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Surface reaction of polycrystalline Fe3Si alloys with oxygen and water vapor
IntemtetaIlics 6 (1998)315-322 0 1998Elsevier Science Limited PII:
Printed in Great Britain. All rights resewed 0966-9795/98/S19.00 + 0.00
SO966-9795(97)00085-X
ELSEVIER
Surface reaction of polycrystalline FesSi alloys with oxygen and water vapor G. L. Chen,” J. H. Peng” & W. X. Xd aUniversity of Science and Technology Beijing, State Key Lab for Advanced Metal Materials, BeQing 100083, People’s Republic of China bShanghai Institute of Iron and Steel Research, Shanghai, 200940, People’s Republic of China
(Received 30 June 1997; accepted 21 August 1997)
The surface reaction of polycrystalline FesSi alloy with oxygen and water vapor was studied by means of Auger electron spectrum (AES) and X-ray photo-electron spectroscopy (XPS). The results showed that the oxygen and water vapor were first absorbed on the surface of specimen, then they preferentially reacted with the alloy constituent of Si to form SiOz. At room temperature, both the molecular absorption and dissociated absorption of water vapor as well as oxygen occurred, but of the molecular absorption was relatively larger. Increasing temperature from 20 to 200°C deferred the surface reaction. The surface reaction of forming SiOa at room temperature was stoichiometry sensitive. Based on the study of on the surface reaction, this investigation predicts that the FeaSi alloys are prone to environmental embrittlement in water vapor and that the severity of environmental embrittlement may be stoichiometry sensitive. 0 1998 Elsevier Science Limited. All rights reserved Key words: A. intermetallics,
A. silicides, B. environmental surface properties, C. vapor deposition
embrittlement,
B.
reaction between Al in aluminides and moisture in air can take place rapidly.2 Zhu et al3 have directly proved that atomic hydrogen can be produced in the bulk when FeAl is exposed to water.3 Silicides such as NisSi and Nis(Si,Ti) also exhibit the environmental embrittlement.4-10 A surface reaction: Si + 2H20+ SiO;?+ 4H, has been suggested for the environmental embrittlement. However, the surface reaction has not been studied in detail. This paper investigated the surface reaction of FesSi polycrystal with water vapor and oxygen at room temperature and elevated temperature.
1 INTRODUCTION Many ordered intermetallics have been found to be susceptible to the hydrogen-induced embrittlement. It is well known that hydrogen-induced embrittlement of ordered intermetallics is caused by moisture in air, and water as well as by hydrogen gas exposure and residual hydrogen in alloys.’ A key to understand this behavior is the chemical nature of the intermetallics. For most technological intermetallics, one component is an active element such as Al, Si, Ti, etc., which can rapidly react with moisture in air. Atomic hydrogen produced by the reaction between the active element and Hz0 diffuses into the materials, finally resulting in embrittlement. However, hydrogen-induced embrittlement involves many reaction steps such as surface absorption and surface chemical reaction of active elements with water vapor in air to produce hydrogen, hydrogen adsorption-desorption process on the surface, diffusion of hydrogen atom in materials, condensation of hydrogen to associated defects such as grain boundary or other lattice defects, crack initiation and propagation. Recently, it has been proved that the surface
2 EXPERIMENTAL The compositions of the Fe3Si alloy are Fe-14Si3Al and Fe-l ISi-3Al (at%). The alloys were prepared by vacuum induction melting using high purity iron (carbon content was less than 800 ppm), aluminum (99.99%) and silicon (99.99%). The 5 kg ingot was rolled at temperatures of 1050-600”C to a sheet with the thickness of 1.5 mm through multiple steps. In order to obtain a B2 structure, The sheets 315
316
G. L. Chen et al.
were given the following treatment: annealing at 700°C 1 hour, followed by a quench into oil. Prior to AES analysis a small FesSi specimen was fractured within the vacuum system and the AES analysis was performed immediately following the fracture, or after a reaction with a selected environment for different reaction times had taken place. The XPS specimen was a sheet of 10x 10 mm2. The clean surface required for the XPS analysis was prepared by in-situ argon-ion sputtering in the vacuum system. The kinetics of the surface reaction of Fe$i alloy with oxygen and water vapor were determined by both AES and XPS. The kinetic curve of the reaction was determined from the Auger peak-to-peak height of oxygen as a function of an exposure parameter, pt (p denotes the pressure of the chamber filled with leaked gas, and t is the exposure time). The XPS analysis allowed the chemical bonding shift to be detected which are caused by the differences in the bonding energy of pure elements and their oxides (such as Si and SiO2). Water vapor was produced by a reservoir of triple distilled water. The whole system was degassed by alternatively freezing and thawing the triple-distilled water and pumping away the dissolved gases at least three times. The inlet valves were used to control the gas pressure. The pressure of water vapor and oxygen was in the range of 10-7-10-5 Pa. When carrying on the elevated temperature test, the specimens were heated to 200°C and held for 5min, then reacted with water vapor. A primary electron beam with 5 keV energy was used. The background pressure in the spectrometer chamber was typical of the order of 1 x low9 Pa (-lo-” Torr).
Fe 0
+-Q-f b.
b-t
C.
1
Binding Energy (ev)
740
Binding Energy (ev)
Fig. 1. Augre electron spectra of Fe-14SC3Al (a) in water vapor; a., fresh surface; b., after exposure of 5x10-’ Pasec; and c., after exposure of 7x lop4 Pasec and (b) in oxygen a., fresh surface; b., after exposure of 5x lo-’ Pasec; and c., after exposure of 7.2x 10e4 Pasec.
the oxygen content on the surface of the FesSi specimen increases; Fig. l(b) shows the Auger spectra obtained from the fresh surface and from the surface after exposures of 5x low7 Pasec and 7.2x 10m4 Pasec in oxygen at room temperature. Again, the oxygen content on the surface of FesSi increases with increasing exposure (pt). Figure 2 shows the kinetic curves of oxygen on the surface water vapor and oxygen atmospheres. It can be seen that the surface adsorption both in water
3 RESULTS Exposure (Pa.%)
3.1 Oxygen on surfaces after exposure In order to study the absorption behavior of water vapor and oxygen, we measure the oxygen peak on the surface of the specimen by AES. Figure l(a) shows the Auger spectra obtained from the fresh fracture surface and from the surface after exposures of 5x 1O-7 Pasec and 7x 10e4 Pasec in water vapor at room temperature. It can be seen that the oxygen peak becomes discernible at a kinetic energy of 510 eV after exposure of 5 x 10h7 Pasec. No oxygen peak can be detected on the fresh fracture surface. With increasing of the exposure to 7x low4 Pasec,
ii 3
(b) 14000
Em
(paSe@
Fig. 2. The kinetic curve of surface reaction of Fe-14Si-3Al: (a) with water vapor; and (b) with oxygen.
