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γ′-δ eutectic alloy
Scripta
METALLURGICA
Vol. I0, pp. 8 8 9 - 8 9 3 , 1 9 7 6 P r i n t e d in the U n i t e d S t a t e s
Pergamon
Press,
Inc.
HYDROGEN ]~4BRITTLEMENT OF AN ALIGNED y/y'-6 EUTECTIC ALLOY
N. S. Stoloff, L. Klein and J. E. Grossman Rensselaer Polytechnic Institute Troy, New York 12181 and H. L. Marcus University of Texas Austin, Texas 78712 ( R e c e i v e d J u l y 2, 1976) ( R e v i s e d J u l y 22, 1976)
Introduction Nickel and its alloys (i), including Monel (Ni,Cu,AI,Fe) (2), Inconel 718 (3), TD-Niekel (4,5) and Ni-Co (6) are known to be embrittled by either internal or external (gaseous) hydrogen to varying degrees. Embrittlement is affected by the presence and distribution of second phases, particularly in the case of Inconel 718 (3), by strength level, and by the presence of impurities such as sulfur at interfaces (6). While there is no generally accepted mechanism of reversible hydrogen embrittlement in either nickel alloys or other structural metals, much attention has recently been directed towards microstructural features at which hydrogen may be trapped (7-9), thereby affecting embrittlement. The role of interfaces may be central to the question of the relation between internal and external hydrogen susceptibility of metals. The regular, continuous microstructure of aligned eutectics renders them particularly suitable for studies of trapping mechanisms at semi-coherent interfaces, where hydrogen may be concentrated to enhance embrittlement (8), either by itself or in conjunction with impurities (6) or alternatively may be rendered innocuous (4) and thereby prevent emhrittlement. This paper is concerned with the influence of internal hydrogen, introduced by cathodic charging, on the unnotched room temperature tensile behavior of an aligned Ni,A1,Nb (y/y'-~) eutectic alloy. Materials The y/y'-6 system consists of alternating plates of NiBNb intermetallic compound (6 phase) with a Ni,AI solid solution (y phase) from which y' NiBAI precipitates subsequent to solidification. This alloy system was chosen for the following reasons: a) the NiBNb phase in Inconel 718, a precipitation-hardened Ni,Cr,Nb,Fe alloy, is known to be particularly susceptible to hydrogen cracking when it is present as a semi-continuous phase at grain boundaries (3), b) temperature gradients necessary to produce aligned microstructures in this system are relatively low, thereby simplifying production of the aligned microstrueture (8) and the properties in air of several alloys in this system have been well documented (i0).
889
890
EMBRITTLEMENT
OF
EUTECTIC
ALLOY
Vol.
10,
No.
i0
Experimental Procedure Two ingots were obtained from the General Electric Co. and then directionally solidified in our laboratory in alumina crucibles, utilizing induction heating and a water-cooled pedestal to achieve the required temperature gradient. Ingot PV-2435 contained 76w%Ni,21.5%Nb,2.5%A1; ingot PV-2152 contained 76.5%Ni,21%Nb,2.5%A1. Solidification of 2.6 cm diameter bars was carried out at a rate R = 3 cm/hr. From each ingot eight cylindrical specimen blanks (four abreast from top, four abreast from bottom) were spark cut. Final grinding to nominal gage dimensions of 0.25 cm dia. x 1.25 cm long was achieved on a Tensilgrind machine. Each sample was mechanically polished to remove all grinding marks prior to cathodic charging. The charging solution was 1 N H2SO 4 + 0.002 g As203_ per liter of solution added as a cathodic reaction poison. Charging times varied from 1 day (8.6 x l02 sec) to 8 days (6.9 x 105 sec). Samples were tensile tested parallel to the lamellae, on an Instron machine. Strain rate was 6.7 x l0 -2 sec -1 for all tests. Results Table 1 summarizes all charging time and tensile property data. The results clearly show that the ductility is sharply reduced by cathodically charged hydrogen; embrittlement is particularly severe for the 76.5Ni,21Nb,2.5A1 ingot, which in the uncharged condition experienced a reduction in area of 27.4%; after four days charging ductility was reduced to about 4%. Significant losses in fracture stress and reduction in area for 76Ni,21.5Nb,2.5A1 were experienced after as little as one day charging; after eight days zero reduction in area was measured. Table 1 also shows that embrittlement was reversible. Eight days charging followed by baking for three days at 500°C in air completely restored ductility. No effect of hydrogen on yield or flow stress of the alloys was noted. TABLE 1 Effect of Hydrogen on Tensile Properties of y/y'-6 76.5%Ni,21%Nb,2.5%A1 Char6in~ Time
%RA
Fracture Stress (MN/m 2)
uncharged 4 days, 25°C 4 days, 25°C
27.4 h.0 2.72
1642 1028 1056
(psi) 238,500 lh9,000 153,400
76%Ni,21.5%Nb,2.5%A1 Char6in5 Time
%RA
Fracture Stress (MN/m 2)
uncharged 1 day, 25°C 2 days, 25°C 8 days, 25°C 8 days, 25°C - baked 3 days at 500°C (air)
16.7 4.9 5.9 0 19.8
1311 1104 1018 1035 lh01
(psi) 190,250 160,600 147,800 150,100 203,100
Uncharged samples and the specimen charged for eight days and then baked showed similar fracture characteristics; the primary crack path was oriented at approxlmately 90 ° to the tensile axis, Fig. la), and secondary cracks were seen only rarely near the fracture surface. Instead, small discontinuous cracks in the 6 NiBNb phase were noted, mostly away from fracture
Vol.
