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Amorphization associated with crack propagation in hydrogen-charged steel
Scripta Materialia 49 (2003) 837–842 www.actamat-journals.com
Amorphization associated with crack propagation in hydrogen-charged steel M. Nagumo a
a,*
, T. Ishikawa a, T. Endoh a, Y. Inoue
b
Department of Materials Science and Engineering, Waseda University, Okubo 3-4-1, Shinjuku, Tokyo 169-8555, Japan b Research Department, Nissan ARC Ltd., Matsushima-cho 1, Yokosuka 237-0061, Japan Received 23 June 2003; received in revised form 15 July 2003; accepted 23 July 2003
Abstract Amorphization, following a decrease in the dislocation density and fragmentation of the matrix, has been revealed in a thin layer just below the tensile-fractured surface of a hydrogen-charged ferritic steel. The creation of a high density of vacancies associated with hydrogen is discussed as the origin. 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Microscopy; Metastable materials; Ferritic steel; Hydrogen embrittlement; Vacancies
1. Introduction Ductile crack growth resistance is generally determined by the distribution of nucleating voids and the stress state in the region ahead of a crack. Void nucleation without participation of secondphase particles was observed to take place at microstructural heterogeneities such as twin and grain boundaries [1] or dislocation cell walls [2]. As the mechanism for this, Lyles and Wilsdorf [3] suggested the condensation of vacancies, the feasibility of which was theoretically shown by Cutti no and Ortiz [4]. Experimentally, vacancy clusters leading to nanovoid formation in the dislocation-free zone ahead of a crack tip were observed by Chen et al. [5]
* Corresponding author. Present address: Materials Science and Engineering, Waseda University, Konodai 1-8-10, Ichikawa, Chiba 272-0827, Japan. Tel./fax: +81-47-372-1862. E-mail address: [email protected] (M. Nagumo).
in an austenitic stainless steel by means of in situ transmission electron microscopy (TEM). On the other hand, stress-induced amorphization at a moving crack tip was observed by Okamoto et al. in a NiTi alloy [6], and the mechanism was discussed from a thermodynamics viewpoint. The involvement of strain-induced vacancies in ductile crack growth resistance in low-carbon ferritic steels has been shown by Nagumo et al. [7] using a thermal desorption analysis of hydrogen introduced as a probe of defects, but the microscopic process of fracture has been a subject for which direct evidence has yet to be presented. A closely related issue is the mechanism of hydrogen-related failure. It has been found that a vacancy density far exceeding thermal equilibrium values is created under a high pressure and high temperature hydrogen atmosphere, resulting from hydrogen–vacancy interactions that reduce the effective formation energy of vacancies [8–10]. Cathodic electrolysis also creates a high density of
1359-6462/$ - see front matter 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/S1359-6462(03)00469-X
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M. Nagumo et al. / Scripta Materialia 49 (2003) 837–842
vacancy–hydrogen complexes [11]. Plastic deformation creates a high density of vacancies [12], which is also enhanced in the presence of hydrogen [13]. As a feature of hydrogen-related failure, a decrease in ductile crack growth resistance has been shown to be associated with increased nucleation voids [14]. A new model [13] we have proposed for the mechanism of hydrogen-related failure is that the clustering of strain-induced vacancies, rather than hydrogen itself, leads to premature crack nucleation and a reduction in crack growth resistance. When the atomic ratio of hydrogen to the trapping vacancies is close to unity [11], the normally observed average hydrogen concentration of 5 · 105 in atomic ratio implies the presence of hydrogen and associated vacancy concentrations that may be higher by more than one order of magnitude in a strain concentrated area. It is estimated that vacancies are involved in the fracture process in two ways: one is the formation of microvoids and the other is amorphization due to lattice instability [15]. In this paper, we report the first observation of amorphization associated with crack propagation in hydrogen-related failure in a ferritic steel.
