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Conversion of stacking fault tetrahedra to voids in electron irradiated Fe-Cr-Ni
Journal of Nuclear Materials North-Holland, Amsterdam
141-143
(1986) 763-766
763
CONVERSION OF STACKING FAULT TETRAHEDRA TO VOIDS IN ELECTRON IRRADIATED Fe-Cr-Ni S. KOJIMA,
Y. SANO,
T. YOSHIIE,
N. YOSHIDA
* and M. KIRITANI
Department of Precision Engineering, Faculty of Engineering, Hokkaido Uniuersi(v, Sapporo 060, Japan
Electron irradiations of the austenitic Fe-13Cr-14Ni alloy were performed with a high voltage electron microscope at temperatures between room temperature and 650 K. Formation of stacking fault tetrahedra, voids and dislocation loops was observed as vacancy clusters. At the lower temperatures, the dominant vacancy clusters were tetrahedra and at the higher temperatures, voids were dominant. In the temperature range at which both tetrahedra and voids were coexistent, conversion of tetrahedra to voids were observed. These results are interpreted as the preferable nucleation of voids at the site of tetrahedra. Local effects of dilatation field at the comer of tetrahedra and the segregation of solute atoms are considered to enhance the nucleation, Clustered defects which are considered to be stacking fault tetrahedra that are formed with D-T fusion neutrons in SUS 316 stainless steel are suggested as the preferable site for void nucleation.
1. Introduction
2. Experimental
Clustered vacancy defects, stacking fault tetrahedra, voids and dislocation loops, which can be stably formed in metals and alloys under a supersaturation of vacancies, do not always exist in the configuration of minimum energy. In quenched aluminum for example, faulted dislocation loops are formed in spite of its high stacking fault energy [l] and voids also coexist [2]. The energy of large clusters is not important in determining the type of vacancy cluster, but the type of cluster is believed to be determined at the initial stage of the clustering process. In the present electron irradiation experiment for an Fe-13Cr-14Ni alloy in the high voltage electron microscope, formation of both stacking fault tetrahedra and voids were confirmed. Simultaneous formation of these two types of defect clusters under electron irradiation has been previously reported by one of the present authors in the case of dilute gold alloys [3]. In these alloys, the nucleation mechanism has been investigated in detail by electrical resitivity measurements and positron annihilation experiment by Takamura et al. in quenched alloys [4,5]. These authors came to the conclusion that the development of either a stacking fault tetrahedron or a void is determined at the stage of only a few vacancies, with the difference arising from different combinations of vacancies and solute atoms. However, the present irradiation experiment of an Fe-13Cr-14Ni alloy has revealed the existence of a reaction which seems to be the conversion of a stacking fault tetrahedron to a void. It is important to discuss the relationship between the presently observed conversion and the importance of initial clustering on the determination of the type of clustered vacancy defects.
Material used in this experiment was the austenitic Fe-13Cr-14Ni alloy which is a model alloy for the candidate SUS 316 alloy for fusion reactor application. This alloy was prepared from highly purified component materials. It was rolled to 0.1 mm thickness and annealed at 1323 K for 30 min in vacuum. Discs of 3 mm diameter were punched out to make electron microscope specimens and electropolished. Electron irradiation and in-situ observation of defect structure development were performed with a H-1300 high voltage electron microscope (HVEM) operated at an acceleration voltage of 1000 kV. A heating stage was used for the irradiation at elevated temperatures. The maximum temperatures of present interest was about 600 K, and changes of the materials due to the high temperature annealing was negligible [6]. Direct estimation of the temperature rise by electron beam heating [7,8] was not carried out. Therefore, the actual temperature may have been higher than the nominal temperature indicated in the text due to the poor heat conductivity of stainless steel. Post-irradiation observations of clustered defects were made with a JEM-ZOOCX transmission electron microscope operated at 200 kV.
* Research Institute sity, Kasugakohen,
for Applied Kasuga-shi,
Mechanics, Kyushu UniverFukuoka 816, Japan.