i0,
No.
I0
EMBRITTLEMENT
OF
EUTECTIC
ALLOY
891
surfaces and sometimes parallel to the y-6 interface, as shown in Fig. 12o). Charged samples, on the other hand, revealed a jagged crack front with macroscopic deviation from the 90 ° orientation, with significant areas of interfacial failure, Fig, 2a), and many secondary cracks, Fig. 2b). Often cracks were aligned parallel to deformation markings, which in the case of the 6 phase are most probably {ll2) twins (ll), see Fig. 3. Cleavage on {100} has been noted in uncharged ~ phase (ll); since the interface plane is parallel to (lll)y and (010)6, it is possible that hydrogen lowers the cleavage energy on (010)~. However, interfacial cracks seemed to lie precisely at the interface, rather than in the ~ phase. Consequently, conclusions concerning hydrogen-assisted cleavage on a particular crystallographic plane cannot yet be made. The nature of the cracking in hydrogen-charged 76.5Ni,21Nb,2.5AI is clearly seen in .Fig. 5. Secondary cracking on a longitudinal surface, parallel to b o t h i n t e r f a c e s and deformation markings are clearly visible. The overall brittle nature of the fractures in charged material can be seen in Fig. 5a), which shows cleavage of both phases and a multitude of small secondary crakcs. Uncharged y/y'-~ fails by ductile fracture of the y/y' matrix and cleavage of the 6 phase, see Fig. 5b), in agreement with previous observations (10). In order to verify that embrittlement in these alloys does not arise from damage by charging (prior to stress application), a sample of 76Ni,21.5Nb,2.5A1 was charged for eight days, sectioned longitudinally and transversely, and examined for evidence of cracking. No cracks were observed, consistent with the results of the baking experiment, Table 1. Discussion Embrittlement of this eutectic alloy is reversible and is accomplished by eased crack propagation along slip or twin bands in the y/y' phases of the alloy as well as along y-6 interfaces. The latter observation suggests that hydrogen may be trapped at semi-coherent interfaces and concentrate until, under the action of an applied stress, a crack will form. Interface failure may also be related to the presence of a locally high concentration of a solute element such as sulfur acting together with the hydrogen; Auger experimentation would be required to examine this hypothesis. Since the interfaces are not precisely parallel to the stress axis throughout any sample, it is possible to develop at least small components of stress normal to the boundaries to aid in local propagation of a crack. However, the generally unfavorable macroscopic stress state at the boundaries, together with the observations of occasional interlamellar separation in uncharged samples and the extensive crack growth through the two phases in charged samples indicate that hydrogen-assisted crack growth along interfaces is only .partially responsible for embrittlement. Rather, it appears that the (ll2} twinning mode of the ~ phase plays an important role in the presence of hydrogen. Uncharged samples, at sufficiently high strains, develop twin boundary cracks in the ~ phase, see Fig. l, but these cracks can only link up after additional extensive plastic deformation of the alloyed y matrix, amounting to 15-20% reduction in area of the sample. In the presence of hydrogen the y phase is no obstacle to crack propagation, see Fig. 3, for example, and complete fracture of the sample can occur with virtually no plastic deformation of either phase (0% RA after eight days charging, see Table 1). It has been suggested that hydrogen in solution may lower the stacking fault energy of nickel (12) amd Monel (2). Increased planarity of slip in the presence of hydrogen could facilitate development of high stress concentrations in nickel-rich lamellae and allow crack propagation through the y phase at much lower applied stress levels than for uncharged samples. The extent to which the aligned microstructure contributes to observed embrittlement has not been established, since as-cast polycrystalline samples have not yet been hydrogen charged and tested. However, in the case of the ordinarily ductile Ni-W fibrous eutectic, catastrophic embrittlement of aligned material in hydrogen gas has been noted (13). In that case the embrittlement was particularly severe at the Ni-W interface. Consequently, the role of semi-coherent interfaces in hydrogen embrittlement of eutectics would seem to be a fertile subject for those interested both in interracial trapping phenomena and possible hydrogenimpurity interactions.
