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Studies on hydrogen embrittlement in Zr- and Ni-based amorphous alloys
Materials Science and Engineering A 449–451 (2007) 920–923
Studies on hydrogen embrittlement in Zr- and Ni-based amorphous alloys S. Jayalakshmi a , S.O. Park a , K.B. Kim a , E. Fleury a,∗ , D.H. Kim b a
Advanced Materials Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, South Korea b Center for Nanocrystalline Materials, Department of Metallurgical Engineering, Yonsei University, Seoul, South Korea Received 21 August 2005; received in revised form 16 February 2006; accepted 17 February 2006
Abstract Mechanical and structural properties of Zr50 Ni27 Ni18 Co5 (at.%) and Ni59 Zr16 Ti13 Nb7 Sn3 Si2 (at.%) amorphous alloys were studied before and after hydrogenation. In both alloys, upon hydrogenation, a significant reduction in mechanical properties was evident. The occurrence of hydrogen embrittlement was supported by fracture morphology and is further explained based on the corresponding structural changes induced by hydrogenation. © 2006 Elsevier B.V. All rights reserved. Keywords: Amorphous alloys; Hydrogenation; Embrittlement; Structural analysis
1. Introduction In recent times, amorphous alloys have been widely considered in fuel cell technology as hydrogen permeable membranes for reformers, in order to separate hydrogen from gas mixtures [1]. This necessitates studies pertaining to the susceptibility of hydrogenated amorphous alloys to hydrogen embrittlement. A detailed review on various embrittlement mechanisms occurring due to hydrogen interaction with amorphous alloys can be found in Ref. [2]. The purpose of the present work was to study the hydrogen embrittlement behaviour of cathodically charged Zr50 Ni27 Nb18 Co5 (numbers indicate at.%) [3] and Ni59 Zr16 Ti13 Nb7 Sn3 Si2 [4] amorphous alloys. Mechanical tests were conducted to determine the proneness of the alloys to hydrogen embrittlement and were examined by fractographic analysis. The structural changes that occur due to hydrogenation were investigated, which would be directive in understanding the phenomenon of hydrogen embrittlement. 2. Experimental methods Melt spun Zr50 Ni27 Nb18 Co5 (15 mm wide and 45 m thick) and Ni59 Zr16 Ti13 Nb7 Sn3 Si2 (7 mm wide and 32 m thick) ∗
Corresponding author. Tel.: +82 2 958 5456; fax: +82 2 958 5449. E-mail address: [email protected] (E. Fleury).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.02.417
amorphous ribbons were cathodically charged with hydrogen in 1 N H2 SO4 electrolyte using a platinum anode, at a constant current density of ∼25 mA/cm2 . Variation in hydrogen concentration (achieved by varying the charging time [5]) was measured using a thermo-gravimetric analyzer coupled with mass spectroscope, at a heating rate of 0.33 K s−1 . Hydrogen embrittlement was studied using bend testing [6], by measuring the critical bending strain for fracture before and after hydrogenation [7–9], with an average of three repetitive values. Microstructural investigation, amorphicity and fracture surface morphology were investigated using X-ray diffractometry (XRD, Cu K␣), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). 3. Results and discussion In order to study the embrittlement occurring due to hydrogen, bend ductility tests were conducted on un-charged and charged specimens. The strain to fracture (εf ) was determined by measuring the radius of curvature at which fracture occurs in a bend test between two parallel plates [7–9]. The strain required for fracture is given by: εf (%) = [t/(2rf − t)] × 100, where t is the specimen thickness and rf is the separation of the plates at fracture [7–9]. It is worthwhile to note that both the uncharged Zr-alloy and Ni-alloy do not fracture during complete bending up to 180◦ , indicating good ductility of the ribbons. Similar behaviour was reported earlier in Fe-base amorphous alloy [7].
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Fig. 1. (a) Representative photographs of (a) un-charged Ni59 Zr16 Ti13 Nb7 Sn3 Si2 alloy and (b) Zr50 Ni27 Nb18 Co5 alloy with ∼13 at.% of hydrogen, after bend ductility test, showing no fracture.
