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Transient effects under light ion irradiation
jourual of
Journal of Nuclear Materials 206 (1993) 335-340 North-Holland
Transient
nudl!ar matorlal!i
effects under light ion irradiation
P. Jung, W. Kesternich and H. Klein Association EUR4TOM-KFA,
Institut fiir Festkiirperforschung,
Forschungszentrum Jiilich, D-52425 Jiilich, Germany
Results on changes of microstructure and irradiation creep at low doses are given and creep results during flux transients and pulsing of light ion irradiation are critically reviewed.
1. Introduction
Irradiation effects will influence dimensional stability and mechanical properties of the first wall and blanket materials in future fusion reactors. Transient effects, i.e. deviations from steady state behaviour may occur during start up and shut down periods and quasi-permanent under pulsed operation. In contrast to fission reactors, light ion irradiation allows for better experimental control and sensitivity to investigate transient effects and especially to simulate pulsed operation. An overview will be given on microstructural and mechanical property changes during the initial period and during pulsing of light ion irradiation. The data are compared to steady state behaviour and to neutron irradiation.
working and stress. Although the data scatter in fig. 1 allows no safe conclusion about the effect of alloying, fig. 2 indicates that higher loop densities than in pure Ni are obtained in FeCrNi- and FeCrNiMo-alloys and in type 316 austenitic stainless steel. Also these materials show a strong reduction of loop number densities with increasing irradiation temperature. For example the loop density of 316L at 0.44 dpa is reduced from = 1022/m3 at 573 K (fig. 2) to = 1019/m3 at 773 K. Much higher cluster densities are observed during low dose neutron irradiation, e.g. displacement doses of
. k ,a 22+1 L
The first effects of irradiation becoming visible in the transmission electron microscope are black dots which finally become resolvable as small faulted dislocation loops. In fig. 1 loop number densities in nickel and in a Ni-4wt%W alloy are plotted as a function of displacement dose. Loop densities are much higher at the lower temperature; for comparison data from refs. [1,2] are included. A previous investigation [3] showed a strong reduction of loop density also from 573 to 693 K. Higher densities under I-MZM irradiation [4] at an even higher temperature (758°C) may be influenced by surface effects of the very thin TEM specimens. The loop densities show no significant dependence on cold 0022-3115/93/$06.00
_I
--o
I
1
~ lf_” +!
102’
2.1. Microstructure
I
v
.‘a
m2. Low dose irradiation
I
I
l
F
0
J91
0
l
a
--__
--._
-_
I
I
I
0.1
0.2
0.3
#a
Fig. 1. Loop number densities in annealed (573 K, O) and 20% cold worked Ni (483 K v, 573 K A) and 20% cold worked Ni-4wt%W (573 K o) as a function of displacement dose under proton irradiation. Open, half-filled and full symbols indicate tensile stresses up to 20 MPa, up to 100 MPa and above 100 MPa, respectively. Included are cluster densities in annealed Ni produced by 16 MeV-protons at room temperature (+ 121) and by 22 MeV-d and 70 MeV-a in cold-worked Ni at 497 K (0 [l]).
0 1993 - Elsevier Science Publishers B.V. All rights reserved
336
P. Jung et al. / Transient effects under light ion irradiation
102’ ’ 0
A
0.2
‘v
I
0.4
0.6
0.8
+a
Fig. 2. Loop number densities in 20% cold worked FeCrNi (A), FeCrNiMo (v) and type 316 stainless steel (01, as a function of displacement dose under proton irradiation at 573 K. Open, half-filled and full symbols indicate tensile stresses of 0, up to 150 and above 150 MPa, respectively.
0.03 dpa at 563 K produced = 10z4/m3 clusters in OWR and > 10z3/m3 in RTNS-II [5]. The loop num-
ber densities under light ion irradiation at 573 K in figs. 1 and 2 show broad maxima around 0.1 dpa (Ni) and around 0.25 dpa (stainless steels), respectively. At higher doses loop growth, unfaulting and interaction with the network become predominant. The resulting higher sink strength of the material eventually causes the observed reduction of the loop density. The density of the network dislocations in Ni and Ni-4wt%W is shown in fig. 3. The densities of cold worked and annealed material drop, respectively increase already at doses below 0.05 dpa to almost constant values with only a slight decrease at further increasing dose. The FeCrNi- and FeCrNiMo-alloys and 316 tend to a quasi stationary dislocation density as shown in fig. 4. The present data do not show any significant dependence of dislocation densities on stress and temperature, see also data from ref. [l] in fig. 3. The insensitivity of the saturation dislocation density on temperature is in agreement with other experimental [6] as well as theoretical examinations [7]. The dislocation densities of annealed and cold worked nickel obviously do not converge to the same value, the dislocation density of the cold worked material remaining always above that of the annealed material. The dislocation density of the present, only 50 u.rn thick, 20% cold worked, type 316 stainless steel specimens is much lower than for thicker material used for neutron irradiation, where values before irradiation from 1.5 to
IO'+
0
I
I
0.1
0.2
I
0.3
+a Fig. 3. Dislocation densities in annealed (573 K, O) and 20% cold worked Ni (483 K v , 573 K A) and 20% cold worked Ni-4wt%W (573 K 01 as a function of displacement dose under proton irradiation. Open, half-filled and full symbols indicate tensile stresses below 20 MPa, below 100 MPa and above 100 MPa, respectively. Included are cluster densities in cold-worked Ni produced by 22 MeV-d and 70 MeV-a at 497 K(+ [II).
