Effects of gaseous hydrogen and water vapor pressure on environmental embrittlement of Ni3Al

Intermetallics 5 (1997) 48349-O 0 1997 Elsevier Science Limited Printed in Great Britain. All rights reserved

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Effects of gaseous hydrogen and water vapor pressure on environmental embrittlement of Ni3Al K.H. Lee,4* J. T. Lukowskib & C. L. White” uDepartment of Metallurgical and Materials Engineering, Michigan Technological University, 1400 Townsend Dr., Houghton, MI 49931, USA hDepartment of Electrical Engineering Technology, Michigan Technological University, 1400 Townsend Dr., Houghton, MI 49931, USA

(Received 9 July 1996; accepted 11 February

1997)

Tensile ductilities of undoped and boron-doped NisAI have been determined under Hz and Hz0 pressures in the range of 10e3 N 1O’Pa. In agreement with other studies, boron-doped Ni3Al was found to be insensitive to water vapor, while undoped Ni3AI is severely embrittled by water vapor at pressures greater than about 1OW3Pa. This observation is consistent with previous conclusions that the ductilizing effect of boron additions in Ni3Al is at least partly due to its remedial effect on embrittlement by environmental water vapor. For undoped Ni3AI in Hz and HzO, and for boron-doped Ni3AI in Hz, ductilities increased as the pressure of the embrittling gas was decreased. In each case, however, a critical gas pressure was observed, below which no further improvement in tensile ductility was observed. The limiting ductility of each alloy at very low pressures (i.e., under ultrahigh vacuum) is taken to represent the intrinsic ductility of the alloy. Comparison of intrinsic ductilities for boron-doped and undoped Ni3Al indicates that boron additions also exert an intrinsically ductilizing effect, independent of the remedial effect associated with embrittlement by environmental water vapor. 0 1997 Elsevier Science Limited Key words: A. nickel aluminide, based on Ni3Al, B. environmental

INTRODUCTION

Atomic hydrogen produced by such a reaction has been implicitly assumed to segregate at grain boundaries and cause intergranular embrittlement. Significant improvements in ductility have been observed in vacuum, at low temperatures, and in dry oxygen environments.3*4 Vacuum and low temperature environments presumably reduce both the driving force and kinetics of reactions such as the one in eqn (1). The beneficial role of oxygen has not been examined in detail, but it may involve competition between 02 and Hz0 for unreacted aluminum, via a reaction of the form:

In recent years, there have been several convincing reports indicating that the intergranular brittleness normally observed in N&Al polycrystals at ambient temperature and pressure is at least partially due to an environmental effect associated with trace levels of moisture present in many testing environments.‘y2 Although careful elimination of water vapor from testi:ng environments does not eliminate intergranular fracture, significant improvements in tensile ductility of Ni3Al polycrystals, as high as 23’s elongation in ultrahigh vacuum, have been reported.3 Liu first proposed that the reaction between moisture in ordinary testing environments and aluminum in Ni3Al generates atomic hydrogen., possibly according to the reaction in eqn (I).’

2Al+ 3H&is) *To whom correspondence

= A1203+

6H(,d,)

embrittlement.

2Al+ ; 02(&,)= Al203

(2)

In contrast to Ni3A1 (without boron), borondoped ductile N&Al is not very sensitive to the moisture-induced environmental influences that embrittle the undoped material. Boron-doped Ni3Al, for example, exhibits the same high level of ductility in water, vacuum, and oxygen.5,6 Because

(1)

should be addressed. 483

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484

the ductilizing effect of boron additions is associated with its segregation to N&Al grain boundaries, these observations have led to a hypothesis that the principal role of boron in NisAl may be to suppress environmental embrittlement of its grain boundaries.‘,3,6 Because boron and hydrogen are both interstitial solutes and may compete for site occupation at grain boundaries, several investigators have further speculated that the mechanism by which boron enrichment at grain boundaries suppresses moisture-induced environmental embrittlement involves blocking or slowing down hydrogen penetration along grain boundaries.“’ In a previous study,’ we found that undoped Ni3Al is susceptible to environmental embrittlement in either moist Ar or Ar + 5% H2 gas, indicating that both HZ0 and H2 are embrittling gas species for undoped Ni3Al. Both HZ0 and H2 can be potential sources of atomic hydrogen for grain boundary embrittlement. Molecular hydrogen can dissociate when it adsorbs on certain clean metallic surfaces, such as those that form near the tip of a propagating crack. Nickel in Ni3Al is expected to play an important role in embrittlement by gaseous hydrogen because it is known to dissociate molecular hydrogen as in eqn (3), while aluminum does not.‘O Ni + &cads) = Ni + 2H(,d,)

