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Microchemical evolution in PCA under dual ion irradiation
Journal of Nuclear
Materials
525
133&134 (1985) 525-529
MICROCHEMICAL EVOLUTION IN PCA UNDER DUAL ION IRRADIATION N. SEKIMURA, Depur~ment
of Nucleur
H. KAWANISHI Engineerrng,
and S. ISHINO
University of Tokyo, Hongo,
Bunk_vo- ku, Tok_vo 113. Japan
The Prime Candidate Alloy (PCA) for a near term fusion reactor is irradiated with dual ions to investigate the microchemical evolution in this alloy under fusion irradiation conditions. Radiation induced solute segregation or depletion. depending on the atomic size factor, is observed by EDS analysis on cavities and grain boundaries. At 830 K, grain boundary migration is induced by irradiation and results in a row of cavities and Ni concentration peaks at the original position of the grain boundary. At 830 and 870 K, irradiation causes the precipitation of a Ni. Ti, Si and Al rich phase and the decrease of Ni concentration in the matrix, leading to another peak in the swelling variation with temperature.
1. Introduction Ti-modified austenitic stainless steel is considered to be a promising candidate material for a near term fusion reactor. The high helium production rate, coupled with heavy displacement damage in the first wall material subjected to 14 MeV neutrons can cause cavity swelling, which is a major source of dimensional instability. Recently it has become widely recognized that the swelling behaviour is strongly related to microchemical evolution under irradiation. Point-defect flow causes segregation or depletion of alloying elements to the sinks such as grain boundaries, cavities, dislocations and the specimen surface. Irradiation also affects the phase stability in austenitic stainless steel, which in turn changes the mechanical properties of the alloys. The understanding of these phenomena in such complex alloys as type 316 steel is far from complete. In addition, it has been pointed out that the presence of helium also influences the phase stability [l]. In the present study, the Japanese prime candidate alloy (PCA) is irradiated with dual ion beams to study the microchemical evolution near the point-defect sinks and the precipitation behaviour under the high He production rate. This irradiation procedure enables the irradiation response of the material to be studied under well defined conditions that are relevant to the fusion environment.
2. Experimental The composition of the Japanese PCA is given in table 1. The as-received specimens are solution-annealed for 30 min at 1323 K in a vacuum, followed by furnace
cooling. The sample disks of 3 mm in diameter and 0.15 mm in thickness are irradiated with 0.4 MeV Al+ ions to a dose of 50 dpa at the damage peak, which is 160 nm from the specimen surface, with simultaneous injection of 50 keV He+ ions to simulate any synergistic effects of the atomic displacement and helium production. The helium injection rate is 15 appm He/dpa, which corresponds to the expected ratio in the fusion reactor environment. The irradiation temperature is between 750 and 870 K and the damage rate is about 8 x 10m3 dpa/s. Irradiated specimens are backthinned by an automatic Tenupole electro-polishing technique. A quantitative analysis of the microchemical compositions is performed using an energy dispersive X-ray spectroscopy (EDS) apparatus equipped with an electron microscope (JEM-200CX) which is operated at 200 kV. Microstructural observation and selected area electron diffraction (SAD) are also performed. The electron beam probe for the excitation of characteristic X-rays is less than 20 nm in diameter. The specimens are supported by a graphite holder to avoid extraneous X-ray signals. The measured X-ray spectrum is converted to compositional information by taking the ratio of K, or L, intensities for each element, as described by Cliff and Lorimer [2].
3. Results In an earlier paper [3], we have compared the microstructural changes in the PCA after irradiation with dual ions with the changes arising during single beam irradiation and discussed the effect of helium on the cavity formation. In the solution annealed condition. the results of dual beam irradiation up to 50 dpa between 750 and 870 K have shown that the presence of
Table 1 Chemical composition of PCA (wt%) (Fe: balance) Ni
Cr
16.22
14.57
MO 2.37
Mn 1.79
0022-3115/85/$03.30 Q Elsevier Science Publishers (North-Holland Physics Publishing Division)
Si 0.53
B.V.