317
Surface reaction of polycrystallineFe+% alloys
vapor and in oxygen are fast. The oxygen peak is already detected after an exposure of 5 x IO-’ Pasec in either environment. With increasing the exposure (pt) the slope of the oxygen peak curve first increases, then decreases and finally reaches a maximum. The results is consistent with the results shown in the surface reaction studies on Fe3Al*0 and N&Fe.’ 1 It should be noted that the oxygen peak to peak height in the oxygen atmosphere is much higher than that observed in the water vapor atmosphere. It seems that the surface absorption rate in oxygen is faster than that in water vapor. 3.2 Silicon oxide on surfaces after exposure at room temperature Figure 3 shows the Si 2p and Fe 2p photoelectron peaks obtained from the surface of the alloy after exposure to water vapor at room temperature. Only Si in elementary state can be found on the surface of Fe$i until an exposure of 1.8~ lop3 Pasec (not shown in figure). After exposure of 6.3~ 10e3 Pasec, the Si in silicon oxide SiOz state can be identified in terms of an associated photoelectron peak shift. The results directly prove that Si in the alloy can preferentially react with water vapor to form SiOz at room temperature. However, it can be seen from the XPS measurement shown in Fig. 3 that the Fe in the alloy keeps its elementary state on the surface of FesSi alloy regardless of the exposure conditions. That means that Fe in the alloy under the given test condition does not react with water vapor to form any iron oxides. Similar to the AES kinetic curve, the Si 2p peak height in the Si02 state increases with increasing exposure (pt).
Figure 4 shows the Si 2p and Fe 2p photoelectron peaks obtained from the surface of the Fe3Si alloy after exposure to oxygen at room temperature. Silicon oxide is found on the specimen surface after exposure of 4.5x 10e5 Pasec. The surface reaction of Si in oxygen atmosphere is faster than in water vapor. It is worth pointing out that the iron in the alloy can also react with oxygen, unlike with water vapor. The XPS measurement reveals that an iron oxide appears after exposure of 4.74x low4 Pasec. It is obvious that the surface reaction of Si with oxygen, as with water vapor, is preferential. 3.3 Temperature dependence of surface reaction Figure 5 shows the Si 2p and Fe 2p photoelectron peaks obtained from the surface of the alloy after
*a si2p
FeZp
a.
b.
c.
1 104
99
-721
716
711
706
Biding Energy(w) Fig. 4. The X-ray electron spectra of Fe-14Si-3Al
in oxygen at RT: (a) fresh surface; (b) after exposure of 4.5~ lop5 Pasec; and (c) after exposure of 4.7x 10M4Pasec.
\ I
Bii
Energy(w)
Fig. 3. The X-ray electron spectra of Fe-14Sk3Al in water vapor at RT: (a) fresh surface; (b) after exposure of 6.3 x 1O-3 Pasec; and (c) after exposure of 8.13 x 1O-2 Pasec.
99
721
716
711
706
Biidiig Energy(cv)
Fig. 5. The X-ray electron spectra of Fe-14%3Al in water vapor at 200°C: (a) fresh surface; (b) after exposure of 1.71 x 1O-2 Pasec; and (c) after exposure of 7.71 x 10-l Pasec.
318
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exposure in water vapor at 200°C. It shows that the silicon oxide SiOz is absent even after exposure of 1.71 x 10d2 Pasec in water vapor. In comparison with the surface reaction in water vapor at room temperature shown in Fig. 3, the reaction rate at 200°C is thus lower at least by about one order than that at room temperature. Figure 6 shows the photoelectron peaks of oxygen on the surface of Fe-14Si-3Al alloy obtained after exposure in water vapor at 200°C (Fig. 6(b)) and at room temperature (Fig. 6(a)). It can be seen from both figures that the oxygen content at surface increases with increasing exposure pt. However, the oxygen peak for the test at 200°C is much lower than that for the test at room temperature with same exposure by comparison with the reference peak of MgKa peak (occurred from the Auger transition of iron excited by X-ray). The results clearly indicate the adsorption of water vapor is reduced by the increase of the surface temperature from room temperature to 200°C. It implies a possible relationship between the decrease in surface absorption and surface reaction rate (Fig. 3 and Table 1). 3.4 Stoichiometry sensitivity of surface reaction Figure 7 shows the Si 2p and Fe 2p photoelectron peaks for the alloy of Fe-l lSi-3Al obtained after a reaction with water vapor at room temperature. The figure shows that the silicon oxide SiO2 can be clearly identified after exposure of 1.8 x 10d3 Pasec. When the reaction temperature increases from room temperature to 200°C (Fig. 8) the silicon oxide SiOz only can be identified after exposure of
Table 1. Minimum exposure (Pa-see) in water vapor for the formation of SiOs Alloy (at%)
Minimum exposure (Pasec) in water vapor for the formation of SiO2 Room temperature
Fe-14Si-3Al Fe-l 1Si-3AI
200°C
6.3~ 1O-3 1.8x 1O-3
1.71 x10-2 1.8x 1O-2
1.8 x lop2 Pasec. Similar to the alloy with higher Si content, the reaction rate is reduced by increasing temperature from room temperature to 200°C. Table 1 shows a comparison between the results of two alloys with different Si contents. It indicates that the surface reaction at room temperature exhibits stoichiometry sensitivity, but not for the surface reaction at 200°C. Table 1 shows again that when reaction temperature increases from room temperature to 200°C the reaction rate of Fe-l lSi3Al alloy is decreased about one order.
