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The effects of different microstructures on the formation of dislocation loops in aluminium during electron irradiation
Micron, 1972, 3:51-61 with III plates
51
The effects of different microstructures on the formation of dislocation loops in aluminium during electron irradiation * A. WOLFENDEN Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, U.S.A.
Manuscript received May 25, 1971
Exposure to the beam in a 200k V electron microscope caused defect clusters to form in highpurity aluminium foils. The damage was due to the direct displacement of atoms by electrons. The point defect clusters nucleated randomly and grew to form dislocation loops. The microstructure of the aluminium was varied by quenching or alloying. In high-purity foils containing previously quenched-in Frank sessile vacancy loops the subsequent electron irradiation-induced defects were seen to interact with the pre-existing defects. In 99% aluminium and in 2024 aluminium (4.5%Cu, 1.5%Mg, 0.6%Mn, balance Al) no electron-induced damage was seen. However, when the 99% aluminium specimens werefirst neutron irradiated, subsequent bombardment by electrons caused the development of visible dislocation loops. The important conclusion of this electron microscope study is that the presence and distribution of electron irradiation damage in aluminium is extremely sensitive to microstructure. L'exposition au saisceau d' un microscope glectronique de 200k V, a causg la formation de groupes de dgsauts dans les lames d'aluminium de haute puretg. Ces dlgdts ont gtd attribuables au dgplacement direct des atomes par les glectrons. Les groupes de points de ddfauts ont formd des noyaux au hasard et se sont augmentgesjusqu'gz former des boucles de dislocation. On a fait varier la microstructure de l'aluminium en le trempant ou en l'alliant. Dans des lames de haute puretg contenant des boucles de vides sessile Frank, produites en les trempant, on a observg que les dgfauts subsgquents causgs par l'irradiation glectronique rgagissaient rgciproquement sur les dgfauts prgexistants. En 99°//o aluminium et en 2024 aluminium (4,5%Cu, 1,5%Mg, 0,6% Mn, balance Al) aucun dgfaut causg par les glectrons n'a apparu. Cependant, quand les spgcimens de 99% aluminium ont gtg irradigs d'abord par des neutrons, le bombardement subsdquent par des dlectrons a causg le dgveloppement de boucles de dislocation visibles. La conclusion importante de cette gtude au microscope glectronique est que la prgsence et la distribution des dggdts de l'irradiation glectronique dans l'aluminium sont trks impressionables gz la microstructure. Bestrahlung hochedler Alumiumfolien in einem 200kV Elektronenmikroskop hatte zur Folge, dass Defektstellen in der Probe auftraten. Dies ist eine unmittelbare Folge der Atomverschiebung durch den Beschuss mit Elektronen. Die punktfb'rmigen Ifn6tchen bildeten statistisch verteilte Kristallkeime und wuchsen zu Versetzungsschleifen heran. Variationen in der Mikrostruktur des Aluminiums wurden dutch Abschrecken oder Legieren erzielt. In hochedlen Folien mit zuvor eingelOschten nichtgleitfdhigen Frankschen Leerstellenschleifen reagierten neue Elektronenstrahlungsdefekte mit den schon vorhandenen Schiiden. In 99% Aluminium und in 2024Aluminium (4,5%Cu, 1,5%Mg, 0,6%Mn, Differenz At) verursachte Elektronenbestrahlung keine Defekte. Allerdings bei einer Vorbestrahlung des 99% Aluminiums mit Neutronen bewirkte eine Nachbestrahlung mit Elektronen das Auftreten sichtbarer Versetzungsschleifen. Die wichtigste Schlussfolgerung aus dieser iibermikroskopischen Studie besteht darin, dass Bestrahlungsdefekte stark yon der Mikrostruktur abhdngen. * Research sponsored by the U.S. Atomic Energy Commission under contract with the Union Carbide Corporation.