Ti 0.24
C 0.06
P 0.027
B 0.0035
helium strongly affects the cavity nuleation. Thus. the PCA is very resistant to cavity formation in the absence of helium in contrast to type 316 stainless steel. The cavity density increases with increasing He/dpa ratio. At high temperatures (> 833 K), the cavity diameter increases with increasing irradiation temperature and injected helium level, leading to the formation of another peak in the swelling-temperature variation. The EDS analysis around cavities is performed in the specimens irradiated at 830 and 870 K. At the lower temperatures. even the largest cavities in the cavity size distribution are to small for the changes of the alloying elements too be detectable around them. Fig. 1 shows the distribution of elements at every 25 nm along a line passing through the centers of two matrix cavities created after irradiation at 830 K. It is clearly observed that Ni is enriched at the cavities, while Cr tends to be depleted there. Mn and MO are also found to be depleted and Si is enriched. The separation of the Mn K, X-ray from the Cr K,j X-ray has previously been found to be difficult because of the limited energy resolution of the Si(Li) detector at about 150 eV. However in the present EDS system the powerful software and the establishment of a proper reference X-ray peak for every element enable us to separate these two peaks and to perform a quantitative analysis of the Mn content. The enrichment of Ni and Si and the depletion of Cr and Mn on the cavities are more significant at 830 K compared with results after the irradiation at X70 K. Takahashi et al. [4] reported that the maximum segregation of Ni around a cavity formed in a Ti-modified 316 stainless steel irradiated with electrons in a HVEM occurred at the irradiation temperature of 820 K, which is consistent with the present results. However. Mo appears to be more depleted at the irradiati~~n temperature of 870 K than that of 830 K. The results of ED‘S analysis around the grain boundary in the solution annealed PCA show no discernible changes in the content at the grain boundary. The content of MO is found to be variable from &rain to grain in the solution annealed PCA. We also find that
“-’0
the error in the present analysis with the EDS system i\ less than 1 wt%, Irradiation causes enrichment of Ni and Si and the depletion of Cr and Mn at the grain boundary at the irradiation temperature of 750 K as shown in fig. 2. This is the same tendency as observed around the cavities. The level of the enrichment and the depletion of the alloying elements are larger in the specimen irradiated at X00 K than at 750 K. No which seems to he enriched at the grain boundary. is not considered to be affected by the irradiation; the observed enrichment merely rcfleets the concentration difference initially present in each grain prior to irradiation. At the irradiation temperature of X70 K. Mo 1s found to be slightly depleted at the gram boundary. ‘f‘~p~col results of the EDS analysis are presented in fig. 3. The segregation of Ni at the grain boundary 15 decreased compared with that at 800 K. Fig. 3 indicates that the matrix Ni level is decreased. This phenomenom IS also observed in the specimens irradiated at 830 K. At both
,_’
0.4
I1 2 ii:
siatxe
0.6
,
/.Pl >
Fig. 1. Concentration profiles near cavities in PCA irradiated al X30 K.
i
.-
/.-
1..
.
.