4 DISCUSSION The above results clearly indicate that the behavior of water vapor adsorption and desorption is a significant part of the surface catalytic reaction. It can be seen that the oxygen peak on the surface was already detected after a very short exposure of 5x 10e7 Pasec in both water vapor and oxygen environment, while an oxygen peak on the fresh fracture fresh surface cannot be detected by AES. Such short exposure is much less than the minimum exposure for the formation of silicon oxide on the surface (6.3 x 10e3 Pasec in water vapor and
01s
I
,l
536
531
541
536
531 J
Bidiig
Energy (ev)
Biding
Energy (ev)
721 shdiig
Fig. 6. The oxygen content on the surface of Fe-14Si-3Al at different temperature in water vapor: (a) at room temperature; and (b) at 200”C-a., fresh surface; b., after exposure of 1.7x lop3 Pasec; c., after exposure of 7.71 x10-’ Pasec.
716
Energy(ev)
Fig. 7. The X-ray electron spectra of Fe-l lSi-3Al in water vapor at RT: (a) fresh surface; (b) after exposure of 3x10-’ Pasec; and (c) after exposure of 1.8x 10Y3Pasec.
Surface reaction of polycrystallineFe+% alloys
a.
b.
c.
I
104
-
99
1
1
721
b
8
716
I
711
I
I
706
Binding Energy (ev) Fig. 8. The X-ray electron spectra of Fe-1 lSi-3Al in water
vapor at 200°C: (a) fresh surface; (b) after exposure of 1.8x lop2 Pasec; and (c) after exposure of 7.8 x 10e2 Pasec.
4.5 x 10V5 Pasec in oxygen). Obviously, therefore,
this oxygen peak on the surface first occurs due to the surface adsorption of water vapor or oxygen. The isothermal kinetic curve of absorption can be fitted by the well known Langmuir adsorption isotherm. The isotherm can be used to gain insight into the adsorption process as a molecule adsorption or as a dissociative adsorption.12 To accomplish this, a model system is first proposed and then the isotherm obtained from the model is compared with the experimental data. If the curve predicted by the model agrees with the experimental one, the model may reasonably describe what is occurring physically in the real system. Here, two models will be postulated-one in which water vapor or oxygen is absorbed as molecules on the surface, and the other in which dissociative water vapor is absorbed as H+ and OH-, or dissociative oxygen is absorbed as atomic 0. Because the surface reaction is a catalytic process, the observed oxygen contents on the surface represent the adsorption of water vapor (or oxygen) on the active sites of surface. If it is an adsorption of molecules, the following equation should be used:13
Pt= cH2O.S
1 KACS
319
number of active sites per unit mass divided by Avogadro’s number (mol.g.cat-I), & is the adsorption equilibrium constant. The data in Fig. 2(a) and (b) are replotted in Fig. 9(a) and (b) in the form of eqn (1) and in Fig. 10(a) and (b) in the form of eqn (2). The curves in Fig. 10(a) and (b) show a slight but definite curvature. Therefore, there is a doubt as to whether these data really conform to a model of the dissociative adsorption. In contrast, Fig. 9(a) and (b) indicates two good straight lines, providing a support to the postulate of the adsorption of molecules either in water vapor or in oxygen. The coefficients of correlation of the regressions, R2 , in both cases correspond to 0.999. However, it should be noted that the curvature in Fig. 10 is not large enough to provide an unequivocal conclusion. Therefore, additional precise spectroscopic and tracer experiments are required to further confirm the molecular adsorption model. Gleason14 studied the water absorption behavior on NiAl, FeAl and TiAl surfaces at various temperatures by valence-band spectra. He proved some water was molecularly bound to these surfaces at temperature near 200°K in terms
2.0x104
Fe3Si. 02, RT, Molecular
adsorption
1 _,’
1.5x104-
0.0
2.0~10~
I . I . 0 4.0~10~ 6.0~10~ 8.0x106
.
I s 1.0~10~
PT (Pa.Sec) 3.0X1041
Fe3Si. H20, RT, Molecular
adsorption
,’
/,’
+p” cs
If it is a dissociative adsorption, equation should be used:13
the following 0.0
(2)
2.0~10~
4.0~10~
Fig. 9. The molecule adsorption
Where C~~0.s is the water content on the surface, i.e. the oxygen content on the surface; Cs is the
6.0~10~
8.0X10”
LOxlO
PT (Pa.Sec)
from kinetic data in Fig. 2: (a) with oxygen; and (b) with water vapor. PT, exposure; I(O/ Fe), the relative content of oxygen on the surface; dash line, regression line.
320
G. L. Chen (a)
6X10-*
et al.
Fe3Si, 02, RT, Dissociative adsorption 1
o.oY . 0.0
M
S.0xI04
.
I
LOXlO-
.
I
.
1.5X10-,
I
*
I
’
1
-
I
’
2.0x10-32.5X10--‘3.0x10-33.5x10-’
PTA12 (Pa Sec)ll2 (b)
1.0x10-’
Fe3Si, H20, RT, Dissociative adsorption
6.0x10-2-
PT/1/2 (Pa Sec)l/2 Fig. 10. The
adsorption curves from the kinetic data in Fig. 2: (a) with oxygen; and (b) with water vapor. PT, exposure; I(O/Fe), the relative content of oxygen on the surface.