From Fig. 1a it can be seen that uncharged Ni-ribbon exhibit good ductility and did not fracture. A similar behaviour was observed in the uncharged Zr-alloy after bend test (figure not shown). In addition, charged Zr-alloys do not show any embrittlement till ∼13 at.% of hydrogen, in agreement with an earlier report [5]. Fig. 1b shows the ribbon made of Zr-alloy with ∼13 at.% of hydrogen that exhibit good ductility and did not fracture after bend test. Fig. 2a shows the dependence of fracture strain, with hydrogen charging time, for both the hydrogenated alloys. For both the uncharged alloys, and those of Zr-alloy with ∼8 at.% and ∼13 at.% which do not fracture during bending, the data points could not be presented. It can be observed from Fig. 2a that the bending strain decreases with hydrogen concentration for both the alloys. The strain to fracture is a measure of ductility and hence gives a measure of embrittlement [7]. The decrease in fracture strain with Hconc indicates that the addition of hydrogen results in reduced ductility. In contrast to Zr-alloy which shows a gradual decrease, the Ni-alloy shows drastic reduction in ductil-
ity (Fig. 2a). This indicates that the Ni-alloy is highly susceptible to hydrogen embrittlement. Fractographic analysis of both the alloys (inset, Fig. 2b and c) show flat brittle morphology that is typical of hydrogen embrittlement [5,10,11]. Fig. 3 shows the XRD analysis of both the alloy ribbons before and after hydrogenation. Both the alloys exhibit amorphous nature and do not show crystalline peaks even at high hydrogen concentrations. However, the diffused peaks of hydrogenated specimens are shifted considerably to lower angles, which indicate the expansion of the amorphous structure [12,13]. The absence of crystalline peaks in the hydrogenated alloys would possibly be due to the limited resolution of conventional XRD, and hence the microstructures of the hydrogenated samples were examined using TEM. From Fig. 4a it can be seen that nanocrystals of order ∼5 nm are formed in Zr50 Ni27 Nb18 Co5 specimen with Hconc ∼32 at.%. The inset shows the selected area electron diffraction pattern (SAED) of Zr50 Ni27 Nb18 Co5 alloy, the analysis of which shows the formation of ␥-ZrH tetragonal ˚ and c = 4.9686 A, ˚ phase with lattice parameter, a = b = 4.5957 A
Fig. 2. (a) Variation of bending strain with charging time for Zr50 Ni27 Nb18 Co5 () and Ni59 Zr16 Ti13 Nb7 Sn3 Si2 () alloys. Inset (b and c) shows fracture surfaces of the alloys after embrittlement.
Fig. 3. X-ray diffraction pattern of Zr50 Ni27 Nb18 Co5 Ni59 Zr16 Ti13 Nb7 Sn3 Si2 alloys before and after hydrogenation.
and
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Fig. 4. TEM images of (a) Zr50 Ni27 Nb18 Co5 alloy and selected area diffraction pattern (inset) and (b) Ni59 Zr16 Ti13 Nb7 Sn3 Si2 alloy and selected area diffraction pattern (inset).