52
WOLFENDEN
INTRODUCTION When a metal foil is electron bombarded in the beam of a high voltage electron microscope, point defects are created in the specimen if the operating voltages are greater than a threshold voltage. These defects can agglomerate to form loops visible in the microscope. Specimens of copper (Makin, 1968; Ipohorski and Spring, 1969, 1970) and aluminium (Makin, 1968) have been bombarded at voltages above 500kV to study the loop formation. Nickel foils have been studied considerably (Norris, 1970a,b, 1971a,b; Urban, 1970, 1971a,b; Urban and Wilkens, 1970, 1971). Shoaib and Segall (1970) have recently investigated the dependence of electron irradiation damage at 200kV on deviation from the Bragg angle by observing the shrinkage of small faulted vacancy loops quenched into 99.9999wt% aluminium. The present work is concerned with the variety of electron bombardment damage observable at voltages as low as 200kV in aluminium specimens with various microstructures. MATERIALS AND M E T H O D S High-purity aluminium specimens were made from 99.996wt% pure aluminium from Alcoa, subsequently three-pass zone refined. The ingot was rolled to 0.005cm thickness, recrystallization occurring readily during rolling. Some of the rolled sheet was cut into strips (12.5 × 1.5cm) for quenching to introduce Frank sessile vacancy loops. Details of the quenching procedure were given elsewhere (Wolfenden, 1970). Other materials, namely 99wt% aluminium and 2024 aluminium (4.5wt%Cu, 1.5wt%Mg, 0.6wt%Mn, balance AI), were swaged to rods and were ground to 0.24cm diameter. The 99wt% aluminium was annealed at 375°C for 30min; the rods of 2024 aluminium (a precipitation hardening alloy) were either annealed at 413°C for 2hr and furnace cooled (2024-0) or annealed at 493°C for 30min and water quenched (2024-Q). Some of the 99wt% aluminium, 2024-0 and 2024-Qspecimens were neutron irradiated to a fast ( E > 1MeV) fluence of 4 × 1020n/cm2 at 125 ° and 150°C. Details of the reactor irradiation have been reported elsewhere (Farrell, Wolfenden and King, 1971). Electron microscope foils of all the specimens were prepared by the usual electrolytic technique. The electron microscopy was done with a Hitachi 200E, operated at either 175 or 200kV. Further details of the microscope operation have been given (Wolfenden, 1971). RESULTS AND DISCUSSION
High-Purity Aluminium The general microstructure of the zone-refined aluminium (unquenched) consisted of large recrystallized grains essentially free of dislocations. After about 20min exposure to the 200kV beam in the electron microscope the radiation damage in the foil became visible (Figures la and Ib). The damage consisted of dislocation loops distributed uniformly which could be tilted in and out of contrast. At certain sizes the loops showed black-white contrast whose analysis indicates that the loops have Burgers vectors in either {111} or {110} planes (Wolfenden, 1971). At longer electron irradiation times, the loop density was > 1015 loops/cm3 and the size had increased up to about 200A. No electron damage was seen at 175kV. The threshold voltage thus appears to be between 175 and 200kV, i.e. the threshold displacement energy in
ELECTRON IRRADIATION DAMAGE IN A L U M I N I U M
53
aluminium is between 17 and 19eV, in good agreement with the experimentally determined displacement energy of about 16eV (Iseler, Dawson, Mehner and Kauffman, 1966; Simpson and Chaplin, 1969).
Quenched High-Purity Aluminium Many Frank sessile vacancy loops were observed in the as-quenched microstructure. Some of the loops had single-layer stacking faults; others had double-layer faults. These loops have been seen previously by other authors (see, for example, Edington and West, 1967). During exposure of the foil to the 200kV beam the displacement defects interacted with the pre-existing vacancy loops, as shown in Figures 2a and 2b. Often, the vacancy loops were partially removed by the beam-generated point defects (Figure 3), demonstrating that some of the latter were interstitials and were attracted to the vacancy loops. Sometimes there was a shape change of the quenched-in hexagonal vacancy loops (Figure 4). The six-sided star-shaped configuration of the loop was apparently caused by the induced motion of opposite sides (dislocation climb) of the loop towards the loop centre. The six corners of the 'star' were evidently either pinned or moving slower than the sides. Shimomura (1965) observed the formation of a single cusp during annealing of vacancy loops in quenched aluminium. He noticed that the corner of a loop near a dislocation or near another loop did not shrink in a rounded form (as did isolated originally-hexagonal loops) but conserved its original polygonal corner (see his Figure 7d). The two observations are remarkably similar, although in his case the shape of the loop was presumably due to vacancy movements during annealing in the temperature range 140-150°C, while the present change in shape