of the occurrence of 1b2 molecular orbital emission of water (valence binding energy, 13ev15T16). As temperature was increased, dissociative absorption on the surfaces gradually occurred instead of molecular absorption. Considering both our results and the data from literature, we assume that the dissociative absorption occurs at some active sites on the catalyst surface of Fe& alloy while the molecular absorption of water vapor on FesSi may be the main absorption behavior at room temperature. The active sites on the surface may be grain boundaries, defects areas and Si sites, etc. Figure 6 indicated the influence of temperature on the surface absorption of water vapor. It indicated that the oxygen peak for the test at 200°C is much lower than that for the test at room temperature (with same exposure). The results imply the adsorption of water vapor on the surface is decreased by desorption due to the increase of the surface temperature. It is well know that the forces of attraction between the gas molecules and the solid surface are weak, and the amount of gas physically absorbed decreases rapidly with increasing temperature. However, as indicated by
Ref. 14, increasing temperature should accelerate the dissociation absorption of water vapor. The change in the absorption mechanism should also influence on the amount of absorption of water vapor. The water vapor absorbed on the surface preferentially reacts with Si in FesSi alloy to form Si02. Under the experimental exposure condition the water vapor could not react with Fe in FesSi alloys to form iron oxide. However, the oxygen absorbed on the surface not only reacts with Si to form SiO*, but also reacts with Fe in Fe$i to form iron oxide (the valence of iron is hard to determine). The preferential oxidation of Si in both atmosphere is true. However, it should be noted that the SiOz could be found after relatively shorter exposure in oxygen. Temperature has a large influence on the rate of the surface reaction of forming SiO2 in water vapor. The increase in reaction temperature from room temperature to 200°C deferred the surface reaction by one order of exposure for two FesSi alloys. If we integrate this fact with the temperature dependence of absorption of water vapor on the surface, it implies that the absorption step is
Surface reaction of polycrystalline Fe$‘i alloys
the controlling step of the surface reaction at 200°C. That means that the decrease in the adsorption of water vapor at 200°C leads to a slow-down of the surface reaction of forming SiO2. The surface reaction at room temperature appears to be composition sensitive. The silicon oxide Si02 can be identified after an exposure of 6.3 x 10m3 Pasec for the Fe-14Si-3Al alloy, but only after an exposure of 1.8 x 1O-3 Pasec for the Fe-l ISi-3Al. Decreasing Si content from 14 to 11% accelerates the surface reaction of forming Si02. Recently, it has been found that the severity of environmental embrittlement of N&(Si,Ti) alloys is strongly stoichiometry sensitive.4 A small change in composition from Ni-9*5Ti-1 laOSi to Ni-9*5TilO.OSi reduced the tensile ductility from 37.2 to 16.3% in water at room temperature and strain rate of 8.3 x lop4 s-l. Decreasing Si content only from 11 to 10% increased the severity of environmental embrittlement of N&(Si,Ti) alloys. It looks as if these two phenomena are correlated to each other in terms of reaction sensitivity. Other reaction products of the surface reaction may be atomic H and molecular Hz. It is because another reversible surface catalytic reaction H+2H can happen on the surface. The atomic hydrogen produced by the surface reactions, then, should diffuse into the bulk, leading to embrittlement of Fe3Si alloys. Based on the surface reaction study, we predict that Fe3Si alloys are prone to environmental embrittlement in water vapor and the severity of environmental embrittlement may be stoichemetry sensitive. However, limited ductility in Fe3Si alloys exhibit the brittleness in vacuum as in air. This may be due to severe intrinsic brittleness because of a large amount of Si. We believe the improvement of both intrinsic brittleness and environmental embrittlement are both important for the development of Fe3Si based alloys.
2.
3.
4. 5.
321
behavior is more important. Increasing temperature from room temperature to 200°C decreased the surface absorption of water vapor. A Si 2p binding energy shift which points towards the formation of SiOz in a water vapor atmosphere can be observed after exposure of 6.3 x 1O-3 Pasec. Increasing temperature from room temperature to 200°C deferred the surface reaction, leading to an increase of the minimum exposure pt of forming SiOz. Si in Fe3Si based alloys preferentially reacted with water vapor absorbed on the surface. The Fe in the alloy under the given test condition did not react with water vapor to form any iron oxides. For the oxygen atmosphere, besides the preferential oxidation behavior of Si in the alloys, the Fe in the alloys also can react with oxygen to form iron oxide. The minimum exposure pt for the forming Si02 was shorter in oxygen than that in water vapor. The surface reaction for forming Si02 at room temperature was stoichiometry sensitive. Based on the surface reaction study, we predict that Fe3Si alloys are prone to environmental embrittlement in water vapor and the severity of environmental embrittlement may be stoichiometry sensitive.
ACKNOWLEDGEMENTS The authors are grateful to Professor Wagner for carefully reviewing the manuscript. This research was supported by the NNSFC.
REFERENCES 5 CONCLUSIONS 1. Water vapor and oxygen rapidly absorbed on the surface of Fe3Si based alloys at room temperature and 200°C. The isothermal kinetic curve of absorption can be fitted by Langmuir adsorption isotherm. The isotherm can be used to gain insight into the adsorption process as a molecule adsorption or as a dissociative adsorption. At room temperature, both the molecular absorption and dissociated absorption of water vapor (or oxygen) occurred, but probably the molecular absorption
1. Tagkasugi, T., Misawa T. and Saitoh H., Mater. Sci. Eng., 1995, A192/193,413. 2. Liu, C. T., Lee, E. H. and McKamey, C. G., Scripta Metall, 1989, 23, 875. 3. Zhu, Y. F., Liu, C. T. and Chen, C. H., Scripta Metall., 1996,35, 1435. 4. Kumar, K. S., Liu, C. T. and Wright, J. L., Zntermetallics, 1996, 4, 309. 5. Takasugi, T., Zntermetallics,1996, 4, S 181. 6. Tanabe, H., Douki, T., Takasugi, T. and Misawa, T., Intermetallics, 1996, 4, S 189. 7. Liu, C. T., George, E. P. and Oliver, C., Zntermetallics, 1996, 4, 77. 8. Takasugi, T., Ma, C. L. and Hanada, S., Mater. Sci. Eng., 1995, A192/193,407. 9. Takasugi, T. and Yoshida, M., J. Nater. Sci., 1991, 26, 3032.
