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Hydrogen induced void nucleation of 310 stainless steel
~
Acta metall, mater. Vol. 43, No. 10, pp. 3727-3732, 1995
Pergamon
0956-7151(95)00073-9
HYDROGEN
ElsevierScienceLtd Copyright© 1995Acta MetallurgicaInc. Printed in Great Britain.All rights reserved 0956-7151/95 $9.50+ 0.00
I N D U C E D VOID N U C L E A T I O N OF 310 STAINLESS STEEL
XING-GANG J I A N G t , WU-YANG CHU and J I - M E I XIAO Department of Materials Physics, University of Science and Technology Beijing, Beijing 100083, P.R. China (Received 6 May 1994; in revised form 16 January 1995)
Abstract--Hydrogen induced void nucleation of 310 stainless steel was investigated. Experimental results indicated that hydrogen promoted void nucleation. A new model of hydrogen induced void nucleation was proposed. The basic idea of this model is that hydrogen induces void nucleation not only by promoting microcrack nucleation but also by promoting the transition of microcrack to microvoid; hydrogen also increases the stability of a microvoid by forming hydrogen pressure in the microvoid and by decreasing the void surface energy.
1. INTRODUCTION Many experiments observed up to now have shown that hydrogen may induce ductile failure of intermediate and low strength steels [1-4]. Although there are many researches about the mechanism of hydrogen induced ductile failure, the mechanism of hydrogen induced void nucleation is not clear. In the first, there are two different viewpoints about whether hydrogen promotes void nucleation. On the one hand, Thompson et al. [5-7] suggested that hydrogen promoted void growth and interlinkage but it had almost no effect on void nucleation. On the other hand, Kwon and Asaro [8] observed hydrogen effects on void nucleation in tensile specimens of spheroidized 1518 steel. Unlike Thompson et al., however, they concluded that hydrogen promoted void nucleation at average-sized carbide particles by reducing the critical interfacial strength. Lee et al. [1] and Lee and Bernstein [3] also found that hydrogen enhanced void initiation along the characteristic slip line field. In the second, Kwon and Asaro [8] have proposed a model, based on the assumption that all the voids were nucleated at the second phase particles, to explain the reason that hydrogen promotes void nucleation. However, many experimental observations indicate that the second phase particles may facilitate but may not be essential for nucleation of voids. The above different reports about the hydrogen effect on void nucleation indicate that further investigation of this problem should be carried out. The first objective of the present investigation is to do an experimental observation of the effect of tPresent address: School of Materials Science & Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245, U.S.A.
hydrogen on void nucleation behavior during the tensile test of 310 stainless steel. In-situ TEM observation was performed on the thin foils of 310 stainless steel that precharged hydrogen to observe the process of hydrogen induced void nucleation. As is well known, a number of different models of hydrogen-induced cracking have been proposed by numerous investigators. For example, hydrogen induced local deformation theories [9, 10], which have been developed from an original idea proposed by Beachem [9] that hydrogen increases the crack tip ductility. The key points of this mechanism are that hydrogen promotes the dislocation movement; crack growth in the presence of hydrogen is because of the reduction of local flow stress and the increase in activity of dislocations in the high stress regions ahead of the crack tip. The mechanisms were supported by a series of in situ TEM observation evidences [11-15]; the decohesion theory [16], proposed originally by Oriani and Josephic [16], suggests that hydrogen may concentrate in a very high stress region of a few atomic distances ahead of cracks and lower the cohesive strength of the lattice or interface thereby facilitating "decohesion"; the adsorption theory [17], proposed by Petch et al. suggests that adsorbed hydrogen at the crack tip lowers the surface energy of newly formed fracture surfaces so that the overall fracture surface energy is lowered [17]; the pressure theory [18] was based on the fact that hydrogen precipitated at internal surfaces or voids thereby producing high gas pressures. Up to now, these theories have successfully explained many experimental results. However, in some cases, it is quite possible that these mechanisms are cooperative. The second objective of the paper is to propose a new model of hydrogen induced void nucleation by cooperation of all these theories.