2. Experimental procedures The material used was a low-carbon ferritic steel (C:0.06, Si:0.49, Mn:0.07, Ni:2.01, Al:0.05, S:0.0028 in wt%), the fracture behavior of which has been precisely examined previously [7]. Flat specimens 2 mm in thickness and 6 mm in width were tensile-strained to fracture at a strain rate of 3 · 105 s1 under concurrent cathodic hydrogencharging with a current density of 1 mA/cm2 in a 3% NaCl + 3 g/l NH4 SCN aqueous solution after pre-charging for 12 h in order to give the specimens a uniform distribution of hydrogen. After depositing a C coating on the fracture surface, specimens for transmission electron microscopy (TEM) were prepared from just beneath the fracture surface by means of a focused ion beam (FIB) method using Ga ions energized at 30 kV. The orientation of a TEM specimen is illustrated in Fig. 1. Prior to the FIB thinning, an additional coating of W was de-
Fig. 1. Directions of sampling TEM specimens using an FIB method.
posited in the FIB chamber. TEM observations were conducted with a 300 kV electron microscope (Hitachi H-9000). For the observation of lattice images, a Tecnai G2 F20S-TWIN (FEI Co.) field emission transmission electron microscope operated at 200 kV was used. The microscope was equipped with a Gatan imaging filter (GIF) set to zero-loss filtering with a 10 eV energy window to obtain good lattice images.
3. Experimental results The yield stress of 310 MPa and following flow stress of a hydrogen-charged specimen were almost coincident with those of a non-charged specimen except the elongation to fracture decreased from 33% to 19%, confirming degradation by hydrogen. Fig. 2 shows TEM micrographs of an area just beneath the fracture surface of a hydrogen-charged specimen. Fig. 2(a) shows a layer with grey contrast along the surface over a thickness of less than 1 lm. In the neighboring internal area, the dislocation density was substantially high, developing cell structures parallel to the surface. The selected area electron diffraction (SAD) from the encircled area in the grey layer showed halo rings as shown in Fig. 2(b). The distance from the center to the first strong halo ring was 0.250 nm in terms of the atomic distance, but the value varied by sites in the grey contrast area from 0.247 to 0.250 nm. The next outer ring was coincident with the diffraction from f1 1 0g planes of bcc Fe. A magnified view shown in Fig. 2(c) revealed featureless blocky areas associated with
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Fig. 2. (a) Amorphous zone that developed beneath the fracture surface; (b) the electron diffraction pattern from the encircled area in (a); (c) magnified image of (a); (d) dark-field image of (c) from the part indicated in the diffraction pattern.
random-oriented dislocation lines, indicating a disappearance of cell structures and a decrease in dislocation density. The sites and contrast of the blocky areas hardly changed on tilting the specimen. A dark-field image, Fig. 2(d), from a part on the halo diffraction ring, indicated in Fig. 2(b), showed fine nm-scale bright patches within the grey background. Different parts on the ring gave similar bright patches in different sites. The dark areas in Fig. 2(d) correspond to the featureless areas in Fig. 2(c) and are likely due to thinning of the specimen. The lattice image thereof using the central spot and all halo rings revealed a disordered distribution of atoms as shown in Fig. 3. The findings imply that amorphization occurred associated with the crack growth. Fig. 4(a) is a bright-field image of another area in the extension of the grey contrast layer in Fig. 2(a), and the width of the layer covers a few dislocation cells. A dark-field image, Fig. 4(b), from a part including a spot on the SAD pattern is similar
Fig. 3. Lattice image of the area shown in Fig. 2(c).
to that in Fig. 2(d), showing dispersed bright crystalline patches. In this case, the distance to the first ring consisting of halo and spotty patterns was coincident with d1 1 0 of bcc Fe. The SAD pattern in Fig. 4 implies a stage in which fragmentation of the Fe matrix takes place and
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Fig. 4. (a) Bright-field image of an area in the extension of the grey zone shown in Fig. 1(a) and (b) dark-field image from the part indicated in the diffraction pattern.