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10. Takasugi, T., Suenaga, H. and Izumi, O., J. Muter. Sci., 1991,26, 1179. 11. Wan, X. J., Zhu, J. H., Jing, K. L. and Liu, C. T., Scripta Metall., 1994, 31, 677. 12. Fogler, H. S., Elements of Chemical Reaction Engineering. Prentice-Hall, Englewood Cliffs, NJ, 1986, p. 238.
13. Chia, W. J. and Chung, Y. W., Intermetallics, 1996,4,283. 14. Gleason, N. R., Gerken, C. A. and Strongin, D. R., App. Surf. Sci., 1993,12, 215. 15. Szalkowski, S. J., J. Chem. Phys., 1982, 7, 5224. 16. Thiel, P. A. and Madey, T. E., Surf. Sci. Rep., 1987, 7, 271.
Printed in Great Britain. All rights resewed 0966-9795/98/S19.00 + 0.00
SO966-9795(97)00085-X
ELSEVIER
Surface reaction of polycrystalline FesSi alloys with oxygen and water vapor G. L. Chen,” J. H. Peng” & W. X. Xd aUniversity of Science and Technology Beijing, State Key Lab for Advanced Metal Materials, BeQing 100083, People’s Republic of China bShanghai Institute of Iron and Steel Research, Shanghai, 200940, People’s Republic of China
(Received 30 June 1997; accepted 21 August 1997)
The surface reaction of polycrystalline FesSi alloy with oxygen and water vapor was studied by means of Auger electron spectrum (AES) and X-ray photo-electron spectroscopy (XPS). The results showed that the oxygen and water vapor were first absorbed on the surface of specimen, then they preferentially reacted with the alloy constituent of Si to form SiOz. At room temperature, both the molecular absorption and dissociated absorption of water vapor as well as oxygen occurred, but of the molecular absorption was relatively larger. Increasing temperature from 20 to 200°C deferred the surface reaction. The surface reaction of forming SiOa at room temperature was stoichiometry sensitive. Based on the study of on the surface reaction, this investigation predicts that the FeaSi alloys are prone to environmental embrittlement in water vapor and that the severity of environmental embrittlement may be stoichiometry sensitive. 0 1998 Elsevier Science Limited. All rights reserved Key words: A. intermetallics,
A. silicides, B. environmental surface properties, C. vapor deposition
embrittlement,
B.
reaction between Al in aluminides and moisture in air can take place rapidly.2 Zhu et al3 have directly proved that atomic hydrogen can be produced in the bulk when FeAl is exposed to water.3 Silicides such as NisSi and Nis(Si,Ti) also exhibit the environmental embrittlement.4-10 A surface reaction: Si + 2H20+ SiO;?+ 4H, has been suggested for the environmental embrittlement. However, the surface reaction has not been studied in detail. This paper investigated the surface reaction of FesSi polycrystal with water vapor and oxygen at room temperature and elevated temperature.
1 INTRODUCTION Many ordered intermetallics have been found to be susceptible to the hydrogen-induced embrittlement. It is well known that hydrogen-induced embrittlement of ordered intermetallics is caused by moisture in air, and water as well as by hydrogen gas exposure and residual hydrogen in alloys.’ A key to understand this behavior is the chemical nature of the intermetallics. For most technological intermetallics, one component is an active element such as Al, Si, Ti, etc., which can rapidly react with moisture in air. Atomic hydrogen produced by the reaction between the active element and Hz0 diffuses into the materials, finally resulting in embrittlement. However, hydrogen-induced embrittlement involves many reaction steps such as surface absorption and surface chemical reaction of active elements with water vapor in air to produce hydrogen, hydrogen adsorption-desorption process on the surface, diffusion of hydrogen atom in materials, condensation of hydrogen to associated defects such as grain boundary or other lattice defects, crack initiation and propagation. Recently, it has been proved that the surface
2 EXPERIMENTAL The compositions of the Fe3Si alloy are Fe-14Si3Al and Fe-l ISi-3Al (at%). The alloys were prepared by vacuum induction melting using high purity iron (carbon content was less than 800 ppm), aluminum (99.99%) and silicon (99.99%). The 5 kg ingot was rolled at temperatures of 1050-600”C to a sheet with the thickness of 1.5 mm through multiple steps. In order to obtain a B2 structure, The sheets 315
316
G. L. Chen et al.
were given the following treatment: annealing at 700°C 1 hour, followed by a quench into oil. Prior to AES analysis a small FesSi specimen was fractured within the vacuum system and the AES analysis was performed immediately following the fracture, or after a reaction with a selected environment for different reaction times had taken place. The XPS specimen was a sheet of 10x 10 mm2. The clean surface required for the XPS analysis was prepared by in-situ argon-ion sputtering in the vacuum system. The kinetics of the surface reaction of Fe$i alloy with oxygen and water vapor were determined by both AES and XPS. The kinetic curve of the reaction was determined from the Auger peak-to-peak height of oxygen as a function of an exposure parameter, pt (p denotes the pressure of the chamber filled with leaked gas, and t is the exposure time). The XPS analysis allowed the chemical bonding shift to be detected which are caused by the differences in the bonding energy of pure elements and their oxides (such as Si and SiO2). Water vapor was produced by a reservoir of triple distilled water. The whole system was degassed by alternatively freezing and thawing the triple-distilled water and pumping away the dissolved gases at least three times. The inlet valves were used to control the gas pressure. The pressure of water vapor and oxygen was in the range of 10-7-10-5 Pa. When carrying on the elevated temperature test, the specimens were heated to 200°C and held for 5min, then reacted with water vapor. A primary electron beam with 5 keV energy was used. The background pressure in the spectrometer chamber was typical of the order of 1 x low9 Pa (-lo-” Torr).
Fe 0
+-Q-f b.
b-t
C.