3727
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JIANG et al.: HYDROGEN INDUCED VOID NUCLEATION 2. EXPERIMENTAL PROCEDURE
The material used in this study is AISI 310 stable austenitic stainless steel. The chemical compositions of the alloy are: 24.48 Cr, 21.33 Ni, 0.23 Nb, 0.50 Si, 0.60 Mn, 0.08 Mo, 0.08 C (wt%). The tensile specimens were prepared from cold rolled foils (about 0.25 mm thick). The specimens were annealed in an argon atmosphere at 1050°C for about 20rain and quenched into oil. The anneal established the grain size of 15/~m. Some specimens were precharged in a 1 N H 2SO 4 solution containing 0.25 g/1 ofAs203, for 15 h, others were not charged with hydrogen during tensile test. The current density was 103 A/m 2. The average concentration of hydrogen in the specimen is about 80 wppm. The hydrogen contents were determined by hot extraction and volumetric measurement using specimens taken from the same charged specimen. After precharging, the specimens were tensile tested at room temperature and at strain rate of 2 x 10 -4 s -1, while the same hydrogen charged condition as used in the precharging was maintained. In order to demonstrate that these effects of hydrogen charging were reversible, the specimens were charged at the same condition and outgassed prior to testing. The stress-strain curves were identical to those of uncharged specimens, within the normally observed scatter for such tests ( + 5%). Because the width of the specimen is 8 mm, which is much larger than the thickness (0.25 mm) of the specimen, buckling or out-of-plane bending can be avoided in tensile tests by fixing two pairs of back-up alloy plates at the two heads of specimens. An optical microscope was used to observe void formation after testing. The $250 M K I I type scanning electron microscope was used to observe the fracture surface. In order to observe the detail of the physical process of microvoid nucleation, in-situ high voltage TEM observation was accomplished on the thin foils of the alloy precharged hydrogen under the same condition as above. The voltage is I000 kV, the use of high accelerating voltages permitted the observation of foils up to 1.5 # m thick without a loss in resolution [19]. Thin foils were prepared by using an electrolytic polishing solution containing 5 parts perchloric acid and 95 parts methanol at 243 K. After perforation, the foils were stored in liquid nitrogen to keep the hydrogen content in the specimen. The foils were deformed in a tensile specimen holder that was displacement controlled. As the stress was applied, cracks were initiated at the edge of the hole, which served as a notch, and propagated in the direction perpendicular to the tensile stress. Some hydrogen will move out from the surface of the foil due to high vacuum environment. So in-situ TEM observation of the void nucleation should be finished as fast as possible. In the present experiment, we finished the in-situ test within 2 h. Because 310
stainless steel has f.c.c, structure, the diffusion coefficient is very low, even though some hydrogen will move out from the surface of the foil, we can make sure that there is still a lot of hydrogen in the thin foil specimen within 2 h.
3. EXPERIMENTAL RESULTS
3.1. Observation failure.
of hydrogen promoting
ductile
Figures l(a) and (b) are the optical observations of void formation after 18% of strain for the undercharged and charged specimens respectively. It is shown that very few voids were nucleated for the uncharged specimen. However, a lot of voids were formed for the charged specimen. Figure 1 demonstrates that hydrogen promotes void nucleation and growth during tensile deformation. Figures 2(a) and (b) are the SEM observations of fracture surfaces of the uncharged and charged specimens. It is shown that both charged and uncharged specimens were fractured in a ductile manner, there
Fig. 1. Optical observation of void nucleation after 18% of strain for (a) the uncharged specimen and (b) the charged specimen.
JIANG et al.: HYDROGEN INDUCED VOID NUCLEATION
3729
Fig. 4. TEM observation of microstructure before tensile stress was applied.
Fig. 2. SEM observation of fracture surface of (a) the uncharged specimen and (b) the charged specimen. is not a significant difference in the morphology of the fracture surfaces of the specimens. The final elongation for the specimen uncharged and charged with hydrogen was 40 and 20% respectively. It is difficult to estimate whether hydrogen promotes void nucleation from the fracture surface of the specimen. However, from the optical observation results, we found that hydrogen may promote premature ductile failure of the specimen by promoting void nucleation and growth.
3.2. In situ T E M observation o f microvoid nucleation The above observations indicate that all the specimens which were charged or uncharged with hydrogen fractured in a ductile manner. In the following, we will observe in detail the physical process of
Fig. 3. TEM observation of the perforation before tensile stress was applied.
microvoid nucleation during the deformation of 310 stainless steel pre-charged with hydrogen. Figure 3 is the low magnification TEM observation of the thin foil before tensile stress is applied. When the tensile stress is applied, we will pay much more attention to the cracks which are almost perpendicular to the tensile stress. Figure 4 is the microstructural observation of the specimen before deformation. Very few particles were observed and no pre-initiated microvoids caused by hydrogen charging were found. Figure 5 is the observation of the initial stage of microcrack nucleation when the tensile stress is applied. It is shown that a microcrack was nucleated ahead of a main crack. Figure 6 is the observation of microvoid nucleation. The microvoids were formed by blunting the microcrack tips. The above TEM observations indicate that a microvoid may nucleate from a microcrack when the crack tip is blunted; many void nucleation sites appear not correlated with the second phase particles. 4. DISCUSSION
4.1. Experimental observations o f hydrogen promoting void nucleation 4.1.1. The effect o f second phase particles on void
Fig. 5. TEM observation ofmicrocrack nucleation ahead of a main crack.