6 x 10’5/m2 have been reported [g-lo]. Also the saturation values after light ion irradiation are lower than after high dose neutron irradiation, which range from
‘OY1:
I 0.4
.__..a 0.6
0.8
dpa Fig. 4. Dislocation densities in 20% cold worked FeCrNi (A ), FeCrNiMo ( v ) and type 316 stainless steel (o), as a function of displacement dose under proton irradiation at 573 K. Included are results for annealed FeCrNiMo (0) and 316L (0) at 773 K [17] and of 20% cold worked FeCrNiMo (*) under cyclic irradiation at 773 K. Open, half-filled and full symbols indicate tensile stresses of 0 MPa, below 150 MPa and above 150 MPa, respectively.
P. Jung et al. / Transienteffects under lightion irradiation
5.5 to 9 x 1014/m2 [g-lo]. An intermediate value of 3 x 10’4/m2 was observed in 316 after Ni+ irradiation at 873 K [ll]. Investigations on void swelling of unstressed AISI 316 under light charged particle irradiation have so far been performed only using 1 MeV electrons and 0.2-l MeV protons. These particles allow only irradiation to very low depth beneath the surface. At temperatures around 873 K, incubation periods around 1.3-2 dpa for electron irradiation and 2-3 dpa for proton irradiation [12] were observed. For comparison incubation periods for swelling of Fe-15Cr-25Ni alloys amount to above 3 dpa for 2.8-5.0 MeV Ni+-ion [lo] and to about 10 dpa for fast neutron irradiation [13]. The incubation period can be shortened by the application of tensile [14] or compressive [15,16] stresses. A pure, annealed FeCrNiMO alloy, irradiated with protons at 773 K under 100 MPa tensile stress, showed an incubation period of about 0.2 dpa [17]. The incubation period and the eventual swelling rate of about 1.5%/dpa were within experimental error equal to values of similar materials in cold worked condition irradiated under tensile [14] and compressive stress [15], respectively. Also, very short transients were found for hardening and softening of nickel [2] and copper [18] by 16 and 18 MeV proton irradiation at room temperature, repectively. For a comparison to neutron data see ref. [19]. 2.2. Irradiation creep It has been shown that microstructural evolution can explain transient irradiation creep rates in nickel in the framework of a climb enabled dislocation glide model [201. But a variety of transient effects has been observed in irradiation creep measurements on various materials. In stainless steels [21-241 and in a NiW-alloy [25] initially negative irradiation creep was observed, i.e. contraction against the applied tensile stress. Strains of about 5 X 10e5 were accumulated during this contraction period at temperatures from 573 to 773 K. This negative creep is tentatively ascribed to some segregation or precipitation of solutes, a notation which is corrobarated by the absence of negative transients under torsional stress [26]. Some pure metals show constant strain rates under irradiation from the very beginning of irradiation, i.e. no transients [27]. But most metals and ceramics show a “conventional” transient of enhanced creep rate which contributes an extra strain from about 1 to 6 X 1O-4 [25,27-291. Some experiments also have shown a transient of enhanced creep rate after shut down of neutron as well as proton irradiation [30,31]. But subsequent attempts to repro-
331
duce this effect failed [321 or could not rule out the possibility of thermal transients [33]. The most important question with respect to these transient effects is whether they reappear during repeated on/off cycles and thus may accumulate significant amounts of extra strain. This is especially important for devices with cyclic operation such as rotating target- or spallation neutron sources and for some fusion reactor designs like tokamacs. This question has been addressed in a series of investigations using electron irradiation, for references see ref. [34]. But these investigations suffered from the fact that only a few on/off cycles were performed and that a temperature/stress regime was investigated in which thermal creep was not negligible. Under these conditions thermal transients and/or stresses from temperature excursions may cause extensive deformation during beam switching. The effect of temperature excursions can be avoided by measuring far away from the thermal creep regime, while the effect of thermal transients can be avoided by using continuously pulsed irradiation by which quasi stationary conditions are eventually achieved.
