Elevated-temperature environmental embrittlement and alloy design of L12 ordered intermetallics

538

Materials Science and Engineering, A 153 ( 1992 ) 538-547

Elevated-temperature environmental embrittlement and alloy design of L 12 ordered intermetallics M. T a k e y a m a a n d C. T. Liu*

3rd Research Group, National Research Institutefor Metals, Nakameguro, Meguro-ku, Tokyo 153 (Japan)

Abstract Ordered intermetallic alloys with L 12 crystal structure such as Ni3Ai and Ni3Si exhibit environmental embrittlement with much lower ductilities in air than in vacuum when being tested at elevated temperatures. The environmental embrittlement is sensitive to test temperature, alloy composition and grain geometry. The embrittling behavior at elevated temperatures is a dynamic phenomenon, i.e. gaseous oxygen in test environments is chemically absorbed at the tip of cracks induced on alloy surfaces by deformation and then drives into the metal along grain boundaries, leading to a reduction of the grain boundary cohesion. The embrittlement can be alleviated by formation of protective oxide films on alloy surfaces, control of grain shape or addition of beneficial elements such as chromium. In contrast, ordered intermetallics based on b.c.c, crystal structures such as FeAI (B2) and Fe3AI (D03) do not show environmental embrittlement at elevated temperatures. The embrittling behavior as well as alloy design to alleviate the embrittlement is discussed in this paper.

1. Introduction

Many ordered intermetallic alloys show poor room temperature and intermediate temperature tensile ductilities. The brittleness of the ordered intermetallic alloys at ambient temperatures has been considered mainly from intrinsic effects, such as a lack of sufficient slip systems, grain boundary weakness, poor cleavage strength and low mobility of dislocations. Single crystal Ni3AI has an excellent room temperature ductility, but it exhibits a ductility drop in the temperature range of 600-1000 °C [1]. The reduction in ductility was also considered to be associated with the intrinsic effects. However, recent studies have indicated that environmental embrittlement (an extrinsic effect), strongly affects the mechanical properties of ordered intermetallics. There has been a number of cases where ordered intermetallic alloys based on f.c.c, and b.c.c. crystal structures show unusual brittle behavior, depending on test environments, not only at elevated temperatures [2-18] but also at ambient temperatures [19-34], as summarized in Table 1. Such embrittlement is caused by an absorption of embrittling agents present in test environments. The room temperature embrittlement is associated with chemisorption of hydrogen at crack tips, whereas the embrittling agent at *Present address: Metals and Ceramics Division, Oak Ridge National Laboratory, PO Box 2008, Oak Ridge, TN 378316115, USA. 0921-5093/92/$5.00

elevated temperatures is oxygen, which penetrates along grain boundaries and reduces their cohesion. Thus, the environmental embrittlement is becoming one of the key issues that has to be satisfactorily solved in order to use intermetallics as engineering materials. This paper focuses mainly on elevated temperature environmental embrittlement of ordered intermetallic alloys based on L12 materials, particularly NiaAI and Ni3Si, and covers what causes the embrittlement and how to alleviate this problem. We also briefly discuss environmental embrittlement of other intermetallic alloys and compare this phenomenon with embrittlement observed at ambient temperatures.

2. Environmental embrittlement in LI 2 ordered intermetallics

2.1. Ni3Alalloys Environmental embrittlement of Ni3A1 alloys is a strong function of test temperature, test environments and alloy composition. Figure 1 compares the tensile elongation of IC-145 (Ni-21.5AI-0.5Hf-0.1B, at.%) tested in air and vacuum (10-3 Pa) as a function of test temperature [2]. The alloy tested in air shows distinctly lower ductility than that tested in vacuum (10-3 Pa) at temperatures above 300 °C, and the severe embrittlement occurs in the temperature range of 600-850 °C, despite the fact that Ni3AI alloys exhibit good oxidation resistance. The similarity in the profile of the two © 1992--Elsevier Sequoia. All fights reserved

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Embrittlement and alloy design of ordered intermetallics

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TABLE 1. Nickel aluminides and other ordered intermetallic alloys showing environmental embrittlement at elevated and ambient temperatures Ambient temperature embrittlement

Elevated temperature embrittlement Alloy

Crystal structure

Environmental embrittlement a

Ni3AI + Hf + B Ni3AI + Cr (Ni,Co)3AI Ni3Si Ni3Si + Cr Ni~(Si,Ti)