0022-3115/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
3. General aspects of point defect cluster formation Fig. 1 compares the clustered defects produced at three representative temperatures. At 373 K, a large number of small interstitial clusters were observed. The existence of very small vacancy clusters (< 2 nm) was confirmed in the 200 kV microscope observations, but the clusters were too small to be observed in the bright field image of the HVEM during irradiation. At the higher temperatures, the number density of clustered defects decreased and their sizes became larger. At 523 K, well developed vacancy clusters and interstitial loops could be seen. And at 623 K, no interstitial loops were formed in the thinner parts of a specimen and only vacancy clusters were observed.
B.V.
764
S. Kojima et al. / Conversion of stacking fault tetrahedra to voids
Fig. 1. Point defect clusters produced at three representative temperatures at 1.8X102 s after the start of the irradiation. Left side is the edge of the thickness tapered specimen and their thickness increase toward right side (6 X 1O23 electrons/ m2s).
The observed vacancy clusters were stacking fault tetrahedra, voids, and dislocation loops. Fig. 2 shows those vacancy clusters formed at 573 K in two different imaging conditions, i.e., (a) observations with so-called void contrast far from any strong electron reflection, and (b) those adjusted so as to obtain strong contrast of stacking fault tetrahedra with a smaller deviation from Bragg reflection. The planes of observation were both close to the (110) orientation, i.e., octahedral voids were observed as diamond shapes and stacking fault tetrahedra were observed as triangles. The dislocation loops of the interstitial type that coexisted with smaller vacancy type defect clusters, always grew rapidly under irradiation. The smaller loops, that are known to be converted from stacking fault tetrahedra, did not show a rapid growth, and thus we are confident that the presently observed stacking fault tetrahedra are vacancy type and not interstitial type as reported by Hardy and Jenkins [9]. While the number density of vacancy clusters as a whole gradually decreased with the increase of irradiation temperature, each different type of vacancy cluster showed a different temperature dependence. In fig. 3, the variation of number density of stacking fault tetrahedra (SFT), voids and loops above 473 K is shown. Irradiation times were all 1.8 X lo2 s. Since the number
Fig. 2. Vacancy clusters formed at 573 K observed (a) with a large deviation and (b) with a small deviation from the Bragg condition.
density values change with irradiation time, especially at the condition for conversion among different types of defect clusters, a strict comparison has little meaning. However, general tendencies are obvious. The pronounced decrease of stacking fault tetrahedra at the higher temperatures is remarkable, while the fraction of voids is increasing. At 623 K, most of the vacancy clusters were voids.
Irradiation Temperature (K) Fig. 3. Variation of number densities of stacking fault tetrahedra, voids and vacancy type dislocation loops with irradiation temperature.
S. Kojima et al. / Conversion of stacking fault tetrahedra to voids
765
4. Conversion of stacking fault tetrahedra to voids During the continuous observation of stacking fault tetrahedra under irradiation at higher temperatures, their strong image contrast for the tetrahedra often fades out. After detailed observations, we have confirmed that the stacking fault tetrahedra are changing into voids. Fig. 4 shows an example of the conversion at an irradiation temperature of 573 K. The micrographs were taken under the diffraction condition similar to that used to obtain fig. 2b. The deviation from the Bragg condition was not too large in order not to lose the image contrast of the tetrahedra. In this series, one can see seven tetrahedra that were converted into voids. which should be strongly The observation, emphasized, is the comparison of the size of the two types of vacancy clustered defects before and after the conversion. The size of the void after conversion is comparable to the size of the corresponding stacking fault tetrahedron before the conversion. This means that the conversion is not taking place by the rearrangement of atomic sites, since a void requires many more vacancies than a tetrahedron of the same size. A reasonable interpretation for the above results is that stacking fault tetrahedra have served as the preferable site for the nucleation of voids. For the confirmation of this interpretation, we have performed irradiation experiments utilizing void contrast, as in fig. 2a, for the observations. Fig. 5 shows some collected examples of the results. Although the examples in fig. 5 will not
Fig. 4. Conversion
of stacking
fault tetrahedra
Fig. 5. Preferable nucleation of voids at the sites of the stacking fault tetrahedra at 573 K (6 X 10z3 electrons/m’s).
explain all aspects of the reaction, the nucleation of voids just at the site of the stacking fault tetrahedron is observed. It can be concluded that the presently observed conversion does not force any change in our general understanding that the type of vacancy cluster is determined at the beginning of the clustering process. In other words, the word “conversion”, though it has been used in this text as a convenience of the expression, is no longer adequate. The important point here is
to voids during
irradiation
at 573 K (6 X 1O23 electrons/m2s).