and space group: P42 /n. In the Ni59 Zr16 Ti13 Nb7 Sn3 Si2 alloy (Fig. 4b; Hconc ∼15 at.%), nanocrystals formed are of the order of ∼2 nm. In both the cases, the size of the nanocrystals was measured using the HRTEM image and were comparable to those calculated from the XRD pattern by applying the Scherrer formula [14]. The inset in Fig. 4b shows the SAED pattern of Ni59 Zr16 Ti13 Nb7 Sn3 Si2 alloy, which indicates the formation of ˚ and c = 4.33 A, ˚ Ni2 H phase with lattice parameter a = b = 2.66 A and space group P3m1. Based on the observations made from XRD, it can be seen that the presence of hydrogen affects the structural properties as well, and hence it would be worthwhile to relate these changes to the observed embrittlement behaviour. Considering the role of the alloying elements present, Zr atoms possess highest hydrogen affinity (enthalpy of mixing for Zr with H: −19 kJ/mol and that of Ni with H: 27 kJ/mol) [3]. Hence, initially for lower Hconc (<∼13 at.%), hydrogen would occupy the most stable sites (interstitial sites) [13] without any modification of structure and properties. With increasing Hconc (>∼13 at.%), the metal atoms would move within short range, inorder to accommodate more hydrogen [13], resulting in an increased distance between metal atoms. In Ni59 Zr16 Ti13 Nb7 Sn3 Si2 alloy, the major alloying element Ni has a smaller atomic size, and Zr, which has high hydrogen affinity, is of low atomic percentage. Based on the atomic structural model for metallic glasses proposed by Miracle [15], the formation of dense cluster-packing structure of Ni atoms provide interstitial sites of small size. Therefore, due to the relatively less space available, the introduction of hydrogen would tend to increase the distance between metal atoms even for low concentration and an expansion of the amorphous structure is detected by XRD (Fig. 3). This expansion of the amorphous structure results in the reduction of cohesion between metal atoms (de-cohesion) [10]. Similar observations indicating de-cohesion as the major reason for hydrogen embrittlement in amorphous alloys have been reported earlier [10,11]. Further, the flat fracture morphology observed by SEM is typical of hydrogen embrittlement [5,10,11]. In addition, for both
the alloys, the formation of hydride nanocrystalline phases is believed to promote easy crack initiation [5,16] accelerating the embrittling effect. 4. Conclusion The hydrogen embrittlement behaviour of Zr50 Ni27 Nb18 Co5 and Ni59 Zr16 Ti13 Nb7 Sn3 Si2 alloys was studied. Hydrogen embrittlement occurred in both alloys, at higher concentration for Zr-alloy (>∼13 at.%) and at concentrations as low as ∼6.5 at.% for Ni-alloy. However, in both the alloys, the presence of hydrogen resulted in microstructural evolution and formation of hydrides. Hydrogen embrittlement behaviour was mainly dominated by the mechanism of de-cohesion between metal atoms, and aided by nanocrystalline formation. Acknowledgement This work was funded by KIST Research Program 2E18470. References [1] A. Inoue, Acta Mater. 48 (2000) 279–306. [2] N. Eliaz, D. Eliezer, Adv. Perform. Mater. 6 (1999) 5–31. [3] S. Yamaura, Y. Shinpo, H. Okouchi, M. Nishida, O. Kajita, A. Inoue, Mater. Trans. 45 (2004) 330–333. [4] J.Y. Lee, D.H. Bae, J.K. Lee, D.H. Kim, J. Mater. Res. 18 (2004) 221– 225. [5] S. Jayalakshmi, K.B. Kim, E. Fleury, J. Alloy Compd. 417 (2006) 195– 202. [6] F.E. Luborsky, J.L. Walter, J. Appl. Phys. 47 (1976) 3648–3650. [7] S. Ashok, N.S. Stoloff, M.E. Glicksman, T. Slavin, Scripta Metall. 15 (1981) 331–337. [8] M.A. Munoz-Morris, S. Surinach, L.K. Varga, M.D. Baro, D.G. Morris, Scripta Mater. 47 (2002) 31–37. [9] W. Ding, M.H. Wang, C.M. Haiao, Y. Xu, Z. Tian, Scripta Metall. 21 (1987) 1685–1688. [10] H.W. Schoeder, U. Koster, J. Non-Crystall. Solids 56 (1983) 213–218. [11] S. Yamaura, M. Hasegawa, H. Kimura, A. Inoue, Mater. Trans. 43 (2002) 2543–2547.
S. Jayalakshmi et al. / Materials Science and Engineering A 449–451 (2007) 920–923 [12] D. Zander, H. Leptein, U. Koster, N. Eliaz, D. Eliezer, J. Non-Crystall. Solids 250–252 (1999) 893–897. [13] X.G. Li, T. Otahara, S. Takahashi, T. Shoji, H.M. Kimura, A. Inoue, J. Alloy Compd. 297 (2000) 303–311.
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[14] B.D. Cullity, S.R. Stock, Elements of X-ray Diffraction, 3rd ed., Prentice Hall, NJ, USA, 2001, p. 170. [15] D.B. Miracle, Nat. Mater. 3 (2004) 697–702. [16] M. Yan, J.F. Sun, J. Shen, J. Alloy Compd. 381 (2004) 86–90.