L 1~ L 12 L12

0 zx 0

Ni3(Si,Ti) + B Co;Ti FeAI Fe3AI Ti3AI

L12

O

L 12 L12

/x ob

LI 2 L12 B2 DO3 D019

o x x x o

Reference 2-5 2 6 7 7 8 8 23 17 17 18

Alloy

Crystal structure

Environmental embrittlement"

Ni.~(AI,Ti)(single crystal) Ni3AI + Be Nis(A1,Mn) Ni3Si Ni3(Si,Ti) Ni3(Si,Ti) + B Ni3AI + B Co3Ti Co3Ti + Fe, AI (Co,Fe).~V Ni3Fe TiA1 FeAI Fe3AI

L 12 L 12 LI~ LI 2 L 1~ LI~ L 12 L12 L12 L 12 L 12 L10 B2

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DO s

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Reference 19 20 21 22 8 8 21 23-25 26 27-28 31 29 32-34 33

ao, environmental embrittlement is observed when tested in oxidizing or moist environment; ,5, observed but reduced by alloying; ×, not observed. bNo difference in elevated temperature tensile ductility between air and vacuum was reported; however, we believe that the environmental embrittlement is possibly masked because of a poor vacuum used for the test.

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curves in Fig. 1 further suggests that the e n v i r o n m e n t a l e m b r i t t l e m e n t c a n n o t be c o m p l e t e l y s u p p r e s s e d by a c o n v e n t i o n a l v a c u u m in the range of 1 0 - 3 Pa. T h e e n v i r o n m e n t a l effect in oxidizing e n v i r o n m e n t s is clearly d e m o n s t r a t e d in Fig. 2, w h e r e the tensile ductility of I C - 1 3 6 ( N i - 2 3 A 1 - 0 . 5 H f - 0 . 0 7 B , at.%) tested at 7 6 0 °C is plotted as a function o f air pressure. T h e alloy exhibits only a b o u t 1% elongation w h e n tested in air; however, the elongation c o n t i n u o u s l y increases with d e c r e a s e in air p r e s s u r e a n d it reaches as high as 2 5 % at an air p r e s s u r e lower t h a n 10 -5 Pa [3]. F i g u r e 3 c o m p a r e s the tensile elongation of Ni3AI alloys containing 0 o r 0.5 at.% h a f n i u m as a function of

Fig. 2. Tensile elongation of IC-136 (Ni-23%AI-0.5%Hf0.07%B, at.%) tested at 760°C as a function of air pressure, showing an environmental effect on ductility.

the a l u m i n u m plus h a f n i u m c o n c e n t r a t i o n in air and in v a c u u m at 6 0 0 °C [4]. All the alloys exhibit excellent ductility w h e n tested in v a c u u m , whereas the ductility is dramatically l o w e r e d w h e n tested in air. T h e corres p o n d i n g fracture m o d e changes f r o m a ductile transgranular for the v a c u u m test to a brittle intergranular for the air test. H o w e v e r , in b o t h cases the alloy shows small but steady increases in ductility with decreasing a l u m i n u m concentration. T h e result suggests that the

M. Takeyama, C. T. Liu

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Embrittlement and alloy design of ordered intermetallics

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Ni3A1 alloys are less embrittled in an oxidizing environment when they contain a lower concentration of aluminum. It should be interesting to note that the alloys preoxidized ( 1100 °C/2 h + 850 °C/5 h) prior to the tensile test exhibit better ductility than the unoxidized (bare) specimens when tested in air and again the ductility of preoxidized specimens increases with decreasing the aluminum concentration. Another example of the environmental embrittlement is demonstrated in Fig. 4, where the high cycle fatigue life of a boron-doped Ni3AI (24 at.% A1) prepared by powder metallurgy is plotted as a function of test temperature [5]. The alloy showed a sharp drop in fatigue life at temperatures above 500 °C. Note that in this case the fatigue tests were conducted in vacuum (10 -3 Pa), suggesting again that the conventional vacuum is not good enough to suppress the environmental embrittlement in this alloy. The drop in the fatigue life is also accompanied by a change in fracture mode from transgranular to intergranular. Another example of environmental embrittlement is shown in Fig. 5, where spray-formed Ni66Co10A124 containing 0.25 at.% B was tensile tested in vacuum or argon at the temperature range from room temperature to 1000°C [6]. The alloy showed distinctly reduced ductility at temperatures to 800°C when tested in argon, indicating that even a small amount of oxygen contained in the test environment played a major role in embrittling the alloy. It should be noted that this alloy also exhibited a sharp drop in ductility caused by environmental embrittlement when being tested in a conventional vacuum (10-3 Pa).

