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Direct evidence of hydrogen generation from the reaction of water with FeAl
ScriptaMaterialia,Vol. 35,No. 12,pp. 1435-1439,1996 Elsevia ScienceLtd 1996ActaMetallurgicaInc. Printedin the USA. All rightsreserved 1359~6462/96 $12.00+ .OO
Pergamon
PI1 S1359-6462(96)00316-8
DIRECT EVIDENCE OF HYDROGEN GENERATION FROM THE REACTION OF WATER WITH FeAl Y.F. Zhu’, CT. Liut and C.H. Chen’ *Health Sciences Research Division, Bldg. 5500, MS 6378, Oak Ridge National Laboratory, Oak Ridge, Tennessee 3783 1, U.S.A. ‘Author for correspondence: Metals and Ceramic Division, Oak Ridge National Laboratory, Bldg. 45OOS,MS 6115, Oak Ridge, Tennessee 37831, U.S.A. (Received April 23, 1996) (Accepted July 18, 1996) Introduction
One of the most important findings in the recent study of deformation and fracture in ordered intermetallies is that many intermetallic alloys with high crystal symmetries (such as Ll 2 and B2) are intrinsically quite ductile at ambient temperatures, and that their commonly observed low ductility and brittle fracture are caused mainly by extrinsic factors [l-7]. Environmental embrittlement has been identified as a major exlrinsic factor that embrittles many intermetallic alloys [ 1,4-71. In this case, the alloys showed brittle fracture and low ductilities when tested in moist air, but they exhibited distinctly high ductilities when tested in high-vacuum or dry-oxygen environments. One prominent example is FeAl (36% Al) which showed a tensile ductility of only 2% in air at room temperature and 18% in dry oxygen [ 1,7]. The proposed mechanism involves the chemical interaction of moisture in air with a fresh metal surface to generate atomic hydrogen that penetrates into the bulk of the alloy to cause severe embrittlement.. At present, there are only limited studies of the water/intermetallic interactions. Gleason et al. [8] fist reported the possible release of hydrogen from water decomposition on intermetallic alloy surfaces using a temperature-programmed desorption technique. Chia and Chtmg [9] found that water molecules dissociate on the surface but remain intact on the cl1 l> surface of N&(Al,Ti) single crystals. Their observation suggests that the reaction is highly dependent on chemical composition and atomic arrangement on crystallographic planes. Nevertheless, direct measurements of trapped hydrogen atoms in the bulk of intermetallic alloys have not been reported. In order to have a mechanistic understanding of moisture-induced hydrogen embrittlement, it is essential to have a direct measurement of trapped hydrogen atoms with position resolution. In this study, we try 1:o develop a laser desorption mass spectrometric method to measure hydrogen atoms in a
*“The submittedmanuscript has been authorized by a contractor of the U.S. Government under contract No. DE-ACOS96OR22464.Accordingly, the U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for the U.S. Government purposes.”
1435
1436
DIRECT EVIDENCE OF HYDROGEN GENERATION
Vol. 35, No. 12
position-sensitive way. We also compare the amount of trapped hydrogen, with and without exposure to water, for FeAl containing 35.8 at. % Al. Experimental Procedures A laser desorption mass spectrometer facility [lo] was set up to measure the trapped hydrogen in intermetallic alloys. An experimental schematic is shown in Fig. 1. A pulsed Nd-Yag laser (Model DCR2, Spectra Physics, Mountain View, CA, USA) with a wavelength of 355 nm produced from third harmonic generation was used for desorption and ionization. The laser energy used ranged from 50 to 800 pI per pulse with a pulse duration of 5 ns. The laser beam was focussed down to about 125 pm by a lens with 25 cm focal length. A time-of-flight (TOF) mass spectrometer with 5-kV acceleration voltage was used to obtain mass spectra. This facility can be used as a linear TOF or reflectron TOF since two separate detectors are installed. However, only linear TOF was used for this work since resolution of hydrogen is not a major concern. During the laser ablation process, trapped hydrogen and metal atoms, as well as electron and ion pairs, are produced due to strong absorption of laser light. The desorbed ions are separated in the drift tube and subsequently impinge on an aluminum foil to produce secondary electrons which are detected by a channeltron. The signals from the channeltron detector were fed into a digital oscilloscope (HP 54520 Hewlett-Packard Inc., Colorado Springs, CO, USA). The mass spectra obtained are typically an average of 32 laser pulses. From the time interval between the laser pulse and ion detection, the molecular weight of ions can be calculated. The chamber pressure of TOF mass spectrometer during the work is typically 4 x 10” Pa. Alloy specimens used in the present study were prepared from FeAl alloy containing 35.8 at. % Al. The alloy ingot prepared by arc melting and drop casting was clad in stainless steel plate and hot-rolled to 0.025~in-thick sheet at 1100 to 980°C with an intermediate annealing for 1 h at 900°C. Sheet specimens cut from the hot-rolled sheet were mechanically polished using 00-grade Sic paper and given a final heat treatment of 1 h at 900°C to produce a recrystallized grain structure plus 17 h at 500°C to produce fully ordered B2 structure in a vacuum of 3 x lOAPa. The specimens after annealing were dropped into a liquid nitrogen Dewar, waiting for hydrogen measurements. A set of specimens was cathodically charged with hydrogen for 1 d in a 0.1 N sulfuric acid solution [ 111. Another set of specimens was immersed in distilled water for 3 d. In order to remove surface oxide films and enhance the interaction with water, specimen surfaces were rubbed periodically against 00-grade Sic paper in water during water immersion.