crystallinity remains more substantially than in the stage shown in Fig. 2. A similar featureless phase was also occasionally observed apart from the fracture surface. Fig. 5 shows one such phase observed at about 1 lm below the fracture surface, with a successive tilting of the specimen to X:1.88 and Y:)7.00. The size of the phase is coincident with a dislocation cell. The grey featureless image remained on tilting the specimen, while the contrast of neighboring dislocation cells changed noticeably. The SAD pattern from the bright layer was spotty without halo rings. Thus, the grey image may result from a low
dislocation density in the area, prior to fragmentation of the matrix crystal and amorphization. On the side surface of the fractured specimen, many short cracks appeared transverse to the tensile axis near the fracture surface. Fig. 6 shows TEM micrographs of the front area of a side crack. Similar to the fracture subsurface, a grey featureless zone developed in the crack front where the dislocation density was substantially higher. The SAD pattern shown in the inset was with halo rings similar to those in Fig. 2 and a dark-field image clearly shows amorphization therein. The estimated plastic zone size, assuming a crack
Fig. 5. (a) A grey area that appeared at about 1 lm below the fracture surface; (b) magnified image of the grey area, and (c) the same area with a successive tilting of the specimen to X:1.88 and Y:)7.00.
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Fig. 6. (a) A grey area that developed at the front of a small side crack and (b) dark-field image of the same area as (a) from the part indicated in the diffraction pattern.
length of 10 lm, is about 2 lm, indicating that the grey zone is within the plastic zone. The appearance of the featureless phase in restricted areas as shown in Figs. 5 and 6 implies that the formation of the phase is not due to damage or deposition of the W coating associated with sample preparation using FIB. Such formations of bright layers beneath the fracture surface or along dislocation cell walls were not observed in specimens tensile-fractured without hydrogencharging.
4. Discussion The structural changes observed in the present study show that fragmentation of the Fe matrix and subsequent amorphization take place in a strain-concentrated area, associated with crack propagation in the presence of hydrogen. That the halo diffraction pattern in Fig. 2(b) is not solely due to grain refinement is supported by the lattice image shown in Fig. 3. It is noticed, however, that the location of the first strong halo ring was not coincident with d1 1 0 of bcc Fe. It implies that the nearest neighbor configuration in bcc Fe was not conserved in the progress of amorphization, but some lattice expansion might have taken place. The atomic configuration producing the halo is not definite at present, but it might evolve from hydrogen–vacancy interactions. A substantial volume expansion we have previously observed in some Fe-base amorphous alloys when hydrogen was introduced [16] presumably took place in the
present case. However, the strong halo ring shown in Fig. 2(b) contained some spotty patterns, and fine nm-scale bright patches were observed in the dark-field image in Fig. 2(d). Iron hydrides observed under high pressure environments [17,18] accompanying expansion of molar volume, are not feasible in the present case, but some ordering involving hydrogen might have taken place. Solid-state amorphization of metallic materials has been observed in mechanical alloying [19,20], hydrogenation of intermetallic compounds [21,22] or during the eutectoid reaction of a Ti–Al alloy [23]. In the last case, an increased vacancy density associated with the phase separation accompanying different Al stoichiometric concentrations has been assumed as the cause of amorphization. Amorphization associated with vacancy generation has been observed in various intermetallic compounds by high energy electron or ion irradiation [24]. If we assume that the increase in vacancy density to a critical level causes lattice instability, the increased vacancy density due to plastic straining and binding with hydrogen could reasonably induce amorphization. The mechanism of vacancy generation associated with plastic straining is due to interactions between dislocations, such as dipole annihilation [25] or drag of jogs [4], which may take place preferentially in sites of a high dislocation density such as dislocation cell walls or crack front. Further, dislocation dynamics associated with crack growth may alter local dislocation configurations, destructing cell structures. The fragmentation of the Fe-matrix
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and reduced dislocation density revealed in Figs. 4 and 5 are likely to show early structural changes prior to amorphization. A characteristic of the mechanical properties of the amorphous phase is plastic instability leading to strain localization, and the ductility is much reduced in the presence of hydrogen [16]. In the fracture process, the formation of the amorphous phase in the strain-concentrated area in front of the crack is expected to reduce crack growth resistance. The present findings support the vacancyclustering model [13] for hydrogen-related failure of steel.