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[6]. 2.2. Ni~Si alloys The alloys based on NiaSi also show severe embrittlement in air at elevated temperatures. Figure 6 shows the temperature dependence of yield strength and ductility in Ni-18.9at.%A1 tested in air. A sharp drop in ductility was observed at 300 °C [7], similar to that observed in Ni3AI alloys. The ratio of 600°C ductilities obtained in air and vacuum was plotted in Fig. 7 as a function of chromium content [7]. The result clearly demonstrates that the binary alloy exhibits ductility in vacuum 20 times higher than that in air. The embrittlement in air can be most effectively reduced by addition of chromium, resulting in a highest ductility ratio for the 4% chromium alloy.

M. Takeyama, C. T. Liu

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Both boron-free and boron-doped Ni3(Si,Ti ) alloys exhibited environmental embrittlement at elevated temperatures [8], as shown in Fig. 8. The data suggest, however, that boron is quite effective in reducing embrittlement in the temperature range of 600-800 K, and that the beneficial effect of boron apparently disappears above 900 K; all the alloys lost their tensile ductility above this temperature. It is important to point out that all the alloys showed environmental embrittlement above 7 0 0 K even when tested in vacuum, although the boron-doped alloy had tensile ductilities better in vacuum than those in air at 6 0 0 - 9 0 0 K . The most dramatic effect of boron, however, is to reduce the ambient temperature embrittlement in air at 300 K. In this case, the embrittlement is caused by moisture-induced hydrogen when

3. Embrittling mechanisms

The environmental embrittlement is caused by a dynamic effect simultaneously involving a high localized stress concentration, elevated temperature and gaseous oxygen. Although the embrittlement occurs when samples are exposed in oxidizing environments, it is not necessarily associated with formation of oxide scale on the specimen surfaces or with grain boundary oxidation. This is because, in some cases, the preoxidation causes a slight increase rather than a decrease in ductility of Ni3AI when tested in air at 600 °C (see Fig. 4). Also, the specimens fractured in a brittle manner in air at elevated temperatures (600-760°C) remain ductile by subsequent bend tests at room temperature [5]. Such a dynamic effect might involve repeated weakening and cracking of the grain boundary as a result of oxygen adsorption and penetration at crack tips. The reduction in ductility is accompanied by a change in fracture mode from ductile transgranular to brittle intergranular. Since the embrittlement becomes less severe with decreasing aluminum concentration in Ni3AI, the decohesion of atomic bonding may be asso-

542

M. Takeyarna, C. T. Liu

/ Embrittlement and alloy design of ordered intermetallics

ciated with a weakening of either Ni-AI bonds or AI-AI bonds rather than Ni-Ni bonds [5]. Hippsley and De Van [9] recently studied high temperature crack growth in Ni3AI alloys tested in oxidizing environments. They have proposed a fracture mechanism of stress assisted grain boundary oxygen penetration (SAGBO), similar to the grain boundary oxygen embrittlement observed in superalloys, to explain the dynamic embrittlement in Ni3A1. According to their model (see Fig. 9), the dynamic embrittlement consists of the following four sequential steps: ( 1 ) occurrence of surface cracks at the initial stage of deformation, (2) chemisorption of gaseous oxygen to the crack tips where a high localized stress field is involved, (3) oxygen penetration in its atomistic form to the stress field ahead of tips and (4) inward development of surface cracks preferentially along the grain boundaries, leaving some secondary cracks. Steps (2)-(4) proceed continuously and repeatedly during deformation, leading to premature fracture and severe loss in ductility at elevated temperatures in oxidizing environments. Environmental embrittlement is critically dependent on oxygen penetration along grain boundaries at crack tips (step (3)). Takeyama and Liu [10-12] found that