Studies on hydrogen embrittlement in Zr- and Ni-based amorphous alloys S. Jayalakshmi a , S.O. Park a , K.B. Kim a , E. Fleury a,∗ , D.H. Kim b a
Advanced Materials Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, South Korea b Center for Nanocrystalline Materials, Department of Metallurgical Engineering, Yonsei University, Seoul, South Korea Received 21 August 2005; received in revised form 16 February 2006; accepted 17 February 2006
Abstract Mechanical and structural properties of Zr50 Ni27 Ni18 Co5 (at.%) and Ni59 Zr16 Ti13 Nb7 Sn3 Si2 (at.%) amorphous alloys were studied before and after hydrogenation. In both alloys, upon hydrogenation, a significant reduction in mechanical properties was evident. The occurrence of hydrogen embrittlement was supported by fracture morphology and is further explained based on the corresponding structural changes induced by hydrogenation. © 2006 Elsevier B.V. All rights reserved. Keywords: Amorphous alloys; Hydrogenation; Embrittlement; Structural analysis
1. Introduction In recent times, amorphous alloys have been widely considered in fuel cell technology as hydrogen permeable membranes for reformers, in order to separate hydrogen from gas mixtures [1]. This necessitates studies pertaining to the susceptibility of hydrogenated amorphous alloys to hydrogen embrittlement. A detailed review on various embrittlement mechanisms occurring due to hydrogen interaction with amorphous alloys can be found in Ref. [2]. The purpose of the present work was to study the hydrogen embrittlement behaviour of cathodically charged Zr50 Ni27 Nb18 Co5 (numbers indicate at.%) [3] and Ni59 Zr16 Ti13 Nb7 Sn3 Si2 [4] amorphous alloys. Mechanical tests were conducted to determine the proneness of the alloys to hydrogen embrittlement and were examined by fractographic analysis. The structural changes that occur due to hydrogenation were investigated, which would be directive in understanding the phenomenon of hydrogen embrittlement. 2. Experimental methods Melt spun Zr50 Ni27 Nb18 Co5 (15 mm wide and 45 m thick) and Ni59 Zr16 Ti13 Nb7 Sn3 Si2 (7 mm wide and 32 m thick) ∗
Corresponding author. Tel.: +82 2 958 5456; fax: +82 2 958 5449. E-mail address: [email protected] (E. Fleury).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.02.417
amorphous ribbons were cathodically charged with hydrogen in 1 N H2 SO4 electrolyte using a platinum anode, at a constant current density of ∼25 mA/cm2 . Variation in hydrogen concentration (achieved by varying the charging time [5]) was measured using a thermo-gravimetric analyzer coupled with mass spectroscope, at a heating rate of 0.33 K s−1 . Hydrogen embrittlement was studied using bend testing [6], by measuring the critical bending strain for fracture before and after hydrogenation [7–9], with an average of three repetitive values. Microstructural investigation, amorphicity and fracture surface morphology were investigated using X-ray diffractometry (XRD, Cu K␣), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). 3. Results and discussion In order to study the embrittlement occurring due to hydrogen, bend ductility tests were conducted on un-charged and charged specimens. The strain to fracture (εf ) was determined by measuring the radius of curvature at which fracture occurs in a bend test between two parallel plates [7–9]. The strain required for fracture is given by: εf (%) = [t/(2rf − t)] × 100, where t is the specimen thickness and rf is the separation of the plates at fracture [7–9]. It is worthwhile to note that both the uncharged Zr-alloy and Ni-alloy do not fracture during complete bending up to 180◦ , indicating good ductility of the ribbons. Similar behaviour was reported earlier in Fe-base amorphous alloy [7].
S. Jayalakshmi et al. / Materials Science and Engineering A 449–451 (2007) 920–923
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Fig. 1. (a) Representative photographs of (a) un-charged Ni59 Zr16 Ti13 Nb7 Sn3 Si2 alloy and (b) Zr50 Ni27 Nb18 Co5 alloy with ∼13 at.% of hydrogen, after bend ductility test, showing no fracture.
