micrographs between the as-annealed specimens and those charged with hydrogen at room temperature but not quenched. Repeated charging at room temperature caused no observable change either. Optical microscopic observation of the surface revealed no blister in the specimens singly charged or repeatedly charged b u t not quenched. By contrast, when the charged specimens were quenched to 4.2 K and warmed up to room temperature, an increase in the dislocation density was observed near grain boundaries and sub-boundaries as shown in Fig. 7. Little change in the dislocation density was found inside the sub-grain. When the warmed-up specimen was charged again, blisters were found on the surface. If the charging, quenching and warming cycle was repeated, more and larger blisters were found.
Kn-~
where n is the fraction of quenched-in resistivity remaining after time t, and K is a constant. It is obvious that the reaction cannot be represented by a single 7; 3' varies from a large value to about 3 with time. It also depends on the aging temperature. The slope-change method was applied to estimate the apparent activation energy for the first stage. The aging temperature was changed by 10 K in the range between 95 and 125 K as shown in Fig. 6(a). The calculated activation energy for four specimens falls in the range from 0.15 to 0.30 eV and gives an average of 0.22 eV, as shown in Fig. 6(b).
&Pc= RRRM (I0"I° ~,cm] (ram) IO 5.5 4710 0,10 • 5.8 4740 010
°I
0.8
I 0.6 L ~
Io
4~9 0
91~
~.o.4 <3
0 085
Oq ~ 0
'
20
L
40
[
~lO
I
8'0
I O0
[
Aging Time (K) Fig. 4. Isothermal resistivity recovery curves at 105 K.
(iv) Cold-worked specimens Figure 8 shows isochronal recovery curves of pre-strained and hydrogen-charged specimens. The pre-straining is 1% in tension at room temperature. The two-stage recovery is no longer observed b u t a single, broad stage appears. The two-stage recovery is observed for the second run, as shown in the Figure. The temperatures of the two stages are a b o u t the same as in annealed specimens. The first stage in the pre-strained specimen is larger than that in the annealed specimens. It was also found that the relative amount of the second stage to the first stage, Apii/Api , in the second run, became larger as the degree of deformation increased. The slope-change method was applied to the first stage in the second run, and an activation energy of about 0.2 eV was obtained. Electron microscopy revealed no difference in microstructure between the as-cold-worked
1.0 0.5 ,--
~
, ~
~
~.
~
-
-
-
.
a
~
• • lOOK
• • O.l
A * IO5 K =
I IO -
=
l I00
=
dnld
t (arbitrary unit)
I 1000
=
Fig. 5. Chemical reaction rate analysis o f 100 and 105 K isotherms. T h e relation between In n and in n o t linear, so that the order o f reaction is n o t unique.
(--dn/dt)
is
75
20.C
27 ~26
0"22 eV ~,5,%K
$25 T 0
16C
Pre-straJn 1% • I st Run
0.21eV ~ I 5 K
24 O.15eV~
12.C
23
22
?-
0
'
I'O
2;0
'
3~0
Time(rain)
(a)
_o 8c (3
4C
030 f
_~
~ 0.20 ILJJ
--I .... ;___
-
overage
o
/ 010
tF
=
I
I
I
I
I
I
I
J
95 ~105~115 ~125 T (K) (b)
Fig. 6. (a) Determination of the activation energy for the 110 K stage with the slope-change method. (b) The observed activation energies (their scatter and the average).
9 4.o 2.0
o
5'o ,6o ,;o 2;0 2go Aging
Temp,(K)
Fig. 8. Isochronal recovery in cold-worked specimens. Note the significant difference between the 1st and the 2nd runs.
4. DISCUSSION
(i) Interpretation of the two stages
Fig. 7. Electron microscopy of a charged, quenched and aged specimen. Note the dislocations existing outside but near the sub-boundary. No dislocation is observed outside sub-boundaries in charged but unquenched specimens.
specimens and those cold worked and charged but not quenched. After charging, quenching and warming up, no appreciable increase in the dislocation density was observed by electron microscopy, but cracks were sometimes observed at grain boundaries in optical microscopy.
