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Void swelling and defect cluster formation in reactor-irradiated copper
262
Journal
of Nuclear
Materials 168 (1989) 262-267 North-Holland, Amsterdam
VOID SWELLING AND DEFECT CLUSTER FORMATION IN REACTOR-IRRADIATED COPPER * S.J. ZINKLE
and K. FARRELL
Metals and Ceramics Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6376, USA Received
17 February
1989; accepted
2 June 1989
Copper disks were irradiated at temperatures of 182 to 500 * C with moderated fission neutrons under carefully controlled conditions to a damage level of 1.1 to 1.3 dpa at a damage rate of 2 X10-’ dpa/s. The lower temperature limit for void formation was found to lie between 182 and 220 o C, with a maximum swelling of about 0.5% at 300 to 350 o C and no swelling at 500 o C. At 182O C, vacancies produced stacking fault tetrahedra instead of voids.
confirm studies.
1. Inaction There have been hundreds of studies on the effects of irradiation on copper over the past 30 years [l]. However, there have been relatively few attempts to carefully characterize the microstructure of neutronirradiated copper over an extended temperature range. There are less than 20 known studies of void formation in neutron-irradiated copper, with the bulk of these studies performed at a single irradiation temperature [l]. In addition, many of these experiments were performed with large uncertainties in the irradiation temperature and displacement dose due to the lack of adequate in-reactor temperature m~surement and few or no dosimetry foils. There are no known neutron irradiation studies of copper conducted over an extended temperature range with damage levels > 0.5 dpa. Most of the proposed applications for copper and copper alloys in fusion reactors involve irradiation temperatures of 50 to 470 QC. The most extensive study to date in this temperature range was performed by a group of French scientists [2-41, where copper subjected to damage levels ranging from 0.3 to 0.5 dpa and estimated irradiation temperatures of 220 to 600 o C was examined. The present investigation was intended to
* Research sponsored by the Division of Materials Sciences and by the Office of Fusion Energy, US Department of Energy under contract DE-AC05840R21400 with Martin Marietta Energy Systems, Inc.
~22-3115/89/$03.50 (North-Holland
Physics
Division)
the results of these earlier low-dose
procedures
2. Experimental
Polycrystalline ~~-purity (zone-refined) copper produced by the Solid State Division at the Oak Ridge National Laboratory (ORNL) was used in the experiment. Its chemical composition is given in table 1. Disks of dimensions 3.2 mm diameter by 0.28 mm thick were annealed for 1 h at 550° C in vacuum prior to the irradiation. The disks werre irradiated in nine separate helium-filly capsules in the BS site of the Oak Ridge Research (ORR) reactor for a period of 14 weeks. Neutron fluences and irradiation temperatures were carefully measured and controlled. Fourteen packages of flux monitor wires placed at the capsule site and elsewhere in the experimental assembly provided a thorough map of the neutron environment. Each capsule
Table 1 Chemical
analysis
of copper
Impurity
(wt ppm)
Impurity
(wt ppm)
Ag Al B Cr Fe
50 1 < 0.5 5 10
Ni Si Zr C 0 N
10 1 70 7 35 6
0 Elsevier Science Publishers B.V. Publishing
and extend
S.J. Zinkle, K. Farrell / Void swelling and defect cluster formation in Cu
Table 2 Irradiation data for copper specimens Temperature
Neutron fluence (1O25 n/m2)
(“C)
Total
Thermal
> 0.1 MeV
Damage level (+a) ”
182 220 250 275 300 350 400 450 500
3.6 3.6 4.3 4.0 3.6 3.1 3.1 3.5 3.3
1.2 1.2 1.2 1.2 1.1 1.2 1.2 1.2 1.0
1.1 1.1 1.2 1.3 1.0 1.2 1.3 1.3 1.0
1.2 1.2 1.3 1.3 1.1 1.2 1.3 1.2 1.1
the size and density of voids at these versus cavities created by oxide particles of the foil during electropolishing.