.~
N. Sekimura
et al. / Microchemical
temperatures, platelet precipitateswhich are considered to be radiation-induced, are formed in the matrix. Stereo analysis of these precipitates shows that they are formed in the thin region near the specimen surface, which is consistent with the displacement distribution of the incident ions [5], and it is very difficult to extract them in order to identify the composition directly. We apply the subtraction method to estimate the precipitate composition. First the X-ray spectrum is taken at a point including one of the precipitates and then another spectrum is taken at the point of matrix near the precipitate. The subtraction of two spectra defines the approximate composition of the precipitate. Fig. 4 shows an example of subtracted spectra under the assumption that the precipitate does not contain Fe. The spectra indicate that the precipitates formed at both 830 and 870 K are rich in Ni, Ti, Si and Al. Al, originating from the injected ions, amounts to about 4 at.% at the range peak, situated 200 nm from the surface, in the specimen irradiated to 50 dpa. Ti is not detected in all the specimens by the EDS analysis of the matrix. This suggests that most of the Ti atoms are located in the TIC precipitates. Such precipitates are observed mainly in the matrix of the unirradiated specimens in spite of the solution treatment at 1323 K. Irradiation at lower temperatures promotes the precipitation of fine TIC particles in the matrix. A direct relationship between KC precipitates and cavity formation is not observed. At higher temperatures, irradiation does not seem to promote the precipitation of Tic, however, EDS analysis near the cavities shows that some cavities have a Ti-enriched zone around them. When the irradiation temperature is 830 K, unusual
0
(keV).
lo
Fig. 4. EDS spectrum of the precipitate in PCA irradiated at 870 K.
521
evolutron in PCA
011 ’ 1.0 0.8
1.0
0.8
0.6
’
’
’
’
’
’
0.6
0.4
a.2
0
0.2
0.L
0.4
Distance
0.2
0
from Grain
0.2
0.4
Boundary
u
t
L
0.6
0.6
’
3.8
0.8
”
l.G
1.0
( vm)
Fig. 5. (a) Concentration profiles near the grain boundary in PCA irradiated at 830 K. (b) Cavity number profile near the grain boundary in PCA irradiated at 830 K.
phenomena are observed near the grain boundary. A peak in the Ni content appears on one side of the grain boundary and lies typically about 300 nm from the grain boundary as shown in fig. 5(a). In the same side, cavities are observed parallel to the grain boundary. Fig. 6 shows a micrograph of the cavity and grain boundary structure. Grain boundaries which are accompanied by such a row of cavities are observed to be irregular in shape, while most of grain boundaries are straight in the
1.0
0.8
036
Fig. 6. Microstructure ted at 830 K.
0.4
0.2
near the grain boundary
0
crm
in PCA irradia-
solution annealed specimens or those irradiated at temperatures other than 830 K. The cavity density variation with distance from the grain boundary is measured and is as shown in fig. 5(b). The peak of this variation lies slightly outside the point of maximum Ni content and only a few cavities are formed between the density peak and the grain boundary. Cr depletion around the grain boundary is observed at other temperatures, and detailed analysis shows that the minimum point of the Cr content variation does not exist at the grain boundary.
4. Discussion In the present study. radiation induced segregation into both types of sinks. namely the cavity and grain boundary. exhibits the same tendency. The facts can be accounted for by the following two mechanisms, which have already been clarified by a number of studies of simple binary [6] or FeeCr--Ni alloys [7]. Firstly, an undersize atom migrates as a mixed dumbbell interstitial. The flow of the defects to the interstitial sinks causes the enrichment of the undersize atoms, which are Ni and Si in the present case. For over-sized atoms such as Cr. Mn and MO. an exchange of atoms with vacancies causes the atoms to flow in the opposite direction to the vacancy flow giving rise to the depletion of atoms around the vacancy sinks. However. because of the low mobility of MO atoms in the austenitics, Mo does not necessarily follow the above tendency and its behaviour depends on the irradiation temperature. At lower temperatures, MO is not mobile enough for the detection of the microchemical changes around the grain boundaries, where the pre-irradiation differences in the MO content between neighbourillg grains make the detection of any MO