The quenched hydrogen, at least in large part, is considered to condense at appropriate sites inside the crystal during the 110 K stage. The sites may be dislocations, sub-boundaries and/or other defects. The condensation of hydrogen may decrease the resistivity as in other precipitation and segregation processes. The 170 K stage is considered to be due to the release of such condensed hydrogen from the defects to escape from the surface. In the following, these considerations will be shown to be reasonable. If the surface were the main sink for hydrogen during the 110 K stage, there would be two possibilities. One is that the diffusion of hydrogen to the surface controls the rate o f the 110 K stage reaction. In this case, the isotherms should be first order, except for the initial part o f recovery [5, 6 ] . In this argument, the diffusion constant is assumed to be a constant with time. This assumption will be discussed later, and it will be shown that possible variation of the apparent diffusion constant with time does not change the
76 conclusion below. The other possibility is that the reaction at the surface controls the rate. If the escape o f hydrogen through the surface layer is the rate-controlling process, the reaction should be first order. If the reaction of hydrogen atoms to form molecules at the surface is rate-controlling, the reaction should be second order. The observed order o f reaction is, however, larger than 3. Thus, the surface is ruled o u t as the main sink for hydrogen at the 110 K stage. It has been shown that various kinetic laws other than the first order are possible in precipitation processes [7]. The result of the size-effect experiment shown in Fig. 4 is also unfavorable to the surface being the main sink. If the hydrogen diffusion to the surface were rate-controlling, the rate of isothermal recovery in the specimen 0.085 mm in diameter should be 4 times faster than that in the 0.17 mm specimen. Although the rate o f isothermal recovery scatters within a factor of about 2, the difference of a factor o f 4 should be detected. {If the rate in the thinner specimen is observed within the range R to 2R, the rate in the thicker specimen should be observed within the range R/4 to R/2.) The experimental result shows that the isotherms for the 0.085 mm and 0.17 mm specimens agree with each other at the early stage and differ only by a factor of about 1.5 at about 60% recovery. There are some experimental results showing that the quenched hydrogen causes some damage inside the crystal during the low temperature aging; the dislocation density increases near grain boundaries and sub-boundaries, and some resistivity remains unrecovered in specimens after more than three cycles o f quenching and aging. These facts imply that hydrogen atoms condense to sites, perhaps dislocations and sub-boundaries, during the low temperature aging. Simple diffusion of single hydrogen atoms, even though they interact with traps, can hardly cause such changes. R e m e m b e r that the mere charging and de-gassing treatment at room temperature causes no observable damage. It should be noted here that the damage caused during the low temperature aging does not affect the recovery stage. The damage is not detectable by resistivity measurement, at least in freshly charged and aged specimens. Even when the damage is resistometrically detect-
able after repeated cycles, the recovery stage temperatures remain unchanged. The condensed hydrogen dissolves again and escapes from the surface as the temperature is raised. This is the 170 K stage. The relative amount o f recovery in the 170 K stage to that in the 110 K stage in the second run of the deformed specimens increases with the amount of strain, i.e., the dislocation density. This fact seems to support the consideration that the site for hydrogen condensation at the 110 K stage is dislocations, and the 170 K stage is due to the release of hydrogen from the dislocations. Changes in the subboundary structure shown in Fig. 7 may imply that the sub-boundaries are also the sites for hydrogen condensation. It should be noted that the amount o f recovery in the 170 K stage, APH, in annealed specimens is almost independent of ApQ. This would be reasonable because AP~I should be mainly determined by the dislocation density including sub-boundary dislocations.
(ii) Activation energy o f the 110 K stage The activation energy for the 110 K stage is determined to be about 0.2 eV. If the crosscut method gives the same value, the observed activation energy may be considered to be the activation energy for the solute atom diffusion [8]. The cross-cut method is impossible in the present experiment because o f the p o o r reproducibility of the isotherms. Nevertheless, we consider the observed activation energy, 0.2 eV, to be that for the diffusion o f hydrogen, as is often done in many other precipitation processes. The activation energy for the diffusion o f hydrogen in iron and steels is strongly influenced by the presence o f traps, such as impurities and defects [4]. However, the activation energy determined above about 450 K is considered to be that for the free migration of hydrogen in iron, since the effect of traps is no longer important at these temperatures. The activation energy for the free migration has been reported to be about 0.1 eV, which is appreciably smaller than the present value. Hence, we consider that impurity atoms act as traps for hydrogen around 110 K, even in the present high-purity, zonemelted iron. The concentration of impurities in the zone-melted iron is estimated to be of order o f 2 X 10 -5 atomic fraction with the residual
77
resistivity at 4.2 K (2 X 1 0 - 9 ~ cm) and an assumed resistivity contribution of impurities (1 X 10 -6 ~2 cm per at.%). The impurity concentration is estimated to be about 6 X 10-5 in the Johnson-Matthey iron. (Interstitial impurities are considered to have been removed below a level of 10 -6 [2] .) However, the kind o f impurity is not known with the residual resistivity. Assuming that all the impurities have the same binding energy, B, with hydrogen atoms, we estimate B with the following equation for the trapped diffusion, D Doff - - -
(1)
~'2
I+--
T1
Here Deaf is the effective diffusion constant for hydrogen, D the diffusion constant w i t h o u t traps, rx the time for which hydrogen atoms are free, and 72 the time for which hydrogen atoms are trapped. 71 and 72 are given by T1 =
c i - l p f I exp
,
estimated to be about 1 X 10 -5, from the quenched-in resistivity of about 5 X 1 0 - 1 ° cm for specimens used in the slope-change experiment and the resistivity contribution of hydrogen is assumed to be 0.5 X 10 -6 I2 cm per at.%. For B o f various values between 0.10 and 0.15 eV, Def~ is calculated for temperatures of 95, 105, 115 and 125 K. The apparent activation energy is then calculated for temperature changes from 95 to 105, 105 to 115 and 115 to 125 K. The apparent activation energy is about 0.22 eV for B of 0.13 eV except for the 115 - 125 K change. (E D is taken to be 0.11 eV as given below.) If Ct = Ht, the apparent activation energy is close to 0.11 eV at the beginning of recovery, but soon approaches 0.22 eV as Ht decreases. F o r B = 0.13 eV, Def~ -~ 3 X 10-2D, except for Ht -~ Ct. This is a crude estimate, but it is safely concluded that, if the apparent activation energy is 0.2 eV, Def~ is about 10-2D around 110 K for various combinations of C t and H t. Here, Ct and H t are of the order of 10- 5 and Ct > Hr. Louthan, Derrick, Donovan and Caskey [9] reported
(2) D=2.3X10 -3exp
-
0.11 eV) 1 ~ cm 2s-
and T 2 --
Pb 1 exp
ED +B 1 , ~ /
(3)
where C i is the concentration of free impurity atoms, v~ and Vb are the frequency factors for free and detrapping jumps o f hydrogen atoms, respectively, and E D is the activation energy for the free migration o f hydrogen. For simplicity, we put vf = vb. If the trapping and detrapping reactions are in equilibrium during the hydrogen condensation at the 110 K stage, Ci is calculated with
C p = m ( C t - - C p ) ( H t - - Cp)
exp
~-~
.