Figure 1. Experimental schematic for laser desorption mass spectrometer to probe trapped hydrogen.
Vol. 35, No. 12
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Figure 2. Comparison of trapped hydrogen atoms for samples cathodically charged with and without hydrogen. The same laser energy (257 pJ/pulse) was used for both samples.
Results and Discussion In order to test the capability of detecting trapped hydrogen in FeAl, a sample of the FeAl alloy was cathodically chiarged with hydrogen. Experimental results are shown in Fig. 2. Figure 2(a) is the TOF mass spectrum from the sample which has been charged with hydrogen for an extensive period of time. Figure 2(b) gives the results from the FeAl sample without hydrogen treatment. Since the hydrogen ion level in Fig. 2(a) is much higher than that of Fig. 2(b), it clearly indicates that charged hydrogen can penetrate into Fe-Al alloy without difficulty. Note that most trapped hydrogen is in the atomic form instead of the molecular form. The small hydrogen ion signal in Fig. 2(b) is probably due to the residual background gas absorbed on the surface in the chamber. Since the chamber pressure was 4 x 10” Pa, some background from hydrocarbons absorbed on the FeAl surface is expected. Since the trapped hydrogen compared to bulk materials is expected to be less than a few parts per million, the ion signals due to Fe and ,41 are overwhemingly saturating’the detector. The very broad spectrum for high masses in Fig. 2 is mostly due to the detector saturation and space charge effect due to the high density of electron and ion pairs.
Figure 3. Hydrogen depth distributicn measure.ments. The laser repetition rate is 10 Hz with laser fluence at 380 mJ/cm*.
1438
DIRECTEVIDENCEOF HYDROGENGENERATION
Vol. 35. No. 12
--L~_-_L-._~.~II
0
10
20
30
40
50
60
70
80
90
IO
Mass (M/Z)
Figure 4. Comparison of trapped hydrogen for samples exposed to water and not exposed to water. After samples were put into the mass spectrometer chamber, laser ablation of surface was pursued to clean up any possible hydrocarbon or water absorption on surfaces before data were taken. The laser energy used in this work was 504 pJ/pulse for both samples.
Since a high-power laser is efficient for welding and drilling, we also tried to measure the depth distribution of trapped hydrogen atoms in Fe-Al. Experimental results are shown in Fig. 3. The volume removed by each laser pulse was estimated to be 2 x lo-” cm3 with a depth probe of 2 nm. With this approach, the depth resolution of 2 nm for selected species can possibly be obtained. Since this approach can be used to probe hydrogen concentration on the surface and different depths, this facility can be applied to image the three-dimensional distribution of trapped gaseous atoms and selected elements. Since water induced embrittlement by producing hydrogen has been speculated as the cause of poor ductility, we tried to measure the trapped hydrogen. Two FeAl samples produced with identical heat treatment were tested. One sample was placed in the water for 3 d with periodic removal to polish the surface in order to enhance the possible water interaction with newly created surface. The other sample was put in liquid nitrogen to prevent any contact with water before hydrogen measurements. Surfaces of both samples were polished to the same degree of smoothness to ensure the same absorption efficiency for both samples. Experimental results are shown in Fig. 4. It clearly shows that the sample treated with water has a much larger amount of hydrogen. More than 10 different spots on the sample surfaces have been tested with the ratio of hydrogen level for water-treated to non-treated samples varying between 4 to 20. The variation is mostly due to different surface conditions which can have different laser absorption coefficients. The broad spectra at high mass region are due to the much stronger signals from metal ions and serious space charge effect which also occurs in Fig. 2. During the work, the laser fluence needs to be kept nearly the same since the quantity of desorbed hydrogen is a strong function of laser energy per pulse. These results provide direct evidence that hydrogen is produced when FeAl is exposed to high humidity. Conclusions