3. Pulsed irradiation The possible effect of pulsing on microstructural evolution [35], segregation [36] and property changes [37] has been the topic of a series of theoretical investigations using rate theoretical approaches. But so far the variety of conceivable mechanisms and the variability of parameters has only allowed rather vague predictions.
3.1. Irradiation creep Three effects may change the response of materials to pulsed compared to stationary irradiation. Firstly, there may be a dose rate effect, as equal average dose rate means higher instantaneous dose rate during the pulse. Secondly, thermal effects may contribute during the off cycle when temperature remains high. Thirdly, there may be a real pulsing effect, which means unbalanced point defect fluxes (interstitials and vacancies) to sinks during a transient period, caused by their different mobilities. To a first approximation a maximum effect of pulsing is expected for a duty factor = 0.5 and for pulsing frequencies v close to the reciprocal lifetime of the slower defect species, presumably the vacancies, i.e., (1)
338
P. Jung et al. / Transient effects under light ion irradiation
of pulsing. The fact that even those experiments showed no effect which were in the regime of the most probable H,“-value (1.09 eV) must be ascribed to the fact, that the maximum available displacement rate K = 4 x 10e6 dpa/s was too small to suffice eq. (2). Generally the available cooling power in irradiation creep experiments with light ions and gas cooling, sets a limiting relation between maximum displacement rate K,,, and irradiation temperature T [40], e.g. for 50 pm thick specimens:
-1EMPERATUREIY) 600 400 I I I
200 I
I
K,,,
1o-6I 1.0
I 1.5
II I' II, I I I II, 2.0 2.5 l/T (103Km')-
,
Fig. 5. Frequency v and temperatures used in pulsing experiments on 20% cold worked stainless steel (0) and FeCrNiMo (0) with a duty factor of 0.5 [38]. The relation between ~1and T was calculated according to eq. (1) with D, = 6 x 10m5 m2/s and H,” = 1.09 eV (hatched) and 1.3 eV (shaded), respectively. The regime of typical tokamak designs is shown for comparison. The right-hand side scale shows minimum displacement rates Kmin which are necessary at a given frequency u to initiate climb-glide processes according to eq. (2).
D, = D, exp( - H.J”/kT)
is the vacancy diffusion coefficient and fd the density of dislocations, which are assumed to be the predominant sinks for point defects. The data points in fig. 5 indicate u and T values used in irradiation creep experiments under pulsing condition [38]. Due to the limited duration of the transients, the unbalanced flux of the faster species will only produce a persisting strain if some irreversible process is triggered. The most prominent example is dislocation glide enabled by climb. This is only possible if sufficient defects are accumulated during one pulse to promote unpinning. A crude estimate [39] gives a relation between u and the minimum displacement rate K,,(see right-hand side ordinate in fig. 5): Kmin = Ubds>
(2)
where b is Burger’s vector. The shaded and hatched areas indicate u,T-regimes in which pulsing effects on irradiation creep may be expected for vacancy migration energies H,” of 1.3 and 1.09 eV, respectively. The experiments (0, 0) did not show any significant effect
[dpa/s]
= 10p8.(T-
T,),
(3)
where T, is the temperature of the cooling gas. Recently rather high irradiation creep strains were reported after reactor irradiation to 8 dpa at 333 K [41]. A comparison to fig. 5 shows that at such low temperatures unbalanced flux transients may last a very long time (l/o) and even dose rates in the 10mh dpa/s range may be sufficient to produce climb-glide processes by unbalanced defects, i.e. fulfill eq. (2). On the other hand, irradiation creep measurements under proton irradiation at 338 K indeed showed also an enhanced transient compared to irradiation at 673 K, but only during a rather short period, thus accumulating only an extra strain in the range of 10m4 [42]. Also irradiation creep measurements on coldworked copper at 290 K showed transient behaviour, but with creep rates [49] not significantly exceeding those above 423 K 1501. Pulsed operation of a tokamak reactor will not only produce flux pulsing, but temperature, stress and magnetic fields will be cycled as well. Therefore an experiment was set up which simulated simultaneous cycling of beam, temperature and stress. As a guide line the design of the Next European Torus (NET) was used which assumes for the first wall during the on-cycle a temperature of 793 K and compressive stress and during the off-cycle 523 K and tensile stress. Irradiation creep rates of a 20% cold worked, pure FeCrNiMo alloy under these cyclic conditions [43] are slightly lower than creep rates of the same alloy under static tensile or compressive load [15]. This comparison excludes any enhancement of irradiation creep by pulsed irradiation. 3.2. Swelling and microstructure Extensive investigations on the effect of pulsing on microstructure, mainly void formation and precipitation, have been performed using heavy ion irradiation. A recent review [44] reports as the most prominent effects of pulsing enhanced void coarsening near the
P. Jung et al. / Transient effects under light ion irradiation
peak swelling temperature and reduced void nucleation at higher temperatures, enhanced precipitation of thermally stable phases and retardation of irradiation induced phases. Altogether the results of heavy ion experiments on effects of pulsing on void swelling were not unequivocal, some giving smaller [4.5], some larger [46,47] voids compared to continuous irradiation. Actually one experiment showed larger voids under pulsed irradiation, but smaller voids, when beam and temperature were cycled simultaneously [48]. The present proton irradiations at 773 K under cyclic conditions gave void swelling of a 20% cold worked FeCrNiMO alloy only slightly below the results under stationary condition. However, the irradiation under cyclic conditions produced significantly larger number densities and correspondingly smaller void sizes than under stationary irradiation with constant tensile or compressive stress [15]. The dislocation density under pulsed irradiation at 793 K was slightly above the values of the stationary irradiations at 573 K, see fig. 4.