(4)
Here, Cp is the concentration of impurity a t o m - h y d r o g e n atom pairs, and Ct and H t are the total concentrations o f impurities and hydrogen atoms, m is the number o f pairs of different orientations for one impurity atom. If hydrogen atoms are in the tetrahedral sites, m = 6. Ct is estimated to be 2 X 10 -5. H t is
for the Marz grade iron. This gives D = 1.2 X 10 -8 cm 2 s-1 at 105 K. Hence, Deaf ~ 10 - l ° cm 2 s-1. With this value of De~ at 105 K and the 50% recovery time of about 5 min, the average diffusion distance is calculated to be about 2 X 10 -4 cm. Since the specimen radius is 5 X 10-3 cm, the estimated Deaf is compatible with the conclusion of hydrogen condensation inside the crystal at the 110 K stage. Other high temperature measurements [10] give larger D's than the above because of smaller activation energies. If we take these measurements, B should be larger than 0.13 eV and again, Deaf is expected to be of the order o f 1 0 - 1 ° c m 2 s - 1 . Thus, the average diffusion distance is smaller than the specimen radius. It should be mentioned here that the deviation of the isotherms from a first order reaction cannot be explained in terms of the time dependent Deff. As seen from eqns. (1), (2), (3) and (4), D e f t decreases with H t a s aging proceeds. Calculations with Ct of 2 X 10 -5 and the initial Ht of 1 X 10 -5 show that Deff decreases to 3/5 o f the initial value as Ht decreases to 2 X 10 - s . The decrease in Deff by
78 a factor o f 2 cannot make the isotherms shown in Fig. 2 be of a first order.
hydrogen is much larger than the specimen radius.
(iii) Effect o f silicon contamination on the recovery stages In the previous paper [1], the present authors reported two recovery stages o f quenched-in hydrogen at 150 and 230 K. The specimens in these preliminary experiments were drawn to 0.15 mm diameter and then hydrogen-treated but not etched after the hydrogen treatment, and, hence, contaminated with silicon. The effect of silicon contamination is confirmed in the present research (Fig. 2). The average concentration of silicon in the present zone-melted iron specimen, which was hydrogen-treated but n o t etched, is estimated to be about 20 at. ppm with the residual resistivity and the resistivity contribution of 6 X 10 -8 ~2 cm per at.% silicon [11]. Since the diffusion distance of silicon during this treatment is comparable with the specimen diameter, the silicon contamination is not confined to a thin surface layer b u t spreads throughout the specimen cross-section. The specimens in the previous report [1] are also contaminated throughout the cross-section. Although the mechanism of the effect of silicon on the hydrogen recovery is not known at present, it may be speculated that the silicon atoms interact strongly with the hydrogen atoms and retard their diffusion. The measurement of the activation energy for the hydrogen recovery in a silicon-contaminated specimen would give useful information on the silicon-hydrogen interaction. Research is now in progress to this end. Yamakawa, Tada and Fujita [12] reported two-stage recovery at 170 and 240 K due to hydrogen introduced by heating and quenching specimens in hydrogen gas. Their result is very similar to our previous result, although it is not known whether their specimens were contaminated with silicon. Assuming a first order reaction for the 170 K stage, they calculated the activation energy for the stage to be 0.1 eV, and concluded the stage to be due to free migration of hydrogen. However, their activation energy (0.1 eV) would need to be re-examined, since the assumption of the first order reaction was not confirmed and, as they described, the average diffusion distance calculated with the diffusion constant of free
5. CONCLUSIONS
High-purity iron specimens (RRRH = 1 800 - 5 000) were electrolytically charged with hydrogen under a mild condition (0.1 N NaOH solution and 10 - 30 mA cm -2) which caused no observable damage in the specimens. They were quenched into liquid helium and aged at below room temperature. The change in the resistivity due to hydrogen diffusion was investigated isochronally and isothermally. The conclusions are: (1) The resistivity recovers at two stages centering a b o u t 110 K and 170 K. The 110 K stage is larger than the 170 K stage. (2) A large part of the quenched-in hydrogen condenses inside the crystal during the 110 K stage, perhaps around dislocations and sub-boundaries. (3) The isotherms for the 110 K recovery process are n o t a first order reaction. The activation energy for the process is a b o u t 0.2 eV; the hydrogen diffusion is influenced by impurities even in the present high-purity specimens. (4) Solute silicon affects the recovery stages very strongly; phenomenologically, the 110 K stage tends to disappear, a large stage appears around 150 K and a small, new stage appears above 200 K. Even only a b o u t 20 at. ppm of silicon almost suppresses the 110 K stage. This effect is tentatively interpreted in terms of strong binding between silicon atoms and hydrogen atoms. (5) The 170 K stage was possibly due to the release of trapped hydrogen from dislocations and sub-boundaries. (6) The recovery occurs at a single stage in cold-worked specimens. The two-stage recovery, however, is observed in coldworked specimens in the second run of charging, quenching, and aging at the same temperatures as in the annealed specimens.
ACKNOWLEDGEMENT
This work was partly supported financially by the Ishihara-Asada Research Fund (1974) of the Iron and Steel Institute of Japan.
79 REFERENCES
1 S. Moriya, S. Takaki and H. Kimura, Scr. Metall., 8 (1974) 937. 2 S. Takaki and H. Kimura, Scr. Metall., 10 (1976) 1095. 3 S. Takaki and H. Kimura, Scr. Metall., 10 (1976) 701. 4 For example, H. K. Birnbaum and C. A. Wert, Bet. Bunsenges. Phys. Chem., 76 (1972) 806. 5 J. H. Evans and B. L. Eyre, Acta Metall., 17 (1969) 1109. 6 K. Yamakawa, M. Tada and F. E. Fujita, Jpn. J. Appl. Phys., 15 (1976) 769.