1
Binding Energy (ev)
740
Binding Energy (ev)
Fig. 1. Augre electron spectra of Fe-14SC3Al (a) in water vapor; a., fresh surface; b., after exposure of 5x10-’ Pasec; and c., after exposure of 7x lop4 Pasec and (b) in oxygen a., fresh surface; b., after exposure of 5x lo-’ Pasec; and c., after exposure of 7.2x 10e4 Pasec.
the oxygen content on the surface of the FesSi specimen increases; Fig. l(b) shows the Auger spectra obtained from the fresh surface and from the surface after exposures of 5x low7 Pasec and 7.2x 10m4 Pasec in oxygen at room temperature. Again, the oxygen content on the surface of FesSi increases with increasing exposure (pt). Figure 2 shows the kinetic curves of oxygen on the surface water vapor and oxygen atmospheres. It can be seen that the surface adsorption both in water
3 RESULTS Exposure (Pa.%)
3.1 Oxygen on surfaces after exposure In order to study the absorption behavior of water vapor and oxygen, we measure the oxygen peak on the surface of the specimen by AES. Figure l(a) shows the Auger spectra obtained from the fresh fracture surface and from the surface after exposures of 5x 1O-7 Pasec and 7x 10e4 Pasec in water vapor at room temperature. It can be seen that the oxygen peak becomes discernible at a kinetic energy of 510 eV after exposure of 5 x 10h7 Pasec. No oxygen peak can be detected on the fresh fracture surface. With increasing of the exposure to 7x low4 Pasec,
ii 3
(b) 14000
Em
(paSe@
Fig. 2. The kinetic curve of surface reaction of Fe-14Si-3Al: (a) with water vapor; and (b) with oxygen.
317
Surface reaction of polycrystallineFe+% alloys
vapor and in oxygen are fast. The oxygen peak is already detected after an exposure of 5 x IO-’ Pasec in either environment. With increasing the exposure (pt) the slope of the oxygen peak curve first increases, then decreases and finally reaches a maximum. The results is consistent with the results shown in the surface reaction studies on Fe3Al*0 and N&Fe.’ 1 It should be noted that the oxygen peak to peak height in the oxygen atmosphere is much higher than that observed in the water vapor atmosphere. It seems that the surface absorption rate in oxygen is faster than that in water vapor. 3.2 Silicon oxide on surfaces after exposure at room temperature Figure 3 shows the Si 2p and Fe 2p photoelectron peaks obtained from the surface of the alloy after exposure to water vapor at room temperature. Only Si in elementary state can be found on the surface of Fe$i until an exposure of 1.8~ lop3 Pasec (not shown in figure). After exposure of 6.3~ 10e3 Pasec, the Si in silicon oxide SiOz state can be identified in terms of an associated photoelectron peak shift. The results directly prove that Si in the alloy can preferentially react with water vapor to form SiOz at room temperature. However, it can be seen from the XPS measurement shown in Fig. 3 that the Fe in the alloy keeps its elementary state on the surface of FesSi alloy regardless of the exposure conditions. That means that Fe in the alloy under the given test condition does not react with water vapor to form any iron oxides. Similar to the AES kinetic curve, the Si 2p peak height in the Si02 state increases with increasing exposure (pt).
Figure 4 shows the Si 2p and Fe 2p photoelectron peaks obtained from the surface of the Fe3Si alloy after exposure to oxygen at room temperature. Silicon oxide is found on the specimen surface after exposure of 4.5x 10e5 Pasec. The surface reaction of Si in oxygen atmosphere is faster than in water vapor. It is worth pointing out that the iron in the alloy can also react with oxygen, unlike with water vapor. The XPS measurement reveals that an iron oxide appears after exposure of 4.74x low4 Pasec. It is obvious that the surface reaction of Si with oxygen, as with water vapor, is preferential. 3.3 Temperature dependence of surface reaction Figure 5 shows the Si 2p and Fe 2p photoelectron peaks obtained from the surface of the alloy after
*a si2p
FeZp
a.
b.
c.
1 104
99
-721
716
711
706
Biding Energy(w) Fig. 4. The X-ray electron spectra of Fe-14Si-3Al
in oxygen at RT: (a) fresh surface; (b) after exposure of 4.5~ lop5 Pasec; and (c) after exposure of 4.7x 10M4Pasec.
\ I
Bii
Energy(w)
Fig. 3. The X-ray electron spectra of Fe-14Sk3Al in water vapor at RT: (a) fresh surface; (b) after exposure of 6.3 x 1O-3 Pasec; and (c) after exposure of 8.13 x 1O-2 Pasec.
99
721
716
711
706
Biidiig Energy(cv)
Fig. 5. The X-ray electron spectra of Fe-14%3Al in water vapor at 200°C: (a) fresh surface; (b) after exposure of 1.71 x 1O-2 Pasec; and (c) after exposure of 7.71 x 10-l Pasec.
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exposure in water vapor at 200°C. It shows that the silicon oxide SiOz is absent even after exposure of 1.71 x 10d2 Pasec in water vapor. In comparison with the surface reaction in water vapor at room temperature shown in Fig. 3, the reaction rate at 200°C is thus lower at least by about one order than that at room temperature. Figure 6 shows the photoelectron peaks of oxygen on the surface of Fe-14Si-3Al alloy obtained after exposure in water vapor at 200°C (Fig. 6(b)) and at room temperature (Fig. 6(a)). It can be seen from both figures that the oxygen content at surface increases with increasing exposure pt. However, the oxygen peak for the test at 200°C is much lower than that for the test at room temperature with same exposure by comparison with the reference peak of MgKa peak (occurred from the Auger transition of iron excited by X-ray). The results clearly indicate the adsorption of water vapor is reduced by the increase of the surface temperature from room temperature to 200°C. It implies a possible relationship between the decrease in surface absorption and surface reaction rate (Fig. 3 and Table 1). 3.4 Stoichiometry sensitivity of surface reaction Figure 7 shows the Si 2p and Fe 2p photoelectron peaks for the alloy of Fe-l lSi-3Al obtained after a reaction with water vapor at room temperature. The figure shows that the silicon oxide SiO2 can be clearly identified after exposure of 1.8 x 10d3 Pasec. When the reaction temperature increases from room temperature to 200°C (Fig. 8) the silicon oxide SiOz only can be identified after exposure of
Table 1. Minimum exposure (Pa-see) in water vapor for the formation of SiOs Alloy (at%)
Minimum exposure (Pasec) in water vapor for the formation of SiO2 Room temperature
Fe-14Si-3Al Fe-l 1Si-3AI
200°C
6.3~ 1O-3 1.8x 1O-3
1.71 x10-2 1.8x 1O-2
1.8 x lop2 Pasec. Similar to the alloy with higher Si content, the reaction rate is reduced by increasing temperature from room temperature to 200°C. Table 1 shows a comparison between the results of two alloys with different Si contents. It indicates that the surface reaction at room temperature exhibits stoichiometry sensitivity, but not for the surface reaction at 200°C. Table 1 shows again that when reaction temperature increases from room temperature to 200°C the reaction rate of Fe-l lSi3Al alloy is decreased about one order.