4. Conclusions - Transients
in the evolution of dislocation structure are shorter under light ion irradiation as compared to neutron irradiation. - The transient for the onset of linear void swelling is reduced by tensile as well as compressive stresses. - In the transient regime void swelling is only slightly if at all lower under cycling conditions than under stationary irradiation. But cycling produces a higher density of smaller voids than stationary irradiation. - In a frequency and temperature regime typical for tokamak fusion reactors, pulsed irradiation does not enhance irradiation creep significantly.
References [l] D.J. Michel, P.L. Hendrick and A.G. Pieper, ASTM-STP 611 (1976) p. 284. [2] R.H. Jones, E.R. Bradley and D.L. Styris, J. Nucl. Mater. 116 (1983) 297. [3] P. Jung, T.C. Reiley and W. Kesternich, in: Fusion Technology 1980, XIth Symposium, Oxford, CEC (Pergamon, 1980) B-1287. [4] B. Cochrane, S.B. Fisher, K.M. Miller and P.J. Goodhew, J. Nucl. Mater. 120 (1984) 79. 151 N. Yoshida, H.L. Heinisch, T. Muroga, K. Araki and M. Kiritani, Ann. Progr. Report Monbusho-DOE 1988, p. 105. [6] C. Brown and G. Linekar, UKAEA TRG-Memo-6571 (1974).
339
[7] W.G. Wolfer and B.B. Glasgow, Acta Metall. 33 (1985)
1997. [8] R.A. Weiner, J.P. Foster and A. Boltax, in: Radiation Effects in Breeder Reactor Structural Materials, eds. M.L. Bleiberg and J.W. Bennett (AIME, New York, 1977) p. 865. [9] J.I. Bramman, C. Brown, J.S. Watkins, C. Cawthorne, E.J. Fulton, P.J. Barton and E.A. Little, in: Radiation Effects in Breeder Reactor Structural Materials, eds. M.L. Bleiberg and J.W. Bennett (AIME, New York, 1977) p. 479. [lo] H.R. Brager, F.A. Garner, E.R. Gilbert, J.E. Flinn and W.G. Wolfer, in: Radiation Effects in Breeder Reactor Structural Materials, eds. M.L. Bleiberg and J.W. Bennett (AIME, New York, 1977) p. 727. [ll] N. Azam, J. Delaplace and L. LeNaour, in: Comportement sous Irrad. des Mater. Met. et des Composants des Coeurs des Reacteurs Rapides, eds. J. Poirier and J.M. Dupouy (CEA-DMECN, Gif-sur-Yvette, France, 1979) p. 177. [12] F.A. Garner et al., Proc. Workshop on Correlation of Neutron and Charged Particle Damage, Oak Ridge, 1976, ERDA CONF-760673 (1976) p. 147. [13] F.A. Garner and H.R. Brager, ASTM-SIP 870 (1985) p. 187. I141S.K. Khera, C. Schwaiger and H. Ullmaier, J. Nucl. Mater. 92 (1980) 299. [151H.K. Sahu and P. Jung, J. Nucl. Mater. 136 (1985) 154. [161T. Lauritzen, S. Vaidyanathan, W.L. Bell and W.J.S. Yang, ASTM-SIP 955 (1987) 101. I171P. Jung and H. Klein, J. Nucl. Mater. 179-181 (1991) 503. K31 M. Appello and P. Fenici, J. Nucl. Mater. 152 (1988) 348. I191P. Jung, Radiat. Eff. and Defects in Sol. 113 (1990) 109. DO1 C.H. Henager Jr., E.P. Simonen, E.R. Bradley and R.G. Stang, J. Nucl. Mater. 117 (1983) 250. Dll P. Jung, J. Nucl. Mater. 113 (1983) 133. D21 V.K. Sethi, A.P.L. Turner and F.V. Nolfi, in: Phase Stability during Irradiation, eds. J.R. Holland, L.K. Mansur and D.I. Potter (TMS-AIME, 1981) p. 437. 1231A. Hishinuma and T. Aruga, JAERI Tandem Annual Report (1983) p. 52. 1241J.A. Hudson, R.S. Nelson and R.J. McElroy, J. Nucl. Mater. 65 (1977) 279. 1251P. Jung and H. Klein, J. Nucl. Mater. 159 (1988) 360. D61 E.K. Opperman, J.L. Straalsund, G.L. Wire and R.H. Howell, Nucl. Technol. 42 (1979) 71. [271P. Jung and MI. Ansari, J. Nucl. Mater. 138 (1986) 40. LB1 D.J. Michel, P.L. Hendrick and A.G. Pieper, J. Nucl. Mater. 75 (19781 1. E91 C.H. Henager, E.P. Simonen, E.R. Bradley and R.G. Stang, J. Nucl. Mater. 122 & 123 (1984) 413. I301S.R. McEwen and V. Fidleris, Philos. Mag. 31 (1975) 1149. [311J. Nagakawa, J. Nucl. Mater. 116 (1983) 10. [321D. Faulkner and R.J. McElroy, ASTM-SIP 683 (1979) p. 329.