7 For example, J. W. Christian, The Theory of Transformations in Metals and Alloys, Pergamon Press, Oxford, 1975, p. 525. 8 K. Yoshioka and H. Kimura, Acta Metall., 23 (1975) 1009. 9 M. R. Louthan, Jr., R. G. Derrick, J. A. Donovan and G. R. Caskey, Jr., in A. W. Thompson and I. M. Bernstein (eds.), Proc. Int. Conf. Effect of Hydrogen on Behavior of Metals, AIME, New York, 1976, p. 337. 10 For example, Th. Heumann and E. Domke, Ber. Bunsenges. Phys. Chem., 76 (1972) 825. 11 S. Arajs, F. C. Schwerer and R. M. Fisher, Phys. Status Solidi, 33 (1969) 731. 12 K. Yamakawa, M. Tada and F. E. Fujita, Scr. Metall., 9 (1975) 1.
71
© Elsevier S e q u o i a S.A., L a u s a n n e -- P r i n t e d in t h e N e t h e r l a n d s
Resistivity Recovery at Low Temperatures in High-purity Iron Charged with Hydrogen
S. M O R I Y A
Graduate School o f Engineering, Tohoku University, Sendai (Japan) S. T A K A K I a n d H. K I M U R A
Research Institute for Iron, Steel and Other Metals, Tohoku University, Sendai (Japan) (Received in revised f o r m J u l y 6, 1 9 7 7 )
SUMMARY
High-purity iron specimens (RRRH = 1 800 - 5 000) were electrolytically charged with hydrogen so as to avoid any observable damage in the specimens. They were quenched into liquid helium and aged at below room temperature. The change in resistivity was measured to investigate the behavior of hydrogen. The resistivity recovers in two stages, a large recovery at 110 K and a small recovery at 170 K, in specimens without silicon contamination, but at 150 K and above 200 K (with a small stage around 100 K in less contaminated specimens) in specimens contaminated with silicon during dry hydrogen treatment. The two-stage (110 K and 170 K) recovery is considered to be the representative behavior of hydrogen in iron for this range of purity. A large part of the quenched hydrogen is likely to condense at sites (most likely dislocations) in the crystal in the 110 K stage. The activation energy for the 110 K stage is measured to be about 0.2 eV, which implies that the hydrogen diffusion around 100 K is affected by impurity trapping, even in this high-purity iron. The 170 K stage seems to be due to the release of hydrogen from dislocations and its escape to the surface.
in iron below room temperature [ 1] ; hydrogen was frozen into specimens by electrolytic charging and quenching into liquid helium, and the resistivity due to quenched-in hydrogen was found to recover at two stages below room temperature as long range migration of hydrogen occurred. This is a new technique for studying hydrogen diffusion below room temperature and is expected to be useful in investigating the interactions of hydrogen with impurities and defects, since the interactions are more readily observed at lower temperatures. Recently, Takaki and Kimura have succeeded in preparing very pure iron [2]. It has also been found that iron is contaminated by silicon during dry hydrogen treatment to remove interstitial impurities if iron is placed in a silica tube and heated by a furnace surrounding the tube [3]. Since the diffusion of hydrogen is strongly influenced by impurities [4], high-purity iron treated so as to avoid silicon contamination should be used to clarify the mechanism of the two stages reported in the previous paper. The purpose o f the present research is to investigate the low temperature behavior o f hydrogen in " p u r e " iron of various degrees of purity and perfection by the new technique, and to discuss the mechanism of the recovery stages.
1. I N T R O D U C T I O N 2. E X P E R I M E N T A L
Studies o f the diffusion of hydrogen in pure iron are important steps in clarifying the effects o f hydrogen in steels, e.g., hydrogen embrittlement. The present authors have reported preliminary results of the investigation o f the long range migration of hydrogen
(i) Specimens A high-purity iron was prepared from Johnson-Matthey iron by electron-beam, floating-zone-melting (10 or 12 passes) under ultra-high vacuum (about 10 -7 Pa) [2].
72 The pure iron was swaged and drawn at room temperature to wire 0.3 mm in diameter. Contamination during fabrication was avoided as much as possible by cleaning with trichloroethylene and acetone and by etching. The wire was then annealed in a quartz t u b e for 1 h at 1 200 K in dry hydrogen, 24 h in wet hydrogen and 20 h in dry hydrogen, both at 1 070 K. The dry hydrogen is purified by palladium foil permeation, and the wet hydrogen was produced by passing the dry hydrogen through distilled water at room temperature. After hydrogen treatment, the wire was heavily etched with a solution o f HF, H20 and H202 (3, 6 and 50 by volume, respectively) to a diameter o f 0.10 mm to remove the surface layer contaminated by silicon [3]. After these treatments, the grain structure o f the wire was b a m b o o t y p e with an average grain length of about 1 mm. The wire was cut into pieces about 40 mm long and potential leads of the same wire were spot-welded with a gage length of about 25 mm to make specimens for resistivity measurement. The original Johnson-Matthey iron was also used for comparison after exactly the same process of fabrication, hydrogen treatment and etching. The grain structure is again b a m b o o t y p e with an average grain length o f about 0.2 mm. The purity o f the specimens was estimated using the residual resistivity ratio, RRRH, where the resistivity at 4.2 K was measured with a magnetic field of about (2/n) × 10 s A/m. The RRRH'S of the zone-melted iron specimens were a b o u t 5 000 and that of the original Johnson-Matthey iron specimen was about 1 800.
(ii) Hydrogen charging and resistivity measurement Hydrogen was charged electrolytically in 0.1 N NaOH solution, with a small a m o u n t o f NaAsO2 added, with a current density between 10 and 30 mA cm -2. It has already been shown that after charging under these conditions the resistivity recovers to the precharged value by aging at below room temperature [1]. After charging, the specimen was removed from the bath, quickly plunged into liquid helium, and aged either isochronally or isothermally in a furnace placed in the cryostat with the temperature controlled to -+0.2 K. The resistivity was measured at 4.2 K with a magnetic field of
(2/n) × 105 A/m with an accuracy of 1 × 10 -zz ~2 cm.