presumably involved diffusion of interstitials to the loops to give dislocation climb. Removal of contrast fringes (i.e. removal of stacking fault) during the electron bombardment of quenched-in faulted loops was also seen (Figures 5a and 5b). This observation was noted usually for the originally double-faulted vacancy loops, although the unfaulting did occur in the single-faulted loops too. Edington and Smallman (1965) have estimated the stress required to produce a prismatic loop nucleus within the Frank sessile loop (i.e. to remove the stacking fault) and suggested that both thermal activation and mechanical stress (in their case--quenching stress) were necessary to reach the high unfaulting stress. In the present case, stresses in the foil could have arisen due to carbon contamination of the foil, though precautions were taken to minimize this (Wolfenden, 1971) ; no estimates of these stresses could be made. The beam heating was probably small, ,~10°C (Fisher, 1970). It is possible that the conditions of electron irradiation occurring in the foil may also have aided the stacking fault removal. In Figure 5b zones denuded of visible electron displacement damage surround the affected loops. The loops are seen both face-on and edge-on. The denuded zone probably corresponds to the capture radius of the vacancy loop for the electron irradiationinduced interstitials. This observation gives additional evidence that the electron beam-induced loops are interstitial in character. Previous authors have either suspected (Makin, 1968) or determined (Ipohorski and Spring, 1969; Urban and Wilkens, 1970) that the loops are interstitials. Apparently the electron beam-produced vacancies are less mobile than the interstitials; no evidence of their aggregation to form voids was seen at the present resolution.
54
W()I,I'ENIIEN
99wt'l l, Aluminium The annealed specimens had a recrystallized microstructure but contained many inclusions and some grown-in dislocations. In this grade of aluminium no displacement damage due to the electron beam was observed at 200kV. However, after neutron irradiation subsequent electron bombardment in the microscope caused the development of visible dislocation loops where none was previously visible. These loops formed preferentially near grain boundaries (Figure 6), precipitates (Figures 7a and 7b) and grown-in dislocations. The loops possibly denote the locations of defects introduced by the prior neutron irradiation which are too small to be resolved in the microscope but can influence subsequent loop formation. These pre-existing defects could be tiny gas bubbles (transmuted helium or hydrogen), small precipitates of transmuted silicon or tiny dislocation loops resulting from neutron irradiation damage. Thus, electron bombardment of foils may find use as a decoration technique for locating sub-microscopic defects. 2024 Aluminium The alloy was chosen so that the effects on the electron bombardment damage of a fine-grained microstructure with many precipitates could be studied. No electron irradiation damage was seen at 200kV in the 2024-0 or 2024-Q specimens, whether neutron irradiated prior to the electron bombardment or not. The precipitate spacing in the 2024 aluminium was less than the loop spacing (and hence interstitial capture radius) in the high-purity aluminium. The alloy has so many sinks for the electron beam-induced defects (vacancies and interstitials) that loops of any visible size have no chance to form. This alloy therefore resists electron irradiation damage. ACKNOWLEDGEMENTS The author is grateful to several members of the Oak Ridge National Laboratory for assistance during the course of this work. Dr. R. A. Vandermeer kindly provided the zone-refined aluminium. Dr. R. T. King and E. Bolling arranged the neutron irradiation work. The electron microscopy specimens were thinned by J. T. Houston. Discussions of the observations with Drs. K. H. G. Ashbee, K. Farrell, and .]. O. Stiegler helped greatly in the interpretation of the study. REFERENCES EDINOTON,J. W. and SMALLMAN,R. E., 1965. Faulted dislocation loops in quenched aluminium. Phil. Mag., 11: 1109-1123. EDIN6TON, J. W. and WEST, D. R., 1967. Four-layer defects in quenched aluminium. Phil. Mag., 15: 229-236. FARRELL, K., WOLFENDEN, A. and KING, R. T., 1971. The effects of preinjected gases on radiation voids in aluminium. Radiation Effects, 8:107-114. FISHER, S. B., 1970. On the temperature rise in electron irradiated foils. Radiation Eff'ects, 5 : 239-243. IPOHORSKI,M. and SPRING, M. S., 1969. Electron radiation damage in a high voltage electron microscope. Phil. Mag., 20: 937-941. IPOHORSKI, M. and SPRING, M. S., 1970. Vacancy tetrahedra in copper due to electron irradiation in the high-voltage microscope. Phil. Mag., 22 : 1279-1283. ISELER, G. W., DAWSON,H. I., MEHNER, A. S. and KAUFFMAN,J. W., 1966. Production rates of electrical resistivity in copper and aluminum induced by electron irradiation. Phy.~ Rev., 146: 468-471.
