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Creep cavity observation using liquid metal embrittlement
Scripta
METALLURGICA
Vol. 18, p p . 4 9 7 - 5 0 0 , 1981 Printed in t h e U . S . A .
CREEP CAVITY OBSERVATION
Pergamon P r e s s Ltd. All rights reserved
USING LIQUID METAL EMBRITTLEMENT
T. C. Reiley Oak Ridge National
Laboratory,
(Received February (Revised March
Oak Ridge,
TN
37830
17, 1981) 3, 1 9 8 1 )
Introduction Grain boundary cavities, which form during high-temperature deformation and which may be considered incipient intergranular cracks, have been difficult to observe at the early growth stages. To examine grain boundaries in unfailed specimens one is normally limited to metallographic or TEM techniques, the first having resolution limitations (rcavity ~ l~m) and the second having limitations in the probability of seeing a cavity (especially for large-grained, low-density specimens) or of seeing a cavity which is undisturbed by the thinning process required for TEM. Other observational techniques involving hydrogen attack (I) or lowtemperature impact loading (2) have been used to create brittle intergranular failures to observe grain boundary features, such as creep cavities on pre-crept specimens. These techniques are not, however, applicable to most metals, and as such, the need remains for a more general observational technique. Such a new technique employing liquid metal embrittlement (LME) is described below, and, given that most metals are severely embrittled by at least one liquid metal, this approach may satisfy requirements of general applicability. Liquid Metal Embrittlement
Technique
The approach taken in these experiments entails the development of a cavitated microstructure by creeping a chosen alloy to a given strain at a given stress and temperature. The specimen is then exposed under low stress at a much lower temperature to a particular liquid metal which, as it penetrates the grain boundary causes brittle intergranular failure. Once the liquid metal is removed, the grain boundary details of interest may be examined in the scanning electron microscope (SEH). The phenomenology of LME can be quite varied, ranging from a slight reduction in strainto-failure to complete grain boundary decohesion under essentially zero load (e.g., gallium on aluminum) -- see refs. 3--5 for reviews of this subject. The basic phenomenon involves a surface-energy-driven process in which grain boundary decohesion (or sometimes cleavage) at a crack tip proceeds at a lower stress than would be possible without the liquid metal. The stress and temperature-dependent kinetics of liquid metal delivery and, perhaps, adsorption at the crack tip are also important factors in LME. For LME to occur, the following relationships between solid and liquid metal are usually observed in ~IE couples: (i) the solid and liquid metals have low mutual intersolubilities and (ii) these metals do not form intermetallic compounds. [A set of criteria based on heats of solution having fewer exceptions than the above criteria has been proposed by Old (6).] Note, this noninteractive behavior is quite convenient for the proposed LME fracture process, given that the exposure of the intergranular fracture surface to liquid metal should lead to minimal dissolution or alteration of fine-scale grain boundary features. It is also expected that the greater the degree of embrittlement, or the smaller the strain-to-failure under liquid metal, the better will be the preservat[on of these grain boundary details. Results The alloy chosen for study is Ni--4 wt % W, a low stacking-fault-energy, fcc, solid solution, whose preparation and impurities are described in refs. 7 and 8, along with its proclivity for creep cavitation. TEH examination showed no evidence of a second phase. Tensile
497 0036-9748/81/050497-04502.00/0
498
CREEP CAVITY OBSERVATION
Vol.