We have used laser desorption TOF mass spectrometry to successfully prove that atomic hydrogen can be produced in the bulk when FeAl is exposed to water. This result provides direct support of the proposed mechanism that moisture-induced hydrogen is the cause of severe embrittlement in iron aluminides and other intermetallic alloys [ 1,4-71 when tested in moisture-containing environments at ambient temperatures. By reducing hydrogen generation through exposure to low levels of moisture in test environments, the tensile ductility of these intermetallic alloys is expected to be significantly improved. This is enhanced from the measurement of the room-temperature tensile ductility of Fe3A1as a
Vol. 35, No. 12
DIRECT EVIDENCE
OF HYDROGEN
GENERATION
1439
function of partial pressure of water vapor in test environments [12]. The ductility increased from 5% obtained in a water vapor pressure of -1300 Pa to 24% in a vacuum of lOAPa. Acknowledgements This research was sponsored by the Division of Material Sciences, U.S. Department of Energy, under contract DE-AC05-960R22464 with Oak Ridge National Laboratory, managed by Lockheed Martin Energy Research Corporation. The authors thank Kathy Spence for editorial review. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
C. T. Liu, E. II. Lee, and C. G. McKamey, Ser. Metall. 23,875 (1989). J. E. Hack, J. M. Brzeski, and R. Darolia, Ser. Metall. 27, 1259 (1992). C. T. Liu and J. A. Horton, Mater. Sci. Eng. A19U193, 170 (1995). C. T. Liu, Ser. Metah. 27,25 (1992) P. E. George, C. T. Liu, and D. P. Pope, Ser. MetaIl. 28, 857 (1993). R. J. Lynch and L. A. Heldt, J. Met. 45,177 (1993). N. S. Stoloff ;andC. T. Liu, Intermetallics, 2,75 (1994). N. R. Gleason, C. A. Gerken, and D. R. Strongin, Appl. Surf. Sci. 72,215 (1993). W. J. Chia and Y. W. Chung, Intermetallics 3,505 (1995). C. H. Chen, T. M. Murphy, and R. C. Phillips, Appl. Phys. Lett. 57,937 (1990). A. K. Kuruvilla and N. S. Stoloff, Ser. Metall. 19,83 (1985). C. G. McKamey and E. H. Lee, p. 983 in High-Temperature Ordered Intermetallic Alloys V, MRS Proc. Vol. 288, Materials Rer,earch Society, Pittsburgh, Pa. 1993.
Pergamon
PI1 S1359-6462(96)00316-8
DIRECT EVIDENCE OF HYDROGEN GENERATION FROM THE REACTION OF WATER WITH FeAl Y.F. Zhu’, CT. Liut and C.H. Chen’ *Health Sciences Research Division, Bldg. 5500, MS 6378, Oak Ridge National Laboratory, Oak Ridge, Tennessee 3783 1, U.S.A. ‘Author for correspondence: Metals and Ceramic Division, Oak Ridge National Laboratory, Bldg. 45OOS,MS 6115, Oak Ridge, Tennessee 37831, U.S.A. (Received April 23, 1996) (Accepted July 18, 1996) Introduction
One of the most important findings in the recent study of deformation and fracture in ordered intermetallies is that many intermetallic alloys with high crystal symmetries (such as Ll 2 and B2) are intrinsically quite ductile at ambient temperatures, and that their commonly observed low ductility and brittle fracture are caused mainly by extrinsic factors [l-7]. Environmental embrittlement has been identified as a major exlrinsic factor that embrittles many intermetallic alloys [ 1,4-71. In this case, the alloys showed brittle fracture and low ductilities when tested in moist air, but they exhibited distinctly high ductilities when tested in high-vacuum or dry-oxygen environments. One prominent example is FeAl (36% Al) which showed a tensile ductility of only 2% in air at room temperature and 18% in dry oxygen [ 1,7]. The proposed mechanism involves the chemical interaction of moisture in air with a fresh metal surface to generate atomic hydrogen that penetrates into the bulk of the alloy to cause severe embrittlement.. At present, there are only limited studies of the water/intermetallic interactions. Gleason et al. [8] fist reported the possible release of hydrogen from water decomposition on intermetallic alloy surfaces using a temperature-programmed desorption technique. Chia and Chtmg [9] found that water molecules dissociate on the
*“The submittedmanuscript has been authorized by a contractor of the U.S. Government under contract No. DE-ACOS96OR22464.Accordingly, the U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for the U.S. Government purposes.”