ELECTRON IRRADIATION DAMAGEIN ALUMINIUM
55
MAKIN,M.J., 1968. Electron displacement damage in copper and aluminium in a high voltage electron microscope. Phil. Mag., 18 : 637-653. NORRIS, D. I. R., 1970a. Voids in nickel electron irradiated after previous argon ion bombardment. Nature, Lond., 227: 830-831. NORRIS, D. I. R., 1970b. Dislocation loop growth in an electron irradiated thin foil. Phil. Mag., 22: 1273-1278. NORRIS,D. I. R., 1971a. The growth of voids in nickel in a high voltage electron microscope. Phil. Mag., 23: 135-152. NORRIS, D. I. R., 1971b. The dose dependence of swelling in electron irradiated nickel. Phys. Star. Sol. a, 4: KS-K8. SHIMOMUr~, Y., 1965. Annealing of secondary defects in quenched aluminum. 07. Phys. Soc. japan, 20: 965-979. SnoAm, K. A. and S~GALL,R. L., 1970. Dependence of radiation damage on deviation from the Bragg angle. Phil. Mag., 22: 1269-1272. SIMPSON, H. M. and CHAPLIN,R. L., 1969. Damage and recovery of aluminum for low-energy electron irradiations. Phys. Rev., 185: 958-961. URBAN, K., 1970. Voids in nickel after electron irradiation. Phys. Stat. Sol. a, 3: K167-K168. URBAN, K., 1971a. Growth of defect clusters in thin nickel foils during electron irradiation (I). Phys. Stat. Sol. a, 4: (in press). URBAN, K., 1971b. Growth of interstitial and vacancy agglomerates in nickel during electron irradiation. In : Proceedings of the European Conference on Voids Formed by Irradiation of Reactor Materials, University of Reading, England, British Nuclear Energy Society, (in press). URBAN, K. and WIL~ENS, M., 1970. Radiation damage in nickel in a high voltage electron microscope. In: Proceedings of the 7th International Congress of Electron Microscopy, Grenoble, France, P. Favard (ed.), Soci6t6 Fran~ais de Microscopie Electronique, Paris, 2: 217-218. URBAN, K. and WILKENS, M., 1971. Growth of defect clusters in thin nickel foils during electron irradiation (II). Phys. Stat. sol. a, (to be published). WOLFENDEN,A., 1970. Effects ofpreinjected helium on void formation in quenched aluminum. 07. Nucl. Mater., 116: 218-222. WOLFENDF.N,A., 1971. Damage in aluminum by 200kV electrons. 07. Nucl. Mater., 118: 114-115.
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FIGURES 1 and 2
Electron damage in unquenched and quenched high-purity aluminium Figure I. Electron damage in unquenched high-purity aluminium after about 20min exposure to 200kV electrons. The damage appears in the form of dislocation loops distributed randomly. (a) Electron irradiation time, 0rain. ×55,000. (b) Electron irradiation time, 22min. x 55,000.
Figure 2. Interaction of electron beam-induced displacement defects with previously quenchedin vacancy loops in quenched high-purity aluminium. (a) Electron irradiation time, 0rain. x50,000. (b) Electron irradiation time, 60rain. x 50,000.
E L E C T R O N I R R A D I A T I O N DAMAGE IN A L U M I N I U M
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58
W()I,FEN1)EN
F I G U R E S 3-5
Effects of electron irradiation on faulted vacancy loops in quenched high-purity aluminium Figure 3. An effect of electron irradiation on pre-existing vacancy loops. The hexagonal loops are 'eaten-away' by the beam-induced interstitials. × 50,000.
Figure 4. Quenched-in vacancy loops sometimes have a change in shape from hexagonal to star-shaped during electron bombardment, x 175,000.
Figure 5. Removal of contrast fringes denoting removal of the stacking fault during electron irradiation of quenched-in faulted loops. After about 40min zones denuded of visible electron displacement damage surround the loops. (a) Electron irradiation time, 8min. × 112,500. (b) Electron irradiation time, 38min. × 112,500.