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the grain size in Ni3AI strongly affects the formation of protective oxide films, which, in turn, influences oxygen penetration along the grain boundaries. In their study, Ni3A1 specimens with various grain sizes (17-200 pm) were preoxidized at 1000 °C for 10 min and then tensile tested at 600 and 760 °C in vacuum [10]. The results are compared with those from the bare specimens tested in vacuum [13], as shown in Fig. 10. The preoxidation causes no embrittlement in the fine-grained boron-doped Ni3AI; however, the ductility of the preoxidized specimens decreases with increasing grain size, despite the fact that the ductility of bare specimens is nearly insensitive to the grain size. A severe embrittlement occurs at 760 °C for the largest grained material. This is accompanied by a fracture mode change from ductile grain boundary fracture for the bare specimen to completely brittle grain boundary fracture for the preoxidized specimen, as shown in Figs. 1 l(a) and 1 l(b) respectively. The loss of ductility is a result of oxygen penetration along the grain boundary during the preoxidation treatment [10]. Auger analyses revealed a large amount of oxygen on the grain boundaries in the preoxidized large grained specimen but little oxygen in the bare specimen, as shown in Fig. 12. The analyses also revealed an oxygen gradient from the surface to the specimen center, indicating a diffusion of oxygen along the grain boundaries. The most interesting result in connection with Ni3AI preoxidation is the finding of no oxygen on the grain boundaries for the preoxidized fine grained specimens. Such grain size dependence of oxygen penetration is found to be attributed to the difference in surface oxide films between the fine grained and large grained specimens: a continuous, thin aluminum-rich oxide layer on the specimen surfaces of the fine grained material, and a nickel-rich oxide dominated on the large grained samples. The former is protective enough

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Embrittlement and alloy design of ordered intermetallics

543

Fig. 11. Scanning electron micrographs showing the fracture surfaces of (a) bare and (b) preoxidized specimens having a grain size of 193 pm tested at 760 °C in vacuum.

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to shut out oxygen from the alloy, preventing any loss of ductility, whereas the latter allows oxygen to penetrate along the grain boundary, causing severe embrittlement. Formation of aluminum-rich oxide in the fine grained specimen is attributed to a short circuit path for the rapid diffusion of aluminum atoms to the surface along the grain boundaries. Preoxidation to produce a thin adherent oxide film should be responsible for the reduction of the high temperature embrittlement in the bare specimens tested in air (see Fig. 3). These results suggest that a key factor controlling the ductility of the preoxidized specimens is the oxidation product on specimen surfaces rather than grain size itself. In other words, embrittlement owing to oxygen penetration for the large grained samples can be stopped if a protective aluminum-rich oxide layer could be formed on the surface. In an attempt to verify the presumption of the combined effect of grain size and surface oxide film on high temperature embrittlement, Takeyama and Liu [11, 12] conducted a complex heat treatment (CHT) to obtain large grained specimens covered with aluminum-rich oxides: ( 1 ) annealing

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Embrittlement and alloy design of ordered intermetallics

at 1000 °C/20 min in a vacuum to adjust the grain size to about 20 pm, (2) oxidation at 1000 °C/30 rain in air to produce aluminum-rich oxide on surfaces, (3) reannealing at 1050°C/7 d a y s + 1 0 0 0 ° C / 2 days in vacuum to coarsen the grain size to about 200 p m and (4) oxidizing again at 1000 °C/10 min in air to simulate exactly the same final preoxidation condition prior to the tensile test in vacuum at 760 °C. As expected, the CHT specimens showed ductility nearly equal to that of the bare specimens and no high temperature embrittlement was observed (see Table 2). Thus, it is the surface oxide layer, not the grain size itself, that is responsible for the ductility loss of the large grained specimens exposed to the oxidizing environment at elevated temperatures.

zone remelting (DLZR) using rod materials. The grains in the D L Z R material are essentially aligned along the solidification direction. As shown in Table 3, the specimens with an equiaxed grain structure showed good tensile ductility in vacuum at both 600 and 760 °C, but distinctly lower ductility in air. The loss of ductility is accompanied by a change in fracture mode from ductile transgranular to brittle grain boundary fracture,