15, No,
5
creep specimens, having rectangular cross sections of 2 ~ x 1 mm, were machined from recrystallized sheet, then annealed in high vacuum at 900°C for I h. Grain size was approximately 100 ~m. In the first stage of these experiments it was necessary to find a suitable embrittling agent. Two liquid metals were found which satisfy the ~IE criteria for nickel. These are pure lead and pure thallium, both of which reduced the tensile elongation at 625 K from ~ 30% to <1% with reductions in ultimate tensile strength from 275 MPa to <60 MPa when exposed to either liquid (see Fig. i). This author is not aware of thallium being reported as an embrittling agent for nickel or nickel-base alloys; it is expected that thallium would embrittle nickel alloys other than Ni--4 W. After a certain period of exposure at temperature (50--75 K higher than the melting temperature of the liquid) and stress, the specimen failed and the liquid metal was drained into a reservoir, leaving a small amount of liquid metal to freeze on the fracture surface. This remaining metal was dissolved using a solution of I part acetic acid, 2 parts H202 for lead and concentrated sulfuric acid for thallium. Neither of these acids seemed to attack Ni--4 W during the short exposures. An example of the cleaned fracture surface of a lead-embrittled specimen is shown in Fig. 2. Lead was chosen as a more convenient embrittling agent, given that exposure periods leading to failure at 625--675 K and 50 ~ a were only i--2 h for lead and 10-15 h for thallium, with the quality of the LME fracture surface being roughly the same. Also, lead has considerably lower toxicity than thallium. One technique which aids in the SEM observation of the fracture surface is the use of backscattered electron imaging. This SEM mode is used to observe any remaining liquid metal, when the liquid metal has a significantly different atomic number from the solid metal. An example is given in Fig. 3 of a recrystalllzed aluminum (99.99%) specimen which has been embrittled by gallium at 320 K and low stress. It is viewed using conventional secondary electron imaging and backscattered electron imaging, exposing those areas of the fracture surface covered with gallium. The same technique is applicable to lead or thallium on Ni--4 W. Once the embrittling agents were established, Ni--4 W specimens were pre-crept under constant load at 1075--1175 K in argon at nominal stresses of 14--21 MPa. Typical creep strains-to-failure were 5-7%. The failure mode was intergranular. Creep cavities were observed in metallographic cross sections and on a fracture surface of a specimen failed during thermal creep (see Fig. 4). Several pre-crept specimens were tested under dead-weight tensile loading in contact with liquid lead at 650 K. The specimens were given a light sanding to remove oxide before testing in the liquid metal, since the oxide was found to inhibit the LME attack significantly. On some of the specimens the creep cavities were clearly exposed on the LME
Fig. I. (a) Tensile fracture surface of Ni--4 W tested at 625 K in vacuum, 30% elongation, 275 MPa ultimate tensile strength, [40 x]. (b) tensile fracture surface of Ni--4 W tested at 625 K in the presence of liquid thallium, <1% elongation, <60 MPa ultimate tensile strength.
[80x].
Vo].
~5,
No.
5
CREEP CAVITY OBSERVATION
499
•!i
Fig. 2. F r a c t u r e surface of Ni--4 W tensile tested in the p r e s e n c e of liquid lead at 625 K, <1% e l o n g a t i o n <60 MPa u l t i m a t e tensile s t r e n g t h (680x).
l'i.~,. 3. F r a c t u r e surface of AI (99.99), tensile tested in the p r e s e n c e or ]iquid gallium, examin~Jd in the SEM using (a) secondary electrons (500×), (b) b a c k - s c a t t e r e d ele~-trons, with the ~ a l l i u m remaining on the fracture surface a p p e a r i n g bright. (500×).
Fi~. 4(a). U n e t c h e d m e t a l l o g r a p h i c cross section of a Ni--4 W s p e c i m e n crept to 7% strain at 1075 K and 21 MPa (500×). (b) F r a c t u r e surface of Ni--4 W crept to failure at 1175 K and 14 MPa, w i t h 4.7% strain (600×).
500
CREEP CAVITY OBSERVATION
Vol.
1S,
No.
S
fracture surface, as shown in Fig. 5. However, many of tile internal grain boundaries observed showed no cavities, indicating heterogeneous cavitation, with a gradient in cavity concentration from tile specimen surface inward. This greater cavitation at the surface is likely to arise from one of two factors: (i) an environmental effect from impurities in the argon atmosphere, or (ii) a greater degree of grain boundary sliding in the relatively less constrained grains near the surface. Given the reduction in fracture strains observed for Ni-W alloys for creep tests conducted in argon versus vacuum (9), it is more likely that the heterogeneity in cavity nucleation is an environmental effect. The impurities and mechanism for such an effect have not been established. Summary I. To study grain boundary cavitation during creep, especially at early stages of cavity growth, new techniques are required. Fig. of Ni--4 lead at at 1075 cleaning
5. Tensile fracture surface W, pulled to failure in liquid 650 K, after pre-creeping 7% K, 21 MPa in argon [2000x]. away the I#IE agent,
examining
2. A potential technique which, given the general susceptibility of most metals to ~IE attack, may be applicable to most metals involves (a) the preparation of a given microstructure, (b) exposing this material under load to liquid metal, (c) after the intergranular fracture surface in the SEM.