(iii) Electron microscopy A wire of the zone-melted iron, 1 mm in diameter, was cold rolled to a thickness of a b o u t 160 pm. The width of the foil was a b o u t 1.7 mm. The foil specimens were treated in the same way as the resistivity specimens and finally chemically polished to about 100 pm thickness. After the desired treatment they were polished and examined with an electron microscope operated at 200 or 1 000 kV.
3. R E S U L T S
(i) Isochronal aging and recovery stages The isochronal aging program was 10 min aging at every 10 K from 50 K. Figure 1 shows the resistivity recovery curves in the zone-melted iron specimens together with their RRRH's. Their derivative curves are also shown in the lower part of the Figure. The resistivity recovers in two stages, one centering at about 110 K and the other at about 170 K. The recovery is complete at about 200 K. In these specimens, the charging, quenching and aging were repeated, and it was found that the recovery characteristics were the same for each run. In the fourth run, however, the recovery was not
7
RRR. 6
---o- 4850
-~ 5 u oc~ 4
--e- 4 8 8 0 4890 4910
b 2 I
,
5~o ,oo ,~o 260 2~o Aging Temp.(K)
Fig. 1. I s o c h r o n a l r e s i s t i v i t y r e c o v e r y c u r v e s a n d t h e i r d e r i v a t i v e s in h y d r o g e n c h a r g e d specimens.
73
5.0 o
As annealed
40
u. o i 0
s.o
E 3.0[ u
Temp.
2.o
o.. <~
%
I'0 I 0 o
I 50
I
~oo Aging
I I rso 200 Temp.(K)
[ "~. "9 I 250 500
Fig. 2. Isoehronal resistivity recovery curves in hydrogen-charged specimens. The specimens are of zonemelted iron, but the silicon-contaminated layer was not removed.
complete at 200 K and a small fraction of the quenched-in resistivity remained. Figure 2 shows the recovery in zone-melted specimens which were hydrogen-treated after being drawn to 0.10 mm diameter and not etched to remove the silicon contamination. There are three regions of recovery in an as-annealed specimen; a small recovery below 100 K, a distinct stage at 150 K, and a small and gradual recovery above 200 K. If the silicon contaminated specimen was deformed 1.5% in tension at room temperature before charging, the 150 K stage became larger and a small but clear stage appeared at 230 K. The recovery curves were obtained also on the original Johnson-Matthey iron specimens. The recovery occurs at two stages, of which the temperatures are the same as in the zonemelted iron. However, when the charging and aging were repeated the incomplete recovery at 200 K appeared in the second run, in contrast to the fourth run in the zone-melted iron specimens. It should be noted that the stage temperatures, especially that for the first stage, are independent of the specimen purity, ranging from RRRH of 1 800 to 5 000, provided that the specimens are heavily etched after hydrogen treatment to remove the silicon contaminated layer. The a m o u n t of quenched-in resistivity had no correlation with the charging time or the charging current density. It seems to depend, at least partly, on the quenching procedure; a part of the hydrogen seems to be lost from solution during the time involved in transferring a specimen from the charging
2:0 2.0
4.0
6.0
8.0
&pa( I0-'° D..cm)
Fig. 3. The resistivity remaining after the first stage, ApIi, against the total quenched-in resistivity, ApQ.
bath to the liquid helium. Figure 3 shows the relation between the resistivity remaining after the first stage, ~Pn, and the total quenched-in resistivity, ApQ. It should be noted that APH is almost independent of ApQ.
(ii) Isothermal aging and the activation energy for the recovery Since the a m o u n t of recovery at the second stage is too small for quantitative investigation, only the first stage was investigated further for kinetic analysis. Isothermal aging at 105 K was performed for specimens with various initial hydrogen concentrations, which were given by the quenched-in resistivity, ApQ, and varied from 3.4 X 10 -1° ~2 cm to 11.7 × 10 -1° ~2 cm. The specimen purities are about 5 000 in R R R n . Because of an experimental limitation, isothermal aging could not be completed. Hence, each isotherm is normalized with Apl corresponding to its Apq shown in Fig. 3*. Figure 4 shows examples taken from fifteen normalized isotherms for specimens 0.10 mm in diameter. It should be noted that while isochronal aging gives a peak temperature at 110 K with little scatter from specimen to specimen, the rate of isothermal recovery considerably depends on the specimens. The largest rate is about twice the smallest one. In the Figure, isotherms for specimens of 0.085 mm and 0.17 mm in diameter are also shown. The isothermal recovery is never represented by a first order reaction. Figure 5 shows chemical reaction rate analysis o f the isotherms at 100 and 105 K. The slope of the
* A p I for A p Q of 11.7 X 10 -1° ~ c m was estimated by extrapolating the curve in Fig. 3.
74 curves in the Figure gives the order of reaction, 7, in the following equation dn -
dt
(iii) Electron- and optical microscopy No difference was found in the electron micrographs between the as-annealed specimens and those charged with hydrogen at room temperature but not quenched. Repeated charging at room temperature caused no observable change either. Optical microscopic observation of the surface revealed no blister in the specimens singly charged or repeatedly charged b u t not quenched. By contrast, when the charged specimens were quenched to 4.2 K and warmed up to room temperature, an increase in the dislocation density was observed near grain boundaries and sub-boundaries as shown in Fig. 7. Little change in the dislocation density was found inside the sub-grain. When the warmed-up specimen was charged again, blisters were found on the surface. If the charging, quenching and warming cycle was repeated, more and larger blisters were found.
