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On the formation of interstitial — Hydrogen clusters in iron
Scripta METALLURGICA
Vol. IS, pp. 941-943, 1981 Printed in the U.S.A.
Pergamon Press Ltd. All rights reserved
ON THE FOR~t~TIONOF INTERSTITIAL - HYDROGENCLUSTERS IN IRON
J. Au* and H.K. Birnbaum University of I l l i n o i s at Urbana Champaign Department of Metallurgy and Mining Eng. and Materials Research Laboratory Urbana, IL 61801
(Received June 8, 1981)
The interaction of H with other i n t e r s t i t i a l solutes to form complexes has been well established in a number of bcc metals (1-4). In the~ Nb-H systems the H-O and H-N binding enthalpy was shown to be of the order of 10 kJ/mole (3, 5) and while the binding enthalpy has not been as well established in other systems, i t is expected to be of this order of magnitude. Although this type of trapping has not been as extensively studied in the Fe-H system, due p r i n c i p a l l y to the small s o l u b i l i t y of H in Fe, i t has been recognized as a significant factor in determining the low temperature d i f f u s i v i t y of H (6). In addition to trapping at solutes, H interactions with dislocations have also been well established (4, 7). In general these interactions have been considered to result in dislocation pinning and solution strengthening (8). In a series of recent publications however, i t was shown that the introduction of H into high purity iron leads to a decrease of the flow stress, i . e . solution softening in the temperature range 170-300 K (9-11). While the mechanism of this solid solution softening is not established, i t has been suggested that i t is caused by a decrease of the Peierls stress or a decrease in the energy to form double kinks on the dislocations (9). In contrast to these i n t r i n s i c softening mechanisms the phenomena could also be accounted for by extrinsic softening due to a decrease in the effectiveness of solutes as dislocation pinning points which may result from the interaction between H and other solutes (8, 12). In order to establish whether i n t r i n s i c or extrinsic softening mechanisms are important i t is necessary to understand the interactions between solutes and H and the effect of H on the solute interactions with dislocations. In the present paper we report the results of a study of the interaction of H with C and N i n t e r s t i t i a l solutes in high purity Fe using the technique of magnetic disaccommodation (13). This method, which is the magnetic analog of the better known internal f r i c t i o n methods, has the s e n s i t i v i t y to allow the study of these i n t e r s t i t i a l solutes at the ppm (atomic) concentration level. The Fe used in these experiments had a total impurity concentration of less than 100 ppm impurities (by spark source mass spectroscopic analysis) with the major substitutional impurities being Mn, Mg, and Si. The C and N levels were reduced by purification in wet and dry hydrogen atmospheres to less than one ppm as measured by the magnetic disaccommodation method. In these pure specimens, containing about 10 ppm H, only low temperature relaxations in the temperature range 4-40 K which were due to hydrogen clusters were observed (13). The addition of 50 ppm C solutes to these Fe°H specimens reduced the relaxation amplitudes of these H cluster relaxations and introduced a new low temperature relaxation w h i c h was identified as resulting f r o m the tunneling of H around a C interstitial. The addition of 200 ppm of N i n t e r s t i t i a l s to the solid solution completely removed the H cluster relaxations without introducing any new relaxations in the temperature range 4-300 K. These observations were interpreted as resulting from the trapping of H i n t e r s t i t i a l s by C and N i n t e r s t i t i a l s (13). The magnetic times (1 *Present
effect of this trapping on the C and N relaxations can also be studied using the disaccommodation technique. Figure 1 shows the r e l u c t i v i t y difference at two fixed and 15 minutes) after randomization of the magnetic domains, a (1, 15) as a function address: Sundstrand Engineering, Rockford, IL.
941 0036-9748/81/080941-03502.00/0 Copyright (c) 1981 Pergamon Press Ltd.
942
INTERSTITIAL
H CLUSTERS
IN IRON
Vol.
15, No.