3. An fcc solid-solution alloy, Ni--4 W, was chosen for study. Two liquid metals, lead and thallium, were found to be very strong embrittling agents for this alloy, and, perhaps other nickel-base alloys. 4. This alloy was cavitated during creep deformation, then exposed to liquid lead, allowing the grain boundary cavities to be examined, indicating promise as a quantitative technique to study the creep cavitation process. 5. Cavitation was found to be heterogeneous, with cavity concentration decreasing away from the specimen surface. This is consistent with an environmental, impurity-induced enhancement of cavity nucleation and/or growth near the surface. Acknowledgment The author wishes to acknowledge the experimental assistance received at KFA-Julich during his guest appointment there, and, as well, the help of C. E. Zachary, T. J. Henson, R. W. Swindeman and B. C. Williams at ORNL. The donation of the Ni--4 W alloy by Professor W. D. Nix of Stanford University is also appreciated. This research is sponsored by the Division of Materials Sciences, U.S. Department of Energy, under contract No. W-7405-eng-26 at Oak Ridge National Laboratory. References i. 2. 3. 4. 5. 6. 7. 8. 9.
R. J. M. C. M. C. W. W. W.
Lagneborg, J. Iron & Steel Inst. 207 (1969) 363. Y. Park and S. Danyluk, Corrosion 38 (1977) 304. G. Nicholas and C. F. Old, J. Mat. Sci. 14 (1979) i. F. Old, Metal Sci. 14 (1980) 433. H. Kamdar, Prog. Mat. Sci. 1 5 (1973) 289. F. Old, to be published. R. Johnson, C. R. Barrett, and W. D. Nix, Met. Trans. 3 (1972) R. Johnson, Ph.D. thesis, Stanford University, 1969. R. Johnson, C. R. Barrett, and W. D. Nix, ,Met. Trans. ~ (1972)
963. 695.
METALLURGICA
Vol. 18, p p . 4 9 7 - 5 0 0 , 1981 Printed in t h e U . S . A .
CREEP CAVITY OBSERVATION
Pergamon P r e s s Ltd. All rights reserved
USING LIQUID METAL EMBRITTLEMENT
T. C. Reiley Oak Ridge National
Laboratory,
(Received February (Revised March
Oak Ridge,
TN
37830
17, 1981) 3, 1 9 8 1 )
Introduction Grain boundary cavities, which form during high-temperature deformation and which may be considered incipient intergranular cracks, have been difficult to observe at the early growth stages. To examine grain boundaries in unfailed specimens one is normally limited to metallographic or TEM techniques, the first having resolution limitations (rcavity ~ l~m) and the second having limitations in the probability of seeing a cavity (especially for large-grained, low-density specimens) or of seeing a cavity which is undisturbed by the thinning process required for TEM. Other observational techniques involving hydrogen attack (I) or lowtemperature impact loading (2) have been used to create brittle intergranular failures to observe grain boundary features, such as creep cavities on pre-crept specimens. These techniques are not, however, applicable to most metals, and as such, the need remains for a more general observational technique. Such a new technique employing liquid metal embrittlement (LME) is described below, and, given that most metals are severely embrittled by at least one liquid metal, this approach may satisfy requirements of general applicability. Liquid Metal Embrittlement
Technique
The approach taken in these experiments entails the development of a cavitated microstructure by creeping a chosen alloy to a given strain at a given stress and temperature. The specimen is then exposed under low stress at a much lower temperature to a particular liquid metal which, as it penetrates the grain boundary causes brittle intergranular failure. Once the liquid metal is removed, the grain boundary details of interest may be examined in the scanning electron microscope (SEH). The phenomenology of LME can be quite varied, ranging from a slight reduction in strainto-failure to complete grain boundary decohesion under essentially zero load (e.g., gallium on aluminum) -- see refs. 3--5 for reviews of this subject. The basic phenomenon involves a surface-energy-driven process in which grain boundary decohesion (or sometimes cleavage) at a crack tip proceeds at a lower stress than would be possible without the liquid metal. The stress and temperature-dependent kinetics of liquid metal delivery and, perhaps, adsorption at the crack tip are also important factors in LME. For LME to occur, the following relationships between solid and liquid metal are usually observed in ~IE couples: (i) the solid and liquid metals have low mutual intersolubilities and (ii) these metals do not form intermetallic compounds. [A set of criteria based on heats of solution having fewer exceptions than the above criteria has been proposed by Old (6).] Note, this noninteractive behavior is quite convenient for the proposed LME fracture process, given that the exposure of the intergranular fracture surface to liquid metal should lead to minimal dissolution or alteration of fine-scale grain boundary features. It is also expected that the greater the degree of embrittlement, or the smaller the strain-to-failure under liquid metal, the better will be the preservat[on of these grain boundary details. Results The alloy chosen for study is Ni--4 wt % W, a low stacking-fault-energy, fcc, solid solution, whose preparation and impurities are described in refs. 7 and 8, along with its proclivity for creep cavitation. TEH examination showed no evidence of a second phase. Tensile
497 0036-9748/81/050497-04502.00/0
498
CREEP CAVITY OBSERVATION
Vol.