263
temperatures dropping out
3. Results
‘) Calculated from the measured neutron fluence using displacement damage calculations based on IAEA recommendations and the ENDFB/IV nuclear data file (51.
was housed in its own small, Nichrome wire-wound furnace which was attached to a common water-cooled heat sink. Irradiation temperatures were maintained by balancing heat loss to the sink with electrical heat input from the furnaces. Helium gas surrounded the TEM disks in the furnaces. Two thermocouples attached to each capsule measured the temperature and controlled the furnace input. Temperatures were recorded continuously on disk charts; the measured values remained within * 5 o C of the targeted nominal values. The lowest temperature of 182O C was the minimum controllable temperature attainable with this arrangement. The irradiation temperatures ranged from 182 to 500° C, and the damage level ranged from 1.1 to 1.3 dpa. The damage levels were determined from neutron flux measurements and the neutron spectra data in ref. [S]. Calculations indicate that approximately 99% of the displacements in copper were due to fast (E > 0.1 MeV) neutrons. A summary of the irradiation parameters is given in table 2. The calculated [5] gas concentrations for the irradiation conditions of this study were - 0.3 appm He and - 15 appm H. The actual helium concentration was measured after the irradiation to be 0.19 appm 161. The damage microstructure was studied by transmission electron microscopy (TEM). The foil thicknesses of the TEM specimens were measured by stereomicroscopy. A low density of large (- 0.1 to 0.5 pm diameter) oxide particles were observed in both control and irradiated specimens. This produced some uncertainty in the TEM void swelling parameters for specimens irradiated at 2 400 o C due to the similarities in
Irradiation at 182°C resulted in uniform “blackspot” formation, with no evidence of voids or helium bubbles. Fig. 1 shows the defect microstructure for this irradiation condition. About half of the visible spots were resolvable as stacking fault tetrahedra (SFT), seen as triangular features in fig. 1. The black spot density was 1.7 x 10z3/m3 and the mean diameter was 2.5 nm. Further details of the microstructure at 182 o C are given elsewhere [7]. Dislocation loop, SFT, and cavity formation were observed in copper irradiated at 220 to 450” C. The cavity swelling parameters are given in table 3. The swelling due to void formation was determined from TEM measurements and also by measuring the densities of the irradiated disks with a microdensitometer. The standard error in the density changes measured with the densitometer was kO.0551;. Both the TEM and density measurements indicate that void swelling in copper at - 1.2 dpa is maximized for irradiation temperatures near 325OC. Fig. 2 shows the temperature-dependent density changes measured by the densitometer. There was no observable void formation at 182 and 500” C. The void swelling calculated from TEM measurements was generally less than that obtained from the density measurements (table 3). This may be due to an overestimate of the TEM foil thickness (stereomicroscopy) or, more likely, under-measuring the void “diameter.” A network dislocation density of about 2 X 1012/m2 was found for all of the irradiation conditions. A comparable value was observed in non-irradiated copper disks, which suggests that the irradiation dose in this study was too low to produce a significant network dislocation density. The dislocation loops produced by the irradiation increased in size as the irradiation temperature increased, changing from a mean diameter of 2.5 nm at 182OC to -10nmat300°Cand -2Onm at 400 o C (SET were not included in these size measurements). The loop parameters were not accurately measured, but a decrease in loop density was observed with increasing irradiation temperature. A detailed analysis of the loop and SFT microstructure is in progress. The voids exhibited faceting consistent with that of truncated octahedra bounded by (111) planes, in agreement with previous observations [l-4]. The void microstructure in copper following irradiation at 300°C is
S.J. Zinkle, K. Farrell / Void sweiling and defect cluster formation in CM
264
Fig. 1. Weak-beam dark-field (g, 3g) microstructure of copper irradiated at 182°C normal is near [llO] and g = (002).
shown in fig. 3. Some small dislocation loops are also visible in this picture. The voids were in general homogeneously distributed in the grain interiors. However, there were occasional localized spatial variations in the void size and density. This may be due to the presence of copper oxide particles in the disks. (The measured oxygen content was 35 wtppm - table 1.) There was no evidence for any
VOID
o.6
SWELLING
IN NEUTRON-IRRADIAl”ED
showing stacking fault tetrahedra. The foil
periodic spatial fluctuations in the dislocation loop or SFT distribution, in contrast to ion 181 and low-dose neutron [9,10] irradiation observations. Fig. 4 shows a
COPPER
$7
(00
300
200 IRRADIATION
500
400 TEMPERATURE
CC)
Fig. 2. Swelling in copper as determined from density measurements of irradiated disks. The displacement levels range from 1.1 to 1.3 dpa and the damage rate was - 2XlO_rdpa/s.
Fig. 3. Microstructure of copper irradiated at 300 o C.