depletion difficult. At the irradiation temperature of 870 K. however, MO is mobile and the depletion then becomes detectable. As mentioned earlier, it had been very difficult in previous studies to observe the depletion of Mn near the point-defect sinks. despite the large oversize factor of a Mn atom in the austenitic stainless steels. In the present analysis. Mn depletion is clearly observed around cavities and grain boundaries. The aligned cavity formation and the Ni enrichment peak on one side of grain boundary observed at 830 K are considered to have resulted from the migration of the grain boundary induced by irradiation; though the dose dependence experiment required to clarify this notion has not yet been conducted, the serrated nature of the grain boundary compared with the relatively straight boundary in the solution annealed specimens is good evidence for such migration. The distance of the grain boundary migration is considered to correspond to the distance between the observed grain boundary and the row of cavities. At the early stage of the irradiation. introduced helium atoms agglomerate at the
grain boundary to form the cavity nuclei. Strings 01 such cavities along the grain boundary hah been r+ ported in several works. Shimada et al. [X] have ohserved this kind of phenomena in ion irradiated Type 316 stainless steel preinjected with helium. Ffowever. rows of cavities have not been observed in the specimenu irradiated without helium injection. in which cavities are homogeneously formed in the matrix unlike the present results in the PCA. Ohnuki et al. [9] have observed the preferential f~~rrnatjon of voids at the grain boundary and the migration of the grain houndarc together with the voids in a Ti-modified 316 stainless steel irradiated in the HVEM. The difference can he accounted for by the relative immobilty of helium bubbles compared with voids. .4t the irradiation temperature of 830 K. grain boundaries can be the dominant point-defect sinks effectively. At lower temperatures, point defects annihilate preferentially by mutual recombination. while at higher temperatures the specimen surface acts as the most effective sink in experiments such as the present when relatively low energy ions are used. The formation of radiation-induced precipitates at the higher temperatures, which are found to he rich in Ni, is considered to be the main reason for the decrease in the Ni content in the matrix. In addition. TI is collected in these type of precipitates. limiting the radiation-enhanced precipitation of TiC as observed at the lower temperatures. As pointed out in the previous paper [3]. preferential agglomeration of helium on fine TiC precipitates or Ti-C complexes attracted by the misfit strain enhances the cavity nucleation, which is certified by some EDS analysis data at the cavity. The present facts indicate that cavity swelling at higher temperatures is increased by both the decrease in the Ni content in the matrix and the decreased number of cavity nucleation sites, which result from the precipitate formation. The formation of these types of precipitates may be promoted not only hy irradiation hut hy the injected -41 ions. In the present irradiation. the damaged region and the distribution of Injected ions cannot be separated in the microstructural observation. Detailed stereoscopic observation must he of great help to interpret the results from the relativjely low energy damaging ions [5].
5. Summary The irradiation-induced segregation is detected hy EDS analysis on two types of point-defect sinks, i.e. cavities and grain boundaries, in the PCA irradiated with dual ions. Ni and Si are enriched at the sinks. while Cr and Mn are depleted at the sinks. MO is also found to be depleted at the cavities and grain boundaries at the high temperatures. In the case of irradiation at X30 K. strings of cavities and a Ni concentration peak are
N. Sekimuru er al. / M~crochemical ervlution ,n PCA
observed on one side of the grain boundary. They are considered to have resulted from the migration of grain boundaries. At higher temperatures. radiation-induced precipitates which are rich in Ni. Ti, Si and Al are formed, resulting in an increase in cavity swelling by both the decrease of Ni in the matrix and the decrease of cavity nucleation sites.
References [I] P.J. Maziasz, J.A. Horak and B.L. Cox. in: Conf. Proc. Irradiation Effects on Phase Stability (AIME. New York. 1981) p. 271. [2] G. Cliff and G.W. Lorimer, J. Microscopy 103 (1975) 203.