15, No,
5
creep specimens, having rectangular cross sections of 2 ~ x 1 mm, were machined from recrystallized sheet, then annealed in high vacuum at 900°C for I h. Grain size was approximately 100 ~m. In the first stage of these experiments it was necessary to find a suitable embrittling agent. Two liquid metals were found which satisfy the ~IE criteria for nickel. These are pure lead and pure thallium, both of which reduced the tensile elongation at 625 K from ~ 30% to <1% with reductions in ultimate tensile strength from 275 MPa to <60 MPa when exposed to either liquid (see Fig. i). This author is not aware of thallium being reported as an embrittling agent for nickel or nickel-base alloys; it is expected that thallium would embrittle nickel alloys other than Ni--4 W. After a certain period of exposure at temperature (50--75 K higher than the melting temperature of the liquid) and stress, the specimen failed and the liquid metal was drained into a reservoir, leaving a small amount of liquid metal to freeze on the fracture surface. This remaining metal was dissolved using a solution of I part acetic acid, 2 parts H202 for lead and concentrated sulfuric acid for thallium. Neither of these acids seemed to attack Ni--4 W during the short exposures. An example of the cleaned fracture surface of a lead-embrittled specimen is shown in Fig. 2. Lead was chosen as a more convenient embrittling agent, given that exposure periods leading to failure at 625--675 K and 50 ~ a were only i--2 h for lead and 10-15 h for thallium, with the quality of the LME fracture surface being roughly the same. Also, lead has considerably lower toxicity than thallium. One technique which aids in the SEM observation of the fracture surface is the use of backscattered electron imaging. This SEM mode is used to observe any remaining liquid metal, when the liquid metal has a significantly different atomic number from the solid metal. An example is given in Fig. 3 of a recrystalllzed aluminum (99.99%) specimen which has been embrittled by gallium at 320 K and low stress. It is viewed using conventional secondary electron imaging and backscattered electron imaging, exposing those areas of the fracture surface covered with gallium. The same technique is applicable to lead or thallium on Ni--4 W. Once the embrittling agents were established, Ni--4 W specimens were pre-crept under constant load at 1075--1175 K in argon at nominal stresses of 14--21 MPa. Typical creep strains-to-failure were 5-7%. The failure mode was intergranular. Creep cavities were observed in metallographic cross sections and on a fracture surface of a specimen failed during thermal creep (see Fig. 4). Several pre-crept specimens were tested under dead-weight tensile loading in contact with liquid lead at 650 K. The specimens were given a light sanding to remove oxide before testing in the liquid metal, since the oxide was found to inhibit the LME attack significantly. On some of the specimens the creep cavities were clearly exposed on the LME
Fig. I. (a) Tensile fracture surface of Ni--4 W tested at 625 K in vacuum, 30% elongation, 275 MPa ultimate tensile strength, [40 x]. (b) tensile fracture surface of Ni--4 W tested at 625 K in the presence of liquid thallium, <1% elongation, <60 MPa ultimate tensile strength.
