Barlow, P.

Leffers, Torben

Singh, Bachu Narain

The apparent activation energy for the growth of interstitial dislocation loops in copper, Cu-l%Ni, Cu-2%Ni, and Cu-5%Ni during high voltage electron microscope irradiation was determined. The apparent activation energy for loop growth in all these materials can be taken to be 0.34eV±0.02eV. This value together with the corresponding value of 0.44eV±0.02eV determined earlier for Cu-10%Ni is discussed with reference to the void growth rates observedin these materials. The apparent activationenergy for loop growth in copper (and in Cu-l%Ni that has a void growth rate similar to that in pure copper) is interpreted as twice the vacancy migration energy (indicating that divacancies donot play any significant role). For the materials with higher Ni content (in which the void growth rate is much lower than that in Cu and Cu-l%Ni) the measured apparent activation energy is interpreted to be characteristic of loops positioned fairly close to the foil surface and not of loops in "bulk material". Prom the present results in combination with the earlier results for Cu-10%Ni it is concluded that interstitial trapping is the most likely explanation of the reduced void growth rate in Cu-Ni alloys

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Downloaded from orbit.dtu.dk on: Dec 20, 2017The activation energy for loop growth in Cu and in Cu-Ni alloysBarlow, P.; Leffers, Torben; Singh, Bachu NarainPublication date:1978Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Barlow, P., Leffers, T., & Singh, B. N. (1978). The activation energy for loop growth in Cu and in Cu-Ni alloys.(Risø-M; No. 2129).Risø Risø-M-CHD Titte and author(s) THE ACTIVATION ENERGY FOR LOOP GROWTH IN Cu AND Cu-Ni ALLOYS by P. Barlow, T. Leffers, and B.N. Singh 1 3 paeti 4- 1 table -+ 5 illustrations Du, August 1978 Department or group Metallurgy Group's own rcgntration ») Abstract The apparent activation energy for the growth of interstitial dislocation loops in copper, Cu-l%Ni, Cu-2%Ni, and Cu-5%Ni during high voltage electron microscope irradiation was determined. The apparent activation energy for loop growth in all these materials can be taken to be 0.34eV±0.02eV. This value together with the corresponding value of 0.44eV±0.02eV determined earlier for Cu-10%Ni is discussed with reference to the void growth rates observed in these ...acerials. The apparent activation energy for loop growth in copper (and in Cu-l%Ni that has a void growth rate similar to that in pure copper) is interpreted as twice the vacancy migration energy (indicating that divacancies do not olay any significant role). For the mater-ials with higher Ni content (in which the void growth rate is much lower than that in Cu and Cu-l%Ni) the measured apparent activation energy is interpreted to be characteristic of loops positioned fairly close to the foil surface and not of loops in "bulk material". Prom the present results in combination with the earlier results for Cu-10%Ni it is concluded that interstitial trapping is the most likely explanation of the reduced void growth rate in Cu-Ni alloys. Copies to Available or. '•'.•qu^ Rt, from Riea Libr&ry, Risø National Laboratory ("•::..-••• bibliotek, Forsøgsanls-g Risai), DK-*»ooo Rc;:ki - 1 . , Denmark Teleyii one: •>•) 51 01. ext, 33*». telex; i»31l6 ISBN 87-550-0574-8 ISSN 0418-6435 3 CONTENTS Page 1. Introduction 5 2. Experimental Procedures 5 3. Experimental Results 9 4. Discussion 9 References 13 Table 14 Figures 15 5 1. INTRODUCTION Barlow and Leffers (1977) have determined the apparent activation energy for the growth of interstitial dislocation loops in Cu-10%Ni during HVEM (high voltage electron microscope) irradiation to be 0.43eV±0.O2eV. This result was suggested to reflect vacancy binding to clusters of Ni atoms with a binding energy of ~0.3eV. Leffers, Singlv and Barlow (1977) have pointed out that an alternative interpretation in terms of an inter-stitial binding energy of ~0.9eV to the Ni clusters is possible. Leffers et al. and Singh, Leffers, and Barlow (1978) used this vacancy or interstitial binding to explain the reduced void growth rate in Cu-Ni alloys: the clusters of Ni atoms were con-sidered to act as recombination centres by trapping either vacancies or interstitials. It is the aim of the present work to extend the investi-gation of point-defect binding in Cu-Ni by the addition of loop-growth experiments in Cu-Ni alloys with Ni contents below 10%. The new results will be interpreted (and the results on Cu-10%Ni reinterpreted) in the light of an investigation of the relation between the apparent activation energy for loop growth during HVEM irradiation and the activation energy for point-defect migration (Leffers and Singh 1978). The question whether interstitial or vacancy trapping is responsible for the reduced void growth in Cu-Ni allys will be considered in view of this new knowledge. 