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JIANG et aL: HYDROGEN INDUCED VOID NUCLEATION movement and void nucleation process. Further detailed investigations should be done in future. 4.2. A theoretical model for hydrogen promoting void nucleation
Fig. 6. TEM observation of microvoid nucleation ahead of a main crack. nucleation. Kwon and Asaro [8] have analyzed the reason of hydrogen promoting void nucleation. Their theory is based on Argon's early idea [20] that voids were generally nucleated at particles. They believed that hydrogen promoted void nucleation at the carbide particles by reducing the critical interfacial strength. However, the present TEM observations and other observations [11, 21-23] indicated that void nucleation is not correlated with the second phase particles, and voids were found to nucleate at either the intersection of active slip bands or at the dislocation cell walls which act as dislocation sources. Detailed observation of void nucleation process indicated that the second phase particles were not a prequisite for void nucleation; microcracks first nucleated because of stress concentration; microvoids were often formed by blunting the microcrack tips. The observations of void nucleation in other materials also indicated the above behavior [24, 25]. 4.1.2. The effect o f hydrogen on void nucleation. Birnbaum et al. [11-15] have observed systematically the effect of hydrogen on deformation and void nucleation in nickel, high purity A1, etc. They found the advance of transgranular cracks in a hydrogen environment occurred by either direct emission of dislocations from the crack tip or by a complex process of void nucleation and growth ahead of the crack. The effect of hydrogen is to decrease the stress required for crack advance and to localize the deformation. Their observations are strong supports to the hydrogen induced local deformation theory. They also found that hydrogen promoted void nucleation. In the present investigation, the TEM observation technique is different from Birnbaum's environmental cell TEM techniques, and the material observed is also different. However, the observation result that hydrogen promotes microvoid nucleation is basically consistent with their results. It should be mentioned that the present experimental condition is limited, and because some hydrogen may move out during in-situ TEM observation, so it is very difficult to give the exact content of hydrogen in the thin foil that affects dislocation
4.2.1. Hydrogen promotes microcrack nucleation. The present and many other TEM observations showed that the microvoid was nucleated by blunting the microcrack tip. For a smooth tensile specimen, the dislocation pile-ups are formed by local deformation. Figure 7 is the TEM observation of a dislocation pile-ups after 10% of strain of the charged specimen during tensile deformation of 310 stainless steel. When the stress concentration ahead of dislocation pile-ups is sufficiently large, the atomic bonds will be ruptured. The initiation condition for a microcrack coinciding with the slip plane at the head of pile-ups is [26] zc = zo + [2G~ /n (1 - v )L ]1/2
(1)
where z¢ is the critical stress of microcrack nucleation, z0 is the lattice resistance of dislocation moving, G is the shear modulus, 7 is the surface energy, v is Poisson's ratio and L is the length of dislocation pile-ups. When the specimen was charged with hydrogen during deformation, the hydrogen promotes, the local plastic deformation and decreases the atomic bond strength, which results in a decrease of ~c, that is zc (H) < zc. First, the strain energy of dislocations is reduced because the segregation of hydrogen and dislocations will move along with a hydrogen atmosphere [27], as a result, the friction stress necessary to operate the F r a n k - R e a d source and forming pile-ups of dislocations will be decreased from z0 to z0 (H); secondly, an additional force will act on dislocations and help the external stress to make the dislocations multiply and move. The resultant stress on the
Fig. 7. The formation of a dislocation pile-up during deformation of the charged specimen.
JIANG et al.: HYDROGEN INDUCED VOID NUCLEATION dislocations surrounded by the hydrogen atmosphere is r (H) = k%, where k > 1, k is a function of hydrogen concentration, yields strength and temperature [28]. Therefore, the local effective shear stress will be z(H) - z0 = k z c - r0(H), instead of r~-- %. In the third aspect, hydrogen will decrease 7 to 7(H) [16, 17]. So the critical stress of microcrack initiation for specimens charged with hydrogen is rc(n) = ¼{z0(H) + [2GT(H)/~(1 - v)L]m}.
(2)
Comparing equations (1) and (2), we know that %(H) < zc, which indicates that hydrogen promotes microcrack nucleation.
4.2.2. Hydrogen promotes the transition of microcrack to microvoid. When the microcrack is nucleated, if it propagates in a stable manner and the crack tip does not blunt, then the specimen will be fractured in a brittle manner; if the microcrack tip is blunted by emitting edge dislocations, then a microvoid will be formed and the specimen will be fractured in a ductile manner. In the present investigation, TEM observations indicated that the microcrack tip was blunted during deformation of 310 stainless steel either charged or uncharged with hydrogen, which is consistent with many other TEM observation results [11-13]. In the following, we analyze the reason that hydrogen promotes microcrack bluntness in the present deformation condition. In simplicity, suppose the microcrack is a type I crack, then according to literature [29, 30] we obtain the critical stress intensity factor for edge dislocations emitted from the crack tip, which blunted the crack tip
2
[
Gb
3731
formation of an equilibrium shaped microvoid as follows
AG = - V i a + ( A l y -- A2YB)-- V2a2n/2E
(4)
where V~ is the volume of the microvoid, tr is the applied stress, A1 is the surface area of the microvoid, A2 is the interphase area of the void and the second phase particle, ?a is the boundary or interface energy per unit area, V: is the volume within which the strain energy is relaxed on the formation of the microvoid, trn is the local stress on the void by the group of dislocation piled-up, and E is the elastic modulus. The first term in equation (4) is the work done by the applied stress on the system. The second and third terms are respectively the change in surface and interface energies, and the fourth term the reduction of strain energy in the system. According to Stroh's calculation [34], the maximum stress of the group of piled-up dislocations is
am
x/3 \ x J
(5)
where x is a distance from the head of the piled up group to the microvoid nucleation site, L is the length of the dislocation occupy. Assuming r is the microvoid radius for nucleation, then the average value of the stress may be written as 6m= r
0amdx
:
x/3\r]
.