[80x].
Vo].
~5,
No.
5
CREEP CAVITY OBSERVATION
499
•!i
Fig. 2. F r a c t u r e surface of Ni--4 W tensile tested in the p r e s e n c e of liquid lead at 625 K, <1% e l o n g a t i o n <60 MPa u l t i m a t e tensile s t r e n g t h (680x).
l'i.~,. 3. F r a c t u r e surface of AI (99.99), tensile tested in the p r e s e n c e or ]iquid gallium, examin~Jd in the SEM using (a) secondary electrons (500×), (b) b a c k - s c a t t e r e d ele~-trons, with the ~ a l l i u m remaining on the fracture surface a p p e a r i n g bright. (500×).
Fi~. 4(a). U n e t c h e d m e t a l l o g r a p h i c cross section of a Ni--4 W s p e c i m e n crept to 7% strain at 1075 K and 21 MPa (500×). (b) F r a c t u r e surface of Ni--4 W crept to failure at 1175 K and 14 MPa, w i t h 4.7% strain (600×).
500
CREEP CAVITY OBSERVATION
Vol.
1S,
No.
S
fracture surface, as shown in Fig. 5. However, many of tile internal grain boundaries observed showed no cavities, indicating heterogeneous cavitation, with a gradient in cavity concentration from tile specimen surface inward. This greater cavitation at the surface is likely to arise from one of two factors: (i) an environmental effect from impurities in the argon atmosphere, or (ii) a greater degree of grain boundary sliding in the relatively less constrained grains near the surface. Given the reduction in fracture strains observed for Ni-W alloys for creep tests conducted in argon versus vacuum (9), it is more likely that the heterogeneity in cavity nucleation is an environmental effect. The impurities and mechanism for such an effect have not been established. Summary I. To study grain boundary cavitation during creep, especially at early stages of cavity growth, new techniques are required. Fig. of Ni--4 lead at at 1075 cleaning
5. Tensile fracture surface W, pulled to failure in liquid 650 K, after pre-creeping 7% K, 21 MPa in argon [2000x]. away the I#IE agent,
examining
2. A potential technique which, given the general susceptibility of most metals to ~IE attack, may be applicable to most metals involves (a) the preparation of a given microstructure, (b) exposing this material under load to liquid metal, (c) after the intergranular fracture surface in the SEM.
3. An fcc solid-solution alloy, Ni--4 W, was chosen for study. Two liquid metals, lead and thallium, were found to be very strong embrittling agents for this alloy, and, perhaps other nickel-base alloys. 4. This alloy was cavitated during creep deformation, then exposed to liquid lead, allowing the grain boundary cavities to be examined, indicating promise as a quantitative technique to study the creep cavitation process. 5. Cavitation was found to be heterogeneous, with cavity concentration decreasing away from the specimen surface. This is consistent with an environmental, impurity-induced enhancement of cavity nucleation and/or growth near the surface. Acknowledgment The author wishes to acknowledge the experimental assistance received at KFA-Julich during his guest appointment there, and, as well, the help of C. E. Zachary, T. J. Henson, R. W. Swindeman and B. C. Williams at ORNL. The donation of the Ni--4 W alloy by Professor W. D. Nix of Stanford University is also appreciated. This research is sponsored by the Division of Materials Sciences, U.S. Department of Energy, under contract No. W-7405-eng-26 at Oak Ridge National Laboratory. References i. 2. 3. 4. 5. 6. 7. 8. 9.
R. J. M. C. M. C. W. W. W.
Lagneborg, J. Iron & Steel Inst. 207 (1969) 363. Y. Park and S. Danyluk, Corrosion 38 (1977) 304. G. Nicholas and C. F. Old, J. Mat. Sci. 14 (1979) i. F. Old, Metal Sci. 14 (1980) 433. H. Kamdar, Prog. Mat. Sci. 1 5 (1973) 289. F. Old, to be published. R. Johnson, C. R. Barrett, and W. D. Nix, Met. Trans. 3 (1972) R. Johnson, Ph.D. thesis, Stanford University, 1969. R. Johnson, C. R. Barrett, and W. D. Nix, ,Met. Trans. ~ (1972)
963. 695.