Kn-~
where n is the fraction of quenched-in resistivity remaining after time t, and K is a constant. It is obvious that the reaction cannot be represented by a single 7; 3' varies from a large value to about 3 with time. It also depends on the aging temperature. The slope-change method was applied to estimate the apparent activation energy for the first stage. The aging temperature was changed by 10 K in the range between 95 and 125 K as shown in Fig. 6(a). The calculated activation energy for four specimens falls in the range from 0.15 to 0.30 eV and gives an average of 0.22 eV, as shown in Fig. 6(b).
&Pc= RRRM (I0"I° ~,cm] (ram) IO 5.5 4710 0,10 • 5.8 4740 010
°I
0.8
I 0.6 L ~
Io
4~9 0
91~
~.o.4 <3
0 085
Oq ~ 0
'
20
L
40
[
~lO
I
8'0
I O0
[
Aging Time (K) Fig. 4. Isothermal resistivity recovery curves at 105 K.
(iv) Cold-worked specimens Figure 8 shows isochronal recovery curves of pre-strained and hydrogen-charged specimens. The pre-straining is 1% in tension at room temperature. The two-stage recovery is no longer observed b u t a single, broad stage appears. The two-stage recovery is observed for the second run, as shown in the Figure. The temperatures of the two stages are a b o u t the same as in annealed specimens. The first stage in the pre-strained specimen is larger than that in the annealed specimens. It was also found that the relative amount of the second stage to the first stage, Apii/Api , in the second run, became larger as the degree of deformation increased. The slope-change method was applied to the first stage in the second run, and an activation energy of about 0.2 eV was obtained. Electron microscopy revealed no difference in microstructure between the as-cold-worked
1.0 0.5 ,--
~
, ~
~
~.
~
-
-
-
.
a
~
• • lOOK
• • O.l
A * IO5 K =
I IO -
=
l I00
=
dnld
t (arbitrary unit)
I 1000
=
Fig. 5. Chemical reaction rate analysis o f 100 and 105 K isotherms. T h e relation between In n and in n o t linear, so that the order o f reaction is n o t unique.
(--dn/dt)
is
75
20.C
27 ~26
0"22 eV ~,5,%K
$25 T 0
16C
Pre-straJn 1% • I st Run
0.21eV ~ I 5 K
24 O.15eV~
12.C
23
22
?-
0
'
I'O
2;0
'
3~0
Time(rain)
(a)
_o 8c (3
4C
030 f
_~
~ 0.20 ILJJ
--I .... ;___
-
overage
o
/ 010
tF
=
I
I
I
I
I
I
I
J
95 ~105~115 ~125 T (K) (b)
Fig. 6. (a) Determination of the activation energy for the 110 K stage with the slope-change method. (b) The observed activation energies (their scatter and the average).
9 4.o 2.0
o
5'o ,6o ,;o 2;0 2go Aging
Temp,(K)
Fig. 8. Isochronal recovery in cold-worked specimens. Note the significant difference between the 1st and the 2nd runs.
4. DISCUSSION
(i) Interpretation of the two stages
Fig. 7. Electron microscopy of a charged, quenched and aged specimen. Note the dislocations existing outside but near the sub-boundary. No dislocation is observed outside sub-boundaries in charged but unquenched specimens.
specimens and those cold worked and charged but not quenched. After charging, quenching and warming up, no appreciable increase in the dislocation density was observed by electron microscopy, but cracks were sometimes observed at grain boundaries in optical microscopy.
The quenched hydrogen, at least in large part, is considered to condense at appropriate sites inside the crystal during the 110 K stage. The sites may be dislocations, sub-boundaries and/or other defects. The condensation of hydrogen may decrease the resistivity as in other precipitation and segregation processes. The 170 K stage is considered to be due to the release of such condensed hydrogen from the defects to escape from the surface. In the following, these considerations will be shown to be reasonable. If the surface were the main sink for hydrogen during the 110 K stage, there would be two possibilities. One is that the diffusion of hydrogen to the surface controls the rate o f the 110 K stage reaction. In this case, the isotherms should be first order, except for the initial part o f recovery [5, 6 ] . In this argument, the diffusion constant is assumed to be a constant with time. This assumption will be discussed later, and it will be shown that possible variation of the apparent diffusion constant with time does not change the
76 conclusion below. The other possibility is that the reaction at the surface controls the rate. If the escape o f hydrogen through the surface layer is the rate-controlling process, the reaction should be first order. If the reaction of hydrogen atoms to form molecules at the surface is rate-controlling, the reaction should be second order. The observed order o f reaction is, however, larger than 3. Thus, the surface is ruled o u t as the main sink for hydrogen at the 110 K stage. It has been shown that various kinetic laws other than the first order are possible in precipitation processes [7]. The result of the size-effect experiment shown in Fig. 4 is also unfavorable to the surface being the main sink. If the hydrogen diffusion to the surface were rate-controlling, the rate of isothermal recovery in the specimen 0.085 mm in diameter should be 4 times faster than that in the 0.17 mm specimen. Although the rate o f isothermal recovery scatters within a factor of about 2, the difference of a factor o f 4 should be detected. {If the rate in the thinner specimen is observed within the range R to 2R, the rate in the thicker specimen should be observed within the range R/4 to R/2.) The experimental result shows that the isotherms for the 0.085 mm and 0.17 mm specimens agree with each other at the early stage and differ only by a factor of about 1.5 at about 60% recovery. There are some experimental results showing that the quenched hydrogen causes some damage inside the crystal during the low temperature aging; the dislocation density increases near grain boundaries and sub-boundaries, and some resistivity remains unrecovered in specimens after more than three cycles o f quenching and aging. These facts imply that hydrogen atoms condense to sites, perhaps dislocations and sub-boundaries, during the low temperature aging. Simple diffusion of single hydrogen atoms, even though they interact with traps, can hardly cause such changes. R e m e m b e r that the mere charging and de-gassing treatment at room temperature causes no observable damage. It should be noted here that the damage caused during the low temperature aging does not affect the recovery stage. The damage is not detectable by resistivity measurement, at least in freshly charged and aged specimens. Even when the damage is resistometrically detect-
able after repeated cycles, the recovery stage temperatures remain unchanged. The condensed hydrogen dissolves again and escapes from the surface as the temperature is raised. This is the 170 K stage. The relative amount o f recovery in the 170 K stage to that in the 110 K stage in the second run of the deformed specimens increases with the amount of strain, i.e., the dislocation density. This fact seems to support the consideration that the site for hydrogen condensation at the 110 K stage is dislocations, and the 170 K stage is due to the release of hydrogen from the dislocations. Changes in the subboundary structure shown in Fig. 7 may imply that the sub-boundaries are also the sites for hydrogen condensation. It should be noted that the amount o f recovery in the 170 K stage, APH, in annealed specimens is almost independent of ApQ. This would be reasonable because AP~I should be mainly determined by the dislocation density including sub-boundary dislocations.