892
EMBRITTLEMENT OF EUTECTIC ALLOY
Vol.
10,
No.
10
Conclusions The aligned y/y'-6 eutectic alloy is susceptible to reversible hydrogen embrittlement. Cracking along interphase boundaries and along slip or twin bands in the nickel-rich y phase is facilitated by hydrogen. Acknowledgements This work was carried out at the Rockwell Science Center, Thousand Oaks, California and at Rensselaer Polytechnic Institute. The authors are grateful tO Rockwell International Corp. and to the Office of Naval Research, under Contract N00014-75-C-0503, for financial support, to W. A. Johnson of Rensselaer for assistance in the experimental program, and to Prof. D. J. Duquette for helpful discussions. References
1. 2. 3. 4. 5. 6. 7. 8. 9. lO. ll. 12. 13.
G. C. Smith, in Hydrogen in Metals, I. M. Bernstein and A. W. Thompson, Editors, p. 485, ASM, Metals Park, Ohio (1974). J. D. Frandsen, W. L. Morris and H. L. Marcus, in Hydrogen in Metals, I. M. Bernstein and A. W. Thompson, Editors, p. 633, ASM, Metals Park, Ohio (1974). R. J. Walter and W. T. Chandler, in Hydrogen in Metals, I. M. Bernstein and A. W. Thompson, Editors, p. 515, ASM, Metals Park, Ohio (1974). A. W. Thompson and B. A. Wilcox, Scripta Met. 6, 689 (1972). J. D. Frandsen, N. E. Paton and H. L. Marcus, Scripta Met° 7, 409 (1973). J. D. Frandsen, H. L. Marcus and A. S. Tetelman, in Effect of Hydrogen on Behavior of Materials, A. W. Thompson and I. M. Bernstein, Editors, p. 299, AIME, New York (1976). H. H. Johnson, in Hydrogen in Metlas, I. M. Bernstein and A. W. Thompson, Editors, p. 35, ASM, Metals Park, Ohio (1974). J. R. Rice, in Effect of Hydrogen on Behavior of Materials, A. W. Thompson and I. M. Bernstein, Editors, p. 445, AIME, New York (1976). J. P. Laurent, G. Lapasset, M. Aucouturier and P. Lacombe, in Hydrogen in Metals, I. M. Bernstein and A. W. Thompson, Editors, p. 599, ASM, Metals Park, Ohio (1974). F. D. Lemkey, UARL M911213-15, United Aircraft Research Laboratories, Final Report to NASA Lewis Research Center, Contract No. NAS3-15562, January 1973. R. T. Quinn, R. W. Kraft and R. W. Hertzberg, Trans. ASM 62, 38 (1969). A. H. Windle and G. C. Smith, Met. Sci. 2, 187 (1968). G. Garmong, Rockwell Science Center, private communication.
b)
a) FIG. 1 Cracks in uncharged 76.5Ni,21Nb,2.5A-1, tested to fracture b) twin band cracks and interfacial failure, Xl000
a) twin band cracks, X1000
Vol.
I0, No.
i0
EMBRITTLEMENT
a)
ALLOY
a) fracture surface, X50
FIG. 3
b) secondary and
FIG. 4
Secondary cracks in 76.5Ni,21Nb,2.5AI charged 1 day, )[250
Scanning electron micrograph of 76.5Ni,21Nb, 2.5AI, charged h days
~
a)
893
b)
FIG. 2
Cracks in 76.5Ni,21Nb,2. SAI, charged 4 days interracial cracks, XI000
SI
OF E U T E C T I C
FIG. 5
Fracture surface of 76.5Ni,21Nb,2.5AI, XI000
b) a) charged h days
b) uncharged
METALLURGICA
Vol. I0, pp. 8 8 9 - 8 9 3 , 1 9 7 6 P r i n t e d in the U n i t e d S t a t e s
Pergamon
Press,
Inc.