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P. Jung et al. / Transient effects under light ion irradiation
[33] G.E. Lucas, M. Suprenant, J. DiMarzo and G.J. Brown,
[34] [35] [36] [37] [38] [39] [40]
J. Nucl. Mater. 101 (1981) 78. L.N. Bystrov, L.I. Ivanov and A.B. Tsepelev, Radiat. Eff. 97 (1986) 127. E.P. Simonen, N.M. Ghoniem and N.H. Packan, J. Nucl. Mater. 122 & 123 (1984) 391. N.Q. Lam, G. Leaf and R.A. Johnson, J. Nucl. Mater. 74 (1978) 277. N.M. Ghoniem and G.L. Kulcinski, Nucl. Technol/Fusion 2 (1982) 165. P. Jung, J. Nucl. Mater. 113 (1983) 163. P. Jung and H. Ullmaier, Radiat. Eff. 103 (1987) 21. P. Jung, C. Schwaiger, H. Ullmaier and J. Viehweg, in: Comportement sous Irrad. des Mater. Met. et des Composants des Coeurs des Reacteurs Rapides, eds. J. Poirier and J.M. Dupouy (CEA-DMECN, Gif-sur-Yvette, France, 1979) p. 415.
[41] M.L. Grossbeck and L.K. Mansur, J. Nucl. Mater. 179181 (1991) 130. [42] R. Scholz, report JRC Ispra, EUR 12411 EN (1989) p. 61, unpublished. [43] Z. Zhu, P. Jung and H. Klein, to be published in J. Nucl. Mater (1993). [44] N.H. Packan, Radiat. Eff. 101 (1986) 189. [45] J.A. Sprague and F.A. Smidt Jr., NRL Memo Reports 2555 (1972) 25; and 2629 (1973) 27. [46] A. Taylor et al., report ANL/CTR/TM-39 (19751 18. [47] J.L. Brimhall, E.P. Simonen and L.A. Charlot 117 (1983) 118. [48] N.H. Packan, J. Nucl. Mater. 122 & 123 (19841 644. [49] S.N. Buckley and S.A. Manthorpe, Irrad. Embritt. and Creep in Fuel Cladding and Core Comp. (BNES, London, 1972) p. 253. [50] P. Jung, J. Nucl. Mater. 200 (1993) 138.
Journal of Nuclear Materials 206 (1993) 335-340 North-Holland
Transient
nudl!ar matorlal!i
effects under light ion irradiation
P. Jung, W. Kesternich and H. Klein Association EUR4TOM-KFA,
Institut fiir Festkiirperforschung,
Forschungszentrum Jiilich, D-52425 Jiilich, Germany
Results on changes of microstructure and irradiation creep at low doses are given and creep results during flux transients and pulsing of light ion irradiation are critically reviewed.
1. Introduction
Irradiation effects will influence dimensional stability and mechanical properties of the first wall and blanket materials in future fusion reactors. Transient effects, i.e. deviations from steady state behaviour may occur during start up and shut down periods and quasi-permanent under pulsed operation. In contrast to fission reactors, light ion irradiation allows for better experimental control and sensitivity to investigate transient effects and especially to simulate pulsed operation. An overview will be given on microstructural and mechanical property changes during the initial period and during pulsing of light ion irradiation. The data are compared to steady state behaviour and to neutron irradiation.