529
M. Nodaka and S. Ishino. J. (31 N. Sekrmura. H. Kawamshi. Nucl. Mater. 122-123 (1984) 322. S. Ohnuki. H. Osanai. T. Takeyama and K. 141 H. Takahashi. Shiraishi. J. Nucl. Mater. 1222123 (1984) 327. [51 N. Sekimura and S. Ishino. to be published. f61 L.E. Rehn. P.R. Okamoto. D.I. Potter and H. Wiederaich. J. Nucl. Mater. 74 (197X) 242. 171 W.G. Johnston, W.G. Morrts and A.M. Turkalo. tn: Radtanon Effects m Breeder Reactor Structural Materials. rds.. M.L. Bleiberg and J.W. Bennett (TMSAIME. New York. 1977) p. 421. [Xl M. Shimada and H. Kamet. J. Nucl. Mater. 103-104 (IYXl) 1481. ]91 S. Ohnuki. H. Takahashi and T. Takeyama. tn: Proc. 12th Int. Symp. on Effects of Radiation on Mater& (June. 1984) to be published in ASTM STP 870.
Materials
525
133&134 (1985) 525-529
MICROCHEMICAL EVOLUTION IN PCA UNDER DUAL ION IRRADIATION N. SEKIMURA, Depur~ment
of Nucleur
H. KAWANISHI Engineerrng,
and S. ISHINO
University of Tokyo, Hongo,
Bunk_vo- ku, Tok_vo 113. Japan
The Prime Candidate Alloy (PCA) for a near term fusion reactor is irradiated with dual ions to investigate the microchemical evolution in this alloy under fusion irradiation conditions. Radiation induced solute segregation or depletion. depending on the atomic size factor, is observed by EDS analysis on cavities and grain boundaries. At 830 K, grain boundary migration is induced by irradiation and results in a row of cavities and Ni concentration peaks at the original position of the grain boundary. At 830 and 870 K, irradiation causes the precipitation of a Ni. Ti, Si and Al rich phase and the decrease of Ni concentration in the matrix, leading to another peak in the swelling variation with temperature.
1. Introduction Ti-modified austenitic stainless steel is considered to be a promising candidate material for a near term fusion reactor. The high helium production rate, coupled with heavy displacement damage in the first wall material subjected to 14 MeV neutrons can cause cavity swelling, which is a major source of dimensional instability. Recently it has become widely recognized that the swelling behaviour is strongly related to microchemical evolution under irradiation. Point-defect flow causes segregation or depletion of alloying elements to the sinks such as grain boundaries, cavities, dislocations and the specimen surface. Irradiation also affects the phase stability in austenitic stainless steel, which in turn changes the mechanical properties of the alloys. The understanding of these phenomena in such complex alloys as type 316 steel is far from complete. In addition, it has been pointed out that the presence of helium also influences the phase stability [l]. In the present study, the Japanese prime candidate alloy (PCA) is irradiated with dual ion beams to study the microchemical evolution near the point-defect sinks and the precipitation behaviour under the high He production rate. This irradiation procedure enables the irradiation response of the material to be studied under well defined conditions that are relevant to the fusion environment.
2. Experimental The composition of the Japanese PCA is given in table 1. The as-received specimens are solution-annealed for 30 min at 1323 K in a vacuum, followed by furnace
cooling. The sample disks of 3 mm in diameter and 0.15 mm in thickness are irradiated with 0.4 MeV Al+ ions to a dose of 50 dpa at the damage peak, which is 160 nm from the specimen surface, with simultaneous injection of 50 keV He+ ions to simulate any synergistic effects of the atomic displacement and helium production. The helium injection rate is 15 appm He/dpa, which corresponds to the expected ratio in the fusion reactor environment. The irradiation temperature is between 750 and 870 K and the damage rate is about 8 x 10m3 dpa/s. Irradiated specimens are backthinned by an automatic Tenupole electro-polishing technique. A quantitative analysis of the microchemical compositions is performed using an energy dispersive X-ray spectroscopy (EDS) apparatus equipped with an electron microscope (JEM-200CX) which is operated at 200 kV. Microstructural observation and selected area electron diffraction (SAD) are also performed. The electron beam probe for the excitation of characteristic X-rays is less than 20 nm in diameter. The specimens are supported by a graphite holder to avoid extraneous X-ray signals. The measured X-ray spectrum is converted to compositional information by taking the ratio of K, or L, intensities for each element, as described by Cliff and Lorimer [2].