8
of temperature for Fe-C and Fe-C-H specimens. In the temperature range near 250 K the reluctivity difference exhibits a maximum at a temperature which is a function of the activation enthalpy for the C Snoek relaxation and the two times at which the reluctivity is measured (13). The height of the relaxation peak is proportional to the concentration of the relaxing species ie, C interstitials in this case. The data shown in Fig. 1 shows that H has no significant effect on the C Snoek relaxation in the temperature range near 250 K. (The small additional relaxation observed at 225 K was not studied in detail.) In marked contrast to this behavior, the addition of H to Fe-N alloys (Fig. 2) significantly reduced the magnitude of the N Snoek relaxation observed at about 237 K. The relaxation temperature was not significantly affected and no new N-H relaxation was observed in the temperature range 4300 K. A reduction of the Snoek relaxation strength can result from the formation of C-H or N-H complexes in two ways. I f the C or N motion activation enthalpy is changed by the formation of the complex a new relaxation peak w i l l be formed at a temperature characteristic of the new complex and the Snoek relaxation strength is reduced. Alternatively, the Snoek relaxation strength w i l l decrease i f the formation of a C or N cluster with H changes the symmetry of the defects towards a more cubic distortion f i e l d . In the case of N in Fe the reduction of the Snoek peak due to H i n t e r s t i t i a l s appears to reflect a decrease of the tetragonality of the defect when the N-H complex forms. This is indicated by the lack of any new relaxation peaks. The formation of the N-H complex is shown both by the decrease of the N Snoek relaxation and the removal of the low temperature H relaxations (13) in the Fe-N-H alloys. While the H concentration in these specimens is not known accurately, i t is greater than 10 ppm (measured by vacuum e x t r a c t i o n after the experiments) and less than 80 ppm (estimated from the decrease in the relaxation strength). Using these values the N-H binding enthalpy required to cause the large decrease in the Snoek relaxation at 237 K can be estimated to be BNH~ 25 kJ/mole. In the case of C, the lack of an effect of H on the Snoek relaxation suggests that either the C-H binding enthalpy is too low to form significant concentrations of complexes at temperatures near 250 K or that C behavior is not affected by the presence of H in the complex. At the present time we cannot distinguish between these two p o s s i b i l i t i e s . The decrease of tetragonality of the strain f i e l d around the N i n t e r s t i t i a l (and possibly also around the C i n t e r s t i t i a l ) can result from the structure of the complex. Since the tetragonal axes of the constituent i n t e r s t i t i a l s w i l l l i k e l y have different orientations ( p a r t i c u l a r l y i f nearest neighbor i n t e r s t i t i a l sites are occupied) the overall strain f i e l d can have close to cubic symmetry. Reorientation of the N solute w i l l be accompanied by motion of the H solutes to equivalent sites. Since the H jump frequencies are orders of magnitude larger than that of the N, the result is that the structure and symmetry of the complex do not change even i f the N solute reorients. Observation of a decrease of i n t e r s t i t i a l solute tetragonality as a result of H trapping does not establish the observed solution softening in the Fe-H alloys as an extrinsic phenomena. I t does support the suggestion (8, 12) that solution softening can result from a decrease in effectiveness of i n t e r s t i t i a l solutes due to introduction of a second solute. If this mechanism were applied to the observations made on the Fe-H system (9-11) i t would imply that the high purity Fe contained an effective pinning point (C, N, O) the strength of which was reduced by small additions of H solutes. While this remains conjectural at this time, we have demonstrated the decrease in tetragonality on formation of N-H complexes and therefore the extrinsic mechanism must be seriously considered. Acknowledgements This research was supported by the U.S. Department of Energy contract EY-76-C-02-1198. References 1. C.C. Baker and H.K. Birnbaum, Acta Met. 21, 865 (1973). 2. P. Zapp and H.K. Birnbaum, Acta Met. 2__88~1275 (1980). 3. G. P f e i f f e r and H. Wipf, J. Phys. F. Met. Phys. 6, 167 (1976). 4. J.P. Hirth, Met. Trans. 11A 861 (1980). 5. R.F. Mattas and H.K. B i r ~ u m , Acta Met. 23973 (1975). 6. R.A. Oriani, Acta Met. 18147 (1970). m
Vol.