4 DISCUSSION The above results clearly indicate that the behavior of water vapor adsorption and desorption is a significant part of the surface catalytic reaction. It can be seen that the oxygen peak on the surface was already detected after a very short exposure of 5x 10e7 Pasec in both water vapor and oxygen environment, while an oxygen peak on the fresh fracture fresh surface cannot be detected by AES. Such short exposure is much less than the minimum exposure for the formation of silicon oxide on the surface (6.3 x 10e3 Pasec in water vapor and
01s
I
,l
536
531
541
536
531 J
Bidiig
Energy (ev)
Biding
Energy (ev)
721 shdiig
Fig. 6. The oxygen content on the surface of Fe-14Si-3Al at different temperature in water vapor: (a) at room temperature; and (b) at 200”C-a., fresh surface; b., after exposure of 1.7x lop3 Pasec; c., after exposure of 7.71 x10-’ Pasec.
716
Energy(ev)
Fig. 7. The X-ray electron spectra of Fe-l lSi-3Al in water vapor at RT: (a) fresh surface; (b) after exposure of 3x10-’ Pasec; and (c) after exposure of 1.8x 10Y3Pasec.
Surface reaction of polycrystallineFe+% alloys
a.
b.
c.
I
104
-
99
1
1
721
b
8
716
I
711
I
I
706
Binding Energy (ev) Fig. 8. The X-ray electron spectra of Fe-1 lSi-3Al in water
vapor at 200°C: (a) fresh surface; (b) after exposure of 1.8x lop2 Pasec; and (c) after exposure of 7.8 x 10e2 Pasec.
4.5 x 10V5 Pasec in oxygen). Obviously, therefore,
this oxygen peak on the surface first occurs due to the surface adsorption of water vapor or oxygen. The isothermal kinetic curve of absorption can be fitted by the well known Langmuir adsorption isotherm. The isotherm can be used to gain insight into the adsorption process as a molecule adsorption or as a dissociative adsorption.12 To accomplish this, a model system is first proposed and then the isotherm obtained from the model is compared with the experimental data. If the curve predicted by the model agrees with the experimental one, the model may reasonably describe what is occurring physically in the real system. Here, two models will be postulated-one in which water vapor or oxygen is absorbed as molecules on the surface, and the other in which dissociative water vapor is absorbed as H+ and OH-, or dissociative oxygen is absorbed as atomic 0. Because the surface reaction is a catalytic process, the observed oxygen contents on the surface represent the adsorption of water vapor (or oxygen) on the active sites of surface. If it is an adsorption of molecules, the following equation should be used:13
Pt= cH2O.S
1 KACS
319
number of active sites per unit mass divided by Avogadro’s number (mol.g.cat-I), & is the adsorption equilibrium constant. The data in Fig. 2(a) and (b) are replotted in Fig. 9(a) and (b) in the form of eqn (1) and in Fig. 10(a) and (b) in the form of eqn (2). The curves in Fig. 10(a) and (b) show a slight but definite curvature. Therefore, there is a doubt as to whether these data really conform to a model of the dissociative adsorption. In contrast, Fig. 9(a) and (b) indicates two good straight lines, providing a support to the postulate of the adsorption of molecules either in water vapor or in oxygen. The coefficients of correlation of the regressions, R2 , in both cases correspond to 0.999. However, it should be noted that the curvature in Fig. 10 is not large enough to provide an unequivocal conclusion. Therefore, additional precise spectroscopic and tracer experiments are required to further confirm the molecular adsorption model. Gleason14 studied the water absorption behavior on NiAl, FeAl and TiAl surfaces at various temperatures by valence-band spectra. He proved some water was molecularly bound to these surfaces at temperature near 200°K in terms
2.0x104
Fe3Si. 02, RT, Molecular
adsorption
1 _,’
1.5x104-
0.0
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.
I s 1.0~10~
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Fe3Si. H20, RT, Molecular
adsorption
,’
/,’
+p” cs
If it is a dissociative adsorption, equation should be used:13
the following 0.0
(2)
2.0~10~
4.0~10~
Fig. 9. The molecule adsorption
Where C~~0.s is the water content on the surface, i.e. the oxygen content on the surface; Cs is the
6.0~10~
8.0X10”
LOxlO
PT (Pa.Sec)
from kinetic data in Fig. 2: (a) with oxygen; and (b) with water vapor. PT, exposure; I(O/ Fe), the relative content of oxygen on the surface; dash line, regression line.
320
G. L. Chen (a)
6X10-*
et al.
Fe3Si, 02, RT, Dissociative adsorption 1
o.oY . 0.0
M
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.
I
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.
I
.