3. Results In an earlier paper [3], we have compared the microstructural changes in the PCA after irradiation with dual ions with the changes arising during single beam irradiation and discussed the effect of helium on the cavity formation. In the solution annealed condition. the results of dual beam irradiation up to 50 dpa between 750 and 870 K have shown that the presence of
Table 1 Chemical composition of PCA (wt%) (Fe: balance) Ni
Cr
16.22
14.57
MO 2.37
Mn 1.79
0022-3115/85/$03.30 Q Elsevier Science Publishers (North-Holland Physics Publishing Division)
Si 0.53
B.V.
Ti 0.24
C 0.06
P 0.027
B 0.0035
helium strongly affects the cavity nuleation. Thus. the PCA is very resistant to cavity formation in the absence of helium in contrast to type 316 stainless steel. The cavity density increases with increasing He/dpa ratio. At high temperatures (> 833 K), the cavity diameter increases with increasing irradiation temperature and injected helium level, leading to the formation of another peak in the swelling-temperature variation. The EDS analysis around cavities is performed in the specimens irradiated at 830 and 870 K. At the lower temperatures. even the largest cavities in the cavity size distribution are to small for the changes of the alloying elements too be detectable around them. Fig. 1 shows the distribution of elements at every 25 nm along a line passing through the centers of two matrix cavities created after irradiation at 830 K. It is clearly observed that Ni is enriched at the cavities, while Cr tends to be depleted there. Mn and MO are also found to be depleted and Si is enriched. The separation of the Mn K, X-ray from the Cr K,j X-ray has previously been found to be difficult because of the limited energy resolution of the Si(Li) detector at about 150 eV. However in the present EDS system the powerful software and the establishment of a proper reference X-ray peak for every element enable us to separate these two peaks and to perform a quantitative analysis of the Mn content. The enrichment of Ni and Si and the depletion of Cr and Mn on the cavities are more significant at 830 K compared with results after the irradiation at X70 K. Takahashi et al. [4] reported that the maximum segregation of Ni around a cavity formed in a Ti-modified 316 stainless steel irradiated with electrons in a HVEM occurred at the irradiation temperature of 820 K, which is consistent with the present results. However. Mo appears to be more depleted at the irradiati~~n temperature of 870 K than that of 830 K. The results of ED‘S analysis around the grain boundary in the solution annealed PCA show no discernible changes in the content at the grain boundary. The content of MO is found to be variable from &rain to grain in the solution annealed PCA. We also find that
“-’0
the error in the present analysis with the EDS system i\ less than 1 wt%, Irradiation causes enrichment of Ni and Si and the depletion of Cr and Mn at the grain boundary at the irradiation temperature of 750 K as shown in fig. 2. This is the same tendency as observed around the cavities. The level of the enrichment and the depletion of the alloying elements are larger in the specimen irradiated at X00 K than at 750 K. No which seems to he enriched at the grain boundary. is not considered to be affected by the irradiation; the observed enrichment merely rcfleets the concentration difference initially present in each grain prior to irradiation. At the irradiation temperature of X70 K. Mo 1s found to be slightly depleted at the gram boundary. ‘f‘~p~col results of the EDS analysis are presented in fig. 3. The segregation of Ni at the grain boundary 15 decreased compared with that at 800 K. Fig. 3 indicates that the matrix Ni level is decreased. This phenomenom IS also observed in the specimens irradiated at 830 K. At both
,_’
0.4
I1 2 ii:
siatxe
0.6
,
/.Pl >
Fig. 1. Concentration profiles near cavities in PCA irradiated al X30 K.
i
.-
/.-
1..
.
.