S.J. Zinkle, K. Farreli / Void swelling and defect ~i~ter~~rrn~t~~n in Cu
265
Table 3 Void parameters for irradiated copper Temperature (“C) 182 220 250 275 300 350 400 450 500
Void density (10t8/m3) 146 155 66 65 38 1.8 0.1 < 0.01
Mean void diameter (nm) 19 28 46 45 49 140 400 _
TEM (4%)
Density change (W
0
- 0.05
AV/V
0.06 0.23 0.38 0.37 0.35 0.26 0.2
0.08 0.34 0.56 0.49 0.55 0.26 0.18 0.00
typical distribution of voids for a specimen irradiated at 275OC. Some spatial heterogeneities in the void distribution are visible. A void-denuded zone was observed along grain boundaries and incoherent twin boundaries. The width of this denuded zone varied from - 0.3 pm at 220 and 25o*c to - 0.7 ,nm at 350 o C. This agrees with Adamson et al. [ll] who reported a void-denuded zone width of - 0.7 pm for copper irradiated with fission neutrons
Fig. 5. Void-denuded zone along a grain boundary in copper irradiated at 350 o C.
to - 0.45 dpa at 327 o C. On the other hand, Singh et al. [9] observed a void-denuded zone in copper of - 1 pm following a low-dose (- 0.01 dpa) neutron irradiation at 250 OC. Fig. 5 shows the void microstructure adjacent to a grain boundary in copper irradiated at 350 ’ C. The void size and density reached their maximum values at a few pm from the grain boundary in copper specimens irradiated at 220 to 400 o C. A maximum in the void size and density in the region adjacent to the grain boundary void-denuded zone has been observed in neutron-i~adiated copper [ll] and aluminum 1121. The high density of small voids near the grain boundary in fig. 5 was not observed along other grain boundaries in the present study. and may be due to an oxide particle effect as discussed in the preceding paragraph.
4. Discussion
Fig. 4. General microstructure of copper irradiated at 275OC showing slight spatial heterogeneities in the void population.
Previous neutron irradiation studies on copper (with reIatively large uncertainties in the irradiation temperature) have reported that the lower limit for void formation in copper is i 220°C [l-4,13]. Wolfenden 1131 found that voids did not form in copper irradiated with neutrons to a damage level of 10 dpa at 175 & 25 OC.
266
S.J. Zinkle, K. Farrell / Void swelling and defect cluster formation in Cu
The present investigation, in which the irradiation temperature was measured, shows that substantial void formation occurs at 1.2 dpa for an irradiation temperature of 220°C whereas there is no void formation at 182O C for the damage rate employed in this study. These experimental observations are in agreement with the predictions of a simple model of vacancy cluster stability in the presence of helium [14]. This model predicts that neutron-irradiated copper containing 0.2 appm He will form SFT at temperatures below 200 o C and voids at temperatures above 200 o C. Irradiation to higher doses (or in a fusion neutron spectrum) will produce more helium and should cause cavity formation at temperatures below 200°C, according to the model. Void and helium bubble formation have been observed in a copper-boron alloy irradiated in ORR at 182 o C, where a uniform helium concentration of 100 appm was generated during the 1.2 dpa irradiation due to the “B(n, a)‘Li reaction [15]. Previous neutron irradiation studies of copper conducted at very low doses (lop4 to 10e2 dpa) have noticed the formation of segregation damage regions at temperatures of 250 to 400 ’ C, where irregular walls of dislocation loops and segments are separated by regions of relatively low residual defect densities [9,10,16]. We have not observed this type of damage segregation in our studies. The dislocation density in the irradiated copper specimens was independent of irradiation temperature and equal to the unirradiated value of 2 X 10’2/m2. Other neutron irradiation studies of copper performed at temperatures of 220 to 400°C and doses of 0.3 to 0.5 dpa have not reported evidence of segregation damage regions [1,3,11,17]. This suggests that the heterogeneous damage structure may be a low-fluence or low-damage-rate effect. The experimental results from this study (table 3) indicate that the maximum void swelling in neutronirradiated copper occurs at a temperature near 325°C (044T,), in general agreement with previous lower dose studies [l-4]. The density measurements (fig. 2) show a maximum swelling in this temperature regime of 0.55% The corresponding void swelling rate is therefore 0.46%/dpa. The steady-state swelling rate cannot be determined from these data since the fluence dependence of the void swelling was not studied. Brager and Garner [18] have observed a steady-state swelling rate of - O.SW/dpa for copper and copper alloys irradiated with neutrons at - 450 o C. The steady-state swelling rate of neutron-irradiated copper has not been determined at other temperatures, although data by Livak et al. [19] suggest a similar rate at 385 o C. It should be noted that transient swelling rates in excess of l%/dpa