working and stress. Although the data scatter in fig. 1 allows no safe conclusion about the effect of alloying, fig. 2 indicates that higher loop densities than in pure Ni are obtained in FeCrNi- and FeCrNiMo-alloys and in type 316 austenitic stainless steel. Also these materials show a strong reduction of loop number densities with increasing irradiation temperature. For example the loop density of 316L at 0.44 dpa is reduced from = 1022/m3 at 573 K (fig. 2) to = 1019/m3 at 773 K. Much higher cluster densities are observed during low dose neutron irradiation, e.g. displacement doses of
. k ,a 22+1 L
The first effects of irradiation becoming visible in the transmission electron microscope are black dots which finally become resolvable as small faulted dislocation loops. In fig. 1 loop number densities in nickel and in a Ni-4wt%W alloy are plotted as a function of displacement dose. Loop densities are much higher at the lower temperature; for comparison data from refs. [1,2] are included. A previous investigation [3] showed a strong reduction of loop density also from 573 to 693 K. Higher densities under I-MZM irradiation [4] at an even higher temperature (758°C) may be influenced by surface effects of the very thin TEM specimens. The loop densities show no significant dependence on cold 0022-3115/93/$06.00
_I
--o
I
1
~ lf_” +!
102’
2.1. Microstructure
I
v
.‘a
m2. Low dose irradiation
I
I
l
F
0
J91
0
l
a
--__
--._
-_
I
I
I
0.1
0.2
0.3
#a
Fig. 1. Loop number densities in annealed (573 K, O) and 20% cold worked Ni (483 K v, 573 K A) and 20% cold worked Ni-4wt%W (573 K o) as a function of displacement dose under proton irradiation. Open, half-filled and full symbols indicate tensile stresses up to 20 MPa, up to 100 MPa and above 100 MPa, respectively. Included are cluster densities in annealed Ni produced by 16 MeV-protons at room temperature (+ 121) and by 22 MeV-d and 70 MeV-a in cold-worked Ni at 497 K (0 [l]).
0 1993 - Elsevier Science Publishers B.V. All rights reserved
336
P. Jung et al. / Transient effects under light ion irradiation
102’ ’ 0
A
0.2
‘v
I
0.4
0.6
0.8
+a
Fig. 2. Loop number densities in 20% cold worked FeCrNi (A), FeCrNiMo (v) and type 316 stainless steel (01, as a function of displacement dose under proton irradiation at 573 K. Open, half-filled and full symbols indicate tensile stresses of 0, up to 150 and above 150 MPa, respectively.
0.03 dpa at 563 K produced = 10z4/m3 clusters in OWR and > 10z3/m3 in RTNS-II [5]. The loop num-
ber densities under light ion irradiation at 573 K in figs. 1 and 2 show broad maxima around 0.1 dpa (Ni) and around 0.25 dpa (stainless steels), respectively. At higher doses loop growth, unfaulting and interaction with the network become predominant. The resulting higher sink strength of the material eventually causes the observed reduction of the loop density. The density of the network dislocations in Ni and Ni-4wt%W is shown in fig. 3. The densities of cold worked and annealed material drop, respectively increase already at doses below 0.05 dpa to almost constant values with only a slight decrease at further increasing dose. The FeCrNi- and FeCrNiMo-alloys and 316 tend to a quasi stationary dislocation density as shown in fig. 4. The present data do not show any significant dependence of dislocation densities on stress and temperature, see also data from ref. [l] in fig. 3. The insensitivity of the saturation dislocation density on temperature is in agreement with other experimental [6] as well as theoretical examinations [7]. The dislocation densities of annealed and cold worked nickel obviously do not converge to the same value, the dislocation density of the cold worked material remaining always above that of the annealed material. The dislocation density of the present, only 50 u.rn thick, 20% cold worked, type 316 stainless steel specimens is much lower than for thicker material used for neutron irradiation, where values before irradiation from 1.5 to
IO'+
0
I
I
0.1
0.2
I
0.3
+a Fig. 3. Dislocation densities in annealed (573 K, O) and 20% cold worked Ni (483 K v , 573 K A) and 20% cold worked Ni-4wt%W (573 K 01 as a function of displacement dose under proton irradiation. Open, half-filled and full symbols indicate tensile stresses below 20 MPa, below 100 MPa and above 100 MPa, respectively. Included are cluster densities in cold-worked Ni produced by 22 MeV-d and 70 MeV-a at 497 K(+ [II).
6 x 10’5/m2 have been reported [g-lo]. Also the saturation values after light ion irradiation are lower than after high dose neutron irradiation, which range from
‘OY1:
I 0.4
.__..a 0.6
0.8
dpa Fig. 4. Dislocation densities in 20% cold worked FeCrNi (A ), FeCrNiMo ( v ) and type 316 stainless steel (o), as a function of displacement dose under proton irradiation at 573 K. Included are results for annealed FeCrNiMo (0) and 316L (0) at 773 K [17] and of 20% cold worked FeCrNiMo (*) under cyclic irradiation at 773 K. Open, half-filled and full symbols indicate tensile stresses of 0 MPa, below 150 MPa and above 150 MPa, respectively.