766
S. Kojima et al. / Conversion of stacking fault tetrahedra to voidr
that the existence of one type of vacancy cluster has a strong influence on the nucleation of another type with an entirely different structure. 5. Effect of dilatation field and solute atoms In the ordinary reaction between vacancies and stacking fault tetrahedra, vacancies would result in the growth of tetrahedra. Thus, in order to produce a void, vacancies should remain as vacant sites and not be incorporated into the stacking fault of the tetrahedra. Favored sites which satisfy the above condition, will be the corners of a tetrahedron where three stair rod dislocations join. At these places, the lattice is in dilatation and vacancies have the least probability of being compressed and collapsed. In addition, over-sized solute atoms which have a strong binding to vacancies will prefer those dilatation sites. In the case of dilute gold alloys, it has been reported that the presence of solute atoms, which are strongly bound to vacancies, has a pronounced effect on the formation of voids [4,5]. The transport of these solute atoms to stacking fault tetrahedra will result in more favorable sites for the nucleation of voids. We conclude with a short comment on the expected implication of the present observations and analysis on
Fig. 6. Formation of clustered defects in austenitic stainless steel by D-T fusion neutron irradiation (5.5 X lo** n/m*).
SUS 316 at 563 K
the response to neutron irradiation of a candidate alloy for fusion reactor first wall applications. Fig. 6 shows the clustered defects formed in SUS 316 austenitic stainless steel (16.2Cr-13.6Ni-2.46Mo-l.glMnl.OOSi-O.O89Ti-0.029P) by 14MeV D-T fusion neutron irradiation at 563 K. The clustered defects are so small that the identification of their type has not been successful even with an additional effort after the last report by the present authors [lo], nevertheless, the clustered defects are strongly suspected to be stacking fault tetrahedra. If they are stacking fault tetrahedra, they may have a high probability of becoming the preferential sites for the nucleation of voids during the neutron irradiation. The authors are grateful to Professor Emeritus T. Takeyama, the former Head of High Voltage Electron Microscope Laboratory of Hokkaido University, for his continuous encouragement. They are also thankful to the members of the Laboratory for their help in experiments. References [l] S. Yoshida, M. Kiritani and Y. Shimomura. .I. Phys. Sot. Japan 18 (1963) 175. [2] M. Kiritani, J. Phys. Sot. Japan 19 (1964) 618, 1266. [3] M. Kiritani, in: Proc. 5th Yamada Conf. on Point Defects and Defect Interaction in Metals, Kyoto. 1981, Eds. J. Takamura et al. (University of Tokyo Press, Tokyo, 1982) p. 59. [4] J. Takamura, ibid. ref. [3], p. 431. [5] Y. Shirai, T. Hashimoto, T. Takeshita, K. Furukawa and J. Takamura, ibid. ref. [3], p, 441. [6] T. Yoshiie, S. Kojima, Y. Sato, N. Yoshida and M. Kiritani, J. Nucl. Mater. 133&134 (1985) 390. [7] M. Kiritani, K. Yoshida and H. Fujita, J. Electron Microscopy 24 (1975) 211. [8] F.A. Garner, L.E. Thomas and D.S. Gelles. in: Proc. Symp. on Experimental Methods for Charged-Particle Irradiations, Gatlinburg, 1975, p. 51. [9] G.J. Hardy and M.L. Jenkins, Philos. Mag. A52 (1985) L19. [IO] N. Yoshida, K. Kitajima, E. Kuramoto, H. Kawanishi, S. Ishino and M. Kiritani, First Int. Conf. on Fusion Reactor Materials, Tokyo. 1984, 5p-8.