(3)

While boron-doped Ni3A1 is apparently immune to embrittlement by water vapor, it is quite susceptible to intergranular embrittlement in environments containing gaseous hydrogen (pure Hz and Ar + 5% H2).9 Like moisture-induced embrittlement of undoped Ni3Al, embrittlement of borondoped Ni3Al in H2 also is believed to involve transport and segregation of atomic hydrogen at grain boundaries. If the remedial effect of boron-doping for Ni3Al in moist environments is really due to its effect on segregation and transport of atomic hydrogen at grain boundaries, it is difficult to understand how boron can fail to inhibit embrittlement of NisAl in H2 as well. Given this apparent contradiction, it seems that other mechanisms for boron’s beneficial effect on moisture-induced embrittlement of Ni3Al should be considered. In order to better understand the mechanism(s) for embrittlement of Ni3Al by H2 or H20, and the role of boron additions in mitigating moistureinduced embrittlement, it is useful to determine the dependence of embrittlement on the environmental concentration of the embrittling species. George et

et al. al. have reported the effect of vacuum level on the ductility of undoped Ni3AJ3 and our previous work also indicates significantly less embrittlement of boron-doped Ni3Al in Ar + 5% Hz than in pure H2.9 There have not been any detailed studies, however, where partial pressures of either Hz or HZ0 are varied in a controlled manner. The results presented here show how the ductility of undoped polycrystalline Ni3A1 depends on the partial pressure of both water vapor and gaseous hydrogen in the range from 10e3 to lO”Pa, and how the ductility of boron-doped Ni3A1 depends on hydrogen partial pressure in that same range. A comparison of tensile ductility between undoped and borondoped Ni3A1 in UHV has also been made in order to shed some light on the intrinsic versus extrinsic effects of boron segregation in Ni3Al.

EXPERIMENTAL Directionally solidified [Ol l] single crystals with dimensions of 25.4 mm x 6.4 mm x 160 mm were used as starting materials. Compositions of the ascast ingots were Ni-22_7at% Al (hereafter called ‘undoped N&Al’), and Ni-23.5 at% Al-O.065 wt% B (hereafter called ‘boron-doped Ni3Al’). Impurity levels of both alloys are shown in Table 1. As shown in Figs l(a) and (b), optical microstructures of both as-received materials contained significant dendritic structure, with dendrite axes parallel to the solidification direction. Ingots were homogenized for 24h at 1200°C in a vacuum of 10p3Pa. Rectangular blanks with thickness of 2.4mm were then cut transverse to the solidification direction of the ingots. The blanks were cold-rolled to a reduction of 40%, and tensile specimens having a gage section with nominal dimensions of 1.Omm x 2.3 mm x 7.6 mm were electric-discharge machined from them. Tensile specimens were recrystallized at 1000°C for 1 h in the vacuum, and their flat faces were polished through 0.05 pm alumina following conventional metallographic procedures. As shown in Fig. 1, four equally spaced fiducial marks were scribed on the polished portion of each tensile gage section to facilitate measurement of elongation following tensile fracture. Table 1. Impurity levels of undoped and boron-doped Ni+U (wt%) C Undoped Ni3Al Boron-doped N&AI

s

0.011 <0.0002 0.035 < 0.0002

N

0

0.0002 0~0001

0.0007 0.0008

Effects of gaseous hydrogen and water vapor pressure on enviromental embrittlement of Ni3Al

The grain size for undoped Ni3A1 was approximately 35pm; and for the boron-doped N&Al it was approximately 55 llrn. As shown in Figs l(c) and (d), the grain sizes of both alloys are uniform, but some dendritic structure is present even after homogenization and recrystallization. These optical microstructures are similar to those reported by and it is likely that the denother investigators,‘l dritic structure consists of y (fee) + y ’ (Liz) phases. Room temperature tensile tests were performed in an ultrahigh vacuum (UHV) chamber attached to a PHI 660 scanning Auger microprobe. The UHV tensile fracture chamber was equipped with a turbomolecular pump and an ion pump. After loading a specimen and rough pumping the test chamber, it was baked out at 150°C for 24 h with the ion pump running. The chamber was then cooled to room temper.ature, and after ion pumping for an additional 24 - 48 h, the pressure of the test chamber was usuallly - lop7 Pa where most UHV tests were carried out. Because of reports that hot filaments might produce atomic hydrogen in gaseous H2 environments, and that even the