2. EXPERIMENTAL PROCEDURES The materials used were copper of 99.999% purity and copper--nickel alloys containing 1, 2, and 5% nickel (by weight) produced from copper and nickel of 99.999% purity. Barlow and Leffers (1977) have given a more detailed description of the 6 materials. All Materials were vacuum-annealed for 4 hours at BOO C before specimens for electron microscopy were made. The thin foil specimens were prepared from 3 mm discs by electropolisning (with jet) in a solution of 33% nitric acid in methanol at -20°C and 10V. The experiments were made in the AEI EM7 microscope at Harwell operated at 1MV with a dose rate of about 10* dpa per 24 -2 -1 second (~2*10 electrons m s ). The nominal irradiation temperature was controlled to within ±2°C. The actual irradiation temperature is estimated to be within 1C°C of the nominal _7 temperature. During irradiation a vacuum of about 5-10 torr was maintained in the specimen chamber. The loop growth was recorded on cine film taken from the screen at a speed of one frame every 2 seconds. The screen was tilted 30 from the position perpendicular to the beam. This introduced some distortion in the images of the disloation loops ( the loop growth rates quoted are corrected so that they correspond to a non-tilted screen (the correction depending upon the angle between the tilt axis of the screen ^nd thr direction in which the loop dimension is measured). In Cu-10%Ni Barlow and Leffers (1977) measured the tempe-rature dependence of the growth rate of the "rectangular" loops (with Burgers vector o[100], habit plane (100), and line vectors foil] and [011]) emitted from one specific dislocation climb source. In the present work we could in practice only use rect-angular loops emitted from climb sources in Cu-5%Ni and Cu-2fNi; there were very few rectangular loops and climb sources in Cu-l%Ni and none in Cu. In Cu-l%Ni and in Cu we therefore had to use Frank loops for the measurements. The use of these two 7 different procedures (based on rectangular loops and Prank loops, respectively) is justified by the finding of Barlow and Leffers that there is no significant difference between the apparent activation energy for growth of rectangular loops and Frank loops. Xn Cu-5%Hi and Cu-2%Ni we compared the activation-energy plots obtained frost different climb sources close to one another (foil thickness ~0.5 urn, (100) approximately parallel to the foil), each operating at the different temperatures. Normally the points fell on the same line, i.e. the conditions (e.g. position in the foil) were similar for most climb sources in a given area. In Cu-5%Ni and Cu-2%Ni new loops were continually emitted from the slimb sources in the temperature range 325--500°C (particularly when the area to be investigated was initially irradiated in the temperature range 400-S00°C). In Cu-ltHi and Cu, on the other hand, there was not much loop nucleation after the initial stage of irradiation. It was therefore necessary, as the first stage of each experiment, to necleate a number of loops in a certain area by irradiation at about 250-300°Cj we then recorded the growth of these loops at various temperatures (~250-400°C). This procedure had the disadvantage that we could not keep irradiating exactly the same area because the loop would grow out to the surface and disappear. Thus, one activation--energy plot refers to loops in different (closely spaced) areas, which tends to introduce somewhat more experimental scatter than in the case of Cu-5%Ni and Cu-2INi. The exact procedure adopted for the determination of the apparent activation energy for loop growth was the following; The growth of the loops in a small area (of the order of a few »m in Cu-5%Ni and Cu-2%Ni, Bomewhat larger in Cu-llNi and 8 Cu) with foil thickness -0.5 urn was recorded at different tempe-ratures. The dose rate was kept constant in a given experiment. and the loop for which the growth