have been observed at low doses (< 0.2 dpa) in some studies of neutron-irradiated copper [1,10,17]. Some authors have suggested a modified sigmoidal curve to describe the fluence-dependent void swelling of copper, where a low-swelling nucleation regime is followed by a high-swelling-rate regime and subsequently a steadystate swelling regime. Limited evidence for this type of behavior can be found by inspecting linear plots of void swelling versus fluence in neutron-irradiated copper [10,17,19] and Cu-0.1% Ag [18]. From a practical standpoint, the present results suggest that void swelling will not be of concern for copper exposed to moderate neutron doses at temperatures below 180°C. At these low irradiation temperatures, the vacancies combine to form SFTs and small dislocation loops. Irradiation of copper at temperatures between 220 and 450 o C may cause substantial void swelling. Copper alloys are being considered for a number of applications in fusion reactors, including divertors and normal-conducting magnets. The irradiation environment for these two applications is expected to include doses of 1 to 10 dpa (or higher) at temperatures between 30 and 470°C. Further work may be needed to develop copper alloys that are resistant to void swelling in the temperature range of 220 to 450 o C. Encouraging results have recently been obtained with a dispersionstrengthened copper alloy irradiated at 450’ C to neutron doses of 98 dpa [18]. However, these studies need to be extended to lower irradiation temperatures in a fusion-relevant neutron spectrum.
5. Conclusions Substantial void formation occurs in copper following neutron irradiation to - 1.2 dpa at temperatures from 220 to 450’ C. There was no evidence of void formation at 182OC; instead, stacking fault tetrahedra were observed. The maximum swelling in copper irradiated to 1.2 dpa at a damage rate of 1.7 X lo-’ dpa/s occurs at - 325 o C, and the lower temperature limit for void formation lies between 182 and 220 o C.
Acknowledgements We appreciate the contributions of N.H. Rouse in preparing the TEM specimens and L.J. Turner in making the density measurements.
S.J. Zinkle, K. Farrell / Void swelling and defect cluster formation in Cu
References [l] S.J. Zinkle and R.W. Knoll, A Literature Review of Radiation Damage Data for Copper and Copper Alloys, University of Wisconsin Fusion Technology Institute, Report UWFDM-578 (1984). [2] Y. Adda, in: Radiation-Induced Voids in Metals, Eds. J.W. Corbett and L.C. Ianniello, AEC Symposium Series No. 26, CONF-710601 (1972) pp. 31-81. [3] M. Labbe and J.P. Poirier, J. Nucl. Mater. 46 (1973) 86. [4] M. Labbe, G. Brebec and J.P. Poirier, J. Nucl. Mater. 49 (1973/74) 232. [5] T.A. Gabriel, B.L. Bishop and F.W. Wiffen, Oak Ridge National Laboratory, Report ORNL/TM-6361 (August 1979). [6] B.M. Oliver, Rockwell International, Canoga Park, CA, private communication, 1988. [7] S.J. Zinkle and R.L. Sindelar, J. Nucl. Mater. 155-157 (1988) 1196. [8] W. Jlger et al., Mat. Sci. Forum 15-18 (1987) 881. [9] B.N. Sir&, T. Leffers and A. Horsewell, Philos. Mag. A53 (1986) 233.
267
[lo] C.A. English, B.L. Eyre and J.W. Muncie, Philos. Mag. A56 (1987) 453. [ll] R.B. Adamson, W.L. Bell and P.C. Kelly, J. Nucl. Mater. 92 (1980) 149. [12] A. Horsewell and B.N. Singh, in: Proc. 12th Int. Symp. on Effcts of Radiation on Materials, ASTM STP 870, Eds. F.A. Garner and J.S. Perrin (ASTM, Philadelphia, 1985) pp. 248-261. [13] A. Wolfenden, Radiat. Eff. 15 (1972) 255. [14] S.J. Zinkle, W.G. Wolfer, G.L. Kulcinski and L.E. Seitzman, Philos. Mag. A55 (1987) 127. [15] S.J. Zinkle, to be submitted to J. Nucl. Mater.; also Fusion Reactor Materials Semiannual Progress Report, DOE/ER-0313/5 (March 1989). [16] L.D. Hulett, Jr. et al., J. Appl. Phys. 39 (1968) 3945. [17] J.L. Brimhall and H.E. Kissinger, Radiat. Eff. 15 (1972) 259. [18] H.R. Brager and F.A. Garner, in: Fusion Reactor Materials Semiannual Progress Report, DOE/ ER-0313/3 (March 1988) p. 254; also to be published in ASTM STP 1046 (1989) Eds. N.H. Packan et al. (191 R.J. Livak et al., J. Nucl. Mater. 141-143 (1986) 160.