141-143
(1986) 763-766
763
CONVERSION OF STACKING FAULT TETRAHEDRA TO VOIDS IN ELECTRON IRRADIATED Fe-Cr-Ni S. KOJIMA,
Y. SANO,
T. YOSHIIE,
N. YOSHIDA
* and M. KIRITANI
Department of Precision Engineering, Faculty of Engineering, Hokkaido Uniuersi(v, Sapporo 060, Japan
Electron irradiations of the austenitic Fe-13Cr-14Ni alloy were performed with a high voltage electron microscope at temperatures between room temperature and 650 K. Formation of stacking fault tetrahedra, voids and dislocation loops was observed as vacancy clusters. At the lower temperatures, the dominant vacancy clusters were tetrahedra and at the higher temperatures, voids were dominant. In the temperature range at which both tetrahedra and voids were coexistent, conversion of tetrahedra to voids were observed. These results are interpreted as the preferable nucleation of voids at the site of tetrahedra. Local effects of dilatation field at the comer of tetrahedra and the segregation of solute atoms are considered to enhance the nucleation, Clustered defects which are considered to be stacking fault tetrahedra that are formed with D-T fusion neutrons in SUS 316 stainless steel are suggested as the preferable site for void nucleation.
1. Introduction
2. Experimental
Clustered vacancy defects, stacking fault tetrahedra, voids and dislocation loops, which can be stably formed in metals and alloys under a supersaturation of vacancies, do not always exist in the configuration of minimum energy. In quenched aluminum for example, faulted dislocation loops are formed in spite of its high stacking fault energy [l] and voids also coexist [2]. The energy of large clusters is not important in determining the type of vacancy cluster, but the type of cluster is believed to be determined at the initial stage of the clustering process. In the present electron irradiation experiment for an Fe-13Cr-14Ni alloy in the high voltage electron microscope, formation of both stacking fault tetrahedra and voids were confirmed. Simultaneous formation of these two types of defect clusters under electron irradiation has been previously reported by one of the present authors in the case of dilute gold alloys [3]. In these alloys, the nucleation mechanism has been investigated in detail by electrical resitivity measurements and positron annihilation experiment by Takamura et al. in quenched alloys [4,5]. These authors came to the conclusion that the development of either a stacking fault tetrahedron or a void is determined at the stage of only a few vacancies, with the difference arising from different combinations of vacancies and solute atoms. However, the present irradiation experiment of an Fe-13Cr-14Ni alloy has revealed the existence of a reaction which seems to be the conversion of a stacking fault tetrahedron to a void. It is important to discuss the relationship between the presently observed conversion and the importance of initial clustering on the determination of the type of clustered vacancy defects.
Material used in this experiment was the austenitic Fe-13Cr-14Ni alloy which is a model alloy for the candidate SUS 316 alloy for fusion reactor application. This alloy was prepared from highly purified component materials. It was rolled to 0.1 mm thickness and annealed at 1323 K for 30 min in vacuum. Discs of 3 mm diameter were punched out to make electron microscope specimens and electropolished. Electron irradiation and in-situ observation of defect structure development were performed with a H-1300 high voltage electron microscope (HVEM) operated at an acceleration voltage of 1000 kV. A heating stage was used for the irradiation at elevated temperatures. The maximum temperatures of present interest was about 600 K, and changes of the materials due to the high temperature annealing was negligible [6]. Direct estimation of the temperature rise by electron beam heating [7,8] was not carried out. Therefore, the actual temperature may have been higher than the nominal temperature indicated in the text due to the poor heat conductivity of stainless steel. Post-irradiation observations of clustered defects were made with a JEM-ZOOCX transmission electron microscope operated at 200 kV.
* Research Institute sity, Kasugakohen,
for Applied Kasuga-shi,
Mechanics, Kyushu UniverFukuoka 816, Japan.