4. Alloy design Dynamic embrittlement has to be overcome in order to use aluminide alloys for practical applications in oxidizing environments. Since preoxidation to produce a protective oxide is proven to be effective in eliminating embrittlement, as mentioned in the foregoing section, surface coatings could also be useful to protect underlying alloys from oxygen penetration along the grain boundary. There are two other metallurgical ways to alleviate this problem: (1) control the grain shape, such as a columnar grained structure aligned parallel to the stress axis [15], and (2) add beneficial elements such as chromium to the alloys [2]. Columnar grained structures can substantially eliminate environmental embrittlement in nickel aluminide at elevated temperatures [15]. Figure 13 compares the equiaxed and columnar grained structure of boron-doped Ni3AI. The former is produced by fabrication and recrystallization of drop cast ingot, whereas the latter is produced by directional levitation

TABLE 2. Comparison of tensile properties of bare, preoxidized and CHTa+ preoxidized specimens in boron-doped Ni3AI with a grain size of 193 nun tested at 760 °C in vacuum Specimen

Preoxidation Elongation Y i e l d Ultimate at 1000 °C/ (%) strength strength 10 rain (MPa) (MPa)

Fig. 13. Comparison of grain morphology in Ni3AI (24 at.% AI) doped with 500 p.p.m. B: (a) equiaxed grain material produced by fabrication and recrystallization and (b) columnar grained material produced by directional levitation-zone remelting.

TABLE 3. Tensile elongations of boron-doped Ni3AI with equiaxed grains and columnar grains tested at 600 and 760 °C in air and in vacuum Grain shape

Bare no Preoxidized yes CHTa+ yes preoxidized

14.8 1.1 13.5

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Tensile elongation 600 °C

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Vacuum

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M. Takeyama, C. T. Liu

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Embrittlement and alloy design of ordered intermetallics

as the test environment changes from vacuum to air. However, the specimens with the columnar grained structure exhibited good tensile ductility even when tested in air at both temperatures and their fracture mode is mainly ductile transgranular. The beneficial effect of the columnar grained structure with grain boundaries parallel to the stress axis is attributed to minimizing the normal stress across grain boundaries and, consequently, suppressing nucleation and propagation of cracks along the boundaries even weakened by oxygen penetration. Alloying with 8 at.% Cr to nickel aluminides is also effective in eliminating dynamic embrittlement occurring in the temperature range of 600-800 °C in oxidizing environments, as shown in Fig. 14, where the ductility increases from about 6% to 20%. However, the chromium containing alloy, when tested in vacuum, exhibits better ductility than that in air, indicating that embrittlement at elevated temperatures is reduced by chromium addition but not completely eliminated. The beneficial effect is believed to be associated with rapid formation of chromium oxide films that reduce the penetration of oxygen into grain boundaries. The chromium effect is sensitive to the alloy composition and the best result was obtained from the alloy containing about 10 vol.% of the disordered V phase.

5. General discussion and remarks

Environmental embrittlement is complicated and associated with a number of factors. In addition to the test environment, aluminum level and grain geometry, elevated temperature embrittlement is also sensitive to

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minor alloying elements such as the boron level for Ni3A1 [8, 16], processing methods [6] and residual impurity levels such as oxygen and sulfur [16]. This phenomenon is also affected by the crystal structure of ordered intermetallics [17, 18]. Conventionally cast and thermomechanically processed boron-doped Ni3AI alloys are less susceptible to environmental embrittlement than alloys prepared by a rapidly solidified process [6]. Both materials show similar embrittlement at 600 °C when the test environment is changed from vacuum to air, whereas the conventionally cast and thermomechanically processed alloys exhibited a ductility much higher than that of the rapidly solidified material when tested at 760 °C in vacuum. It is also reported that the spray formed Ni-10Co-24A1 (at.%) alloy with boron showed no ductility at 760 °C in vacuum but subsequent thermomechanical processing substantially improved the ductility [6]. Thus, processing differences and the thermal history of alloys also affects the embrittling behavior at elevated temperatures. Since alloys prepared by powder metallurgy usually have an oxygen concentration higher than that in conventional cast materials, susceptibility to environmental embrittlement could be associated with the internal oxygen level. Boron plays a certain role in reducing environmental embrittlement of the ordered intermetallic alloys. As shown in Fig. 8, the boron-doped Ni3(Si,Ti ) alloys show much higher ductilities at 600-800 K than those of the undoped alloys. The tensile ductility is slightly better in vacuum than in air for the boron-doped alloys at elevated temperatures, whereas there is little difference in elevated temperature ductility between vacuum and air tests for the undoped alloys. The result suggests that the ductility enhancement due to boron is more effective in vacuum than in air. Although the detailed mechanism of the boron effect on environmental embrittlement remains unclear, it is possible that boron, which segregates strongly to grain boundaries [35], makes diffusion of oxygen along grain boundaries sluggish, thereby reducing embrittlement at elevated temperatures. It is also possible that boron segregation enhances grain boundary cohesion and thus reduces oxygen embrittlement. In general, boron is more effective in eliminating ambient temperature embrittlement, as shown in Fig. 8. The ambient temperature embrittlement is also caused by a dynamic effect (similar to elevated temperature embrittlement), but the embrittling agent is hydrogen. In this case, hydrogen atoms are generated from the chemical reaction of moisture with metal at crack tips on specimen surfaces during initial deformation: xM +