ELECTRON IRRADIATION DAMAGE IN ALUMINIUM
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60
WOI,FEN DEN
F I G U R E S 6 and 7
Electron irradiation of 99% aIuminium after irradiation with neutrons Figure 6. After neutron irradiation, the electron b o m b a r d m e n t causes dislocation loops to develop at preferred sites including grain boundaries. × 56,500.
Figure 7. After neutron and electron irradiation, additional preferred sites where dislocation loops develop are at precipitates and grown-in dislocations. (a) Electron irradiation time, 0rain. × 56,500. (b) Electron irradiation time, 32min. x 56,500.
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51
The effects of different microstructures on the formation of dislocation loops in aluminium during electron irradiation * A. WOLFENDEN Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, U.S.A.
Manuscript received May 25, 1971
Exposure to the beam in a 200k V electron microscope caused defect clusters to form in highpurity aluminium foils. The damage was due to the direct displacement of atoms by electrons. The point defect clusters nucleated randomly and grew to form dislocation loops. The microstructure of the aluminium was varied by quenching or alloying. In high-purity foils containing previously quenched-in Frank sessile vacancy loops the subsequent electron irradiation-induced defects were seen to interact with the pre-existing defects. In 99% aluminium and in 2024 aluminium (4.5%Cu, 1.5%Mg, 0.6%Mn, balance Al) no electron-induced damage was seen. However, when the 99% aluminium specimens werefirst neutron irradiated, subsequent bombardment by electrons caused the development of visible dislocation loops. The important conclusion of this electron microscope study is that the presence and distribution of electron irradiation damage in aluminium is extremely sensitive to microstructure. L'exposition au saisceau d' un microscope glectronique de 200k V, a causg la formation de groupes de dgsauts dans les lames d'aluminium de haute puretg. Ces dlgdts ont gtd attribuables au dgplacement direct des atomes par les glectrons. Les groupes de points de ddfauts ont formd des noyaux au hasard et se sont augmentgesjusqu'gz former des boucles de dislocation. On a fait varier la microstructure de l'aluminium en le trempant ou en l'alliant. Dans des lames de haute puretg contenant des boucles de vides sessile Frank, produites en les trempant, on a observg que les dgfauts subsgquents causgs par l'irradiation glectronique rgagissaient rgciproquement sur les dgfauts prgexistants. En 99°//o aluminium et en 2024 aluminium (4,5%Cu, 1,5%Mg, 0,6% Mn, balance Al) aucun dgfaut causg par les glectrons n'a apparu. Cependant, quand les spgcimens de 99% aluminium ont gtg irradigs d'abord par des neutrons, le bombardement subsdquent par des dlectrons a causg le dgveloppement de boucles de dislocation visibles. La conclusion importante de cette gtude au microscope glectronique est que la prgsence et la distribution des dggdts de l'irradiation glectronique dans l'aluminium sont trks impressionables gz la microstructure. Bestrahlung hochedler Alumiumfolien in einem 200kV Elektronenmikroskop hatte zur Folge, dass Defektstellen in der Probe auftraten. Dies ist eine unmittelbare Folge der Atomverschiebung durch den Beschuss mit Elektronen. Die punktfb'rmigen Ifn6tchen bildeten statistisch verteilte Kristallkeime und wuchsen zu Versetzungsschleifen heran. Variationen in der Mikrostruktur des Aluminiums wurden dutch Abschrecken oder Legieren erzielt. In hochedlen Folien mit zuvor eingelOschten nichtgleitfdhigen Frankschen Leerstellenschleifen reagierten neue Elektronenstrahlungsdefekte mit den schon vorhandenen Schiiden. In 99% Aluminium und in 2024Aluminium (4,5%Cu, 1,5%Mg, 0,6%Mn, Differenz At) verursachte Elektronenbestrahlung keine Defekte. Allerdings bei einer Vorbestrahlung des 99% Aluminiums mit Neutronen bewirkte eine Nachbestrahlung mit Elektronen das Auftreten sichtbarer Versetzungsschleifen. Die wichtigste Schlussfolgerung aus dieser iibermikroskopischen Studie besteht darin, dass Bestrahlungsdefekte stark yon der Mikrostruktur abhdngen. * Research sponsored by the U.S. Atomic Energy Commission under contract with the Union Carbide Corporation.