Journal
of Nuclear
Materials 168 (1989) 262-267 North-Holland, Amsterdam
VOID SWELLING AND DEFECT CLUSTER FORMATION IN REACTOR-IRRADIATED COPPER * S.J. ZINKLE
and K. FARRELL
Metals and Ceramics Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6376, USA Received
17 February
1989; accepted
2 June 1989
Copper disks were irradiated at temperatures of 182 to 500 * C with moderated fission neutrons under carefully controlled conditions to a damage level of 1.1 to 1.3 dpa at a damage rate of 2 X10-’ dpa/s. The lower temperature limit for void formation was found to lie between 182 and 220 o C, with a maximum swelling of about 0.5% at 300 to 350 o C and no swelling at 500 o C. At 182O C, vacancies produced stacking fault tetrahedra instead of voids.
confirm studies.
1. Inaction There have been hundreds of studies on the effects of irradiation on copper over the past 30 years [l]. However, there have been relatively few attempts to carefully characterize the microstructure of neutronirradiated copper over an extended temperature range. There are less than 20 known studies of void formation in neutron-irradiated copper, with the bulk of these studies performed at a single irradiation temperature [l]. In addition, many of these experiments were performed with large uncertainties in the irradiation temperature and displacement dose due to the lack of adequate in-reactor temperature m~surement and few or no dosimetry foils. There are no known neutron irradiation studies of copper conducted over an extended temperature range with damage levels > 0.5 dpa. Most of the proposed applications for copper and copper alloys in fusion reactors involve irradiation temperatures of 50 to 470 QC. The most extensive study to date in this temperature range was performed by a group of French scientists [2-41, where copper subjected to damage levels ranging from 0.3 to 0.5 dpa and estimated irradiation temperatures of 220 to 600 o C was examined. The present investigation was intended to
* Research sponsored by the Division of Materials Sciences and by the Office of Fusion Energy, US Department of Energy under contract DE-AC05840R21400 with Martin Marietta Energy Systems, Inc.
~22-3115/89/$03.50 (North-Holland
Physics
Division)
the results of these earlier low-dose
procedures
2. Experimental
Polycrystalline ~~-purity (zone-refined) copper produced by the Solid State Division at the Oak Ridge National Laboratory (ORNL) was used in the experiment. Its chemical composition is given in table 1. Disks of dimensions 3.2 mm diameter by 0.28 mm thick were annealed for 1 h at 550° C in vacuum prior to the irradiation. The disks werre irradiated in nine separate helium-filly capsules in the BS site of the Oak Ridge Research (ORR) reactor for a period of 14 weeks. Neutron fluences and irradiation temperatures were carefully measured and controlled. Fourteen packages of flux monitor wires placed at the capsule site and elsewhere in the experimental assembly provided a thorough map of the neutron environment. Each capsule
Table 1 Chemical
analysis
of copper
Impurity
(wt ppm)
Impurity
(wt ppm)
Ag Al B Cr Fe
50 1 < 0.5 5 10
Ni Si Zr C 0 N
10 1 70 7 35 6
0 Elsevier Science Publishers B.V. Publishing
and extend
S.J. Zinkle, K. Farrell / Void swelling and defect cluster formation in Cu
Table 2 Irradiation data for copper specimens Temperature
Neutron fluence (1O25 n/m2)
(“C)
Total
Thermal
> 0.1 MeV
Damage level (+a) ”
182 220 250 275 300 350 400 450 500
3.6 3.6 4.3 4.0 3.6 3.1 3.1 3.5 3.3
1.2 1.2 1.2 1.2 1.1 1.2 1.2 1.2 1.0
1.1 1.1 1.2 1.3 1.0 1.2 1.3 1.3 1.0
1.2 1.2 1.3 1.3 1.1 1.2 1.3 1.2 1.1
the size and density of voids at these versus cavities created by oxide particles of the foil during electropolishing.