HYDROGEN ]~4BRITTLEMENT OF AN ALIGNED y/y'-6 EUTECTIC ALLOY
N. S. Stoloff, L. Klein and J. E. Grossman Rensselaer Polytechnic Institute Troy, New York 12181 and H. L. Marcus University of Texas Austin, Texas 78712 ( R e c e i v e d J u l y 2, 1976) ( R e v i s e d J u l y 22, 1976)
Introduction Nickel and its alloys (i), including Monel (Ni,Cu,AI,Fe) (2), Inconel 718 (3), TD-Niekel (4,5) and Ni-Co (6) are known to be embrittled by either internal or external (gaseous) hydrogen to varying degrees. Embrittlement is affected by the presence and distribution of second phases, particularly in the case of Inconel 718 (3), by strength level, and by the presence of impurities such as sulfur at interfaces (6). While there is no generally accepted mechanism of reversible hydrogen embrittlement in either nickel alloys or other structural metals, much attention has recently been directed towards microstructural features at which hydrogen may be trapped (7-9), thereby affecting embrittlement. The role of interfaces may be central to the question of the relation between internal and external hydrogen susceptibility of metals. The regular, continuous microstructure of aligned eutectics renders them particularly suitable for studies of trapping mechanisms at semi-coherent interfaces, where hydrogen may be concentrated to enhance embrittlement (8), either by itself or in conjunction with impurities (6) or alternatively may be rendered innocuous (4) and thereby prevent emhrittlement. This paper is concerned with the influence of internal hydrogen, introduced by cathodic charging, on the unnotched room temperature tensile behavior of an aligned Ni,A1,Nb (y/y'-~) eutectic alloy. Materials The y/y'-6 system consists of alternating plates of NiBNb intermetallic compound (6 phase) with a Ni,AI solid solution (y phase) from which y' NiBAI precipitates subsequent to solidification. This alloy system was chosen for the following reasons: a) the NiBNb phase in Inconel 718, a precipitation-hardened Ni,Cr,Nb,Fe alloy, is known to be particularly susceptible to hydrogen cracking when it is present as a semi-continuous phase at grain boundaries (3), b) temperature gradients necessary to produce aligned microstructures in this system are relatively low, thereby simplifying production of the aligned microstrueture (8) and the properties in air of several alloys in this system have been well documented (i0).
889
890
EMBRITTLEMENT
OF
EUTECTIC
ALLOY
Vol.
10,
No.
i0
Experimental Procedure Two ingots were obtained from the General Electric Co. and then directionally solidified in our laboratory in alumina crucibles, utilizing induction heating and a water-cooled pedestal to achieve the required temperature gradient. Ingot PV-2435 contained 76w%Ni,21.5%Nb,2.5%A1; ingot PV-2152 contained 76.5%Ni,21%Nb,2.5%A1. Solidification of 2.6 cm diameter bars was carried out at a rate R = 3 cm/hr. From each ingot eight cylindrical specimen blanks (four abreast from top, four abreast from bottom) were spark cut. Final grinding to nominal gage dimensions of 0.25 cm dia. x 1.25 cm long was achieved on a Tensilgrind machine. Each sample was mechanically polished to remove all grinding marks prior to cathodic charging. The charging solution was 1 N H2SO 4 + 0.002 g As203_ per liter of solution added as a cathodic reaction poison. Charging times varied from 1 day (8.6 x l02 sec) to 8 days (6.9 x 105 sec). Samples were tensile tested parallel to the lamellae, on an Instron machine. Strain rate was 6.7 x l0 -2 sec -1 for all tests. Results Table 1 summarizes all charging time and tensile property data. The results clearly show that the ductility is sharply reduced by cathodically charged hydrogen; embrittlement is particularly severe for the 76.5Ni,21Nb,2.5A1 ingot, which in the uncharged condition experienced a reduction in area of 27.4%; after four days charging ductility was reduced to about 4%. Significant losses in fracture stress and reduction in area for 76Ni,21.5Nb,2.5A1 were experienced after as little as one day charging; after eight days zero reduction in area was measured. Table 1 also shows that embrittlement was reversible. Eight days charging followed by baking for three days at 500°C in air completely restored ductility. No effect of hydrogen on yield or flow stress of the alloys was noted. TABLE 1 Effect of Hydrogen on Tensile Properties of y/y'-6 76.5%Ni,21%Nb,2.5%A1 Char6in~ Time
%RA
Fracture Stress (MN/m 2)
uncharged 4 days, 25°C 4 days, 25°C
27.4 h.0 2.72
1642 1028 1056
(psi) 238,500 lh9,000 153,400
76%Ni,21.5%Nb,2.5%A1 Char6in5 Time
%RA
Fracture Stress (MN/m 2)
uncharged 1 day, 25°C 2 days, 25°C 8 days, 25°C 8 days, 25°C - baked 3 days at 500°C (air)
16.7 4.9 5.9 0 19.8
1311 1104 1018 1035 lh01
(psi) 190,250 160,600 147,800 150,100 203,100
Uncharged samples and the specimen charged for eight days and then baked showed similar fracture characteristics; the primary crack path was oriented at approxlmately 90 ° to the tensile axis, Fig. la), and secondary cracks were seen only rarely near the fracture surface. Instead, small discontinuous cracks in the 6 NiBNb phase were noted, mostly away from fracture
Vol.