5. Conclusions The deformation microstructures associated with crack propagation in a hydrogen-charged ferritic steel have been examined by means of transmission electron microscopy with tensiletested specimens prepared using a Focused Ion Beam method. In the area below the fracture surface, the dislocation density was substantially higher, resulting in the formation of cell structures. A decrease in the dislocation density and fragmentation of the matrix were observed to take place, leading to the evolution of the amorphous phase in a layer less than 1 lm in thickness adjacent to the surface. Similar structural alterations were also observed eventually within the matrix and in the front of a small crack formed on the side surface of the specimen near the fracture surface. The location of the halo rings suggested that the atomic configuration in the amorphous phase is associated with some lattice expansion. The creation of a high density of vacancies with the aid of hydrogen is likely the cause of amorphization due to lattice instability and decreased crack growth resistance. The present results may be the first observations ever obtained of amorphization associated with crack growth in hydrogen-related failure of steel, supporting the vacancy-clustering model.
Acknowledgements The authors would like to thank Dr. H. Morikawa, formerly with Nippon Steel Corp., and Prof. T. Hirotsu, Osaka University, for their valuable discussion. The present study has been conducted as part of a project by the Special Coordination Fund for Promoting Science and Technology of the Ministry of Education, Culture, Sports, Science and Technology of Japan.
References [1] Van Stone RH, Cox TB, Low Jr JR, Psioda JA. Int Met Rev 1985;30:157. [2] Gardner RN, Wilsdorf HGF. Metall Trans A 1980;11A:659. [3] Lyles Jr RL, Wilsdorf HGF. Acta Metall 1975;23:269. [4] Cutti no AM, Ortiz M. Acta Mater 1996;44:427. [5] Chen QZ, Chu WY, Wang YB, Hsiao CM. Acta Mater 1995;43:4371. [6] Okamoto PR, Heuer JK, Lam NQ, Ohnuki S, Matsukawa Y, Tozawa K, et al. Appl Phys Lett 1998;73:473. [7] Nagumo M, Yagi T, Saitoh H. Acta Mater 2000;48:943. [8] Fukai Y, Okuma N. Phys Rev Lett 1994;73:1640. [9] Gavriljuk VG, Bugaev VN, Petrov YuN, Tarasenko AV, Yanchitski BZ. Scr Mater 1996;34:903. [10] McLellan RB, Xu ZR. Scr Mater 1997;36:1201. [11] Birnbaum HK, Buckley C, Zeides F, Sirois E, Rozenak P, Spooner S, et al. J Alloys Compd 1997;253–254:260. [12] Nagumo M, Ohta K, Saitoh H. Scr Mater 1999;40:313. [13] Nagumo M, Nakamura M, Takai K. Metall Mater Trans A 2001;32A:339. [14] Nagumo M, Yoshida H, Shimomura Y, Kadokura T. Mater Trans 2001;42:132. [15] Cahn RW. Nature 1978;273:491. [16] Nagumo M, Takahashi T. Mater Sci Eng 1976;23:257. [17] Badding JV, Hemley RJ, Mao HK. Science 1991;253:421. [18] Fukai Y, Yamakata M. Z Phys Chem 1993;179:119. [19] Schwarz RB, Johnson WL. Phys Rev Lett 1983;51:415. [20] Koch CC, Cavin OB, McKaney CG, Scarbrough JG. Appl Phys Lett 1983;43:1017. [21] Yeh XL, Samwer K, Johnson WL. App Phys Lett 1983;42:242. [22] Aoki K, Li XG, Hirata T, Matsubara E, Waseda Y, Masumoto T. Acta Metall Mater 1993;41:1523. [23] Tanimura M, Inoue Y, Koyama Y. Phys Rev B 1995;52:15239. [24] Limoge Y, Barbu A. Phys Rev B 1984;30:2212. [25] Essmann U, Mughrabi H. Philos Mag 1979;40:731.