263
temperatures dropping out
3. Results
‘) Calculated from the measured neutron fluence using displacement damage calculations based on IAEA recommendations and the ENDFB/IV nuclear data file (51.
was housed in its own small, Nichrome wire-wound furnace which was attached to a common water-cooled heat sink. Irradiation temperatures were maintained by balancing heat loss to the sink with electrical heat input from the furnaces. Helium gas surrounded the TEM disks in the furnaces. Two thermocouples attached to each capsule measured the temperature and controlled the furnace input. Temperatures were recorded continuously on disk charts; the measured values remained within * 5 o C of the targeted nominal values. The lowest temperature of 182O C was the minimum controllable temperature attainable with this arrangement. The irradiation temperatures ranged from 182 to 500° C, and the damage level ranged from 1.1 to 1.3 dpa. The damage levels were determined from neutron flux measurements and the neutron spectra data in ref. [S]. Calculations indicate that approximately 99% of the displacements in copper were due to fast (E > 0.1 MeV) neutrons. A summary of the irradiation parameters is given in table 2. The calculated [5] gas concentrations for the irradiation conditions of this study were - 0.3 appm He and - 15 appm H. The actual helium concentration was measured after the irradiation to be 0.19 appm 161. The damage microstructure was studied by transmission electron microscopy (TEM). The foil thicknesses of the TEM specimens were measured by stereomicroscopy. A low density of large (- 0.1 to 0.5 pm diameter) oxide particles were observed in both control and irradiated specimens. This produced some uncertainty in the TEM void swelling parameters for specimens irradiated at 2 400 o C due to the similarities in
Irradiation at 182°C resulted in uniform “blackspot” formation, with no evidence of voids or helium bubbles. Fig. 1 shows the defect microstructure for this irradiation condition. About half of the visible spots were resolvable as stacking fault tetrahedra (SFT), seen as triangular features in fig. 1. The black spot density was 1.7 x 10z3/m3 and the mean diameter was 2.5 nm. Further details of the microstructure at 182 o C are given elsewhere [7]. Dislocation loop, SFT, and cavity formation were observed in copper irradiated at 220 to 450” C. The cavity swelling parameters are given in table 3. The swelling due to void formation was determined from TEM measurements and also by measuring the densities of the irradiated disks with a microdensitometer. The standard error in the density changes measured with the densitometer was kO.0551;. Both the TEM and density measurements indicate that void swelling in copper at - 1.2 dpa is maximized for irradiation temperatures near 325OC. Fig. 2 shows the temperature-dependent density changes measured by the densitometer. There was no observable void formation at 182 and 500” C. The void swelling calculated from TEM measurements was generally less than that obtained from the density measurements (table 3). This may be due to an overestimate of the TEM foil thickness (stereomicroscopy) or, more likely, under-measuring the void “diameter.” A network dislocation density of about 2 X 1012/m2 was found for all of the irradiation conditions. A comparable value was observed in non-irradiated copper disks, which suggests that the irradiation dose in this study was too low to produce a significant network dislocation density. The dislocation loops produced by the irradiation increased in size as the irradiation temperature increased, changing from a mean diameter of 2.5 nm at 182OC to -10nmat300°Cand -2Onm at 400 o C (SET were not included in these size measurements). The loop parameters were not accurately measured, but a decrease in loop density was observed with increasing irradiation temperature. A detailed analysis of the loop and SFT microstructure is in progress. The voids exhibited faceting consistent with that of truncated octahedra bounded by (111) planes, in agreement with previous observations [l-4]. The void microstructure in copper following irradiation at 300°C is
S.J. Zinkle, K. Farrell / Void sweiling and defect cluster formation in CM
264
Fig. 1. Weak-beam dark-field (g, 3g) microstructure of copper irradiated at 182°C normal is near [llO] and g = (002).
shown in fig. 3. Some small dislocation loops are also visible in this picture. The voids were in general homogeneously distributed in the grain interiors. However, there were occasional localized spatial variations in the void size and density. This may be due to the presence of copper oxide particles in the disks. (The measured oxygen content was 35 wtppm - table 1.) There was no evidence for any
VOID
o.6
SWELLING
IN NEUTRON-IRRADIAl”ED
showing stacking fault tetrahedra. The foil
periodic spatial fluctuations in the dislocation loop or SFT distribution, in contrast to ion 181 and low-dose neutron [9,10] irradiation observations. Fig. 4 shows a
COPPER
$7
(00
300
200 IRRADIATION
500
400 TEMPERATURE
CC)
Fig. 2. Swelling in copper as determined from density measurements of irradiated disks. The displacement levels range from 1.1 to 1.3 dpa and the damage rate was - 2XlO_rdpa/s.