(6)
In simplicity, we assume tr, = 6m, according to Lim [33], Raj and Ashby's results [35] V1
:
rS27r(2 -- 3 cos ~ + cos 3~)/3, = cos-I(~B/2?),
K1e - sin • cos(¢I)/2) (1 - v)(8nr~) 1/2
A~ = r24n(1 -- cos ct), A2 = r2n sin 2:t, + (2rcrc)l/2(o.r_+ r~Z--{~)JB sin ~ ' ~ ]
(3) v2 = 3vl/2.
where B = 2~,fl/g, fl = rce3/2/2, rc is the core radius of a dislocation, • is an angle of the slip plane with the crack plane, b is the Burgus vector, trt is the lattice friction stress. Because hydrogen will decrease 7 [16, 17] and rrf [31], the above equation indicates that KEo(H) < K~e, which means hydrogen promotes edge dislocations emitted from the microcrack tip, and hydrogen promotes the transition from microcrack to microvoid. Li [32] also found that hydrogen will promote the emission of dislocations from the crack tip.
4.2.3. Hydrogen increase the stability of mierovoid. In the above section, we discussed the mechanism of hydrogen promoting microvoid nucleation from the local stress condition. In the following, the energetic condition or the stability of microvoid will be analyzed. Lim [33] calculated the change in Helmholtz free energy of the system on the formation of an equilibrium shaped cavity. In the same way, we give the change in Helmholtz free energy of the system on the
If the microvoid is not nucleated at the second phase particles, the ~'B= 0. Substituting a, = 6m, V~, A,, A2, V2 into equation (4) and let d A G / d r = 0, we can obtain the critical radius of microvoid nucleation as follows rc -
67E - 2Ltr 2 3Err
(7)
If the specimen is charged with hydrogen, then hydrogen will be accumulated by stress induced diffusion at the high stress concentration. Once the microcrack is nucleated and blunted as a microvoid, it will become an irreversible trap capturing a lot of hydrogen atoms, and molecular hydrogens are formed. There is an additional pressure Pn in the microvoid. At this time, the cr in the first term in equation (4) should be replaced by tr + Pn. In another aspect, hydrogen makes ~ decrease, so the ~ in equation (4) should be replaced by ~(H). In the same way, we can obtain the critical microvoid radius for specimens charged with hydrogen as follows
3732
JIANG et al.: HYDROGEN INDUCED VOID NUCLEATION 6), (H)E - 2La 2 re(H) =
3E(tr + P . )
(8)
Comparing equations (7) and (8), ~ , ( H ) < y , and a + PH > tr, so re (H) < r c, which means that hydrogen makes the critical microvoid radius decrease, and hydrogen increases the stability of microvoid. 5. CONCLUSIONS 1. In the present experimental condition, it was observed that hydrogen p r o m o t e d void nucleation during tensile deformation of 310 stainless steel. 2. The model proposed in this paper indicates that hydrogen may induce microvoid nucleation by promoting not only the microcrack nucleation but also the transition from microcrack to microvoid; hydrogen may increase the stability of microvoid by forming hydrogen pressure in the microvoid and by decreasing the void surface energy. The prediction of the model is basically consistent with the present observations and many other experimental observation results. Acknowledgements--This work was supported by National
Science Foundation and Postdoctoral Science Foundation in China. REFERENCES
1. T. D. Lee, T. Goldenberg and J. P, Hirth, Metall. Trans. 10A, 199 (1979). 2. R. A. Oriani and P. H. Josephic, Acta metall. 27, 997 (1979). 3. T. D. Lee and I. M. Bernstein, Acta metall, mater. 39, 363 (1991). 4. J. P. Hirth, in Hydrogen Effects on Material Behavior (edited by N. R. Nvi and A. W. Thompson), p. 677. TMS-AIME (1990). 5. I. G. Park and A. W. Thompson, Metall. Trans. 21A, 465 (1990).