15, No.
7. 8. 9. 10. 11. 12. 13.
8
INTERSTITIAL
H CLUSTERS
IN IRON
943
C.A. Wert, H~dro~en in Metals, G. Alefeld and J. Voik] eds., (1978) voI. 29 pp. 305. E. Pink and R.J. Arsenault, Prog. in Met. Sci. 24 1 (1979). H. Matsui and H. Kimura, Mat. Sci. Eng. 40 207-('i-979). S. Morija, H. Matsui and H. Kimura, Mat. S---ci. Eng. 40 217 (1979). H. Matsui, H. Kimura and A. Kimura, Mat. Sci. Eng. 4~227 (1979). R. Gibala and T.E. Mitchell, Scripta Met. 7 1143 (1-97-3). J.J. Au and H.K. Birnbaum, Acta Met. 26 11~5 (1978).
N 2.0
c
+
1.6
6.4
t+2
¢~ 4.8 0 x
x
~
--" 0.8
3.2
i!
1.6
0.4
220
Fig. l
240
260 T(K)
280
300
-
Fig. 2 E f f e c t •
~(I+5 m i n )
I
220
+
I
I
260
300
T(K)
Effect of H on the C Snoek Peak 1 1 ~(1,15)
AI~,e
180
~ ( I min)
• Fe + 5x]O -3 a t % C z~ Fe + 5xlO -3 a t % C + I x l O - 3 a t % H
o f H on t h e N Snoek Peak
Fe + 2xlO -2 a t % N + 8xlO -3
at+%C A Fe 2xlO-2 at % N + 8xlO-3 at % C + (~5xlO-3 at % H)
Vol. IS, pp. 941-943, 1981 Printed in the U.S.A.
Pergamon Press Ltd. All rights reserved
ON THE FOR~t~TIONOF INTERSTITIAL - HYDROGENCLUSTERS IN IRON
J. Au* and H.K. Birnbaum University of I l l i n o i s at Urbana Champaign Department of Metallurgy and Mining Eng. and Materials Research Laboratory Urbana, IL 61801
(Received June 8, 1981)
The interaction of H with other i n t e r s t i t i a l solutes to form complexes has been well established in a number of bcc metals (1-4). In the~ Nb-H systems the H-O and H-N binding enthalpy was shown to be of the order of 10 kJ/mole (3, 5) and while the binding enthalpy has not been as well established in other systems, i t is expected to be of this order of magnitude. Although this type of trapping has not been as extensively studied in the Fe-H system, due p r i n c i p a l l y to the small s o l u b i l i t y of H in Fe, i t has been recognized as a significant factor in determining the low temperature d i f f u s i v i t y of H (6). In addition to trapping at solutes, H interactions with dislocations have also been well established (4, 7). In general these interactions have been considered to result in dislocation pinning and solution strengthening (8). In a series of recent publications however, i t was shown that the introduction of H into high purity iron leads to a decrease of the flow stress, i . e . solution softening in the temperature range 170-300 K (9-11). While the mechanism of this solid solution softening is not established, i t has been suggested that i t is caused by a decrease of the Peierls stress or a decrease in the energy to form double kinks on the dislocations (9). In contrast to these i n t r i n s i c softening mechanisms the phenomena could also be accounted for by extrinsic softening due to a decrease in the effectiveness of solutes as dislocation pinning points which may result from the interaction between H and other solutes (8, 12). In order to establish whether i n t r i n s i c or extrinsic softening mechanisms are important i t is necessary to understand the interactions between solutes and H and the effect of H on the solute interactions with dislocations. In the present paper we report the results of a study of the interaction of H with C and N i n t e r s t i t i a l solutes in high purity Fe using the technique of magnetic disaccommodation (13). This method, which is the magnetic analog of the better known internal f r i c t i o n methods, has the s e n s i t i v i t y to allow the study of these i n t e r s t i t i a l solutes at the ppm (atomic) concentration level. The Fe used in these experiments had