Fig. 3. Microstructure of copper irradiated at 300 o C.
S.J. Zinkle, K. Farreli / Void swelling and defect ~i~ter~~rrn~t~~n in Cu
265
Table 3 Void parameters for irradiated copper Temperature (“C) 182 220 250 275 300 350 400 450 500
Void density (10t8/m3) 146 155 66 65 38 1.8 0.1 < 0.01
Mean void diameter (nm) 19 28 46 45 49 140 400 _
TEM (4%)
Density change (W
0
- 0.05
AV/V
0.06 0.23 0.38 0.37 0.35 0.26 0.2
0.08 0.34 0.56 0.49 0.55 0.26 0.18 0.00
typical distribution of voids for a specimen irradiated at 275OC. Some spatial heterogeneities in the void distribution are visible. A void-denuded zone was observed along grain boundaries and incoherent twin boundaries. The width of this denuded zone varied from - 0.3 pm at 220 and 25o*c to - 0.7 ,nm at 350 o C. This agrees with Adamson et al. [ll] who reported a void-denuded zone width of - 0.7 pm for copper irradiated with fission neutrons
Fig. 5. Void-denuded zone along a grain boundary in copper irradiated at 350 o C.
to - 0.45 dpa at 327 o C. On the other hand, Singh et al. [9] observed a void-denuded zone in copper of - 1 pm following a low-dose (- 0.01 dpa) neutron irradiation at 250 OC. Fig. 5 shows the void microstructure adjacent to a grain boundary in copper irradiated at 350 ’ C. The void size and density reached their maximum values at a few pm from the grain boundary in copper specimens irradiated at 220 to 400 o C. A maximum in the void size and density in the region adjacent to the grain boundary void-denuded zone has been observed in neutron-i~adiated copper [ll] and aluminum 1121. The high density of small voids near the grain boundary in fig. 5 was not observed along other grain boundaries in the present study. and may be due to an oxide particle effect as discussed in the preceding paragraph.
4. Discussion
Fig. 4. General microstructure of copper irradiated at 275OC showing slight spatial heterogeneities in the void population.
Previous neutron irradiation studies on copper (with reIatively large uncertainties in the irradiation temperature) have reported that the lower limit for void formation in copper is i 220°C [l-4,13]. Wolfenden 1131 found that voids did not form in copper irradiated with neutrons to a damage level of 10 dpa at 175 & 25 OC.
266
S.J. Zinkle, K. Farrell / Void swelling and defect cluster formation in Cu
The present investigation, in which the irradiation temperature was measured, shows that substantial void formation occurs at 1.2 dpa for an irradiation temperature of 220°C whereas there is no void formation at 182O C for the damage rate employed in this study. These experimental observations are in agreement with the predictions of a simple model of vacancy cluster stability in the presence of helium [14]. This model predicts that neutron-irradiated copper containing 0.2 appm He will form SFT at temperatures below 200 o C and voids at temperatures above 200 o C. Irradiation to higher doses (or in a fusion neutron spectrum) will produce more helium and should cause cavity formation at temperatures below 200°C, according to the model. Void and helium bubble formation have been observed in a copper-boron alloy irradiated in ORR at 182 o C, where a uniform helium concentration of 100 appm was generated during the 1.2 dpa irradiation due to the “B(n, a)‘Li reaction [15]. Previous neutron irradiation studies of copper conducted at very low doses (lop4 to 10e2 dpa) have noticed the formation of segregation damage regions at temperatures of 250 to 400 ’ C, where irregular walls of dislocation loops and segments are separated by regions of relatively low residual defect densities [9,10,16]. We have not observed this type of damage segregation in our studies. The dislocation density in the irradiated copper specimens was independent of irradiation temperature and equal to the unirradiated value of 2 X 10’2/m2. Other neutron irradiation studies of copper performed at temperatures of 220 to 400°C and doses of 0.3 to 0.5 dpa have not reported evidence of segregation damage regions [1,3,11,17]. This suggests that the heterogeneous damage structure may be a low-fluence or low-damage-rate effect. The experimental