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Microstructural observations in the vicinity of cavitated grain boundaries in copper bicrystals fatigued at high temperatures
Scripta
~ET~LLURGICA
Vol. 20, p p . 1 6 4 5 - 1 6 5 0 , ].986 Printed in t h e U . S . A .
Pergamon Journals Ltd. All rights reserved
MICROSTRUCTURAL OBSERVATIONS IN THE VICINITY OF CAVITATED GRAIN BOUNDARIES IN COPPER BICRYSTALS FATIGUED AT HIGH TEMPERATURES
J, G. Cabanas-Mot,no* and J. R. Weertman Department of Materials Science & Engineering and Materials Research Center Northwestern University, Evanston, IL 60201
(Received
September
8,
1986)
Recent studies (1-4} of the behavior of grain boundaries (gb's) in a number of pure metals subjected to high temperature fatigue have clearly established that the boundaries can move a small amount during every fatigue cycle. Such studies have been made at relatively low frequencies of cycling ( " 10-3-1 Hz). It was found that the extent of boundary motion per cycle increases with decreasing frequency, but it m a y be expected that even at higher frequencies the migration after a large number of cycles will be appreciable. The migration of grain boundaries is of particular importance to the phenomenon of gb cavitation. It has been shown (5-12) that gb sliding and migration play an active role in the nucleation and growth of cavities on gh's. In most quantitative studies of cavitation (6, 13-17) it is assumed that all or most of the cavities nucleate and remain (growing) on their gb's, although several authors have reported that cavities can be left behind in the matrix by a moving boundary (8,9,18). Thus it is desirable to know more about the behavior of gh's during fatigueinduced cavitation and about how this behavior affects the cavities and, in general, the local mIcrostructure. In this paper we report briefly on observations made by TEM and HVEM on copper bicrystal specimens which were fatigued at a high temperature and frequency. We focus mainly on the microstructural features developed in the vicinity of the gb's. Experimental Pure copper (99.999% pure, as specified by the manufacturer) hicrystals were grown by the Bridgman technique in a graphite crucible under an atmosphere of purified argon. The bicrystals were obtained as 6.5 m m thick sheetse from which tensile specimens for fatigue testing were spark cut. Specimens were fatigued in load control with zero mean stress at 405 ° C (I/2 the melting temperature) in a vacuum of lees than 10 -3 Pa and at a frequency of 17 Hz. Table 1 summarizes the specimen and testing conditions. After fatiguing, TEM and HVEM specimens were obtained by standard procedures. Gener~
Qblervatlonsz
Figure I a wide region structures in numerous foll of the gho
bicrYsta!s
B~
(taken from reference (19)) shows a low surrounding the gb in bicrystal BI. It the immediate vicinity of the gb differ perforations have been produced in both
magnification TEM micrograph covering can be seen that {i} the dislocation markedly from those far away7 and (ii), grains of the bicrystal in the neighborhood
Figures 2 to 4 give some details of the dislocation structure in BI. In Fig. 2 we observe a well-developed subgrain structure adjacent to the gb in both grains (grains A and B). In Figs. 3 (grain A) and 4 (grain B) the dislocation structure near the gb consists of more or less loosely connected dislocation tangles, a number of which run closely parallel to the gbo
*Permanent addreesz Instituto Politechnico Apartado Postal 75-874, Mexlco 14, D.F.
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0036-974g/86 Copyright
(c)
1986
Division de Inginieria
1645 $3.00
+
Pergamon
Journals
.00 Ltd.
Metallurgica,
1646
FATIGUED
Somewhat farther from the parallel to the 9b trace. region located between the the dislocation structure
Cu
BICRYSTALS
Vol.
20,
No.