(ii) Activation energy o f the 110 K stage The activation energy for the 110 K stage is determined to be about 0.2 eV. If the crosscut method gives the same value, the observed activation energy may be considered to be the activation energy for the solute atom diffusion [8]. The cross-cut method is impossible in the present experiment because o f the p o o r reproducibility of the isotherms. Nevertheless, we consider the observed activation energy, 0.2 eV, to be that for the diffusion o f hydrogen, as is often done in many other precipitation processes. The activation energy for the diffusion o f hydrogen in iron and steels is strongly influenced by the presence o f traps, such as impurities and defects [4]. However, the activation energy determined above about 450 K is considered to be that for the free migration of hydrogen in iron, since the effect of traps is no longer important at these temperatures. The activation energy for the free migration has been reported to be about 0.1 eV, which is appreciably smaller than the present value. Hence, we consider that impurity atoms act as traps for hydrogen around 110 K, even in the present high-purity, zonemelted iron. The concentration of impurities in the zone-melted iron is estimated to be of order o f 2 X 10 -5 atomic fraction with the residual
77
resistivity at 4.2 K (2 X 1 0 - 9 ~ cm) and an assumed resistivity contribution of impurities (1 X 10 -6 ~2 cm per at.%). The impurity concentration is estimated to be about 6 X 10-5 in the Johnson-Matthey iron. (Interstitial impurities are considered to have been removed below a level of 10 -6 [2] .) However, the kind o f impurity is not known with the residual resistivity. Assuming that all the impurities have the same binding energy, B, with hydrogen atoms, we estimate B with the following equation for the trapped diffusion, D Doff - - -
(1)
~'2
I+--
T1
Here Deaf is the effective diffusion constant for hydrogen, D the diffusion constant w i t h o u t traps, rx the time for which hydrogen atoms are free, and 72 the time for which hydrogen atoms are trapped. 71 and 72 are given by T1 =
c i - l p f I exp
,
estimated to be about 1 X 10 -5, from the quenched-in resistivity of about 5 X 1 0 - 1 ° cm for specimens used in the slope-change experiment and the resistivity contribution of hydrogen is assumed to be 0.5 X 10 -6 I2 cm per at.%. For B o f various values between 0.10 and 0.15 eV, Def~ is calculated for temperatures of 95, 105, 115 and 125 K. The apparent activation energy is then calculated for temperature changes from 95 to 105, 105 to 115 and 115 to 125 K. The apparent activation energy is about 0.22 eV for B of 0.13 eV except for the 115 - 125 K change. (E D is taken to be 0.11 eV as given below.) If Ct = Ht, the apparent activation energy is close to 0.11 eV at the beginning of recovery, but soon approaches 0.22 eV as Ht decreases. F o r B = 0.13 eV, Def~ -~ 3 X 10-2D, except for Ht -~ Ct. This is a crude estimate, but it is safely concluded that, if the apparent activation energy is 0.2 eV, Def~ is about 10-2D around 110 K for various combinations of C t and H t. Here, Ct and H t are of the order of 10- 5 and Ct > Hr. Louthan, Derrick, Donovan and Caskey [9] reported
(2) D=2.3X10 -3exp
-
0.11 eV) 1 ~ cm 2s-
and T 2 --
Pb 1 exp
ED +B 1 , ~ /
(3)
where C i is the concentration of free impurity atoms, v~ and Vb are the frequency factors for free and detrapping jumps o f hydrogen atoms, respectively, and E D is the activation energy for the free migration o f hydrogen. For simplicity, we put vf = vb. If the trapping and detrapping reactions are in equilibrium during the hydrogen condensation at the 110 K stage, Ci is calculated with
C p = m ( C t - - C p ) ( H t - - Cp)
exp
~-~
.
(4)
Here, Cp is the concentration of impurity a t o m - h y d r o g e n atom pairs, and Ct and H t are the total concentrations o f impurities and hydrogen atoms, m is the number o f pairs of different orientations for one impurity atom. If hydrogen atoms are in the tetrahedral sites, m = 6. Ct is estimated to be 2 X 10 -5. H t is
for the Marz grade iron. This gives D = 1.2 X 10 -8 cm 2 s-1 at 105 K. Hence, Deaf ~ 10 - l ° cm 2 s-1. With this value of De~ at 105 K and the 50% recovery time of about 5 min, the average diffusion distance is calculated to be about 2 X 10 -4 cm. Since the specimen radius is 5 X 10-3 cm, the estimated Deaf is compatible with the conclusion of hydrogen condensation inside the crystal at the 110 K stage. Other high temperature measurements [10] give larger D's than the above because of smaller activation energies. If we take these measurements, B should be larger than 0.13 eV and again, Deaf is expected to be of the order o f 1 0 - 1 ° c m 2 s - 1 . Thus, the average diffusion distance is smaller than the specimen radius. It should be mentioned here that the deviation of the isotherms from a first order reaction cannot be explained in terms of the time dependent Deff. As seen from eqns. (1), (2), (3) and (4), D e f t decreases with H t a s aging proceeds. Calculations with Ct of 2 X 10 -5 and the initial Ht of 1 X 10 -5 show that Deff decreases to 3/5 o f the initial value as Ht decreases to 2 X 10 - s . The decrease in Deff by
78 a factor o f 2 cannot make the isotherms shown in Fig. 2 be of a first order.
hydrogen is much larger than the specimen radius.
