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Irradiation creep of 20% cold-worked copper
jeu.rplof guam' mteu'ials
Journal of Nuclear Materials 200 (1993) 138-140 North-Holland
Letterto the Editors Irradiation creep of 20% cold-worked copper Peter Jung lnstitut fidr Festk&perforschung, ForschungszentrumJiilich, Association Euratom- KFA, Postfach 1913, 1)-5170 Jiilich, Germany Received 19 November 1992; accepted 14 December 1992
Copper has recently gained interest for use as heat sink material in highly loaded parts of future fusion reactor, such as divertors. In a review [1] relevant data of copper and copper alloys were summarized, but no results on irradiation creep were available. Actually copper was one of the first materials on which irradiation creep measurements under light ion irradiation have been performed [2]. However, these measurements at 260°C and 69 MPa gave no enhancement of the deformation rate compared to thermal creep, probably due to the rather low displacement rate of less than 10 - s dpa/s. Also in a later investigation no irradiation creep of copper was detected [3]. In the present work, investigations on creep of 20% coldworked copper foils under 6.2 MeV proton irradiation at temperatures between 100 and 200°C and tensile stresses from 20 to 70 MPa are reported. Studies on transient creep during flux and stress changes are included. The specimens were prepared from 99.999% purity copper by stepwise cold-rolling by 20% with intermediate annealings at 650°C to a final thickness of 50 pro. The specimens showed a grain size of about 25 i~m in the annealed state. Details of the irradiation creep apparatus have been given previously [4]. Irradiations were performed at the Jiilich compact cyclotron with a proton beam, which after passing through an aluminum window, had an energy of 6.2 MeV. The beam current was determined from the energy loss of the beam in the specimen, and displacement rates of (0.73.5) x 10 -6 d p a / s were derived according to ref. [5]. Fig. 1 shows strains during irradiation at 150°C under 70 MPa tensile stress with a displacement rate of about 0.7 x 10 -6 dpa/s. The curve takes off with an enhanced strain rate during a transient period of about 0.005 dpa, corresponding to an irradiation time of about 2 h. The transient gives an extra straining e of
about 1.5 x 10 -4. This transient strain is similar to vaiues observed for nickel and stainless steels [6]. The temperature dependence of creep rates d norrealized to the displacement rates K is given in fig. 2. The Arrhenius type plot shows roughly a straight line, corresponding to a e x p ( - H / k T ) temperature dependence with an activation energy H = 0.12 eV. This value is very similar to a value of 0.15 eV, observed for nickel and anstenitic stainless steels [7]. In fig. 3 normalized irradiation creep rates at 150°C are plotted as a function of tensile stress. At stresses above 50 MPa, creep without irradiation becomes measurable and is subtracted from the creep rates during irradiation ( + ) to obtain the true irradiation creep
O't Cu;
0.3 1500 '~ 0.2 ~ ' 0
. 0 0
1
/ I
I
0.005
0.010
dpa
0.015
Fig. 1. Straining of 20% cold-worked copper at 150°C and 70 MPa during proton irradiation at a displacement rate of 0.7× 10-6 dpa/s.
0022-3115/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
P. ,lung / Irradiation creep of 20% cold-worked copper
T [K] 400
500
I
CU
20 MPa I
(3. "O ',4
"=10-a I
2.0
I
i
I
2.2
I
I
2.4
,
2.6
2.8
10 a / T [K] Fig. 2. Temperature dependence of the normalized creep rates of 20% cold-worked copper at 20 MPa. Displacement rates K ranged from 1.5 to 4× 1(~-6 dpa/s. The straight l~le indicates an Arrhenius behaviour with an activation energy of 0.12 eV.
rates ([]), At stresses below 50 MPa the creep rates can be described by a linear stress dependence, while above 50 MPa the stress exponent rises to a value of at least 4 (solid line). i
i
i
i
i
i
t
+
Cu 10 -2
e,i "D
139
Changing the stress during irradiation caused transient straining which was positive for stress increase, i.e., it supplied extra strain compared to the stationary creep rates, and was negative in the case of stress decrease. A typical extra strain per stress change was 1.6 × 10-12/pa. This value is similar to the value of 1 × 10-12/pa obtained for cold-worked Ni and stainless steels [8]. An explanation of these stress-transient strains has been given in t e l [8] in terms of irradiation-enaEled glide of dislocations. The total straining expected from this process should be ( 6 Y ) - t where Y is Young's modulus [9]. With Y = 125 GPa for copper, a value of 1.3 x 10-12/pa is obtained, which is by a~out a factor of 0.8 smaller than the above experimental value. Almost the same ratio between calculated and experimental values was observed for nickel and stainless steels in ref. [8]. In conclusion, irradiation creep of copper shows essentially the same dependences on stress and temperature as nickel and austenitic stainless steel. Also the behaviour during flux and stress transients is comparable. In the regime of the linear stress dependence a compliance for irradiation creep of d/~yK--6.2 x 10 -11 is obtained at 150°C. Taking into account the temperature dependence in fig. 2, a value of about 14.5× 10 -in P a - l d p a -1 is extrapolated for 300°C. This value is only slightly higher than the compliance for nickel of about 11 x 1 0 - H / ( P a dpa) at 300°C [8]. For both nickel and stainless steels the irradiation creep compliances under neutron irradiation were by factors of about 3 and 9 smaller than for light ions [10], depending whether dpa rates were calculated according to experimental displacement rates [5] or according to the modified Kinchin-Pease N R T [!1] model. Considering the above similarities between the results for nickel and the present results for copper, the same relation may be used to extrapolate from the present light ion data to the creep behaviour of copper under neutron irradiation.