results from this study (table 3) indicate that the maximum void swelling in neutronirradiated copper occurs at a temperature near 325°C (044T,), in general agreement with previous lower dose studies [l-4]. The density measurements (fig. 2) show a maximum swelling in this temperature regime of 0.55% The corresponding void swelling rate is therefore 0.46%/dpa. The steady-state swelling rate cannot be determined from these data since the fluence dependence of the void swelling was not studied. Brager and Garner [18] have observed a steady-state swelling rate of - O.SW/dpa for copper and copper alloys irradiated with neutrons at - 450 o C. The steady-state swelling rate of neutron-irradiated copper has not been determined at other temperatures, although data by Livak et al. [19] suggest a similar rate at 385 o C. It should be noted that transient swelling rates in excess of l%/dpa
have been observed at low doses (< 0.2 dpa) in some studies of neutron-irradiated copper [1,10,17]. Some authors have suggested a modified sigmoidal curve to describe the fluence-dependent void swelling of copper, where a low-swelling nucleation regime is followed by a high-swelling-rate regime and subsequently a steadystate swelling regime. Limited evidence for this type of behavior can be found by inspecting linear plots of void swelling versus fluence in neutron-irradiated copper [10,17,19] and Cu-0.1% Ag [18]. From a practical standpoint, the present results suggest that void swelling will not be of concern for copper exposed to moderate neutron doses at temperatures below 180°C. At these low irradiation temperatures, the vacancies combine to form SFTs and small dislocation loops. Irradiation of copper at temperatures between 220 and 450 o C may cause substantial void swelling. Copper alloys are being considered for a number of applications in fusion reactors, including divertors and normal-conducting magnets. The irradiation environment for these two applications is expected to include doses of 1 to 10 dpa (or higher) at temperatures between 30 and 470°C. Further work may be needed to develop copper alloys that are resistant to void swelling in the temperature range of 220 to 450 o C. Encouraging results have recently been obtained with a dispersionstrengthened copper alloy irradiated at 450’ C to neutron doses of 98 dpa [18]. However, these studies need to be extended to lower irradiation temperatures in a fusion-relevant neutron spectrum.
5. Conclusions Substantial void formation occurs in copper following neutron irradiation to - 1.2 dpa at temperatures from 220 to 450’ C. There was no evidence of void formation at 182OC; instead, stacking fault tetrahedra were observed. The maximum swelling in copper irradiated to 1.2 dpa at a damage rate of 1.7 X lo-’ dpa/s occurs at - 325 o C, and the lower temperature limit for void formation lies between 182 and 220 o C.
Acknowledgements We appreciate the contributions of N.H. Rouse in preparing the TEM specimens and L.J. Turner in making the density measurements.
S.J. Zinkle, K. Farrell / Void swelling and defect cluster formation in Cu
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[lo] C.A. English, B.L. Eyre and J.W. Muncie, Philos. Mag. A56 (1987) 453. [ll] R.B. Adamson, W.L. Bell and P.C. Kelly, J. Nucl. Mater. 92 (1980) 149. [12] A. Horsewell and B.N. Singh, in: Proc. 12th Int. Symp. on Effcts of Radiation on Materials, ASTM STP 870, Eds. F.A. Garner and J.S. Perrin (ASTM, Philadelphia, 1985) pp. 248-261. [13] A. Wolfenden, Radiat. Eff. 15 (1972) 255. [14] S.J. Zinkle, W.G. Wolfer, G.L. Kulcinski and L.E. Seitzman, Philos. Mag. A55 (1987) 127. [15] S.J. Zinkle, to be submitted to J. Nucl. Mater.; also Fusion Reactor Materials Semiannual Progress Report, DOE/ER-0313/5 (March 1989). [16] L.D. Hulett, Jr. et al., J. Appl. Phys. 39 (1968) 3945. [17] J.L. Brimhall and H.E. Kissinger, Radiat. Eff. 15 (1972) 259. [18] H.R. Brager and F.A. Garner, in: Fusion Reactor Materials Semiannual Progress Report, DOE/ ER-0313/3 (March 1988) p. 254; also to be published in ASTM STP 1046 (1989) Eds. N.H. Packan et al. (191 R.J. Livak et al., J. Nucl. Mater. 141-143 (1986) 160.