1435
1436
DIRECT EVIDENCE OF HYDROGEN GENERATION
Vol. 35, No. 12
position-sensitive way. We also compare the amount of trapped hydrogen, with and without exposure to water, for FeAl containing 35.8 at. % Al. Experimental Procedures A laser desorption mass spectrometer facility [lo] was set up to measure the trapped hydrogen in intermetallic alloys. An experimental schematic is shown in Fig. 1. A pulsed Nd-Yag laser (Model DCR2, Spectra Physics, Mountain View, CA, USA) with a wavelength of 355 nm produced from third harmonic generation was used for desorption and ionization. The laser energy used ranged from 50 to 800 pI per pulse with a pulse duration of 5 ns. The laser beam was focussed down to about 125 pm by a lens with 25 cm focal length. A time-of-flight (TOF) mass spectrometer with 5-kV acceleration voltage was used to obtain mass spectra. This facility can be used as a linear TOF or reflectron TOF since two separate detectors are installed. However, only linear TOF was used for this work since resolution of hydrogen is not a major concern. During the laser ablation process, trapped hydrogen and metal atoms, as well as electron and ion pairs, are produced due to strong absorption of laser light. The desorbed ions are separated in the drift tube and subsequently impinge on an aluminum foil to produce secondary electrons which are detected by a channeltron. The signals from the channeltron detector were fed into a digital oscilloscope (HP 54520 Hewlett-Packard Inc., Colorado Springs, CO, USA). The mass spectra obtained are typically an average of 32 laser pulses. From the time interval between the laser pulse and ion detection, the molecular weight of ions can be calculated. The chamber pressure of TOF mass spectrometer during the work is typically 4 x 10” Pa. Alloy specimens used in the present study were prepared from FeAl alloy containing 35.8 at. % Al. The alloy ingot prepared by arc melting and drop casting was clad in stainless steel plate and hot-rolled to 0.025~in-thick sheet at 1100 to 980°C with an intermediate annealing for 1 h at 900°C. Sheet specimens cut from the hot-rolled sheet were mechanically polished using 00-grade Sic paper and given a final heat treatment of 1 h at 900°C to produce a recrystallized grain structure plus 17 h at 500°C to produce fully ordered B2 structure in a vacuum of 3 x lOAPa. The specimens after annealing were dropped into a liquid nitrogen Dewar, waiting for hydrogen measurements. A set of specimens was cathodically charged with hydrogen for 1 d in a 0.1 N sulfuric acid solution [ 111. Another set of specimens was immersed in distilled water for 3 d. In order to remove surface oxide films and enhance the interaction with water, specimen surfaces were rubbed periodically against 00-grade Sic paper in water during water immersion.
Figure 1. Experimental schematic for laser desorption mass spectrometer to probe trapped hydrogen.
Vol. 35, No. 12
DIRECTEVIDENCEOF HYDROGENGENERATION
1437
Figure 2. Comparison of trapped hydrogen atoms for samples cathodically charged with and without hydrogen. The same laser energy (257 pJ/pulse) was used for both samples.
Results and Discussion In order to test the capability of detecting trapped hydrogen in FeAl, a sample of the FeAl alloy was cathodically chiarged with hydrogen. Experimental results are shown in Fig. 2. Figure 2(a) is the TOF mass spectrum from the sample which has been charged with hydrogen for an extensive period of time. Figure 2(b) gives the results from the FeAl sample without hydrogen treatment. Since the hydrogen ion level in Fig. 2(a) is much higher than that of Fig. 2(b), it clearly indicates that charged hydrogen can penetrate into Fe-Al alloy without difficulty. Note that most trapped hydrogen is in the atomic form instead of the molecular form. The small hydrogen ion signal in Fig. 2(b) is probably due to the residual background gas absorbed on the surface in the chamber. Since the chamber pressure was 4 x 10” Pa, some background from hydrocarbons absorbed on the FeAl surface is expected. Since the trapped hydrogen compared to bulk materials is expected to be less than a few parts per million, the ion signals due to Fe and ,41 are overwhemingly saturating’the detector. The very broad spectrum for high masses in Fig. 2 is mostly due to the detector saturation and space charge effect due to the high density of electron and ion pairs.
Figure 3. Hydrogen depth distributicn measure.ments. The laser repetition rate is 10 Hz with laser fluence at 380 mJ/cm*.