From Fig. 1a it can be seen that uncharged Ni-ribbon exhibit good ductility and did not fracture. A similar behaviour was observed in the uncharged Zr-alloy after bend test (figure not shown). In addition, charged Zr-alloys do not show any embrittlement till ∼13 at.% of hydrogen, in agreement with an earlier report [5]. Fig. 1b shows the ribbon made of Zr-alloy with ∼13 at.% of hydrogen that exhibit good ductility and did not fracture after bend test. Fig. 2a shows the dependence of fracture strain, with hydrogen charging time, for both the hydrogenated alloys. For both the uncharged alloys, and those of Zr-alloy with ∼8 at.% and ∼13 at.% which do not fracture during bending, the data points could not be presented. It can be observed from Fig. 2a that the bending strain decreases with hydrogen concentration for both the alloys. The strain to fracture is a measure of ductility and hence gives a measure of embrittlement [7]. The decrease in fracture strain with Hconc indicates that the addition of hydrogen results in reduced ductility. In contrast to Zr-alloy which shows a gradual decrease, the Ni-alloy shows drastic reduction in ductil-
ity (Fig. 2a). This indicates that the Ni-alloy is highly susceptible to hydrogen embrittlement. Fractographic analysis of both the alloys (inset, Fig. 2b and c) show flat brittle morphology that is typical of hydrogen embrittlement [5,10,11]. Fig. 3 shows the XRD analysis of both the alloy ribbons before and after hydrogenation. Both the alloys exhibit amorphous nature and do not show crystalline peaks even at high hydrogen concentrations. However, the diffused peaks of hydrogenated specimens are shifted considerably to lower angles, which indicate the expansion of the amorphous structure [12,13]. The absence of crystalline peaks in the hydrogenated alloys would possibly be due to the limited resolution of conventional XRD, and hence the microstructures of the hydrogenated samples were examined using TEM. From Fig. 4a it can be seen that nanocrystals of order ∼5 nm are formed in Zr50 Ni27 Nb18 Co5 specimen with Hconc ∼32 at.%. The inset shows the selected area electron diffraction pattern (SAED) of Zr50 Ni27 Nb18 Co5 alloy, the analysis of which shows the formation of ␥-ZrH tetragonal ˚ and c = 4.9686 A, ˚ phase with lattice parameter, a = b = 4.5957 A
Fig. 2. (a) Variation of bending strain with charging time for Zr50 Ni27 Nb18 Co5 () and Ni59 Zr16 Ti13 Nb7 Sn3 Si2 () alloys. Inset (b and c) shows fracture surfaces of the alloys after embrittlement.
Fig. 3. X-ray diffraction pattern of Zr50 Ni27 Nb18 Co5 Ni59 Zr16 Ti13 Nb7 Sn3 Si2 alloys before and after hydrogenation.
and
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Fig. 4. TEM images of (a) Zr50 Ni27 Nb18 Co5 alloy and selected area diffraction pattern (inset) and (b) Ni59 Zr16 Ti13 Nb7 Sn3 Si2 alloy and selected area diffraction pattern (inset).