52
WOLFENDEN
INTRODUCTION When a metal foil is electron bombarded in the beam of a high voltage electron microscope, point defects are created in the specimen if the operating voltages are greater than a threshold voltage. These defects can agglomerate to form loops visible in the microscope. Specimens of copper (Makin, 1968; Ipohorski and Spring, 1969, 1970) and aluminium (Makin, 1968) have been bombarded at voltages above 500kV to study the loop formation. Nickel foils have been studied considerably (Norris, 1970a,b, 1971a,b; Urban, 1970, 1971a,b; Urban and Wilkens, 1970, 1971). Shoaib and Segall (1970) have recently investigated the dependence of electron irradiation damage at 200kV on deviation from the Bragg angle by observing the shrinkage of small faulted vacancy loops quenched into 99.9999wt% aluminium. The present work is concerned with the variety of electron bombardment damage observable at voltages as low as 200kV in aluminium specimens with various microstructures. MATERIALS AND M E T H O D S High-purity aluminium specimens were made from 99.996wt% pure aluminium from Alcoa, subsequently three-pass zone refined. The ingot was rolled to 0.005cm thickness, recrystallization occurring readily during rolling. Some of the rolled sheet was cut into strips (12.5 × 1.5cm) for quenching to introduce Frank sessile vacancy loops. Details of the quenching procedure were given elsewhere (Wolfenden, 1970). Other materials, namely 99wt% aluminium and 2024 aluminium (4.5wt%Cu, 1.5wt%Mg, 0.6wt%Mn, balance AI), were swaged to rods and were ground to 0.24cm diameter. The 99wt% aluminium was annealed at 375°C for 30min; the rods of 2024 aluminium (a precipitation hardening alloy) were either annealed at 413°C for 2hr and furnace cooled (2024-0) or annealed at 493°C for 30min and water quenched (2024-Q). Some of the 99wt% aluminium, 2024-0 and 2024-Qspecimens were neutron irradiated to a fast ( E > 1MeV) fluence of 4 × 1020n/cm2 at 125 ° and 150°C. Details of the reactor irradiation have been reported elsewhere (Farrell, Wolfenden and King, 1971). Electron microscope foils of all the specimens were prepared by the usual electrolytic technique. The electron microscopy was done with a Hitachi 200E, operated at either 175 or 200kV. Further details of the microscope operation have been given (Wolfenden, 1971). RESULTS AND DISCUSSION
High-Purity Aluminium The general microstructure of the zone-refined aluminium (unquenched) consisted of large recrystallized grains essentially free of dislocations. After about 20min exposure to the 200kV beam in the electron microscope the radiation damage in the foil became visible (Figures la and Ib). The damage consisted of dislocation loops distributed uniformly which could be tilted in and out of contrast. At certain sizes the loops showed black-white contrast whose analysis indicates that the loops have Burgers vectors in either {111} or {110} planes (Wolfenden, 1971). At longer electron irradiation times, the loop density was > 1015 loops/cm3 and the size had increased up to about 200A. No electron damage was seen at 175kV. The threshold voltage thus appears to be between 175 and 200kV, i.e. the threshold displacement energy in
ELECTRON IRRADIATION DAMAGE IN A L U M I N I U M
53
aluminium is between 17 and 19eV, in good agreement with the experimentally determined displacement energy of about 16eV (Iseler, Dawson, Mehner and Kauffman, 1966; Simpson and Chaplin, 1969).