References 10-3
,
20
L
,
,
40 60 [MPa]
~
'
80
'
100
Fig. 3. N o r m a l i z e d creep rates o f 20% c o l d - w o r k e d copper at
1500C as a function of tensile stress or during proton irradiation ( + ) and after subtraction of thermal creep contributions (El). Displacement rates K were about 0.7× 10 -s dpa/s. The straight lines indicate stress exponents of 1 and 4, respectively.
[1] G.J. Butterworth and C.B.A. Forty, J. Nucl. Mater. 189 (1992) 237. [2] W.F. Witzig, J. AppL Phys. 23 (1952) 1263. [3] S.N. Buckley, in The Interaction between Dislocations and Point Defects, ed. B.L. Eyre, UKAEA Report Hatwell, AERE-R 5944, vol. Ii (1968) p. 517. [4] P. Jung, A. Schwarz and H.K. Sahu, Nucl. Instr. and Meth. A234 (1985) 331. [5] P. Jung, J. Nucl. Mater. 117 (1983) 133.
140
P. Jung / Irradiation creep of 20% cold-worked copper
[6] P. Jung, H. Klein and W. Kesternich, to be pubfished. [7] P. Jung, in Dimensional Stability and Mechanical Behavior of Irradiated Metals and Alloys, vol. 1 (British Nuclear Engineering Society, London, 1983) p. 21. [8] P. Jung, J. Nucl. Mater. 113 (1983) 133.
[9] J. Friedel, Philos. Mag. 44 (1953) 444; Dislocation (pergamon, Oxford, 1964) p. 235. [10] P. Jung, Radiat. Eft. Def. in Solids 113 f1990) 109. [ll] H.J. Norgett, M.T. Robinson and I.M. Torrens, Nucl. Eng. Des. 33 (1975) 50.
Journal of Nuclear Materials 200 (1993) 138-140 North-Holland
Letterto the Editors Irradiation creep of 20% cold-worked copper Peter Jung lnstitut fidr Festk&perforschung, ForschungszentrumJiilich, Association Euratom- KFA, Postfach 1913, 1)-5170 Jiilich, Germany Received 19 November 1992; accepted 14 December 1992
Copper has recently gained interest for use as heat sink material in highly loaded parts of future fusion reactor, such as divertors. In a review [1] relevant data of copper and copper alloys were summarized, but no results on irradiation creep were available. Actually copper was one of the first materials on which irradiation creep measurements under light ion irradiation have been performed [2]. However, these measurements at 260°C and 69 MPa gave no enhancement of the deformation rate compared to thermal creep, probably due to the rather low displacement rate of less than 10 - s dpa/s. Also in a later investigation no irradiation creep of copper was detected [3]. In the present work, investigations on creep of 20% coldworked copper foils under 6.2 MeV proton irradiation at temperatures between 100 and 200°C and tensile stresses from 20 to 70 MPa are reported. Studies on transient creep during flux and stress changes are included. The specimens were prepared from 99.999% purity copper by stepwise cold-rolling by 20% with intermediate annealings at 650°C to a final thickness of 50 pro. The specimens showed a grain size of about 25 i~m in the annealed state. Details of the irradiation creep apparatus have been given previously [4]. Irradiations were performed at the Jiilich compact cyclotron with a proton beam, which after passing through an aluminum window, had an energy of 6.2 MeV. The beam current was determined from the energy loss of the beam in the specimen, and displacement rates of (0.73.5) x 10 -6 d p a / s were derived according to ref. [5]. Fig. 1 shows strains during irradiation at 150°C under 70 MPa tensile stress with a displacement rate of about 0.7 x 10 -6 dpa/s. The curve takes off with an enhanced strain rate during a transient period of about 0.005 dpa, corresponding to an irradiation time of about 2 h. The transient gives an extra straining e of
about 1.5 x 10 -4. This transient strain is similar to vaiues observed for nickel and stainless steels [6]. The temperature dependence of creep rates d norrealized to the displacement rates K is given in fig. 2. The Arrhenius type plot shows roughly a straight line, corresponding to a e x p ( - H / k T ) temperature dependence with an activation energy H = 0.12 eV. This value is very similar to a value of 0.15 eV, observed for nickel and anstenitic stainless steels [7]. In fig. 3 normalized irradiation creep rates at 150°C are plotted as a function of tensile stress. At stresses above 50 MPa, creep without irradiation becomes measurable and is subtracted from the creep rates during irradiation ( + ) to obtain the true irradiation creep
O't Cu;
0.3 1500 '~ 0.2 ~ ' 0
. 0 0
1
/ I
I
0.005
0.010
dpa
0.015
Fig. 1. Straining of 20% cold-worked copper at 150°C and 70 MPa during proton irradiation at a displacement rate of 0.7× 10-6 dpa/s.