i0,
No.
I0
EMBRITTLEMENT
OF
EUTECTIC
ALLOY
891
surfaces and sometimes parallel to the y-6 interface, as shown in Fig. 12o). Charged samples, on the other hand, revealed a jagged crack front with macroscopic deviation from the 90 ° orientation, with significant areas of interfacial failure, Fig, 2a), and many secondary cracks, Fig. 2b). Often cracks were aligned parallel to deformation markings, which in the case of the 6 phase are most probably {ll2) twins (ll), see Fig. 3. Cleavage on {100} has been noted in uncharged ~ phase (ll); since the interface plane is parallel to (lll)y and (010)6, it is possible that hydrogen lowers the cleavage energy on (010)~. However, interfacial cracks seemed to lie precisely at the interface, rather than in the ~ phase. Consequently, conclusions concerning hydrogen-assisted cleavage on a particular crystallographic plane cannot yet be made. The nature of the cracking in hydrogen-charged 76.5Ni,21Nb,2.5AI is clearly seen in .Fig. 5. Secondary cracking on a longitudinal surface, parallel to b o t h i n t e r f a c e s and deformation markings are clearly visible. The overall brittle nature of the fractures in charged material can be seen in Fig. 5a), which shows cleavage of both phases and a multitude of small secondary crakcs. Uncharged y/y'-~ fails by ductile fracture of the y/y' matrix and cleavage of the 6 phase, see Fig. 5b), in agreement with previous observations (10). In order to verify that embrittlement in these alloys does not arise from damage by charging (prior to stress application), a sample of 76Ni,21.5Nb,2.5A1 was charged for eight days, sectioned longitudinally and transversely, and examined for evidence of cracking. No cracks were observed, consistent with the results of the baking experiment, Table 1. Discussion Embrittlement of this eutectic alloy is reversible and is accomplished by eased crack propagation along slip or twin bands in the y/y' phases of the alloy as well as along y-6 interfaces. The latter observation suggests that hydrogen may be trapped at semi-coherent interfaces and concentrate until, under the action of an applied stress, a crack will form. Interface failure may also be related to the presence of a locally high concentration of a solute element such as sulfur acting together with the hydrogen; Auger experimentation would be required to examine this hypothesis. Since the interfaces are not precisely parallel to the stress axis throughout any sample, it is possible to develop at least small components of stress normal to the boundaries to aid in local propagation of a crack. However, the generally unfavorable macroscopic stress state at the boundaries, together with the observations of occasional interlamellar separation in uncharged samples and the extensive crack growth through the two phases in charged samples indicate that hydrogen-assisted crack growth along interfaces is only .partially responsible for embrittlement. Rather, it appears that the (ll2} twinning mode of the ~ phase plays an important role in the presence of hydrogen. Uncharged samples, at sufficiently high strains, develop twin boundary cracks in the ~ phase, see Fig. l, but these cracks can only link up after additional extensive plastic deformation of the alloyed y matrix, amounting to 15-20% reduction in area of the sample. In the presence of hydrogen the y phase is no obstacle to crack propagation, see Fig. 3, for example, and complete fracture of the sample can occur with virtually no plastic deformation of either phase (0% RA after eight days charging, see Table 1). It has been suggested that hydrogen in solution may lower the stacking fault energy of nickel (12) amd Monel (2). Increased planarity of slip in the presence of hydrogen could facilitate development of high stress concentrations in nickel-rich lamellae and allow crack propagation through the y phase at much lower applied stress levels than for uncharged samples. The extent to which the aligned microstructure contributes to observed embrittlement has not been established, since as-cast polycrystalline samples have not yet been hydrogen charged and tested. However, in the case of the ordinarily ductile Ni-W fibrous eutectic, catastrophic embrittlement of aligned material in hydrogen gas has been noted (13). In that case the embrittlement was particularly severe at the Ni-W interface. Consequently, the role of semi-coherent interfaces in hydrogen embrittlement of eutectics would seem to be a fertile subject for those interested both in interracial trapping phenomena and possible hydrogenimpurity interactions.