a total impurity concentration of less than 100 ppm impurities (by spark source mass spectroscopic analysis) with the major substitutional impurities being Mn, Mg, and Si. The C and N levels were reduced by purification in wet and dry hydrogen atmospheres to less than one ppm as measured by the magnetic disaccommodation method. In these pure specimens, containing about 10 ppm H, only low temperature relaxations in the temperature range 4-40 K which were due to hydrogen clusters were observed (13). The addition of 50 ppm C solutes to these Fe°H specimens reduced the relaxation amplitudes of these H cluster relaxations and introduced a new low temperature relaxation w h i c h was identified as resulting f r o m the tunneling of H around a C interstitial. The addition of 200 ppm of N i n t e r s t i t i a l s to the solid solution completely removed the H cluster relaxations without introducing any new relaxations in the temperature range 4-300 K. These observations were interpreted as resulting from the trapping of H i n t e r s t i t i a l s by C and N i n t e r s t i t i a l s (13). The magnetic times (1 *Present
effect of this trapping on the C and N relaxations can also be studied using the disaccommodation technique. Figure 1 shows the r e l u c t i v i t y difference at two fixed and 15 minutes) after randomization of the magnetic domains, a (1, 15) as a function address: Sundstrand Engineering, Rockford, IL.
941 0036-9748/81/080941-03502.00/0 Copyright (c) 1981 Pergamon Press Ltd.
942
INTERSTITIAL
H CLUSTERS
IN IRON
Vol.
15, No.
8
of temperature for Fe-C and Fe-C-H specimens. In the temperature range near 250 K the reluctivity difference exhibits a maximum at a temperature which is a function of the activation enthalpy for the C Snoek relaxation and the two times at which the reluctivity is measured (13). The height of the relaxation peak is proportional to the concentration of the relaxing species ie, C interstitials in this case. The data shown in Fig. 1 shows that H has no significant effect on the C Snoek relaxation in the temperature range near 250 K. (The small additional relaxation observed at 225 K was not studied in detail.) In marked contrast to this behavior, the addition of H to Fe-N alloys (Fig. 2) significantly reduced the magnitude of the N Snoek relaxation observed at about 237 K. The relaxation temperature was not significantly affected and no new N-H relaxation was observed in the temperature range 4300 K. A reduction of the Snoek relaxation strength can result from the formation of C-H or N-H complexes in two ways. I f the C or N motion activation enthalpy is changed by the formation of the complex a new relaxation peak w i l l be formed at a temperature characteristic of the new complex and the Snoek relaxation strength is reduced. Alternatively, the Snoek relaxation strength w i l l decrease i f the formation of a C or N cluster with H changes the symmetry of the defects towards a more cubic distortion f i e l d . In the case of N in Fe the reduction of the Snoek peak due to H i n t e r s t i t i a l s appears to reflect a decrease of the tetragonality of the defect when the N-H complex forms. This is indicated by the lack of any new relaxation peaks. The formation of the N-H complex is shown both by the decrease of the N Snoek relaxation and the removal of the low temperature H relaxations (13) in the Fe-N-H alloys. While the H concentration in these specimens is not known accurately, i t is greater than 10 ppm (measured by vacuum e x t r a c t i o n after the