P. Jung et al. / Transienteffects under lightion irradiation
5.5 to 9 x 1014/m2 [g-lo]. An intermediate value of 3 x 10’4/m2 was observed in 316 after Ni+ irradiation at 873 K [ll]. Investigations on void swelling of unstressed AISI 316 under light charged particle irradiation have so far been performed only using 1 MeV electrons and 0.2-l MeV protons. These particles allow only irradiation to very low depth beneath the surface. At temperatures around 873 K, incubation periods around 1.3-2 dpa for electron irradiation and 2-3 dpa for proton irradiation [12] were observed. For comparison incubation periods for swelling of Fe-15Cr-25Ni alloys amount to above 3 dpa for 2.8-5.0 MeV Ni+-ion [lo] and to about 10 dpa for fast neutron irradiation [13]. The incubation period can be shortened by the application of tensile [14] or compressive [15,16] stresses. A pure, annealed FeCrNiMO alloy, irradiated with protons at 773 K under 100 MPa tensile stress, showed an incubation period of about 0.2 dpa [17]. The incubation period and the eventual swelling rate of about 1.5%/dpa were within experimental error equal to values of similar materials in cold worked condition irradiated under tensile [14] and compressive stress [15], respectively. Also, very short transients were found for hardening and softening of nickel [2] and copper [18] by 16 and 18 MeV proton irradiation at room temperature, repectively. For a comparison to neutron data see ref. [19]. 2.2. Irradiation creep It has been shown that microstructural evolution can explain transient irradiation creep rates in nickel in the framework of a climb enabled dislocation glide model [201. But a variety of transient effects has been observed in irradiation creep measurements on various materials. In stainless steels [21-241 and in a NiW-alloy [25] initially negative irradiation creep was observed, i.e. contraction against the applied tensile stress. Strains of about 5 X 10e5 were accumulated during this contraction period at temperatures from 573 to 773 K. This negative creep is tentatively ascribed to some segregation or precipitation of solutes, a notation which is corrobarated by the absence of negative transients under torsional stress [26]. Some pure metals show constant strain rates under irradiation from the very beginning of irradiation, i.e. no transients [27]. But most metals and ceramics show a “conventional” transient of enhanced creep rate which contributes an extra strain from about 1 to 6 X 1O-4 [25,27-291. Some experiments also have shown a transient of enhanced creep rate after shut down of neutron as well as proton irradiation [30,31]. But subsequent attempts to repro-
331
duce this effect failed [321 or could not rule out the possibility of thermal transients [33]. The most important question with respect to these transient effects is whether they reappear during repeated on/off cycles and thus may accumulate significant amounts of extra strain. This is especially important for devices with cyclic operation such as rotating target- or spallation neutron sources and for some fusion reactor designs like tokamacs. This question has been addressed in a series of investigations using electron irradiation, for references see ref. [34]. But these investigations suffered from the fact that only a few on/off cycles were performed and that a temperature/stress regime was investigated in which thermal creep was not negligible. Under these conditions thermal transients and/or stresses from temperature excursions may cause extensive deformation during beam switching. The effect of temperature excursions can be avoided by measuring far away from the thermal creep regime, while the effect of thermal transients can be avoided by using continuously pulsed irradiation by which quasi stationary conditions are eventually achieved.
3. Pulsed irradiation The possible effect of pulsing on microstructural evolution [35], segregation [36] and property changes [37] has been the topic of a series of theoretical investigations using rate theoretical approaches. But so far the variety of conceivable mechanisms and the variability of parameters has only allowed rather vague predictions.
3.1. Irradiation creep Three effects may change the response of materials to pulsed compared to stationary irradiation. Firstly, there may be a dose rate effect, as equal average dose rate means higher instantaneous dose rate during the pulse. Secondly, thermal effects may contribute during the off cycle when temperature remains high. Thirdly, there may be a real pulsing effect, which means unbalanced point defect fluxes (interstitials and vacancies) to sinks during a transient period, caused by their different mobilities. To a first approximation a maximum effect of pulsing is expected for a duty factor = 0.5 and for pulsing frequencies v close to the reciprocal lifetime of the slower defect species, presumably the vacancies, i.e., (1)
338
P. Jung et al. / Transient effects under light ion irradiation
of pulsing. The fact that even those experiments showed no effect which were in the regime of the most probable H,“-value (1.09 eV) must be ascribed to the fact, that the maximum available displacement rate K = 4 x 10e6 dpa/s was too small to suffice eq. (2). Generally the available cooling power in irradiation creep experiments with light ions and gas cooling, sets a limiting relation between maximum displacement rate K,,, and irradiation temperature T [40], e.g. for 50 pm thick specimens:
-1EMPERATUREIY) 600 400 I I I
200 I
I
K,,,
1o-6I 1.0
I 1.5
II I' II, I I I II, 2.0 2.5 l/T (103Km')-
,
Fig. 5. Frequency v and temperatures used in pulsing experiments on 20% cold worked stainless steel (0) and FeCrNiMo (0) with a duty factor of 0.5 [38]. The relation between ~1and T was calculated according to eq. (1) with D, = 6 x 10m5 m2/s and H,” = 1.09 eV (hatched) and 1.3 eV (shaded), respectively. The regime of typical tokamak designs is shown for comparison. The right-hand side scale shows minimum displacement rates Kmin which are necessary at a given frequency u to initiate climb-glide processes according to eq. (2).