.~
N. Sekimura
et al. / Microchemical
temperatures, platelet precipitateswhich are considered to be radiation-induced, are formed in the matrix. Stereo analysis of these precipitates shows that they are formed in the thin region near the specimen surface, which is consistent with the displacement distribution of the incident ions [5], and it is very difficult to extract them in order to identify the composition directly. We apply the subtraction method to estimate the precipitate composition. First the X-ray spectrum is taken at a point including one of the precipitates and then another spectrum is taken at the point of matrix near the precipitate. The subtraction of two spectra defines the approximate composition of the precipitate. Fig. 4 shows an example of subtracted spectra under the assumption that the precipitate does not contain Fe. The spectra indicate that the precipitates formed at both 830 and 870 K are rich in Ni, Ti, Si and Al. Al, originating from the injected ions, amounts to about 4 at.% at the range peak, situated 200 nm from the surface, in the specimen irradiated to 50 dpa. Ti is not detected in all the specimens by the EDS analysis of the matrix. This suggests that most of the Ti atoms are located in the TIC precipitates. Such precipitates are observed mainly in the matrix of the unirradiated specimens in spite of the solution treatment at 1323 K. Irradiation at lower temperatures promotes the precipitation of fine TIC particles in the matrix. A direct relationship between KC precipitates and cavity formation is not observed. At higher temperatures, irradiation does not seem to promote the precipitation of Tic, however, EDS analysis near the cavities shows that some cavities have a Ti-enriched zone around them. When the irradiation temperature is 830 K, unusual
0
(keV).
lo
Fig. 4. EDS spectrum of the precipitate in PCA irradiated at 870 K.
521
evolutron in PCA
011 ’ 1.0 0.8
1.0
0.8
0.6
’
’
’
’
’
’
0.6
0.4
a.2
0
0.2
0.L
0.4
Distance
0.2
0
from Grain
0.2
0.4
Boundary
u
t
L
0.6
0.6
’
3.8
0.8
”
l.G
1.0
( vm)
Fig. 5. (a) Concentration profiles near the grain boundary in PCA irradiated at 830 K. (b) Cavity number profile near the grain boundary in PCA irradiated at 830 K.
phenomena are observed near the grain boundary. A peak in the Ni content appears on one side of the grain boundary and lies typically about 300 nm from the grain boundary as shown in fig. 5(a). In the same side, cavities are observed parallel to the grain boundary. Fig. 6 shows a micrograph of the cavity and grain boundary structure. Grain boundaries which are accompanied by such a row of cavities are observed to be irregular in shape, while most of grain boundaries are straight in the
1.0
0.8
036
Fig. 6. Microstructure ted at 830 K.
0.4
0.2
near the grain boundary
0
crm
in PCA irradia-
solution annealed specimens or those irradiated at temperatures other than 830 K. The cavity density variation with distance from the grain boundary is measured and is as shown in fig. 5(b). The peak of this variation lies slightly outside the point of maximum Ni content and only a few cavities are formed between the density peak and the grain boundary. Cr depletion around the grain boundary is observed at other temperatures, and detailed analysis shows that the minimum point of the Cr content variation does not exist at the grain boundary.