6. R. Garber, I. M. Bernstein and A. W. Thompson, Scripta metall. 10, 341 (1976). 7. R. Garber, I. M. Bernstein and A. W. Thompson, Metall. Trans. 12A, 225 (1981). 8. Dong-IL Kwon and R. J. Asaro, Acta metall, mater. 38, 1595 (1990). 9. C. D. Beachem, Metall. Trans. 3, 437 (1972). 10. S. P. Lynch, Scripta metall. 13, 1051 (1979). 11. I. M. Robertson and H. K. Birnbaum, Acta metall. 34, 353 (1986). 12. G. M. Bond, I. M. Robertson and H. K. Birnbaum, Acta metall. 35, 2289 (1987). 13. G. M. Bond, I. M. Roberston and H. K. Birnbaum, Acta metall. 36, 2193 (1988). 14. D. S. Shih, I. M. Robertson and H. K. Birnbaum, Acta metall. 36, 111 (1988). 15. G. M. Bond, I. M. Robertson and H. K. Birnbaum, Acta metall. 37, 1407 (1989). 16. R. A. Oriani and P. H. Josephic, Acta metall 22, 1065 (1974). 17. N. T. Petch, Phil. Mag. 1, 331 (1956). 18. A. S. Tetelamn and W. D. Robertson, Trans. AIME 224, 775 (1962). 19. R. Page and J. R. Weetman, Acta metall. 9, 527 (1981). 20. A. S. Argon, J. Im and R. Safoglu, Metall. Trans. 6A, 825 (1975). 21. H. G. F. Wilsdorf, Acta metall. 30, 1247 (1982). 22. I. Y. Chan and H. G. F. Wilsdorf, Acta metall. 29, 1221 (1981). 23. K. Jagannadham, H. G. F. Wilsdorf, Mater. Sci. Engng 81, 273 (1986). 24. K. Nakase and I. M. Bernstein, Metall. Trans. 19A, 2819 (1988). 25. M. S. Loveday and B. F. Dyson, Acta metall. 31, 397 (1983). 26. E. Smith, J. T. Barnby, Met. Sci. J. 1, 56 (1967). 27. Y. B. Wang, W. Y. Chu, J. M. Xiao, Metall. Trans. 19A, 1335 (1988). 28. T. Y. Zhang, W. Y. Chu and J. M. Xiao, Science in China 29, 1157 (1986). 29. S. M. Ohr, Mater. Sci. Engng 72, 1 (1985). 30. J. R. Rice and R. Thompson, Phil. Mag. 29, 73 (1974). 31. Z. M. Ji, Z. Zhou, Z. M. Liu, Mater. Sci. Progr. 3, 331 (1989) (in Chinese). 32. J. C. M. Li, Scripta metall. 20, 371 (1986). 33. L. C. Lim, Acta metall. 35, 1663 (1987). 34. A. N. Stroh, Proc. R. Soc. A232, 548 (1955). 35. R. Raj and M. F. Ashby, Acta metall. 23, 653 (1975).
Acta metall, mater. Vol. 43, No. 10, pp. 3727-3732, 1995
Pergamon
0956-7151(95)00073-9
HYDROGEN
ElsevierScienceLtd Copyright© 1995Acta MetallurgicaInc. Printed in Great Britain.All rights reserved 0956-7151/95 $9.50+ 0.00
I N D U C E D VOID N U C L E A T I O N OF 310 STAINLESS STEEL
XING-GANG J I A N G t , WU-YANG CHU and J I - M E I XIAO Department of Materials Physics, University of Science and Technology Beijing, Beijing 100083, P.R. China (Received 6 May 1994; in revised form 16 January 1995)
Abstract--Hydrogen induced void nucleation of 310 stainless steel was investigated. Experimental results indicated that hydrogen promoted void nucleation. A new model of hydrogen induced void nucleation was proposed. The basic idea of this model is that hydrogen induces void nucleation not only by promoting microcrack nucleation but also by promoting the transition of microcrack to microvoid; hydrogen also increases the stability of a microvoid by forming hydrogen pressure in the microvoid and by decreasing the void surface energy.
1. INTRODUCTION Many experiments observed up to now have shown that hydrogen may induce ductile failure of intermediate and low strength steels [1-4]. Although there are many researches about the mechanism of hydrogen induced ductile failure, the mechanism of hydrogen induced void nucleation is not clear. In the first, there are two different viewpoints about whether hydrogen promotes void nucleation. On the one hand, Thompson et al. [5-7] suggested that hydrogen promoted void growth and interlinkage but it had almost no effect on void nucleation. On the other hand, Kwon and Asaro [8] observed hydrogen effects on void nucleation in tensile specimens of spheroidized 1518 steel. Unlike Thompson et al., however, they concluded that hydrogen promoted void nucleation at average-sized carbide particles by reducing the critical interfacial strength. Lee et al. [1] and Lee and Bernstein [3] also found that hydrogen enhanced void initiation along the characteristic slip line field. In the second, Kwon and Asaro [8] have proposed a model, based on the assumption that all the voids were nucleated at the second phase particles, to explain the reason that hydrogen promotes void nucleation. However, many experimental observations indicate that the second phase particles may facilitate but may not be essential for nucleation of voids. The above different reports about the hydrogen effect on void nucleation indicate that further investigation of this problem should be carried out. The first objective of the present investigation is to do an experimental observation of the effect of tPresent address: School of Materials Science & Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245, U.S.A.
hydrogen on void nucleation behavior during the tensile test of 310 stainless steel. In-situ TEM observation was performed on the thin foils of 310 stainless steel that precharged hydrogen to observe the process of hydrogen induced void nucleation. As is well known, a number of different models of hydrogen-induced cracking have been proposed by numerous investigators. For example, hydrogen induced local deformation theories [9, 10], which have been developed from an original idea proposed by Beachem [9] that hydrogen increases the crack tip ductility. The key points of this mechanism are that hydrogen promotes the dislocation movement; crack growth in the presence of hydrogen is because of the reduction of local flow stress and the increase in activity of dislocations in the high stress regions ahead of the crack tip. The mechanisms were supported by a series of in situ TEM observation evidences [11-15]; the decohesion theory [16], proposed originally by Oriani and Josephic [16], suggests that hydrogen may concentrate in a very high stress region of a few atomic distances ahead of cracks and lower the cohesive strength of the lattice or interface thereby facilitating "decohesion"; the adsorption theory [17], proposed by Petch et al. suggests that adsorbed hydrogen at the crack tip lowers the surface energy of newly formed fracture surfaces so that the overall fracture surface energy is lowered [17]; the pressure theory [18] was based on the fact that hydrogen precipitated at internal surfaces or voids thereby producing high gas pressures. Up to now, these theories have successfully explained many experimental results. However, in some cases, it is quite possible that these mechanisms are cooperative. The second objective of the paper is to propose a new model of hydrogen induced void nucleation by cooperation of all these theories.