and space group: P42 /n. In the Ni59 Zr16 Ti13 Nb7 Sn3 Si2 alloy (Fig. 4b; Hconc ∼15 at.%), nanocrystals formed are of the order of ∼2 nm. In both the cases, the size of the nanocrystals was measured using the HRTEM image and were comparable to those calculated from the XRD pattern by applying the Scherrer formula [14]. The inset in Fig. 4b shows the SAED pattern of Ni59 Zr16 Ti13 Nb7 Sn3 Si2 alloy, which indicates the formation of ˚ and c = 4.33 A, ˚ Ni2 H phase with lattice parameter a = b = 2.66 A and space group P3m1. Based on the observations made from XRD, it can be seen that the presence of hydrogen affects the structural properties as well, and hence it would be worthwhile to relate these changes to the observed embrittlement behaviour. Considering the role of the alloying elements present, Zr atoms possess highest hydrogen affinity (enthalpy of mixing for Zr with H: −19 kJ/mol and that of Ni with H: 27 kJ/mol) [3]. Hence, initially for lower Hconc (<∼13 at.%), hydrogen would occupy the most stable sites (interstitial sites) [13] without any modification of structure and properties. With increasing Hconc (>∼13 at.%), the metal atoms would move within short range, inorder to accommodate more hydrogen [13], resulting in an increased distance between metal atoms. In Ni59 Zr16 Ti13 Nb7 Sn3 Si2 alloy, the major alloying element Ni has a smaller atomic size, and Zr, which has high hydrogen affinity, is of low atomic percentage. Based on the atomic structural model for metallic glasses proposed by Miracle [15], the formation of dense cluster-packing structure of Ni atoms provide interstitial sites of small size. Therefore, due to the relatively less space available, the introduction of hydrogen would tend to increase the distance between metal atoms even for low concentration and an expansion of the amorphous structure is detected by XRD (Fig. 3). This expansion of the amorphous structure results in the reduction of cohesion between metal atoms (de-cohesion) [10]. Similar observations indicating de-cohesion as the major reason for hydrogen embrittlement in amorphous alloys have been reported earlier [10,11]. Further, the flat fracture morphology observed by SEM is typical of hydrogen embrittlement [5,10,11]. In addition, for both
the alloys, the formation of hydride nanocrystalline phases is believed to promote easy crack initiation [5,16] accelerating the embrittling effect. 4. Conclusion The hydrogen embrittlement behaviour of Zr50 Ni27 Nb18 Co5 and Ni59 Zr16 Ti13 Nb7 Sn3 Si2 alloys was studied. Hydrogen embrittlement occurred in both alloys, at higher concentration for Zr-alloy (>∼13 at.%) and at concentrations as low as ∼6.5 at.% for Ni-alloy. However, in both the alloys, the presence of hydrogen resulted in microstructural evolution and formation of hydrides. Hydrogen embrittlement behaviour was mainly dominated by the mechanism of de-cohesion between metal atoms, and aided by nanocrystalline formation. Acknowledgement This work was funded by KIST Research Program 2E18470. References [1] A. Inoue, Acta Mater. 48 (2000) 279–306. [2] N. Eliaz, D. Eliezer, Adv. Perform. Mater. 6 (1999) 5–31. [3] S. Yamaura, Y. Shinpo, H. Okouchi, M. Nishida, O. Kajita, A. Inoue, Mater. Trans. 45 (2004) 330–333. [4] J.Y. Lee, D.H. Bae, J.K. Lee, D.H. Kim, J. Mater. Res. 18 (2004) 221– 225. [5] S. Jayalakshmi, K.B. Kim, E. Fleury, J. Alloy Compd. 417 (2006) 195– 202. [6] F.E. Luborsky, J.L. Walter, J. Appl. Phys. 47 (1976) 3648–3650. [7] S. Ashok, N.S. Stoloff, M.E. Glicksman, T. Slavin, Scripta Metall. 15 (1981) 331–337. [8] M.A. Munoz-Morris, S. Surinach, L.K. Varga, M.D. Baro, D.G. Morris, Scripta Mater. 47 (2002) 31–37. [9] W. Ding, M.H. Wang, C.M. Haiao, Y. Xu, Z. Tian, Scripta Metall. 21 (1987) 1685–1688. [10] H.W. Schoeder, U. Koster, J. Non-Crystall. Solids 56 (1983) 213–218. [11] S. Yamaura, M. Hasegawa, H. Kimura, A. Inoue, Mater. Trans. 43 (2002) 2543–2547.
S. Jayalakshmi et al. / Materials Science and Engineering A 449–451 (2007) 920–923 [12] D. Zander, H. Leptein, U. Koster, N. Eliaz, D. Eliezer, J. Non-Crystall. Solids 250–252 (1999) 893–897. [13] X.G. Li, T. Otahara, S. Takahashi, T. Shoji, H.M. Kimura, A. Inoue, J. Alloy Compd. 297 (2000) 303–311.
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[14] B.D. Cullity, S.R. Stock, Elements of X-ray Diffraction, 3rd ed., Prentice Hall, NJ, USA, 2001, p. 170. [15] D.B. Miracle, Nat. Mater. 3 (2004) 697–702. [16] M. Yan, J.F. Sun, J. Shen, J. Alloy Compd. 381 (2004) 86–90.