Ii
gb we observe rather long dislocation walls (W) which also run It should be noticed that all foil perforations are confined to the two dislocation walls on either side of the gb. Far from the gb is comprised of cells (Figs. 3 and 4). TABLE 1 Test Data on the Copper Bicrystals
Bicrystal
B X 7 5 5
I 1 3 1 2
8 (degrees)
58 60 15 90 43
@ (degrees)
Stress Amplitude (MPa)
25 14.5 14.5 30.5 30.5
24.8 17.0 17.0 17.0 17.0
Fatigue Time (seconds)
600 1180 1140 1180 1180
• - angle between stress axis and gb plane Q - angle of mlsorientation between grains in blcrystals
From these observations 1.
2.
3.
we can point out the following:
At least that part of the gb of bicrystal BI shown in Figs. 1-4 has moved during fatigue. In those regions which have recently seen the passage of the gb, the dislocatlon structure is broken up into tangles. In places where the passage of the gb occurred much earlier (or, quite likely, where the gb never reached}, the structure is in a much more condensed state (dislocation walls, cells, subgrains). The region which a gb has passed through is identified by changes in the dislocation structure, by dislocation walls demarcating the limits of migration, and by foll Perforations between these walls. Based on these indications, it may be concluded that the lower part of the gb in Fig. 1 has reversed direction of migration during the course of fatigue of the specimen. The foil perforations occur as the result of the electrothinning process during foil preparation. The perforations originate from the fatlgue-lnduced cavities which nucleated and grew on the gb's. Most of these cavities were abandoned in the matrix during gb migration. This supposition concerning the origin of the perforations was confirmed by HVEM observations of fully enclosed cavities in thick foils taken from the fatigued bicrystals (Fig. 5).
Effgc ~ of ~
o r i e n t a t i o n and ~
s~dinq:
b i c r v s t a l s 7 1 , ~ 3 ~ 6 ~ and 6~.
The misorientation between the 2 grains A and B is the same in specimens X I and X 3 but the orientation of the gb with respect to the stress axis differs (Table 1). Dislocation structures in the vicinity of the gb show similar features after fatigue in the two bicrystal specimens. Near the gb more or less condensed dislocation tangles tend to form cells. Running parallel to the boundary some few Pm's away are one or more lines of loosely concentrated "clouds" of dislocations (marked mC" in Fig. 6). In some places these clouds have compressed into walls similar to those of Figs. 3 and 4. As before, foil Perforations are confined to the region between the outermost lines of clouds or walls. Thus it is found that changing the orientation of the gb while keeping other crystallographic parameters constant does not lead to evident cha~ges in the dislocation structures which develop during fatigue. Rather these structures seem to depend primarily on the details of the gb movement. If, however, the orientation of the gb is such that little gb sliding occurs, the relationship of the gb to the stress axis becomes important. Figure 7 illustrates the dislocation structures produced near the gb's in fatigued bicrystal 5 i. According to Table 1, very llttle gb sllding is expected in this specimen. It can be seen that the far field
Vol.
20,
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ii
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1647
dislocation structure in 61 persists right up to the gb. In the case of 62, where it is expected that gb sliding accompanied fatigue, there is a disturbed region close to the gb similar to structures seen in fatigued specimens B1, 71 and 73. No foil perforations (and thus, no cavities, or at least no cavities of a size observable under the present TEM conditions) were produced in specimen 61 by fatigue. H o w e v e ~ voids were found in 62 after fatigue. These differences are in agreement with what would be expected from sliding vs non-sliding boundaries. The former may be prone to void nucleation and gb migration (5, 7-11), but not so in the latter case. Conclusions High temperature fatigue can cause gb migration and gb cavitation in copper bicrystals. Such a migrating boundary leaves a disturbed dislocation structure and cavities in its wake. Migration can reverse direction during fatigue. The extreme positions of a migrating boundary are indicated by the presence of dislocation walls which run closely parallel to the boundary. Cavitation is not observed in the absence of gb sliding. Acknowledgments We are grateful to Argonne National Laboratory for the use of the HVEM microscope. This work was supported by the MRL program of the National Science Foundation under grant DMR-7923573 and by the government of Mexico. References i. 2. 3. 4. 5. 6.
7. 8. 9.
i0. 11. 12. 13. 14. 15. 16.
17, 18. 19.