(iii) Effect o f silicon contamination on the recovery stages In the previous paper [1], the present authors reported two recovery stages o f quenched-in hydrogen at 150 and 230 K. The specimens in these preliminary experiments were drawn to 0.15 mm diameter and then hydrogen-treated but not etched after the hydrogen treatment, and, hence, contaminated with silicon. The effect of silicon contamination is confirmed in the present research (Fig. 2). The average concentration of silicon in the present zone-melted iron specimen, which was hydrogen-treated but n o t etched, is estimated to be about 20 at. ppm with the residual resistivity and the resistivity contribution of 6 X 10 -8 ~2 cm per at.% silicon [11]. Since the diffusion distance of silicon during this treatment is comparable with the specimen diameter, the silicon contamination is not confined to a thin surface layer b u t spreads throughout the specimen cross-section. The specimens in the previous report [1] are also contaminated throughout the cross-section. Although the mechanism of the effect of silicon on the hydrogen recovery is not known at present, it may be speculated that the silicon atoms interact strongly with the hydrogen atoms and retard their diffusion. The measurement of the activation energy for the hydrogen recovery in a silicon-contaminated specimen would give useful information on the silicon-hydrogen interaction. Research is now in progress to this end. Yamakawa, Tada and Fujita [12] reported two-stage recovery at 170 and 240 K due to hydrogen introduced by heating and quenching specimens in hydrogen gas. Their result is very similar to our previous result, although it is not known whether their specimens were contaminated with silicon. Assuming a first order reaction for the 170 K stage, they calculated the activation energy for the stage to be 0.1 eV, and concluded the stage to be due to free migration of hydrogen. However, their activation energy (0.1 eV) would need to be re-examined, since the assumption of the first order reaction was not confirmed and, as they described, the average diffusion distance calculated with the diffusion constant of free
5. CONCLUSIONS
High-purity iron specimens (RRRH = 1 800 - 5 000) were electrolytically charged with hydrogen under a mild condition (0.1 N NaOH solution and 10 - 30 mA cm -2) which caused no observable damage in the specimens. They were quenched into liquid helium and aged at below room temperature. The change in the resistivity due to hydrogen diffusion was investigated isochronally and isothermally. The conclusions are: (1) The resistivity recovers at two stages centering a b o u t 110 K and 170 K. The 110 K stage is larger than the 170 K stage. (2) A large part of the quenched-in hydrogen condenses inside the crystal during the 110 K stage, perhaps around dislocations and sub-boundaries. (3) The isotherms for the 110 K recovery process are n o t a first order reaction. The activation energy for the process is a b o u t 0.2 eV; the hydrogen diffusion is influenced by impurities even in the present high-purity specimens. (4) Solute silicon affects the recovery stages very strongly; phenomenologically, the 110 K stage tends to disappear, a large stage appears around 150 K and a small, new stage appears above 200 K. Even only a b o u t 20 at. ppm of silicon almost suppresses the 110 K stage. This effect is tentatively interpreted in terms of strong binding between silicon atoms and hydrogen atoms. (5) The 170 K stage was possibly due to the release of trapped hydrogen from dislocations and sub-boundaries. (6) The recovery occurs at a single stage in cold-worked specimens. The two-stage recovery, however, is observed in coldworked specimens in the second run of charging, quenching, and aging at the same temperatures as in the annealed specimens.
ACKNOWLEDGEMENT
This work was partly supported financially by the Ishihara-Asada Research Fund (1974) of the Iron and Steel Institute of Japan.
79 REFERENCES
1 S. Moriya, S. Takaki and H. Kimura, Scr. Metall., 8 (1974) 937. 2 S. Takaki and H. Kimura, Scr. Metall., 10 (1976) 1095. 3 S. Takaki and H. Kimura, Scr. Metall., 10 (1976) 701. 4 For example, H. K. Birnbaum and C. A. Wert, Bet. Bunsenges. Phys. Chem., 76 (1972) 806. 5 J. H. Evans and B. L. Eyre, Acta Metall., 17 (1969) 1109. 6 K. Yamakawa, M. Tada and F. E. Fujita, Jpn. J. Appl. Phys., 15 (1976) 769.
7 For example, J. W. Christian, The Theory of Transformations in Metals and Alloys, Pergamon Press, Oxford, 1975, p. 525. 8 K. Yoshioka and H. Kimura, Acta Metall., 23 (1975) 1009. 9 M. R. Louthan, Jr., R. G. Derrick, J. A. Donovan and G. R. Caskey, Jr., in A. W. Thompson and I. M. Bernstein (eds.), Proc. Int. Conf. Effect of Hydrogen on Behavior of Metals, AIME, New York, 1976, p. 337. 10 For example, Th. Heumann and E. Domke, Ber. Bunsenges. Phys. Chem., 76 (1972) 825. 11 S. Arajs, F. C. Schwerer and R. M. Fisher, Phys. Status Solidi, 33 (1969) 731. 12 K. Yamakawa, M. Tada and F. E. Fujita, Scr. Metall., 9 (1975) 1.