Amorphization associated with crack propagation in hydrogen-charged steel M. Nagumo a
a,*
, T. Ishikawa a, T. Endoh a, Y. Inoue
b
Department of Materials Science and Engineering, Waseda University, Okubo 3-4-1, Shinjuku, Tokyo 169-8555, Japan b Research Department, Nissan ARC Ltd., Matsushima-cho 1, Yokosuka 237-0061, Japan Received 23 June 2003; received in revised form 15 July 2003; accepted 23 July 2003
Abstract Amorphization, following a decrease in the dislocation density and fragmentation of the matrix, has been revealed in a thin layer just below the tensile-fractured surface of a hydrogen-charged ferritic steel. The creation of a high density of vacancies associated with hydrogen is discussed as the origin. 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Microscopy; Metastable materials; Ferritic steel; Hydrogen embrittlement; Vacancies
1. Introduction Ductile crack growth resistance is generally determined by the distribution of nucleating voids and the stress state in the region ahead of a crack. Void nucleation without participation of secondphase particles was observed to take place at microstructural heterogeneities such as twin and grain boundaries [1] or dislocation cell walls [2]. As the mechanism for this, Lyles and Wilsdorf [3] suggested the condensation of vacancies, the feasibility of which was theoretically shown by Cutti no and Ortiz [4]. Experimentally, vacancy clusters leading to nanovoid formation in the dislocation-free zone ahead of a crack tip were observed by Chen et al. [5]
* Corresponding author. Present address: Materials Science and Engineering, Waseda University, Konodai 1-8-10, Ichikawa, Chiba 272-0827, Japan. Tel./fax: +81-47-372-1862. E-mail address: [email protected] (M. Nagumo).
in an austenitic stainless steel by means of in situ transmission electron microscopy (TEM). On the other hand, stress-induced amorphization at a moving crack tip was observed by Okamoto et al. in a NiTi alloy [6], and the mechanism was discussed from a thermodynamics viewpoint. The involvement of strain-induced vacancies in ductile crack growth resistance in low-carbon ferritic steels has been shown by Nagumo et al. [7] using a thermal desorption analysis of hydrogen introduced as a probe of defects, but the microscopic process of fracture has been a subject for which direct evidence has yet to be presented. A closely related issue is the mechanism of hydrogen-related failure. It has been found that a vacancy density far exceeding thermal equilibrium values is created under a high pressure and high temperature hydrogen atmosphere, resulting from hydrogen–vacancy interactions that reduce the effective formation energy of vacancies [8–10]. Cathodic electrolysis also creates a high density of
1359-6462/$ - see front matter 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/S1359-6462(03)00469-X
838
M. Nagumo et al. / Scripta Materialia 49 (2003) 837–842
vacancy–hydrogen complexes [11]. Plastic deformation creates a high density of vacancies [12], which is also enhanced in the presence of hydrogen [13]. As a feature of hydrogen-related failure, a decrease in ductile crack growth resistance has been shown to be associated with increased nucleation voids [14]. A new model [13] we have proposed for the mechanism of hydrogen-related failure is that the clustering of strain-induced vacancies, rather than hydrogen itself, leads to premature crack nucleation and a reduction in crack growth resistance. When the atomic ratio of hydrogen to the trapping vacancies is close to unity [11], the normally observed average hydrogen concentration of 5 · 105 in atomic ratio implies the presence of hydrogen and associated vacancy concentrations that may be higher by more than one order of magnitude in a strain concentrated area. It is estimated that vacancies are involved in the fracture process in two ways: one is the formation of microvoids and the other is amorphization due to lattice instability [15]. In this paper, we report the first observation of amorphization associated with crack propagation in hydrogen-related failure in a ferritic steel.
2. Experimental procedures The material used was a low-carbon ferritic steel (C:0.06, Si:0.49, Mn:0.07, Ni:2.01, Al:0.05, S:0.0028 in wt%), the fracture behavior of which has been precisely examined previously [7]. Flat specimens 2 mm in thickness and 6 mm in width were tensile-strained to fracture at a strain rate of 3 · 105 s1 under concurrent cathodic hydrogencharging with a current density of 1 mA/cm2 in a 3% NaCl + 3 g/l NH4 SCN aqueous solution after pre-charging for 12 h in order to give the specimens a uniform distribution of hydrogen. After depositing a C coating on the fracture surface, specimens for transmission electron microscopy (TEM) were prepared from just beneath the fracture surface by means of a focused ion beam (FIB) method using Ga ions energized at 30 kV. The orientation of a TEM specimen is illustrated in Fig. 1. Prior to the FIB thinning, an additional coating of W was de-
Fig. 1. Directions of sampling TEM specimens using an FIB method.
posited in the FIB chamber. TEM observations were conducted with a 300 kV electron microscope (Hitachi H-9000). For the observation of lattice images, a Tecnai G2 F20S-TWIN (FEI Co.) field emission transmission electron microscope operated at 200 kV was used. The microscope was equipped with a Gatan imaging filter (GIF) set to zero-loss filtering with a 10 eV energy window to obtain good lattice images.