P. Yavari and T. G. L a n g d o n , A c t a Met. 31, 1595 (1983). V. R a m a n and T. G. L a n g d o n , J. Mater. Sci. Lett. 2, 180 (1983). T . G . L a n g d o n and R. C. G i f k i n s , A c t a Met. 31, 927 (1983). T. G. Langdon, D. Simpson and R. C. Gifkins, Acta Met. 31, 939 (1983). R. Raj and M. F. Ashby, Acta Met. 23, 653 (1975). M. S. Yang, J. R. W e e r t m a n and M. Roth, Proc. 5th R i s ~ Intnl. Syrup. on M e t a l l u r g y and Materials Science, pp. 589-594, N. Hessel Andersen et al., Eds., Ris~ Natl. Lab., Roskilde, D e n m a r k (1984). T. S a e g u s a and J. R. W e e r t m a n , S c r i p t a Met. 12, 187 (1978). R.A. Page and J. R. W e e r t m a n , A c t a Met. 29, 527 (1981). J . G . C a b a ~ a s - M o r e n o , M. S. Yang, J. R. W e e r t m a n , M. Roth, Z. Y. Yang, G. D. W i g n a l l and W. C. Koehler, Fatigue Mechanismss Quantitative Measurement of Physical Damage, ASTM STP 811, pp. 85-114, J. L a n k f o r d , D. L. D a v i d s o n , W. L. M o r r i s and R. P. Wei, Eds., A S T M (1983). M. H. Yoo and B. T r i n k a u s , Met. Trans. A 14A, 547 (1983). T. W a t a n a b e and P. W. Davies, Philos. Mag. A37, 649 (1978). K. U. S n o w d e n , P. A. S t r a t h e r s and D. S. Hughes, Res M e c h a n i c a I, 129 (1980). A. S. Argon, I.-M. Chen and C. W. Lau, Creep-Fatigue-Environment Interactions, pp. 46-83, R. M. P e l l o u x and N. S. S t o l o f f , Eds., T M S - A I M E , N e w York (1980). D. A. M i l l e r and T. G. L a n g d o n , Met. Trans. A I I A , 955 (1980). M. S. Yang, J. R. W e e r t m a n and M. Roth, S c r i p t a Met. 18, 543 (1984). M. S. Yang, J. R. W e e r t m a n and M. Roth, Proc. 2nd Intnl. Conf. on C r e e p and F r a c t u r e of Engineering Materials and Structures, pp. 149-156, B. Wilshire and D. R. J. Owen, Eds., Pineridge Press, Swansea, UK (1984). R. A. Page, J. R. W e e r t m a n and M. Roth, A c t a Met. 30, 1357 (1982). A. Gittens, M e t a l Sci. J., 2, 51 (1968). J. R. Weertman, Proceedings in Physics: Atomic Transport and Defects in Metals by Neutron Scattering. C. Janot, W. Perry, D. Richter and T. Springer, Eds., Springer Verlag, Berlin (1986).
1648
FATIGUED
Cu BICRYSTALS
Figure 1.
Vol.
20, No. 11
Tl~4micrograph of wide area surrounding the gb in fatigued bicrystal B I. From (19).
Figure 2.
Subgrain structure around gb of fatigued b i c r y s t a l B 1.
Vol. 20, No. ii
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Figure 4. Grain B of fatigued bicrystal B I, showing long dislocation walls (W). Field of view~with respect to Fig. I can be determined by matching O's.
O. 5 ~u~n
I ~m
(a)
Figure 5.
(b)
HVl~micrographs of fatigued bicrystal BI showing small gb cavities completely enclosed in foil. Micrographs taken at Argonne National Laboratory.
1650
I=ATIC,IIED Cu BICRYSTALS
Vol.
20, No. 11
Figure 6. Two lines of dislocation clouds [C) in fatigued bicrystal ~I.
(a)
Figure 7.
Fatigued b i c r y s t a l 61. or clouds.