0022-3115/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
P. ,lung / Irradiation creep of 20% cold-worked copper
T [K] 400
500
I
CU
20 MPa I
(3. "O ',4
"=10-a I
2.0
I
i
I
2.2
I
I
2.4
,
2.6
2.8
10 a / T [K] Fig. 2. Temperature dependence of the normalized creep rates of 20% cold-worked copper at 20 MPa. Displacement rates K ranged from 1.5 to 4× 1(~-6 dpa/s. The straight l~le indicates an Arrhenius behaviour with an activation energy of 0.12 eV.
rates ([]), At stresses below 50 MPa the creep rates can be described by a linear stress dependence, while above 50 MPa the stress exponent rises to a value of at least 4 (solid line). i
i
i
i
i
i
t
+
Cu 10 -2
e,i "D
139
Changing the stress during irradiation caused transient straining which was positive for stress increase, i.e., it supplied extra strain compared to the stationary creep rates, and was negative in the case of stress decrease. A typical extra strain per stress change was 1.6 × 10-12/pa. This value is similar to the value of 1 × 10-12/pa obtained for cold-worked Ni and stainless steels [8]. An explanation of these stress-transient strains has been given in t e l [8] in terms of irradiation-enaEled glide of dislocations. The total straining expected from this process should be ( 6 Y ) - t where Y is Young's modulus [9]. With Y = 125 GPa for copper, a value of 1.3 x 10-12/pa is obtained, which is by a~out a factor of 0.8 smaller than the above experimental value. Almost the same ratio between calculated and experimental values was observed for nickel and stainless steels in ref. [8]. In conclusion, irradiation creep of copper shows essentially the same dependences on stress and temperature as nickel and austenitic stainless steel. Also the behaviour during flux and stress transients is comparable. In the regime of the linear stress dependence a compliance for irradiation creep of d/~yK--6.2 x 10 -11 is obtained at 150°C. Taking into account the temperature dependence in fig. 2, a value of about 14.5× 10 -in P a - l d p a -1 is extrapolated for 300°C. This value is only slightly higher than the compliance for nickel of about 11 x 1 0 - H / ( P a dpa) at 300°C [8]. For both nickel and stainless steels the irradiation creep compliances under neutron irradiation were by factors of about 3 and 9 smaller than for light ions [10], depending whether dpa rates were calculated according to experimental displacement rates [5] or according to the modified Kinchin-Pease N R T [!1] model. Considering the above similarities between the results for nickel and the present results for copper, the same relation may be used to extrapolate from the present light ion data to the creep behaviour of copper under neutron irradiation.
References 10-3
,
20
L
,
,
40 60 [MPa]
~
'
80
'
100
Fig. 3. N o r m a l i z e d creep rates o f 20% c o l d - w o r k e d copper at
1500C as a function of tensile stress or during proton irradiation ( + ) and after subtraction of thermal creep contributions (El). Displacement rates K were about 0.7× 10 -s dpa/s. The straight lines indicate stress exponents of 1 and 4, respectively.
[1] G.J. Butterworth and C.B.A. Forty, J. Nucl. Mater. 189 (1992) 237. [2] W.F. Witzig, J. AppL Phys. 23 (1952) 1263. [3] S.N. Buckley, in The Interaction between Dislocations and Point Defects, ed. B.L. Eyre, UKAEA Report Hatwell, AERE-R 5944, vol. Ii (1968) p. 517. [4] P. Jung, A. Schwarz and H.K. Sahu, Nucl. Instr. and Meth. A234 (1985) 331. [5] P. Jung, J. Nucl. Mater. 117 (1983) 133.
140
P. Jung / Irradiation creep of 20% cold-worked copper
[6] P. Jung, H. Klein and W. Kesternich, to be pubfished. [7] P. Jung, in Dimensional Stability and Mechanical Behavior of Irradiated Metals and Alloys, vol. 1 (British Nuclear Engineering Society, London, 1983) p. 21. [8] P. Jung, J. Nucl. Mater. 113 (1983) 133.
[9] J. Friedel, Philos. Mag. 44 (1953) 444; Dislocation (pergamon, Oxford, 1964) p. 235. [10] P. Jung, Radiat. Eft. Def. in Solids 113 f1990) 109. [ll] H.J. Norgett, M.T. Robinson and I.M. Torrens, Nucl. Eng. Des. 33 (1975) 50.