3727
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JIANG et al.: HYDROGEN INDUCED VOID NUCLEATION 2. EXPERIMENTAL PROCEDURE
The material used in this study is AISI 310 stable austenitic stainless steel. The chemical compositions of the alloy are: 24.48 Cr, 21.33 Ni, 0.23 Nb, 0.50 Si, 0.60 Mn, 0.08 Mo, 0.08 C (wt%). The tensile specimens were prepared from cold rolled foils (about 0.25 mm thick). The specimens were annealed in an argon atmosphere at 1050°C for about 20rain and quenched into oil. The anneal established the grain size of 15/~m. Some specimens were precharged in a 1 N H 2SO 4 solution containing 0.25 g/1 ofAs203, for 15 h, others were not charged with hydrogen during tensile test. The current density was 103 A/m 2. The average concentration of hydrogen in the specimen is about 80 wppm. The hydrogen contents were determined by hot extraction and volumetric measurement using specimens taken from the same charged specimen. After precharging, the specimens were tensile tested at room temperature and at strain rate of 2 x 10 -4 s -1, while the same hydrogen charged condition as used in the precharging was maintained. In order to demonstrate that these effects of hydrogen charging were reversible, the specimens were charged at the same condition and outgassed prior to testing. The stress-strain curves were identical to those of uncharged specimens, within the normally observed scatter for such tests ( + 5%). Because the width of the specimen is 8 mm, which is much larger than the thickness (0.25 mm) of the specimen, buckling or out-of-plane bending can be avoided in tensile tests by fixing two pairs of back-up alloy plates at the two heads of specimens. An optical microscope was used to observe void formation after testing. The $250 M K I I type scanning electron microscope was used to observe the fracture surface. In order to observe the detail of the physical process of microvoid nucleation, in-situ high voltage TEM observation was accomplished on the thin foils of the alloy precharged hydrogen under the same condition as above. The voltage is I000 kV, the use of high accelerating voltages permitted the observation of foils up to 1.5 # m thick without a loss in resolution [19]. Thin foils were prepared by using an electrolytic polishing solution containing 5 parts perchloric acid and 95 parts methanol at 243 K. After perforation, the foils were stored in liquid nitrogen to keep the hydrogen content in the specimen. The foils were deformed in a tensile specimen holder that was displacement controlled. As the stress was applied, cracks were initiated at the edge of the hole, which served as a notch, and propagated in the direction perpendicular to the tensile stress. Some hydrogen will move out from the surface of the foil due to high vacuum environment. So in-situ TEM observation of the void nucleation should be finished as fast as possible. In the present experiment, we finished the in-situ test within 2 h. Because 310
stainless steel has f.c.c, structure, the diffusion coefficient is very low, even though some hydrogen will move out from the surface of the foil, we can make sure that there is still a lot of hydrogen in the thin foil specimen within 2 h.
3. EXPERIMENTAL RESULTS
3.1. Observation failure.
of hydrogen promoting
ductile
Figures l(a) and (b) are the optical observations of void formation after 18% of strain for the undercharged and charged specimens respectively. It is shown that very few voids were nucleated for the uncharged specimen. However, a lot of voids were formed for the charged specimen. Figure 1 demonstrates that hydrogen promotes void nucleation and growth during tensile deformation. Figures 2(a) and (b) are the SEM observations of fracture surfaces of the uncharged and charged specimens. It is shown that both charged and uncharged specimens were fractured in a ductile manner, there
Fig. 1. Optical observation of void nucleation after 18% of strain for (a) the uncharged specimen and (b) the charged specimen.
JIANG et al.: HYDROGEN INDUCED VOID NUCLEATION
3729
Fig. 4. TEM observation of microstructure before tensile stress was applied.
Fig. 2. SEM observation of fracture surface of (a) the uncharged specimen and (b) the charged specimen. is not a significant difference in the morphology of the fracture surfaces of the specimens. The final elongation for the specimen uncharged and charged with hydrogen was 40 and 20% respectively. It is difficult to estimate whether hydrogen promotes void nucleation from the fracture surface of the specimen. However, from the optical observation results, we found that hydrogen may promote premature ductile failure of the specimen by promoting void nucleation and growth.