(b)
Note absence of c a v i t i e s , long d i s l o c a t i o n walls
~ET~LLURGICA
Vol. 20, p p . 1 6 4 5 - 1 6 5 0 , ].986 Printed in t h e U . S . A .
Pergamon Journals Ltd. All rights reserved
MICROSTRUCTURAL OBSERVATIONS IN THE VICINITY OF CAVITATED GRAIN BOUNDARIES IN COPPER BICRYSTALS FATIGUED AT HIGH TEMPERATURES
J, G. Cabanas-Mot,no* and J. R. Weertman Department of Materials Science & Engineering and Materials Research Center Northwestern University, Evanston, IL 60201
(Received
September
8,
1986)
Recent studies (1-4} of the behavior of grain boundaries (gb's) in a number of pure metals subjected to high temperature fatigue have clearly established that the boundaries can move a small amount during every fatigue cycle. Such studies have been made at relatively low frequencies of cycling ( " 10-3-1 Hz). It was found that the extent of boundary motion per cycle increases with decreasing frequency, but it m a y be expected that even at higher frequencies the migration after a large number of cycles will be appreciable. The migration of grain boundaries is of particular importance to the phenomenon of gb cavitation. It has been shown (5-12) that gb sliding and migration play an active role in the nucleation and growth of cavities on gh's. In most quantitative studies of cavitation (6, 13-17) it is assumed that all or most of the cavities nucleate and remain (growing) on their gb's, although several authors have reported that cavities can be left behind in the matrix by a moving boundary (8,9,18). Thus it is desirable to know more about the behavior of gh's during fatigueinduced cavitation and about how this behavior affects the cavities and, in general, the local mIcrostructure. In this paper we report briefly on observations made by TEM and HVEM on copper bicrystal specimens which were fatigued at a high temperature and frequency. We focus mainly on the microstructural features developed in the vicinity of the gb's. Experimental Pure copper (99.999% pure, as specified by the manufacturer) hicrystals were grown by the Bridgman technique in a graphite crucible under an atmosphere of purified argon. The bicrystals were obtained as 6.5 m m thick sheetse from which tensile specimens for fatigue testing were spark cut. Specimens were fatigued in load control with zero mean stress at 405 ° C (I/2 the melting temperature) in a vacuum of lees than 10 -3 Pa and at a frequency of 17 Hz. Table 1 summarizes the specimen and testing conditions. After fatiguing, TEM and HVEM specimens were obtained by standard procedures. Gener~
Qblervatlonsz
Figure I a wide region structures in numerous foll of the gho
bicrYsta!s
B~
(taken from reference (19)) shows a low surrounding the gb in bicrystal BI. It the immediate vicinity of the gb differ perforations have been produced in both
magnification TEM micrograph covering can be seen that {i} the dislocation markedly from those far away7 and (ii), grains of the bicrystal in the neighborhood
Figures 2 to 4 give some details of the dislocation structure in BI. In Fig. 2 we observe a well-developed subgrain structure adjacent to the gb in both grains (grains A and B). In Figs. 3 (grain A) and 4 (grain B) the dislocation structure near the gb consists of more or less loosely connected dislocation tangles, a number of which run closely parallel to the gbo
*Permanent addreesz Instituto Politechnico Apartado Postal 75-874, Mexlco 14, D.F.
Nacional,
0036-974g/86 Copyright
(c)
1986
Division de Inginieria
1645 $3.00
+
Pergamon
Journals
.00 Ltd.
Metallurgica,
1646
FATIGUED
Somewhat farther from the parallel to the 9b trace. region located between the the dislocation structure
Cu
BICRYSTALS
Vol.
20,
No.
Ii
gb we observe rather long dislocation walls (W) which also run It should be noticed that all foil perforations are confined to the two dislocation walls on either side of the gb. Far from the gb is comprised of cells (Figs. 3 and 4). TABLE 1 Test Data on the Copper Bicrystals
Bicrystal
B X 7 5 5
I 1 3 1 2
8 (degrees)
58 60 15 90 43
@ (degrees)
Stress Amplitude (MPa)
25 14.5 14.5 30.5 30.5
24.8 17.0 17.0 17.0 17.0
Fatigue Time (seconds)
600 1180 1140 1180 1180
• - angle between stress axis and gb plane Q - angle of mlsorientation between grains in blcrystals
From these observations 1.