0022-3115/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
3. General aspects of point defect cluster formation Fig. 1 compares the clustered defects produced at three representative temperatures. At 373 K, a large number of small interstitial clusters were observed. The existence of very small vacancy clusters (< 2 nm) was confirmed in the 200 kV microscope observations, but the clusters were too small to be observed in the bright field image of the HVEM during irradiation. At the higher temperatures, the number density of clustered defects decreased and their sizes became larger. At 523 K, well developed vacancy clusters and interstitial loops could be seen. And at 623 K, no interstitial loops were formed in the thinner parts of a specimen and only vacancy clusters were observed.
B.V.
764
S. Kojima et al. / Conversion of stacking fault tetrahedra to voids
Fig. 1. Point defect clusters produced at three representative temperatures at 1.8X102 s after the start of the irradiation. Left side is the edge of the thickness tapered specimen and their thickness increase toward right side (6 X 1O23 electrons/ m2s).
The observed vacancy clusters were stacking fault tetrahedra, voids, and dislocation loops. Fig. 2 shows those vacancy clusters formed at 573 K in two different imaging conditions, i.e., (a) observations with so-called void contrast far from any strong electron reflection, and (b) those adjusted so as to obtain strong contrast of stacking fault tetrahedra with a smaller deviation from Bragg reflection. The planes of observation were both close to the (110) orientation, i.e., octahedral voids were observed as diamond shapes and stacking fault tetrahedra were observed as triangles. The dislocation loops of the interstitial type that coexisted with smaller vacancy type defect clusters, always grew rapidly under irradiation. The smaller loops, that are known to be converted from stacking fault tetrahedra, did not show a rapid growth, and thus we are confident that the presently observed stacking fault tetrahedra are vacancy type and not interstitial type as reported by Hardy and Jenkins [9]. While the number density of vacancy clusters as a whole gradually decreased with the increase of irradiation temperature, each different type of vacancy cluster showed a different temperature dependence. In fig. 3, the variation of number density of stacking fault tetrahedra (SFT), voids and loops above 473 K is shown. Irradiation times were all 1.8 X lo2 s. Since the number
Fig. 2. Vacancy clusters formed at 573 K observed (a) with a large deviation and (b) with a small deviation from the Bragg condition.
density values change with irradiation time, especially at the condition for conversion among different types of defect clusters, a strict comparison has little meaning. However, general tendencies are obvious. The pronounced decrease of stacking fault tetrahedra at the higher temperatures is remarkable, while the fraction of voids is increasing. At 623 K, most of the vacancy clusters were voids.
Irradiation Temperature (K) Fig. 3. Variation of number densities of stacking fault tetrahedra, voids and vacancy type dislocation loops with irradiation temperature.
S. Kojima et al. / Conversion of stacking fault tetrahedra to voids
765
4. Conversion of stacking fault tetrahedra to voids During the continuous observation of stacking fault tetrahedra under irradiation at higher temperatures, their strong image contrast for the tetrahedra often fades out. After detailed observations, we have confirmed that the stacking fault tetrahedra are changing into voids. Fig. 4 shows an example of the conversion at an irradiation temperature of 573 K. The micrographs were taken under the diffraction condition similar to that used to obtain fig. 2b. The deviation from the Bragg condition was not too large in order not to lose the image contrast of the tetrahedra. In this series, one can see seven tetrahedra that were converted into voids. which should be strongly The observation, emphasized, is the comparison of the size of the two types of vacancy clustered defects before and after the conversion. The size of the void after conversion is comparable to the size of the corresponding stacking fault tetrahedron before the conversion. This means that the conversion is not taking place by the rearrangement of atomic sites, since a void requires many more vacancies than a tetrahedron of the same size. A reasonable interpretation for the above results is that stacking fault tetrahedra have served as the preferable site for the nucleation of voids. For the confirmation of this interpretation, we have performed irradiation experiments utilizing void contrast, as in fig. 2a, for the observations. Fig. 5 shows some collected examples of the results. Although the examples in fig. 5 will not
Fig. 4. Conversion
of stacking
fault tetrahedra
Fig. 5. Preferable nucleation of voids at the sites of the stacking fault tetrahedra at 573 K (6 X 10z3 electrons/m’s).
explain all aspects of the reaction, the nucleation of voids just at the site of the stacking fault tetrahedron is observed. It can be concluded that the presently observed conversion does not force any change in our general understanding that the type of vacancy cluster is determined at the beginning of the clustering process. In other words, the word “conversion”, though it has been used in this text as a convenience of the expression, is no longer adequate. The important point here is
to voids during
irradiation
at 573 K (6 X 1O23 electrons/m2s).