D, = D, exp( - H.J”/kT)
is the vacancy diffusion coefficient and fd the density of dislocations, which are assumed to be the predominant sinks for point defects. The data points in fig. 5 indicate u and T values used in irradiation creep experiments under pulsing condition [38]. Due to the limited duration of the transients, the unbalanced flux of the faster species will only produce a persisting strain if some irreversible process is triggered. The most prominent example is dislocation glide enabled by climb. This is only possible if sufficient defects are accumulated during one pulse to promote unpinning. A crude estimate [39] gives a relation between u and the minimum displacement rate K,,(see right-hand side ordinate in fig. 5): Kmin = Ubds>
(2)
where b is Burger’s vector. The shaded and hatched areas indicate u,T-regimes in which pulsing effects on irradiation creep may be expected for vacancy migration energies H,” of 1.3 and 1.09 eV, respectively. The experiments (0, 0) did not show any significant effect
[dpa/s]
= 10p8.(T-
T,),
(3)
where T, is the temperature of the cooling gas. Recently rather high irradiation creep strains were reported after reactor irradiation to 8 dpa at 333 K [41]. A comparison to fig. 5 shows that at such low temperatures unbalanced flux transients may last a very long time (l/o) and even dose rates in the 10mh dpa/s range may be sufficient to produce climb-glide processes by unbalanced defects, i.e. fulfill eq. (2). On the other hand, irradiation creep measurements under proton irradiation at 338 K indeed showed also an enhanced transient compared to irradiation at 673 K, but only during a rather short period, thus accumulating only an extra strain in the range of 10m4 [42]. Also irradiation creep measurements on coldworked copper at 290 K showed transient behaviour, but with creep rates [49] not significantly exceeding those above 423 K 1501. Pulsed operation of a tokamak reactor will not only produce flux pulsing, but temperature, stress and magnetic fields will be cycled as well. Therefore an experiment was set up which simulated simultaneous cycling of beam, temperature and stress. As a guide line the design of the Next European Torus (NET) was used which assumes for the first wall during the on-cycle a temperature of 793 K and compressive stress and during the off-cycle 523 K and tensile stress. Irradiation creep rates of a 20% cold worked, pure FeCrNiMo alloy under these cyclic conditions [43] are slightly lower than creep rates of the same alloy under static tensile or compressive load [15]. This comparison excludes any enhancement of irradiation creep by pulsed irradiation. 3.2. Swelling and microstructure Extensive investigations on the effect of pulsing on microstructure, mainly void formation and precipitation, have been performed using heavy ion irradiation. A recent review [44] reports as the most prominent effects of pulsing enhanced void coarsening near the
P. Jung et al. / Transient effects under light ion irradiation
peak swelling temperature and reduced void nucleation at higher temperatures, enhanced precipitation of thermally stable phases and retardation of irradiation induced phases. Altogether the results of heavy ion experiments on effects of pulsing on void swelling were not unequivocal, some giving smaller [4.5], some larger [46,47] voids compared to continuous irradiation. Actually one experiment showed larger voids under pulsed irradiation, but smaller voids, when beam and temperature were cycled simultaneously [48]. The present proton irradiations at 773 K under cyclic conditions gave void swelling of a 20% cold worked FeCrNiMO alloy only slightly below the results under stationary condition. However, the irradiation under cyclic conditions produced significantly larger number densities and correspondingly smaller void sizes than under stationary irradiation with constant tensile or compressive stress [15]. The dislocation density under pulsed irradiation at 793 K was slightly above the values of the stationary irradiations at 573 K, see fig. 4.
4. Conclusions - Transients
in the evolution of dislocation structure are shorter under light ion irradiation as compared to neutron irradiation. - The transient for the onset of linear void swelling is reduced by tensile as well as compressive stresses. - In the transient regime void swelling is only slightly if at all lower under cycling conditions than under stationary irradiation. But cycling produces a higher density of smaller voids than stationary irradiation. - In a frequency and temperature regime typical for tokamak fusion reactors, pulsed irradiation does not enhance irradiation creep significantly.
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