2.
3.
we can point out the following:
At least that part of the gb of bicrystal BI shown in Figs. 1-4 has moved during fatigue. In those regions which have recently seen the passage of the gb, the dislocatlon structure is broken up into tangles. In places where the passage of the gb occurred much earlier (or, quite likely, where the gb never reached}, the structure is in a much more condensed state (dislocation walls, cells, subgrains). The region which a gb has passed through is identified by changes in the dislocation structure, by dislocation walls demarcating the limits of migration, and by foll Perforations between these walls. Based on these indications, it may be concluded that the lower part of the gb in Fig. 1 has reversed direction of migration during the course of fatigue of the specimen. The foil perforations occur as the result of the electrothinning process during foil preparation. The perforations originate from the fatlgue-lnduced cavities which nucleated and grew on the gb's. Most of these cavities were abandoned in the matrix during gb migration. This supposition concerning the origin of the perforations was confirmed by HVEM observations of fully enclosed cavities in thick foils taken from the fatigued bicrystals (Fig. 5).
Effgc ~ of ~
o r i e n t a t i o n and ~
s~dinq:
b i c r v s t a l s 7 1 , ~ 3 ~ 6 ~ and 6~.
The misorientation between the 2 grains A and B is the same in specimens X I and X 3 but the orientation of the gb with respect to the stress axis differs (Table 1). Dislocation structures in the vicinity of the gb show similar features after fatigue in the two bicrystal specimens. Near the gb more or less condensed dislocation tangles tend to form cells. Running parallel to the boundary some few Pm's away are one or more lines of loosely concentrated "clouds" of dislocations (marked mC" in Fig. 6). In some places these clouds have compressed into walls similar to those of Figs. 3 and 4. As before, foil Perforations are confined to the region between the outermost lines of clouds or walls. Thus it is found that changing the orientation of the gb while keeping other crystallographic parameters constant does not lead to evident cha~ges in the dislocation structures which develop during fatigue. Rather these structures seem to depend primarily on the details of the gb movement. If, however, the orientation of the gb is such that little gb sliding occurs, the relationship of the gb to the stress axis becomes important. Figure 7 illustrates the dislocation structures produced near the gb's in fatigued bicrystal 5 i. According to Table 1, very llttle gb sllding is expected in this specimen. It can be seen that the far field
Vol.
20,
No.
ii
FATIGUED
Cu
BICRYSTALS
1647
dislocation structure in 61 persists right up to the gb. In the case of 62, where it is expected that gb sliding accompanied fatigue, there is a disturbed region close to the gb similar to structures seen in fatigued specimens B1, 71 and 73. No foil perforations (and thus, no cavities, or at least no cavities of a size observable under the present TEM conditions) were produced in specimen 61 by fatigue. H o w e v e ~ voids were found in 62 after fatigue. These differences are in agreement with what would be expected from sliding vs non-sliding boundaries. The former may be prone to void nucleation and gb migration (5, 7-11), but not so in the latter case. Conclusions High temperature fatigue can cause gb migration and gb cavitation in copper bicrystals. Such a migrating boundary leaves a disturbed dislocation structure and cavities in its wake. Migration can reverse direction during fatigue. The extreme positions of a migrating boundary are indicated by the presence of dislocation walls which run closely parallel to the boundary. Cavitation is not observed in the absence of gb sliding. Acknowledgments We are grateful to Argonne National Laboratory for the use of the HVEM microscope. This work was supported by the MRL program of the National Science Foundation under grant DMR-7923573 and by the government of Mexico. References i. 2. 3. 4. 5. 6.
7. 8. 9.
i0. 11. 12. 13. 14. 15. 16.
17, 18. 19.