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*
I
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-
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’
2.0x10-32.5X10--‘3.0x10-33.5x10-’
PTA12 (Pa Sec)ll2 (b)
1.0x10-’
Fe3Si, H20, RT, Dissociative adsorption
6.0x10-2-
PT/1/2 (Pa Sec)l/2 Fig. 10. The
adsorption curves from the kinetic data in Fig. 2: (a) with oxygen; and (b) with water vapor. PT, exposure; I(O/Fe), the relative content of oxygen on the surface.
of the occurrence of 1b2 molecular orbital emission of water (valence binding energy, 13ev15T16). As temperature was increased, dissociative absorption on the surfaces gradually occurred instead of molecular absorption. Considering both our results and the data from literature, we assume that the dissociative absorption occurs at some active sites on the catalyst surface of Fe& alloy while the molecular absorption of water vapor on FesSi may be the main absorption behavior at room temperature. The active sites on the surface may be grain boundaries, defects areas and Si sites, etc. Figure 6 indicated the influence of temperature on the surface absorption of water vapor. It indicated that the oxygen peak for the test at 200°C is much lower than that for the test at room temperature (with same exposure). The results imply the adsorption of water vapor on the surface is decreased by desorption due to the increase of the surface temperature. It is well know that the forces of attraction between the gas molecules and the solid surface are weak, and the amount of gas physically absorbed decreases rapidly with increasing temperature. However, as indicated by
Ref. 14, increasing temperature should accelerate the dissociation absorption of water vapor. The change in the absorption mechanism should also influence on the amount of absorption of water vapor. The water vapor absorbed on the surface preferentially reacts with Si in FesSi alloy to form Si02. Under the experimental exposure condition the water vapor could not react with Fe in FesSi alloys to form iron oxide. However, the oxygen absorbed on the surface not only reacts with Si to form SiO*, but also reacts with Fe in Fe$i to form iron oxide (the valence of iron is hard to determine). The preferential oxidation of Si in both atmosphere is true. However, it should be noted that the SiOz could be found after relatively shorter exposure in oxygen. Temperature has a large influence on the rate of the surface reaction of forming SiO2 in water vapor. The increase in reaction temperature from room temperature to 200°C deferred the surface reaction by one order of exposure for two FesSi alloys. If we integrate this fact with the temperature dependence of absorption of water vapor on the surface, it implies that the absorption step is
Surface reaction of polycrystalline Fe$‘i alloys
the controlling step of the surface reaction at 200°C. That means that the decrease in the adsorption of water vapor at 200°C leads to a slow-down of the surface reaction of forming SiO2. The surface reaction at room temperature appears to be composition sensitive. The silicon oxide Si02 can be identified after an exposure of 6.3 x 10m3 Pasec for the Fe-14Si-3Al alloy, but only after an exposure of 1.8 x 1O-3 Pasec for the Fe-l ISi-3Al. Decreasing Si content from 14 to 11% accelerates the surface reaction of forming Si02. Recently, it has been found that the severity of environmental embrittlement of N&(Si,Ti) alloys is strongly stoichiometry sensitive.4 A small change in composition from Ni-9*5Ti-1 laOSi to Ni-9*5TilO.OSi reduced the tensile ductility from 37.2 to 16.3% in water at room temperature and strain rate of 8.3 x lop4 s-l. Decreasing Si content only from 11 to 10% increased the severity of environmental embrittlement of N&(Si,Ti) alloys. It looks as if these two phenomena are correlated to each other in terms of reaction sensitivity. Other reaction products of the surface reaction may be atomic H and molecular Hz. It is because another reversible surface catalytic reaction H+2H can happen on the surface. The atomic hydrogen produced by the surface reactions, then, should diffuse into the bulk, leading to embrittlement of Fe3Si alloys. Based on the surface reaction study, we predict that Fe3Si alloys are prone to environmental embrittlement in water vapor and the severity of environmental embrittlement may be stoichemetry sensitive. However, limited ductility in Fe3Si alloys exhibit the brittleness in vacuum as in air. This may be due to severe intrinsic brittleness because of a large amount of Si. We believe the improvement of both intrinsic brittleness and environmental embrittlement are both important for the development of Fe3Si based alloys.
2.
3.
4. 5.
321
behavior is more important. Increasing temperature from room temperature to 200°C decreased the surface absorption of water vapor. A Si 2p binding energy shift which points towards the formation of SiOz in a water vapor atmosphere can be observed after exposure of 6.3 x 1O-3 Pasec. Increasing temperature from room temperature to 200°C deferred the surface reaction, leading to an increase of the minimum exposure pt of forming SiOz. Si in Fe3Si based alloys preferentially reacted with water vapor absorbed on the surface. The Fe in the alloy under the given test condition did not react with water vapor to form any iron oxides. For the oxygen atmosphere, besides the preferential oxidation behavior of Si in the alloys, the Fe in the alloys also can react with oxygen to form iron oxide. The minimum exposure pt for the forming Si02 was shorter in oxygen than that in water vapor. The surface reaction for forming Si02 at room temperature was stoichiometry sensitive. Based on the surface reaction study, we predict that Fe3Si alloys are prone to environmental embrittlement in water vapor and the severity of environmental embrittlement may be stoichiometry sensitive.
ACKNOWLEDGEMENTS The authors are grateful to Professor Wagner for carefully reviewing the manuscript. This research was supported by the NNSFC.
REFERENCES 5 CONCLUSIONS 1. Water vapor and oxygen rapidly absorbed on the surface of Fe3Si based alloys at room temperature and 200°C. The isothermal kinetic curve of absorption can be fitted by Langmuir adsorption isotherm. The isotherm can be used to gain insight into the adsorption process as a molecule adsorption or as a dissociative adsorption. At room temperature, both the molecular absorption and dissociated absorption of water vapor (or oxygen) occurred, but probably the molecular absorption
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10. Takasugi, T., Suenaga, H. and Izumi, O., J. Muter. Sci., 1991,26, 1179. 11. Wan, X. J., Zhu, J. H., Jing, K. L. and Liu, C. T., Scripta Metall., 1994, 31, 677. 12. Fogler, H. S., Elements of Chemical Reaction Engineering. Prentice-Hall, Englewood Cliffs, NJ, 1986, p. 238.
13. Chia, W. J. and Chung, Y. W., Intermetallics, 1996,4,283. 14. Gleason, N. R., Gerken, C. A. and Strongin, D. R., App. Surf. Sci., 1993,12, 215. 15. Szalkowski, S. J., J. Chem. Phys., 1982, 7, 5224. 16. Thiel, P. A. and Madey, T. E., Surf. Sci. Rep., 1987, 7, 271.