Fig. 1. Optical

micrographs

of (a) as-cast

485

residual Hz gas in UHV environments could cause embrittlement in the presence of such filaments,12 some UHV tests were performed with the ion gage and/or the ion pump turned off. For those tests under partial pressures of H2 or H20, water vapor or H2 gas (99.999% purity) was leaked into the chamber to the desired pressure. High purity water vapor was obtained by alternately freezing, pumping, and thawing deionized water contained in a liquid nitrogen cold finger. The pressure of water vapor was controlled by varying the level of liquid nitrogen in the cold finger. Gas pressure in the chamber was monitored by the ion gage when the testing pressure was lower than 0.13 Pa, and by a thermocouple gage when the testing pressure was greater than 0.13 Pa. In order to perform tests where gas pressure was in the order of 1 kPa, either one atmosphere of Ar + 5% H2 gas was leaked into the chamber (for boron-doped Ni3A1), or separate tests were carried out in an Instron machine (for undoped and boron-doped Ni3Al in moist Ar, and for undoped N&Al in Ar + 5% H&l3 Details of test

Ni22.7A1, (b) as-cast Ni23.5Al-O.O65B, Ni23.5Al-0.065B (marker: 100 wm).

(c) recrystallized

Ni22.7A1,

(d)

recrystallized

K. H. Lee

486

procedures using the Instron machine equipped with an environmental chamber have been reported elsewhere.’ Undoped Ni3A1 test specimens were fractured in tension at an estimated engineering strain rate of 3.4x 10v4 s-l, and at 2.0x 10e5 s-l for boron-doped NisAl specimens. After fracture, the specimens were removed from the chamber and tensile elongations were determined by measuring the change in distance between fiducial lines on the same portion of the broken gauge section as indicated in Fig. 2. Tensile elongations determined by this method agreed well with those for similar specimens tested in a conventionaL screw-driven tester where elongations were measured using an extensometer.13 All tensile fracture surfaces were examined using scanning electron microscopy.

RESULTS Tensile elongations of the undoped and borondoped Ni3Al are plotted as a function of H2 or Hz0 gas pressure in Fig. 3. Tensile elongations obtained in UHV are also included in the figure, and the pressure for these data indicates vacuum level, not gas pressure. Results for undoped Ni3A1 tested in moist Ar and Ar + 5% Hz, which were determined in separate tests in an Instron machine, l 3 are also included. Tensile elongations of undoped Ni3Al were 11.7 and 14.7% in UHV. The elongation in UHV was also measured to be 14.0% when the ion gage was turned off and 14.4% when both the ion gage and the ion pump were turned off, indicating no significant effect of these vacuum instruments on the

1 cm Fig. 2. A photograph of tensile specimens: upper one for untested and lower one for tested (in UHV) specimen. Fiducial marks are shown on the gage section of the specmens.

et al. ductility. The tensile ductility is still high, 12.8% at an Hz pressure of 2.7x low3 Pa, indicating little embrittlement at this level of Hz pressure. The ductility began to decrease as the Hz pressure exceeded about low3 Pa and dropped to 0.7% in Ar + 5% Hz. In the water vapor environment, the tensile ductility of undoped Ni3A1 showed a similar pressure dependence to that in the Hz environment. The tensile elongation at an Hz0 pressure of 1.3x lop3 Pa was 1l-6%, indicating no significant embrittlement below this pressure. For Hz0 pressure above 1.3 x 10v3 Pa, the ductility began to decrease with increasing Hz0 pressure. Although the elongations for undoped Ni3A1 are slightly greater in water vapor than in Hz at comparable pressures, the results generally indicate a similar embrittling effect for both environments, consistent with the previous results.9 The tensile elongation of boron-doped Ni3A1 tested in UHV was 39.9%. As the H2 pressure was increased to 1.3x 10-l Pa, the ductility was still approximately 35%. Above this H2 pressure, however, the ductility decreased rapidly and the test performed in 1 atm. of Ar + 5% Hz indicated a tensile elongation of only 3.7%. As mentioned previously, the ductility of boron-doped NisAl is not sensitive to water vapor (40.6% elongation in moist Ar where estimated water vapor pressure is 2 N 3000Pa), and the effect of Hz0 pressure was not studied explicitly in this work. The dashed line in Fig. 3 indicates the approximate ductility of boron-doped Ni3A1 in water vapor. As shown in Figs 4(a) and (b), the fracture mode of undoped Ni3Al is primarily intergranular, regardless of testing environments. Comparison of the two fractographs taken from the specimens tested in UHV and 6 kPa H2 does not show any significant difference in the appearance of fracture surface. In contrast to this, fracture mode of boron-doped N&Al depends on testing environments as shown in Figs 4(c) and (d). Failure of boron-doped Ni3Al is primarily ductile transgranular in UHV, but it changes to mixed (somewhat more than 50% transgranular) in 67Pa H2 and predominantly intergranular in a 5 kPa H2 environment. The overall fracture appearance seems to correlate well with the tensile ductility in borondoped Ni3A1.