P. Yavari and T. G. L a n g d o n , A c t a Met. 31, 1595 (1983). V. R a m a n and T. G. L a n g d o n , J. Mater. Sci. Lett. 2, 180 (1983). T . G . L a n g d o n and R. C. G i f k i n s , A c t a Met. 31, 927 (1983). T. G. Langdon, D. Simpson and R. C. Gifkins, Acta Met. 31, 939 (1983). R. Raj and M. F. Ashby, Acta Met. 23, 653 (1975). M. S. Yang, J. R. W e e r t m a n and M. Roth, Proc. 5th R i s ~ Intnl. Syrup. on M e t a l l u r g y and Materials Science, pp. 589-594, N. Hessel Andersen et al., Eds., Ris~ Natl. Lab., Roskilde, D e n m a r k (1984). T. S a e g u s a and J. R. W e e r t m a n , S c r i p t a Met. 12, 187 (1978). R.A. Page and J. R. W e e r t m a n , A c t a Met. 29, 527 (1981). J . G . C a b a ~ a s - M o r e n o , M. S. Yang, J. R. W e e r t m a n , M. Roth, Z. Y. Yang, G. D. W i g n a l l and W. C. Koehler, Fatigue Mechanismss Quantitative Measurement of Physical Damage, ASTM STP 811, pp. 85-114, J. L a n k f o r d , D. L. D a v i d s o n , W. L. M o r r i s and R. P. Wei, Eds., A S T M (1983). M. H. Yoo and B. T r i n k a u s , Met. Trans. A 14A, 547 (1983). T. W a t a n a b e and P. W. Davies, Philos. Mag. A37, 649 (1978). K. U. S n o w d e n , P. A. S t r a t h e r s and D. S. Hughes, Res M e c h a n i c a I, 129 (1980). A. S. Argon, I.-M. Chen and C. W. Lau, Creep-Fatigue-Environment Interactions, pp. 46-83, R. M. P e l l o u x and N. S. S t o l o f f , Eds., T M S - A I M E , N e w York (1980). D. A. M i l l e r and T. G. L a n g d o n , Met. Trans. A I I A , 955 (1980). M. S. Yang, J. R. W e e r t m a n and M. Roth, S c r i p t a Met. 18, 543 (1984). M. S. Yang, J. R. W e e r t m a n and M. Roth, Proc. 2nd Intnl. Conf. on C r e e p and F r a c t u r e of Engineering Materials and Structures, pp. 149-156, B. Wilshire and D. R. J. Owen, Eds., Pineridge Press, Swansea, UK (1984). R. A. Page, J. R. W e e r t m a n and M. Roth, A c t a Met. 30, 1357 (1982). A. Gittens, M e t a l Sci. J., 2, 51 (1968). J. R. Weertman, Proceedings in Physics: Atomic Transport and Defects in Metals by Neutron Scattering. C. Janot, W. Perry, D. Richter and T. Springer, Eds., Springer Verlag, Berlin (1986).
1648
FATIGUED
Cu BICRYSTALS
Figure 1.
Vol.
20, No. 11
Tl~4micrograph of wide area surrounding the gb in fatigued bicrystal B I. From (19).
Figure 2.
Subgrain structure around gb of fatigued b i c r y s t a l B 1.
Vol. 20, No. ii
FATIGUED
Cu BICP.YSTALS
1649
Figure 4. Grain B of fatigued bicrystal B I, showing long dislocation walls (W). Field of view~with respect to Fig. I can be determined by matching O's.
O. 5 ~u~n
I ~m
(a)
Figure 5.
(b)
HVl~micrographs of fatigued bicrystal BI showing small gb cavities completely enclosed in foil. Micrographs taken at Argonne National Laboratory.
1650
I=ATIC,IIED Cu BICRYSTALS
Vol.
20, No. 11
Figure 6. Two lines of dislocation clouds [C) in fatigued bicrystal ~I.
(a)
Figure 7.
Fatigued b i c r y s t a l 61. or clouds.
(b)
Note absence of c a v i t i e s , long d i s l o c a t i o n walls