rate was to be recorded was always brought to the central part of the beam. At a given tempe-rature the great majority of the loops in a given area had growth rates close to one anothert we used the average values of these growth rates. The logarithms of the average growth rates (L) were plotted versus 1/kT (k being the Boltzmann constant and T the irradiation temperature in K). The apparent activation energy was found as the absolute slope of the line through those points. The best line through the points was found by a least-square fit that also gave the standard error on the slope. In Cu-5tNi and Cu-2%Ni we measured the growth along one of the edges of rectangular loops in foils with {100} approximately parallel to the foil, which corresponds to a climb direction approximately parallel to the foil surface (as reflected in a constant growth rate with time for a given loop, cf. fig. 1). In Cu-ltNi and Cu we measured the growth along the stacking-fault fringes of the Frank loops, which again corresponds to a growth direction parallel to the foil surface; the growth rate in this direction was constant with time, cf. fig. 2, whereas the growth rate for a given loop in the direction perpendicular to the stacking-fault fringes decreased with time - as reflected in an increasing elongation with increasing size. The selected micrographs in figs. 3 and 4 of rectangular loops and Prank loops in Cu-5%Ni and in Cu, respectively (taken from the cine film) illustrate the behaviour of the two types of loops: the shape of the rectangular loops does not depend on size, whereas the Frank loops become increasingly elongated as they grow in size. 9 3. EXPERIMENTAL RESULTS Plots of lnL (logarithmic loop growth rata) versus 1/kT for the various experiments are shown in fig. S. The lines for 0.5 im foil of Cu-10%*i from larlow and Laffers (1977) are also shown (without individual points). The apparent activation energies E (absolute values of the slopes from the present experiments and from those of Barlow and Laffers on Cu-10%Mi (calculated in the sane way as the present results) are listed in table I with the corresponding standard deviations. Table I shows that the apparent activation energies for loop growth for all the Materials with Hi contents of 5% or less can be represented by one value of the order of 0.34eV, whereas the activation energy in Cu-10%Ni is significantly different (~0.44eV*). 4. DISCUSSION Kiritani, Yoahida, Takata, and Haehara (197S) found an apparent activation energy for loop growth in Cu of —0.30ev t0.02eV. Theoretically they deduced that the apparent activation energy should be 2 tines the vacancy Migration energy E5. The best estimate of E^ seems to be ~0.7eV (Bourassa and Lengeler (1976), Wright and Evans (1966), and Antesbsrger, Sonnenberg, and Wienhold (1978) found E* values of 0.72eV, 0.71eV, and 0.70eV, respectively), which means that the apparent activation energy measured by Kiritani et al. was lower than 1/2 E^. Kiritani at al. acribed this discrepancy to the presence of divacancies with lower migration energy than that of monovacancies. However, * The value of 0.43eV quoted by Barlow and Laffers was estimated directly from the plot 11 Cu-IUM.) is just about bi« enough to account for SOB« reduction in swelling via vacancy or interstitial trapping Ccf. Hansur v Too. and Ooflaa 1977), but it cannot account for the drastic reduction observed. In the cases of Cu-5tNi and Ce-2tRl there is also a very pronounced dec i seas in swelling, but the neasured C values are not significantly different free, those in Cu and Cu-ltBl. This east nean that the present E values especially for Cu~$%Hi and Ce-2Mi, bat also for Cu-10%«i. are net represen-tative for the interior of the thicker foils where the voids are found. This wool« problea is being investlea ted in sore details. If the effect of the Bi clusters is to trap vacancies, even a slight binding/trapping would be detected as a slight increase in Bs loop growth in copper is governed by vacancy stigration (Kiritani et al. 1975), and vacancy binding will cause an isswjdiate increase in E. The thin foils investigated in the present work should then be unrepresentative of the void--relevaat thicker foils to the extent that the Hi clusters (the presence of which are deaonstrated by the operation of the cliab sources, cf. barlow and Laffers (1977)) should not produce any vacancy binding in the thin foils. If, on the other hand, the effect of the Mi clusters is to trap interstitials, such binding/trapping would not change E until it has bacoaia so strong that E£ is bigger than B*, i.e. until the binding energy reaches a Magnitude of ~0.teV. In the case of interstitial trapping the thin foils of Cu-51Hi and Co-2%Mi would thus only have to be unrepresentative of thicker foils in the quantitative way that EJ is still sua Her than Ey -and not in the qualitative way that there is no binding. 