experiments) and less than 80 ppm (estimated from the decrease in the relaxation strength). Using these values the N-H binding enthalpy required to cause the large decrease in the Snoek relaxation at 237 K can be estimated to be BNH~ 25 kJ/mole. In the case of C, the lack of an effect of H on the Snoek relaxation suggests that either the C-H binding enthalpy is too low to form significant concentrations of complexes at temperatures near 250 K or that C behavior is not affected by the presence of H in the complex. At the present time we cannot distinguish between these two p o s s i b i l i t i e s . The decrease of tetragonality of the strain f i e l d around the N i n t e r s t i t i a l (and possibly also around the C i n t e r s t i t i a l ) can result from the structure of the complex. Since the tetragonal axes of the constituent i n t e r s t i t i a l s w i l l l i k e l y have different orientations ( p a r t i c u l a r l y i f nearest neighbor i n t e r s t i t i a l sites are occupied) the overall strain f i e l d can have close to cubic symmetry. Reorientation of the N solute w i l l be accompanied by motion of the H solutes to equivalent sites. Since the H jump frequencies are orders of magnitude larger than that of the N, the result is that the structure and symmetry of the complex do not change even i f the N solute reorients. Observation of a decrease of i n t e r s t i t i a l solute tetragonality as a result of H trapping does not establish the observed solution softening in the Fe-H alloys as an extrinsic phenomena. I t does support the suggestion (8, 12) that solution softening can result from a decrease in effectiveness of i n t e r s t i t i a l solutes due to introduction of a second solute. If this mechanism were applied to the observations made on the Fe-H system (9-11) i t would imply that the high purity Fe contained an effective pinning point (C, N, O) the strength of which was reduced by small additions of H solutes. While this remains conjectural at this time, we have demonstrated the decrease in tetragonality on formation of N-H complexes and therefore the extrinsic mechanism must be seriously considered. Acknowledgements This research was supported by the U.S. Department of Energy contract EY-76-C-02-1198. References 1. C.C. Baker and H.K. Birnbaum, Acta Met. 21, 865 (1973). 2. P. Zapp and H.K. Birnbaum, Acta Met. 2__88~1275 (1980). 3. G. P f e i f f e r and H. Wipf, J. Phys. F. Met. Phys. 6, 167 (1976). 4. J.P. Hirth, Met. Trans. 11A 861 (1980). 5. R.F. Mattas and H.K. B i r ~ u m , Acta Met. 23973 (1975). 6. R.A. Oriani, Acta Met. 18147 (1970). m
Vol.
15, No.
7. 8. 9. 10. 11. 12. 13.
8
INTERSTITIAL
H CLUSTERS
IN IRON
943
C.A. Wert, H~dro~en in Metals, G. Alefeld and J. Voik] eds., (1978) voI. 29 pp. 305. E. Pink and R.J. Arsenault, Prog. in Met. Sci. 24 1 (1979). H. Matsui and H. Kimura, Mat. Sci. Eng. 40 207-('i-979). S. Morija, H. Matsui and H. Kimura, Mat. S---ci. Eng. 40 217 (1979). H. Matsui, H. Kimura and A. Kimura, Mat. Sci. Eng. 4~227 (1979). R. Gibala and T.E. Mitchell, Scripta Met. 7 1143 (1-97-3). J.J. Au and H.K. Birnbaum, Acta Met. 26 11~5 (1978).
N 2.0
c
+
1.6
6.4
t+2
¢~ 4.8 0 x
x
~
--" 0.8
3.2
i!
1.6
0.4
220
Fig. l
240
260 T(K)
280
300
-
Fig. 2 E f f e c t •
~(I+5 m i n )
I
220
+
I
I
260
300
T(K)
Effect of H on the C Snoek Peak 1 1 ~(1,15)
AI~,e
180
~ ( I min)
• Fe + 5x]O -3 a t % C z~ Fe + 5xlO -3 a t % C + I x l O - 3 a t % H
o f H on t h e N Snoek Peak
Fe + 2xlO -2 a t % N + 8xlO -3
at+%C A Fe 2xlO-2 at % N + 8xlO-3 at % C + (~5xlO-3 at % H)