3. Experimental results The yield stress of 310 MPa and following flow stress of a hydrogen-charged specimen were almost coincident with those of a non-charged specimen except the elongation to fracture decreased from 33% to 19%, confirming degradation by hydrogen. Fig. 2 shows TEM micrographs of an area just beneath the fracture surface of a hydrogen-charged specimen. Fig. 2(a) shows a layer with grey contrast along the surface over a thickness of less than 1 lm. In the neighboring internal area, the dislocation density was substantially high, developing cell structures parallel to the surface. The selected area electron diffraction (SAD) from the encircled area in the grey layer showed halo rings as shown in Fig. 2(b). The distance from the center to the first strong halo ring was 0.250 nm in terms of the atomic distance, but the value varied by sites in the grey contrast area from 0.247 to 0.250 nm. The next outer ring was coincident with the diffraction from f1 1 0g planes of bcc Fe. A magnified view shown in Fig. 2(c) revealed featureless blocky areas associated with
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Fig. 2. (a) Amorphous zone that developed beneath the fracture surface; (b) the electron diffraction pattern from the encircled area in (a); (c) magnified image of (a); (d) dark-field image of (c) from the part indicated in the diffraction pattern.
random-oriented dislocation lines, indicating a disappearance of cell structures and a decrease in dislocation density. The sites and contrast of the blocky areas hardly changed on tilting the specimen. A dark-field image, Fig. 2(d), from a part on the halo diffraction ring, indicated in Fig. 2(b), showed fine nm-scale bright patches within the grey background. Different parts on the ring gave similar bright patches in different sites. The dark areas in Fig. 2(d) correspond to the featureless areas in Fig. 2(c) and are likely due to thinning of the specimen. The lattice image thereof using the central spot and all halo rings revealed a disordered distribution of atoms as shown in Fig. 3. The findings imply that amorphization occurred associated with the crack growth. Fig. 4(a) is a bright-field image of another area in the extension of the grey contrast layer in Fig. 2(a), and the width of the layer covers a few dislocation cells. A dark-field image, Fig. 4(b), from a part including a spot on the SAD pattern is similar
Fig. 3. Lattice image of the area shown in Fig. 2(c).
to that in Fig. 2(d), showing dispersed bright crystalline patches. In this case, the distance to the first ring consisting of halo and spotty patterns was coincident with d1 1 0 of bcc Fe. The SAD pattern in Fig. 4 implies a stage in which fragmentation of the Fe matrix takes place and
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Fig. 4. (a) Bright-field image of an area in the extension of the grey zone shown in Fig. 1(a) and (b) dark-field image from the part indicated in the diffraction pattern.
crystallinity remains more substantially than in the stage shown in Fig. 2. A similar featureless phase was also occasionally observed apart from the fracture surface. Fig. 5 shows one such phase observed at about 1 lm below the fracture surface, with a successive tilting of the specimen to X:1.88 and Y:)7.00. The size of the phase is coincident with a dislocation cell. The grey featureless image remained on tilting the specimen, while the contrast of neighboring dislocation cells changed noticeably. The SAD pattern from the bright layer was spotty without halo rings. Thus, the grey image may result from a low
dislocation density in the area, prior to fragmentation of the matrix crystal and amorphization. On the side surface of the fractured specimen, many short cracks appeared transverse to the tensile axis near the fracture surface. Fig. 6 shows TEM micrographs of the front area of a side crack. Similar to the fracture subsurface, a grey featureless zone developed in the crack front where the dislocation density was substantially higher. The SAD pattern shown in the inset was with halo rings similar to those in Fig. 2 and a dark-field image clearly shows amorphization therein. The estimated plastic zone size, assuming a crack
Fig. 5. (a) A grey area that appeared at about 1 lm below the fracture surface; (b) magnified image of the grey area, and (c) the same area with a successive tilting of the specimen to X:1.88 and Y:)7.00.