12 Of these two possibilities the latter, i.e. that the Ni clusters present in Cu-5%Ni and Cu-2%Ni do produce (interstitial) M binding also in the thin foils, but not enough to make E_ bigger than Ey, seems far more likely than the former that the Ni clusters do not produce any binding in thin foils. We therefore consider the present results to support the suggestion that the reduction in swelling in Cu-Ni is produced by interstitial trapping at the Ni clusters - with the addition that the concentration of Ni clusters (or some other parameter ^or the Ni clusters) in the thin foils of Cu-5%Ni and Cu-2%Ni investigated in the present work is insufficient to produce the interstitial binding energy of ~0.6eV necessary to change the apparent activation energy for loop growth. 13 REFERENCES Antesberger, G., Sonnenberg, K., and Wienhold, P., 1978, J. Nucl. Mat. 69 t 70, 660. Antesberger, G., Sonnenberg, K., Wienhold, P., Coltman, R.R., Klabunde, C.E., Williams, J.H., and Chaplin, R.L., 1975, Funda-mental Aspects of Radiation Damage in Metals, edited by M.T. Robinson and F.W. Young (U.S. ERDA Conference No. 751006) p. 561. Barlow, P., and Leffers, T., 1977, Phil. Mag. 36, 565. Bourassa, R.R., and Lengeler, B., 1976, J. Phys. F, 6_, 1405. Kiritani, M., Yoshida, N., Takata, H., and Maehara, Y., 1975, 5 J. Phys. Soc. Japan, 2§, 1677. Leffers, T., and Singh, B.N., 1978, Risø-M-2142 (to be published). Leffers, T., Singh, B.N., and Barlow, P., 1977, Ristf-M-1937. Mansur, L.K., Yoo, M.H., and Coglan, W.A., 1977, Trans. Am. Nucl. Soc. 7H_, 332. Singh, B.N., Leffers, T., and Barlow, P., 1978, Proceedings of f the Fifth International Conference on High Voltage Electron Microscopy, Kyoto, p. 581. Wright, P., and Evans, J.H., 1966, Phil. Mag., 13, 521. 14 Table I Apparent activation energy for loop growth (E) from the different experiments. Material Cu Cu-l%Ni Cu-2%Ni Cu-2%Ni Cu-5%Ni Cu-5%Ni Cu-10%Ni* Cu-10%Ni* Cu-10%Ni* Cu-10%Ni* E(eV) 0.35t0.02 0.34+0.03 0.34±0.01 0.36+0.01 0.34±0.02 0.33+0.02 0.4U0.01 0.45±0.02 0.45+0.02 0.44+0.02 •from Barlow and Leffers (1977) 15 10 20 30 IRRADIATION TIME (s) 40 Fig. 1. The growth of four rectangular loops in Cu-5%Ni at 421°C corrected for the effect of the tilted screen. The starting points are arbitrarily plotted with intervals of 10 seconds. 10 20 30 IRRADIATION TIME I*) Fig. 2. The growth along the stacking-fault fringes of three Frank loops in copper at 400°C corrected for the effect of the tilted screen. The starting points are arbitrarily plotted with intervals of 10 seconds. 16 Fig. 3. A number of rectangular loops in Cu-5%Ni irradiated at 480°C. The shape of the loops (distorted by the tilt of the screen) does not depend on loop size. The distance between two markers along the tilt axis of the screen (horizontal) corresponds to 0.35 pm. 17 (a) (b) Fig. 4. Two Frank loops in copper irradiated at 355°C at different stages of their growth (the tine elapsed between (a) and (b) is 53 seconds). The loops become increasingly elongated along the stacking-fault fringes with increasing size. The distance between two markers along the tilt axis of the screen (horizontal) correspond« to 0.35 pro. 13 TEMPERATURE (°C) 4 V E c .C C 1 0 500 l i N •v . ~ 450 400 350 • 1 ' - ^ C u - 1 0 % Ni <C y-Cu-2% Ni 300 1 ^ / ^ — C u - 5 % Ni Sy» ^ ^ t x*%»>^^* I 1 >s< 1 250 i O Cu A Cu-1%Ni " Cu-2%Ni 0« Cu-5% Ni ~ Cu-1% Ni z - C u ' ' 14 16 IB 20 1/kT feV1) 22 24 Fiq. 5. Logarithmic loop growth rates versus 1/kT for the various experiments. The best line for each experiment is drawn according to a least-square fit. The corresponding lines for 0.5 urn foil of Cu-10IMi from Barlow and Leffers (1977) are also shown (dotted lines without points) 

The activation energy for loop growth in Cu and in Cu-Ni alloys

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The activation energy for loop growth in Cu and in Cu-Ni alloys

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