3.2. In situ T E M observation o f microvoid nucleation The above observations indicate that all the specimens which were charged or uncharged with hydrogen fractured in a ductile manner. In the following, we will observe in detail the physical process of
Fig. 3. TEM observation of the perforation before tensile stress was applied.
microvoid nucleation during the deformation of 310 stainless steel pre-charged with hydrogen. Figure 3 is the low magnification TEM observation of the thin foil before tensile stress is applied. When the tensile stress is applied, we will pay much more attention to the cracks which are almost perpendicular to the tensile stress. Figure 4 is the microstructural observation of the specimen before deformation. Very few particles were observed and no pre-initiated microvoids caused by hydrogen charging were found. Figure 5 is the observation of the initial stage of microcrack nucleation when the tensile stress is applied. It is shown that a microcrack was nucleated ahead of a main crack. Figure 6 is the observation of microvoid nucleation. The microvoids were formed by blunting the microcrack tips. The above TEM observations indicate that a microvoid may nucleate from a microcrack when the crack tip is blunted; many void nucleation sites appear not correlated with the second phase particles. 4. DISCUSSION
4.1. Experimental observations o f hydrogen promoting void nucleation 4.1.1. The effect o f second phase particles on void
Fig. 5. TEM observation ofmicrocrack nucleation ahead of a main crack.
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JIANG et aL: HYDROGEN INDUCED VOID NUCLEATION movement and void nucleation process. Further detailed investigations should be done in future. 4.2. A theoretical model for hydrogen promoting void nucleation
Fig. 6. TEM observation of microvoid nucleation ahead of a main crack. nucleation. Kwon and Asaro [8] have analyzed the reason of hydrogen promoting void nucleation. Their theory is based on Argon's early idea [20] that voids were generally nucleated at particles. They believed that hydrogen promoted void nucleation at the carbide particles by reducing the critical interfacial strength. However, the present TEM observations and other observations [11, 21-23] indicated that void nucleation is not correlated with the second phase particles, and voids were found to nucleate at either the intersection of active slip bands or at the dislocation cell walls which act as dislocation sources. Detailed observation of void nucleation process indicated that the second phase particles were not a prequisite for void nucleation; microcracks first nucleated because of stress concentration; microvoids were often formed by blunting the microcrack tips. The observations of void nucleation in other materials also indicated the above behavior [24, 25]. 4.1.2. The effect o f hydrogen on void nucleation. Birnbaum et al. [11-15] have observed systematically the effect of hydrogen on deformation and void nucleation in nickel, high purity A1, etc. They found the advance of transgranular cracks in a hydrogen environment occurred by either direct emission of dislocations from the crack tip or by a complex process of void nucleation and growth ahead of the crack. The effect of hydrogen is to decrease the stress required for crack advance and to localize the deformation. Their observations are strong supports to the hydrogen induced local deformation theory. They also found that hydrogen promoted void nucleation. In the present investigation, the TEM observation technique is different from Birnbaum's environmental cell TEM techniques, and the material observed is also different. However, the observation result that hydrogen promotes microvoid nucleation is basically consistent with their results. It should be mentioned that the present experimental condition is limited, and because some hydrogen may move out during in-situ TEM observation, so it is very difficult to give the exact content of hydrogen in the thin foil that affects dislocation
4.2.1. Hydrogen promotes microcrack nucleation. The present and many other TEM observations showed that the microvoid was nucleated by blunting the microcrack tip. For a smooth tensile specimen, the dislocation pile-ups are formed by local deformation. Figure 7 is the TEM observation of a dislocation pile-ups after 10% of strain of the charged specimen during tensile deformation of 310 stainless steel. When the stress concentration ahead of dislocation pile-ups is sufficiently large, the atomic bonds will be ruptured. The initiation condition for a microcrack coinciding with the slip plane at the head of pile-ups is [26] zc = zo + [2G~ /n (1 - v )L ]1/2
(1)
where z¢ is the critical stress of microcrack nucleation, z0 is the lattice resistance of dislocation moving, G is the shear modulus, 7 is the surface energy, v is Poisson's ratio and L is the length of dislocation pile-ups. When the specimen was charged with hydrogen during deformation, the hydrogen promotes, the local plastic deformation and decreases the atomic bond strength, which results in a decrease of ~c, that is zc (H) < zc. First, the strain energy of dislocations is reduced because the segregation of hydrogen and dislocations will move along with a hydrogen atmosphere [27], as a result, the friction stress necessary to operate the F r a n k - R e a d source and forming pile-ups of dislocations will be decreased from z0 to z0 (H); secondly, an additional force will act on dislocations and help the external stress to make the dislocations multiply and move. The resultant stress on the
Fig. 7. The formation of a dislocation pile-up during deformation of the charged specimen.
JIANG et al.: HYDROGEN INDUCED VOID NUCLEATION dislocations surrounded by the hydrogen atmosphere is r (H) = k%, where k > 1, k is a function of hydrogen concentration, yields strength and temperature [28]. Therefore, the local effective shear stress will be z(H) - z0 = k z c - r0(H), instead of r~-- %. In the third aspect, hydrogen will decrease 7 to 7(H) [16, 17]. So the critical stress of microcrack initiation for specimens charged with hydrogen is rc(n) = ¼{z0(H) + [2GT(H)/~(1 - v)L]m}.
(2)
Comparing equations (1) and (2), we know that %(H) < zc, which indicates that hydrogen promotes microcrack nucleation.