4. Discussion In the present study. radiation induced segregation into both types of sinks. namely the cavity and grain boundary. exhibits the same tendency. The facts can be accounted for by the following two mechanisms, which have already been clarified by a number of studies of simple binary [6] or FeeCr--Ni alloys [7]. Firstly, an undersize atom migrates as a mixed dumbbell interstitial. The flow of the defects to the interstitial sinks causes the enrichment of the undersize atoms, which are Ni and Si in the present case. For over-sized atoms such as Cr. Mn and MO. an exchange of atoms with vacancies causes the atoms to flow in the opposite direction to the vacancy flow giving rise to the depletion of atoms around the vacancy sinks. However. because of the low mobility of MO atoms in the austenitics, Mo does not necessarily follow the above tendency and its behaviour depends on the irradiation temperature. At lower temperatures, MO is not mobile enough for the detection of the microchemical changes around the grain boundaries, where the pre-irradiation differences in the MO content between neighbourillg grains make the detection of any MO depletion difficult. At the irradiation temperature of 870 K. however, MO is mobile and the depletion then becomes detectable. As mentioned earlier, it had been very difficult in previous studies to observe the depletion of Mn near the point-defect sinks. despite the large oversize factor of a Mn atom in the austenitic stainless steels. In the present analysis. Mn depletion is clearly observed around cavities and grain boundaries. The aligned cavity formation and the Ni enrichment peak on one side of grain boundary observed at 830 K are considered to have resulted from the migration of the grain boundary induced by irradiation; though the dose dependence experiment required to clarify this notion has not yet been conducted, the serrated nature of the grain boundary compared with the relatively straight boundary in the solution annealed specimens is good evidence for such migration. The distance of the grain boundary migration is considered to correspond to the distance between the observed grain boundary and the row of cavities. At the early stage of the irradiation. introduced helium atoms agglomerate at the
grain boundary to form the cavity nuclei. Strings 01 such cavities along the grain boundary hah been r+ ported in several works. Shimada et al. [X] have ohserved this kind of phenomena in ion irradiated Type 316 stainless steel preinjected with helium. Ffowever. rows of cavities have not been observed in the specimenu irradiated without helium injection. in which cavities are homogeneously formed in the matrix unlike the present results in the PCA. Ohnuki et al. [9] have observed the preferential f~~rrnatjon of voids at the grain boundary and the migration of the grain houndarc together with the voids in a Ti-modified 316 stainless steel irradiated in the HVEM. The difference can he accounted for by the relative immobilty of helium bubbles compared with voids. .4t the irradiation temperature of 830 K. grain boundaries can be the dominant point-defect sinks effectively. At lower temperatures, point defects annihilate preferentially by mutual recombination. while at higher temperatures the specimen surface acts as the most effective sink in experiments such as the present when relatively low energy ions are used. The formation of radiation-induced precipitates at the higher temperatures, which are found to he rich in Ni, is considered to be the main reason for the decrease in the Ni content in the matrix. In addition. TI is collected in these type of precipitates. limiting the radiation-enhanced precipitation of TiC as observed at the lower temperatures. As pointed out in the previous paper [3]. preferential agglomeration of helium on fine TiC precipitates or Ti-C complexes attracted by the misfit strain enhances the cavity nucleation, which is certified by some EDS analysis data at the cavity. The present facts indicate that cavity swelling at higher temperatures is increased by both the decrease in the Ni content in the matrix and the decreased number of cavity nucleation sites, which result from the precipitate formation. The formation of these types of precipitates may be promoted not only hy irradiation hut hy the injected -41 ions. In the present irradiation. the damaged region and the distribution of Injected ions cannot be separated in the microstructural observation. Detailed stereoscopic observation must he of great help to interpret the results from the relativjely low energy damaging ions [5].
5. Summary The irradiation-induced segregation is detected hy EDS analysis on two types of point-defect sinks, i.e. cavities and grain boundaries, in the PCA irradiated with dual ions. Ni and Si are enriched at the sinks. while Cr and Mn are depleted at the sinks. MO is also found to be depleted at the cavities and grain boundaries at the high temperatures. In the case of irradiation at X30 K. strings of cavities and a Ni concentration peak are
N. Sekimuru er al. / M~crochemical ervlution ,n PCA
observed on one side of the grain boundary. They are considered to have resulted from the migration of grain boundaries. At higher temperatures. radiation-induced precipitates which are rich in Ni. Ti, Si and Al are formed, resulting in an increase in cavity swelling by both the decrease of Ni in the matrix and the decrease of cavity nucleation sites.
References [I] P.J. Maziasz, J.A. Horak and B.L. Cox. in: Conf. Proc. Irradiation Effects on Phase Stability (AIME. New York. 1981) p. 271. [2] G. Cliff and G.W. Lorimer, J. Microscopy 103 (1975) 203.
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