1438
DIRECTEVIDENCEOF HYDROGENGENERATION
Vol. 35. No. 12
--L~_-_L-._~.~II
0
10
20
30
40
50
60
70
80
90
IO
Mass (M/Z)
Figure 4. Comparison of trapped hydrogen for samples exposed to water and not exposed to water. After samples were put into the mass spectrometer chamber, laser ablation of surface was pursued to clean up any possible hydrocarbon or water absorption on surfaces before data were taken. The laser energy used in this work was 504 pJ/pulse for both samples.
Since a high-power laser is efficient for welding and drilling, we also tried to measure the depth distribution of trapped hydrogen atoms in Fe-Al. Experimental results are shown in Fig. 3. The volume removed by each laser pulse was estimated to be 2 x lo-” cm3 with a depth probe of 2 nm. With this approach, the depth resolution of 2 nm for selected species can possibly be obtained. Since this approach can be used to probe hydrogen concentration on the surface and different depths, this facility can be applied to image the three-dimensional distribution of trapped gaseous atoms and selected elements. Since water induced embrittlement by producing hydrogen has been speculated as the cause of poor ductility, we tried to measure the trapped hydrogen. Two FeAl samples produced with identical heat treatment were tested. One sample was placed in the water for 3 d with periodic removal to polish the surface in order to enhance the possible water interaction with newly created surface. The other sample was put in liquid nitrogen to prevent any contact with water before hydrogen measurements. Surfaces of both samples were polished to the same degree of smoothness to ensure the same absorption efficiency for both samples. Experimental results are shown in Fig. 4. It clearly shows that the sample treated with water has a much larger amount of hydrogen. More than 10 different spots on the sample surfaces have been tested with the ratio of hydrogen level for water-treated to non-treated samples varying between 4 to 20. The variation is mostly due to different surface conditions which can have different laser absorption coefficients. The broad spectra at high mass region are due to the much stronger signals from metal ions and serious space charge effect which also occurs in Fig. 2. During the work, the laser fluence needs to be kept nearly the same since the quantity of desorbed hydrogen is a strong function of laser energy per pulse. These results provide direct evidence that hydrogen is produced when FeAl is exposed to high humidity. Conclusions
We have used laser desorption TOF mass spectrometry to successfully prove that atomic hydrogen can be produced in the bulk when FeAl is exposed to water. This result provides direct support of the proposed mechanism that moisture-induced hydrogen is the cause of severe embrittlement in iron aluminides and other intermetallic alloys [ 1,4-71 when tested in moisture-containing environments at ambient temperatures. By reducing hydrogen generation through exposure to low levels of moisture in test environments, the tensile ductility of these intermetallic alloys is expected to be significantly improved. This is enhanced from the measurement of the room-temperature tensile ductility of Fe3A1as a
Vol. 35, No. 12
DIRECT EVIDENCE
OF HYDROGEN
GENERATION
1439
function of partial pressure of water vapor in test environments [12]. The ductility increased from 5% obtained in a water vapor pressure of -1300 Pa to 24% in a vacuum of lOAPa. Acknowledgements This research was sponsored by the Division of Material Sciences, U.S. Department of Energy, under contract DE-AC05-960R22464 with Oak Ridge National Laboratory, managed by Lockheed Martin Energy Research Corporation. The authors thank Kathy Spence for editorial review. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
C. T. Liu, E. II. Lee, and C. G. McKamey, Ser. Metall. 23,875 (1989). J. E. Hack, J. M. Brzeski, and R. Darolia, Ser. Metall. 27, 1259 (1992). C. T. Liu and J. A. Horton, Mater. Sci. Eng. A19U193, 170 (1995). C. T. Liu, Ser. Metah. 27,25 (1992) P. E. George, C. T. Liu, and D. P. Pope, Ser. MetaIl. 28, 857 (1993). R. J. Lynch and L. A. Heldt, J. Met. 45,177 (1993). N. S. Stoloff ;andC. T. Liu, Intermetallics, 2,75 (1994). N. R. Gleason, C. A. Gerken, and D. R. Strongin, Appl. Surf. Sci. 72,215 (1993). W. J. Chia and Y. W. Chung, Intermetallics 3,505 (1995). C. H. Chen, T. M. Murphy, and R. C. Phillips, Appl. Phys. Lett. 57,937 (1990). A. K. Kuruvilla and N. S. Stoloff, Ser. Metall. 19,83 (1985). C. G. McKamey and E. H. Lee, p. 983 in High-Temperature Ordered Intermetallic Alloys V, MRS Proc. Vol. 288, Materials Rer,earch Society, Pittsburgh, Pa. 1993.