yH20 +

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M. Takeyama, C. T. Liu

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Embrittlement and alloy design of ordered intermetallics

The resulting hydrogen penetrates into the metal through the tips, embrittling grain boundaries or certain crystallographic planes. Such ambient temperature embrittlement has been seen in many ordered intermetallic alloys [19-34] containing large amounts of reactive elements, such as aluminum, titanium and vanadium. The f.c.c.-based ordered intermetallics, such as Ni3AI alloys [19-21], Ni3Si [22], Ni3(Si,Ti ) [8], Co3Ti [23-26], (Co,Fe)3V [27, 28] and TiA1 [29], all show hydrogen embrittlement at ambient temperatures; however, Ni3Fe [30, 31] containing no reactive elements is not embrittled by moist air. Balsone [18] reported that Ti3A1 with the D019 structure was prone to oxygen embrittlement at elevated temperatures. Ambient temperature embrittlement occurs in b.c.c.-based ordered intermetallics such as FeA1 [32-34] and Fe3A1 [33], whereas elevated temperature embrittlement is not seen in these ordered alloys [17] (see Table 1). Figure 15 shows a change in reduction of area (%RA) with temperature in FeAI (Fe-36.9AI-2.2Ni-0.2Mo, at.%) tested in air and vacuum [17]. There is not much difference in %RA between the two testing atmospheres, indicating that the test environment does not affect the ductility of FeA1 at elevated temperatures (above 420 °C). The reason why the alloys based on the b.c.c, ordered structure do not exhibit environmental embrittlement at elevated temperatures is not well known, it is possibly related to no substantial yield anomaly together with rapid formation of protective oxide scales owing to fast diffusion in b.c.c, materials at elevated temperatures. Further studies are required to clear these points.

6. Summarizing remarks Elevated temperature environmental embrittlement has been observed in a number of ordered intermetallic alloys, specifically L12-type Ni3AI and Ni3Si alloys.

N-

O

•

o

•

i

o

t~

Ik TEMPERATURE (K)

Fig. 15. Reduction of area in F e A I alloy (Fe-36.9A1-2.2Ni-0.2Mo, at.%) tested in (o) vacuum and (o) air, showing no environmental embrittlement at elevated temperatures [17].

These alloys exhibit much lower tensile ductility in air than in vacuum. Such embrittlement is sensitive to test temperature, test environment, alloy composition and grain geometry. In most cases, embrittlement is caused by a dynamic effect involving a high localized stress concentration, elevated temperature and gaseous oxygen. Oxygen penetration along the grain boundary either at crack tips during tensile testing or during preoxidation is responsible for the embrittling behavior. The oxygen present at the grain boundary substantially reduces the grain boundary cohesion, resulting in crack propagation easily along the boundary. The embrittlement can be effectively alleviated by (1) formation of protective oxide films on specimen surfaces, (2) control of grain shape (such as columnar grained structures) and (3) alloying with chromium. The b.c.c.-based ordered intermetallic alloys such as FeAI and Fe3AI , however, do not show environmental embrittlement at elevated temperatures. The crystal structural sensitivity of environmental embrittlement is not well understood but may be related to their different deformation and diffusion behaviors.

Acknowledgments This research was partially sponsored by the U. S. Department of Energy, Assistant Secretary for Conservation and Renewable Energy, Office of Industrial Technologies, Advanced Industrial Concept (AIC) Materials Program, under contract DE-AC05-84-OR 21400 with Martin Marietta Energy Systems, Inc. The authors thank C. G. McKamey and S. Miura for carefully reviewing the manuscript and Connie Dowker and Shirin Badlani for manuscript preparation.

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