DISCUSSION Our results indicate that there exist critical gas pressures for both undoped NiJAl in Hz and Hz0

Efects

of gaseous hydrogen and water vapor pressure on enviromental embrittlement of Ni3Al

487

Extrinsic boron effect

H2 or Hz0 Pressure (Pa) Fig.3. Tensile elongations of undoped and boron-doped N&AI as a function of Hz or Hz0 pressure. W/O B and w/B represent undopd and boron-doped Ni,AI, respectively. AES in the bracket indicates that tests were performed using the ultrahigh vacuum

(- low3 Pa) and for b’oron-doped Ni3Al in Hz (- 10 Pa), below which their tensile ductilities are not affected by their respective embrittling environments. Establishment of such a critical pressure is significant because it allows us to evaluate the intrinsic ductility of these alloys with a reasonable degree of confidence. R-educed water vapor pressure in UHV is expected to be responsible for the improved ductility of un’doped Ni3Al, in agreement with the reports of others3 This level of ductility (12 - 15% elongation), therefore, represents the intrinsic ductility of Ni-:!2.7at% Al. For the boron-doped alloys, tensile elongation in UHV (40%) is much greater than that for the undoped alloys (12- 15%). Boron additions are also associated with a change in fracture mode from intergranular to transgranular as indicated in Figs 4(a) and (c). Because the boron effect exhibited in UHV is not related to a suppression of any environmental effect, it should be regarded as an intrinsic effect. The magnitude of the intrinsic effect is indicated to the right of Fig. 3. Above the critical gas pressure, the tensile ductility decreases with increasing pressure. Bowker and Hardie have reported similar pressure dependence of tensile ductility on hydrogen pressure for high strength steel. I4 They interpreted this type of behavior to indicate th,at embrittlement requires the achievement of a critical concentration of

atomic hydrogen at some specific location ahead of the propagating crack, which is analogous to a similar postulation by Troiano.ls As the gas pressure in the environment is increased above the critical pressure, the stress level and/or the time required to achieve this concentration of atomic hydrogen is decreased, leading to brittle crack propagation and diminished tensile elongation. The above interpretation of the pressure dependence of embrittlement is also similar to the assumption of linear degradation often used by other investigators to establish a fracture criterion such that: ‘w* Uj=D+XC~

(4)

where af and af” are fracture strengths in the presence and absence of hydrogen, respectively, cz is constant, and CH is hydrogen concentration. Here, the hydrogen concentration is related to Hz0 and Hz pressure through the equilibrium constants for eqns (1) and (3), respectively. The critical pressures of Hz and Hz0 for undoped Ni3Al seem to be similar, both - 10F3 Pa. Above this critical pressure, both Hz and Hz0 exert similar embrittling effects on undoped Ni3Al. In the context of the above hypothesis, this suggests that the similar pressures of HZ and Hz0 provide similar levels of hydrogen ahead of the

488

K. H. Lee

et al.