766
S. Kojima et al. / Conversion of stacking fault tetrahedra to voidr
that the existence of one type of vacancy cluster has a strong influence on the nucleation of another type with an entirely different structure. 5. Effect of dilatation field and solute atoms In the ordinary reaction between vacancies and stacking fault tetrahedra, vacancies would result in the growth of tetrahedra. Thus, in order to produce a void, vacancies should remain as vacant sites and not be incorporated into the stacking fault of the tetrahedra. Favored sites which satisfy the above condition, will be the corners of a tetrahedron where three stair rod dislocations join. At these places, the lattice is in dilatation and vacancies have the least probability of being compressed and collapsed. In addition, over-sized solute atoms which have a strong binding to vacancies will prefer those dilatation sites. In the case of dilute gold alloys, it has been reported that the presence of solute atoms, which are strongly bound to vacancies, has a pronounced effect on the formation of voids [4,5]. The transport of these solute atoms to stacking fault tetrahedra will result in more favorable sites for the nucleation of voids. We conclude with a short comment on the expected implication of the present observations and analysis on
Fig. 6. Formation of clustered defects in austenitic stainless steel by D-T fusion neutron irradiation (5.5 X lo** n/m*).
SUS 316 at 563 K
the response to neutron irradiation of a candidate alloy for fusion reactor first wall applications. Fig. 6 shows the clustered defects formed in SUS 316 austenitic stainless steel (16.2Cr-13.6Ni-2.46Mo-l.glMnl.OOSi-O.O89Ti-0.029P) by 14MeV D-T fusion neutron irradiation at 563 K. The clustered defects are so small that the identification of their type has not been successful even with an additional effort after the last report by the present authors [lo], nevertheless, the clustered defects are strongly suspected to be stacking fault tetrahedra. If they are stacking fault tetrahedra, they may have a high probability of becoming the preferential sites for the nucleation of voids during the neutron irradiation. The authors are grateful to Professor Emeritus T. Takeyama, the former Head of High Voltage Electron Microscope Laboratory of Hokkaido University, for his continuous encouragement. They are also thankful to the members of the Laboratory for their help in experiments. References [l] S. Yoshida, M. Kiritani and Y. Shimomura. .I. Phys. Sot. Japan 18 (1963) 175. [2] M. Kiritani, J. Phys. Sot. Japan 19 (1964) 618, 1266. [3] M. Kiritani, in: Proc. 5th Yamada Conf. on Point Defects and Defect Interaction in Metals, Kyoto. 1981, Eds. J. Takamura et al. (University of Tokyo Press, Tokyo, 1982) p. 59. [4] J. Takamura, ibid. ref. [3], p. 431. [5] Y. Shirai, T. Hashimoto, T. Takeshita, K. Furukawa and J. Takamura, ibid. ref. [3], p, 441. [6] T. Yoshiie, S. Kojima, Y. Sato, N. Yoshida and M. Kiritani, J. Nucl. Mater. 133&134 (1985) 390. [7] M. Kiritani, K. Yoshida and H. Fujita, J. Electron Microscopy 24 (1975) 211. [8] F.A. Garner, L.E. Thomas and D.S. Gelles. in: Proc. Symp. on Experimental Methods for Charged-Particle Irradiations, Gatlinburg, 1975, p. 51. [9] G.J. Hardy and M.L. Jenkins, Philos. Mag. A52 (1985) L19. [IO] N. Yoshida, K. Kitajima, E. Kuramoto, H. Kawanishi, S. Ishino and M. Kiritani, First Int. Conf. on Fusion Reactor Materials, Tokyo. 1984, 5p-8.