4.2.2. Hydrogen promotes the transition of microcrack to microvoid. When the microcrack is nucleated, if it propagates in a stable manner and the crack tip does not blunt, then the specimen will be fractured in a brittle manner; if the microcrack tip is blunted by emitting edge dislocations, then a microvoid will be formed and the specimen will be fractured in a ductile manner. In the present investigation, TEM observations indicated that the microcrack tip was blunted during deformation of 310 stainless steel either charged or uncharged with hydrogen, which is consistent with many other TEM observation results [11-13]. In the following, we analyze the reason that hydrogen promotes microcrack bluntness in the present deformation condition. In simplicity, suppose the microcrack is a type I crack, then according to literature [29, 30] we obtain the critical stress intensity factor for edge dislocations emitted from the crack tip, which blunted the crack tip
2
[
Gb
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formation of an equilibrium shaped microvoid as follows
AG = - V i a + ( A l y -- A2YB)-- V2a2n/2E
(4)
where V~ is the volume of the microvoid, tr is the applied stress, A1 is the surface area of the microvoid, A2 is the interphase area of the void and the second phase particle, ?a is the boundary or interface energy per unit area, V: is the volume within which the strain energy is relaxed on the formation of the microvoid, trn is the local stress on the void by the group of dislocation piled-up, and E is the elastic modulus. The first term in equation (4) is the work done by the applied stress on the system. The second and third terms are respectively the change in surface and interface energies, and the fourth term the reduction of strain energy in the system. According to Stroh's calculation [34], the maximum stress of the group of piled-up dislocations is
am
x/3 \ x J
(5)
where x is a distance from the head of the piled up group to the microvoid nucleation site, L is the length of the dislocation occupy. Assuming r is the microvoid radius for nucleation, then the average value of the stress may be written as 6m= r
0amdx
:
x/3\r]
.
(6)
In simplicity, we assume tr, = 6m, according to Lim [33], Raj and Ashby's results [35] V1
:
rS27r(2 -- 3 cos ~ + cos 3~)/3, = cos-I(~B/2?),
K1e - sin • cos(¢I)/2) (1 - v)(8nr~) 1/2
A~ = r24n(1 -- cos ct), A2 = r2n sin 2:t, + (2rcrc)l/2(o.r_+ r~Z--{~)JB sin ~ ' ~ ]
(3) v2 = 3vl/2.
where B = 2~,fl/g, fl = rce3/2/2, rc is the core radius of a dislocation, • is an angle of the slip plane with the crack plane, b is the Burgus vector, trt is the lattice friction stress. Because hydrogen will decrease 7 [16, 17] and rrf [31], the above equation indicates that KEo(H) < K~e, which means hydrogen promotes edge dislocations emitted from the microcrack tip, and hydrogen promotes the transition from microcrack to microvoid. Li [32] also found that hydrogen will promote the emission of dislocations from the crack tip.
4.2.3. Hydrogen increase the stability of mierovoid. In the above section, we discussed the mechanism of hydrogen promoting microvoid nucleation from the local stress condition. In the following, the energetic condition or the stability of microvoid will be analyzed. Lim [33] calculated the change in Helmholtz free energy of the system on the formation of an equilibrium shaped cavity. In the same way, we give the change in Helmholtz free energy of the system on the
If the microvoid is not nucleated at the second phase particles, the ~'B= 0. Substituting a, = 6m, V~, A,, A2, V2 into equation (4) and let d A G / d r = 0, we can obtain the critical radius of microvoid nucleation as follows rc -
67E - 2Ltr 2 3Err
(7)
If the specimen is charged with hydrogen, then hydrogen will be accumulated by stress induced diffusion at the high stress concentration. Once the microcrack is nucleated and blunted as a microvoid, it will become an irreversible trap capturing a lot of hydrogen atoms, and molecular hydrogens are formed. There is an additional pressure Pn in the microvoid. At this time, the cr in the first term in equation (4) should be replaced by tr + Pn. In another aspect, hydrogen makes ~ decrease, so the ~ in equation (4) should be replaced by ~(H). In the same way, we can obtain the critical microvoid radius for specimens charged with hydrogen as follows
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JIANG et al.: HYDROGEN INDUCED VOID NUCLEATION 6), (H)E - 2La 2 re(H) =
3E(tr + P . )
(8)
Comparing equations (7) and (8), ~ , ( H ) < y , and a + PH > tr, so re (H) < r c, which means that hydrogen makes the critical microvoid radius decrease, and hydrogen increases the stability of microvoid. 5. CONCLUSIONS 1. In the present experimental condition, it was observed that hydrogen p r o m o t e d void nucleation during tensile deformation of 310 stainless steel. 2. The model proposed in this paper indicates that hydrogen may induce microvoid nucleation by promoting not only the microcrack nucleation but also the transition from microcrack to microvoid; hydrogen may increase the stability of microvoid by forming hydrogen pressure in the microvoid and by decreasing the void surface energy. The prediction of the model is basically consistent with the present observations and many other experimental observation results. Acknowledgements--This work was supported by National
Science Foundation and Postdoctoral Science Foundation in China. REFERENCES
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