Fig. 4. SEM fractographs of Ni22.7Al tested in (a) UHV, (b) 6 kPA Ha, and Ni23.5Al-0.065B tested in (c) UHV, (d) 5 kPa HZ.

crack tip in undoped NisAl and lead to similar degrees of embrittlement as reflected by the tensile ductility. This similarity seems to exist in spite of the different mechanisms by which atomic hydrogen is generated in the two environments. This is a particularly significant observation when we compare the embrittlement behavior of boron-doped N&Al in these same H2 and Hz0 environments. In the above comparison of our results with eqn (4), we have implicitly assumed that the decreased tensile ductilities associated with higher pressures of embrittling gases is associated with decreased fracture strengths. This assumption is justified in the present situation because fracture occurs before necking, and there is no observable secondary cracking along the gage section. Previous studies on strain rate effects in these environments indicate that the tensile ductility correlates well with the tensile strength.9 As discussed earlier, boron-doped alloys are not embrittled in moist At-, whereas Hz environments severely embrittle this alloy above the critical H2

pressure (- 10 Pa). The partial pressure of Hz0 in the moist Ar is estimated to be 2 - 3000 Pa, which is far above the critical H2 pressure. This indicates that, because of either thermodynamic or kinetic limitations, boron additions do not allow 2- 3000 Pa pressure of Hz0 to produce sufficient hydrogen (corresponding to - 1OPa Hz) for embrittlement. This component of the boron effect is extrinsic in nature, mitigating the environmental component of embrittlement. The magnitude of this extrinsic component is also indicated to the right of Fig. 3. One possible mechanism for the extrinsic component of the boron effect involves its influence on the gas/metal reaction near the tip of a propagating crack. Creation of atomic hydrogen via eqn (1) is likely to occur preferentially on newly exposed and previously unreacted surfaces in the immediate vicinity of such crack tips. Boron enrichment at grain boundaries along which the cracks tend to propagate will also be ‘inherited’ by the grain boundary fracture surfaces immediately adjacent

Efects of gaseous hydrogen and water vapor pressure on enviromental embrittlement of Ni3Al

to the crack tip. Boron enrichment on these surfaces could significantly
489

excluded as a possibility of explaining the discrepancy. In our system, a grounded metal screen was located between the specimen and the pumping well (which contains the ionization gage and ion pump). It is possible that any effects of atomic hydrogen production at the filaments in the gage or pump were limited by that screen. There could of course also be differences in the details of specimen fabrication and resulting microstructures (e.g. grain boundary character distribution) that explain these differences in ductility. We note that our tensile ductilities in UHV agree well with those obtained at 77 K13 where we also expect moisture-induced embrittlement to be negligible. With regard to the embrittling effect of Hz gas and its dissociation on Ni3Al surfaces, we observed a fairly clear effect of H2 partial pressure on the ductility of boron-doped Ni3Al or undoped Ni3Al. The fact that we observe the embrittling effects of H2 gas for boron-doped NijAl (which is not susceptible to moisture induced embrittlement) as well as for the undoped alloy, suggests to us that the embrittling effect of Hz gas in our experiments does not result primarily from any trace levels of moisture that might have been present in our H2 gas. Furthermore, it is interesting to note that there is no discontinuity in our tensile ductilities across approximately 10-l Pa Hz, above which the ionization gage was turned off (a thermocouple gage was used) and below which the ionization gage was turned on to monitor the pressure. As with the discrepancy regarding the intrinsic ductility of undoped Ni,Al, we do not have an exact explanation for the difference between our results and those of Cohron et al. with regard to the embrittling effect of H2 gas. Further research is clearly needed in order to address these issues.

CONCLUSIONS The tensile ductility of undoped and boron-doped Ni3Al has been measured as a function of H2 and HZ0 pressure. The results indicate that the ductilizing effect of boron is twofold: there is an intrinsic component of the ductilizing efect that is evident when the pressures of embrittling environmental species are reduced below their critical values; there is an extrinsic (or remedial) component to the ductilizing effect that is associated with suppression of moisture-induced embrittlement. The extrinsic component of boron’s ductilizing effect is specific to embrittlement by moisture, and it is not effective

K. H. Lee

490

in eliminating embrittlement in gaseous hydrogen. These observations are consistent with other research indicating that boron segregation at Ni3Al grain boundaries suppresses the formation of atomic hydrogen via eqn (1).

et al. 7. Takasugi, T., Suenaga, H. and Izumi, O., Journal of Mat. Sci., 1991, 26, 1179.

8. Wang, J.-S., Mat. Sci. Eng., 1995, A192/193, 371. 9. Lee, K. H. and White, C. L., Scripta Metall. et Mater., 1995, 33, 129.

10. Christmann, K., Surface Science Reports, 1988, 9, 1. 11. Hanada, S., Watanabe, S. and Izumi, O., J. Mat. Sci., 1986, 21, 203.

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