892
EMBRITTLEMENT OF EUTECTIC ALLOY
Vol.
10,
No.
10
Conclusions The aligned y/y'-6 eutectic alloy is susceptible to reversible hydrogen embrittlement. Cracking along interphase boundaries and along slip or twin bands in the nickel-rich y phase is facilitated by hydrogen. Acknowledgements This work was carried out at the Rockwell Science Center, Thousand Oaks, California and at Rensselaer Polytechnic Institute. The authors are grateful tO Rockwell International Corp. and to the Office of Naval Research, under Contract N00014-75-C-0503, for financial support, to W. A. Johnson of Rensselaer for assistance in the experimental program, and to Prof. D. J. Duquette for helpful discussions. References
1. 2. 3. 4. 5. 6. 7. 8. 9. lO. ll. 12. 13.
G. C. Smith, in Hydrogen in Metals, I. M. Bernstein and A. W. Thompson, Editors, p. 485, ASM, Metals Park, Ohio (1974). J. D. Frandsen, W. L. Morris and H. L. Marcus, in Hydrogen in Metals, I. M. Bernstein and A. W. Thompson, Editors, p. 633, ASM, Metals Park, Ohio (1974). R. J. Walter and W. T. Chandler, in Hydrogen in Metals, I. M. Bernstein and A. W. Thompson, Editors, p. 515, ASM, Metals Park, Ohio (1974). A. W. Thompson and B. A. Wilcox, Scripta Met. 6, 689 (1972). J. D. Frandsen, N. E. Paton and H. L. Marcus, Scripta Met° 7, 409 (1973). J. D. Frandsen, H. L. Marcus and A. S. Tetelman, in Effect of Hydrogen on Behavior of Materials, A. W. Thompson and I. M. Bernstein, Editors, p. 299, AIME, New York (1976). H. H. Johnson, in Hydrogen in Metlas, I. M. Bernstein and A. W. Thompson, Editors, p. 35, ASM, Metals Park, Ohio (1974). J. R. Rice, in Effect of Hydrogen on Behavior of Materials, A. W. Thompson and I. M. Bernstein, Editors, p. 445, AIME, New York (1976). J. P. Laurent, G. Lapasset, M. Aucouturier and P. Lacombe, in Hydrogen in Metals, I. M. Bernstein and A. W. Thompson, Editors, p. 599, ASM, Metals Park, Ohio (1974). F. D. Lemkey, UARL M911213-15, United Aircraft Research Laboratories, Final Report to NASA Lewis Research Center, Contract No. NAS3-15562, January 1973. R. T. Quinn, R. W. Kraft and R. W. Hertzberg, Trans. ASM 62, 38 (1969). A. H. Windle and G. C. Smith, Met. Sci. 2, 187 (1968). G. Garmong, Rockwell Science Center, private communication.
b)
a) FIG. 1 Cracks in uncharged 76.5Ni,21Nb,2.5A-1, tested to fracture b) twin band cracks and interfacial failure, Xl000
a) twin band cracks, X1000
Vol.
I0, No.
i0
EMBRITTLEMENT
a)
ALLOY
a) fracture surface, X50
FIG. 3
b) secondary and
FIG. 4
Secondary cracks in 76.5Ni,21Nb,2.5AI charged 1 day, )[250
Scanning electron micrograph of 76.5Ni,21Nb, 2.5AI, charged h days
~
a)
893
b)
FIG. 2
Cracks in 76.5Ni,21Nb,2. SAI, charged 4 days interracial cracks, XI000
SI
OF E U T E C T I C
FIG. 5
Fracture surface of 76.5Ni,21Nb,2.5AI, XI000
b) a) charged h days
b) uncharged