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Fig. 6. (a) A grey area that developed at the front of a small side crack and (b) dark-field image of the same area as (a) from the part indicated in the diffraction pattern.
length of 10 lm, is about 2 lm, indicating that the grey zone is within the plastic zone. The appearance of the featureless phase in restricted areas as shown in Figs. 5 and 6 implies that the formation of the phase is not due to damage or deposition of the W coating associated with sample preparation using FIB. Such formations of bright layers beneath the fracture surface or along dislocation cell walls were not observed in specimens tensile-fractured without hydrogencharging.
4. Discussion The structural changes observed in the present study show that fragmentation of the Fe matrix and subsequent amorphization take place in a strain-concentrated area, associated with crack propagation in the presence of hydrogen. That the halo diffraction pattern in Fig. 2(b) is not solely due to grain refinement is supported by the lattice image shown in Fig. 3. It is noticed, however, that the location of the first strong halo ring was not coincident with d1 1 0 of bcc Fe. It implies that the nearest neighbor configuration in bcc Fe was not conserved in the progress of amorphization, but some lattice expansion might have taken place. The atomic configuration producing the halo is not definite at present, but it might evolve from hydrogen–vacancy interactions. A substantial volume expansion we have previously observed in some Fe-base amorphous alloys when hydrogen was introduced [16] presumably took place in the
present case. However, the strong halo ring shown in Fig. 2(b) contained some spotty patterns, and fine nm-scale bright patches were observed in the dark-field image in Fig. 2(d). Iron hydrides observed under high pressure environments [17,18] accompanying expansion of molar volume, are not feasible in the present case, but some ordering involving hydrogen might have taken place. Solid-state amorphization of metallic materials has been observed in mechanical alloying [19,20], hydrogenation of intermetallic compounds [21,22] or during the eutectoid reaction of a Ti–Al alloy [23]. In the last case, an increased vacancy density associated with the phase separation accompanying different Al stoichiometric concentrations has been assumed as the cause of amorphization. Amorphization associated with vacancy generation has been observed in various intermetallic compounds by high energy electron or ion irradiation [24]. If we assume that the increase in vacancy density to a critical level causes lattice instability, the increased vacancy density due to plastic straining and binding with hydrogen could reasonably induce amorphization. The mechanism of vacancy generation associated with plastic straining is due to interactions between dislocations, such as dipole annihilation [25] or drag of jogs [4], which may take place preferentially in sites of a high dislocation density such as dislocation cell walls or crack front. Further, dislocation dynamics associated with crack growth may alter local dislocation configurations, destructing cell structures. The fragmentation of the Fe-matrix
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and reduced dislocation density revealed in Figs. 4 and 5 are likely to show early structural changes prior to amorphization. A characteristic of the mechanical properties of the amorphous phase is plastic instability leading to strain localization, and the ductility is much reduced in the presence of hydrogen [16]. In the fracture process, the formation of the amorphous phase in the strain-concentrated area in front of the crack is expected to reduce crack growth resistance. The present findings support the vacancyclustering model [13] for hydrogen-related failure of steel.
5. Conclusions The deformation microstructures associated with crack propagation in a hydrogen-charged ferritic steel have been examined by means of transmission electron microscopy with tensiletested specimens prepared using a Focused Ion Beam method. In the area below the fracture surface, the dislocation density was substantially higher, resulting in the formation of cell structures. A decrease in the dislocation density and fragmentation of the matrix were observed to take place, leading to the evolution of the amorphous phase in a layer less than 1 lm in thickness adjacent to the surface. Similar structural alterations were also observed eventually within the matrix and in the front of a small crack formed on the side surface of the specimen near the fracture surface. The location of the halo rings suggested that the atomic configuration in the amorphous phase is associated with some lattice expansion. The creation of a high density of vacancies with the aid of hydrogen is likely the cause of amorphization due to lattice instability and decreased crack growth resistance. The present results may be the first observations ever obtained of amorphization associated with crack growth in hydrogen-related failure of steel, supporting the vacancy-clustering model.
Acknowledgements The authors would like to thank Dr. H. Morikawa, formerly with Nippon Steel Corp., and Prof. T. Hirotsu, Osaka University, for their valuable discussion. The present study has been conducted as part of a project by the Special Coordination Fund for Promoting Science and Technology of the Ministry of Education, Culture, Sports, Science and Technology of Japan.
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