Quenched High-Purity Aluminium Many Frank sessile vacancy loops were observed in the as-quenched microstructure. Some of the loops had single-layer stacking faults; others had double-layer faults. These loops have been seen previously by other authors (see, for example, Edington and West, 1967). During exposure of the foil to the 200kV beam the displacement defects interacted with the pre-existing vacancy loops, as shown in Figures 2a and 2b. Often, the vacancy loops were partially removed by the beam-generated point defects (Figure 3), demonstrating that some of the latter were interstitials and were attracted to the vacancy loops. Sometimes there was a shape change of the quenched-in hexagonal vacancy loops (Figure 4). The six-sided star-shaped configuration of the loop was apparently caused by the induced motion of opposite sides (dislocation climb) of the loop towards the loop centre. The six corners of the 'star' were evidently either pinned or moving slower than the sides. Shimomura (1965) observed the formation of a single cusp during annealing of vacancy loops in quenched aluminium. He noticed that the corner of a loop near a dislocation or near another loop did not shrink in a rounded form (as did isolated originally-hexagonal loops) but conserved its original polygonal corner (see his Figure 7d). The two observations are remarkably similar, although in his case the shape of the loop was presumably due to vacancy movements during annealing in the temperature range 140-150°C, while the present change in shape presumably involved diffusion of interstitials to the loops to give dislocation climb. Removal of contrast fringes (i.e. removal of stacking fault) during the electron bombardment of quenched-in faulted loops was also seen (Figures 5a and 5b). This observation was noted usually for the originally double-faulted vacancy loops, although the unfaulting did occur in the single-faulted loops too. Edington and Smallman (1965) have estimated the stress required to produce a prismatic loop nucleus within the Frank sessile loop (i.e. to remove the stacking fault) and suggested that both thermal activation and mechanical stress (in their case--quenching stress) were necessary to reach the high unfaulting stress. In the present case, stresses in the foil could have arisen due to carbon contamination of the foil, though precautions were taken to minimize this (Wolfenden, 1971) ; no estimates of these stresses could be made. The beam heating was probably small, ,~10°C (Fisher, 1970). It is possible that the conditions of electron irradiation occurring in the foil may also have aided the stacking fault removal. In Figure 5b zones denuded of visible electron displacement damage surround the affected loops. The loops are seen both face-on and edge-on. The denuded zone probably corresponds to the capture radius of the vacancy loop for the electron irradiationinduced interstitials. This observation gives additional evidence that the electron beam-induced loops are interstitial in character. Previous authors have either suspected (Makin, 1968) or determined (Ipohorski and Spring, 1969; Urban and Wilkens, 1970) that the loops are interstitials. Apparently the electron beam-produced vacancies are less mobile than the interstitials; no evidence of their aggregation to form voids was seen at the present resolution.
54
W()I,I'ENIIEN
99wt'l l, Aluminium The annealed specimens had a recrystallized microstructure but contained many inclusions and some grown-in dislocations. In this grade of aluminium no displacement damage due to the electron beam was observed at 200kV. However, after neutron irradiation subsequent electron bombardment in the microscope caused the development of visible dislocation loops where none was previously visible. These loops formed preferentially near grain boundaries (Figure 6), precipitates (Figures 7a and 7b) and grown-in dislocations. The loops possibly denote the locations of defects introduced by the prior neutron irradiation which are too small to be resolved in the microscope but can influence subsequent loop formation. These pre-existing defects could be tiny gas bubbles (transmuted helium or hydrogen), small precipitates of transmuted silicon or tiny dislocation loops resulting from neutron irradiation damage. Thus, electron bombardment of foils may find use as a decoration technique for locating sub-microscopic defects. 2024 Aluminium The alloy was chosen so that the effects on the electron bombardment damage of a fine-grained microstructure with many precipitates could be studied. No electron irradiation damage was seen at 200kV in the 2024-0 or 2024-Q specimens, whether neutron irradiated prior to the electron bombardment or not. The precipitate spacing in the 2024 aluminium was less than the loop spacing (and hence interstitial capture radius) in the high-purity aluminium. The alloy has so many sinks for the electron beam-induced defects (vacancies and interstitials) that loops of any visible size have no chance to form. This alloy therefore resists electron irradiation damage. ACKNOWLEDGEMENTS The author is grateful to several members of the Oak Ridge National Laboratory for assistance during the course of this work. Dr. R. A. Vandermeer kindly provided the zone-refined aluminium. Dr. R. T. King and E. Bolling arranged the neutron irradiation work. The electron microscopy specimens were thinned by J. T. Houston. Discussions of the observations with Drs. K. H. G. Ashbee, K. Farrell, and .]. O. Stiegler helped greatly in the interpretation of the study. REFERENCES EDINOTON,J. W. and SMALLMAN,R. E., 1965. Faulted dislocation loops in quenched aluminium. Phil. Mag., 11: 1109-1123. EDIN6TON, J. W. and WEST, D. R., 1967. Four-layer defects in quenched aluminium. Phil. Mag., 15: 229-236. FARRELL, K., WOLFENDEN, A. and KING, R. T., 1971. The effects of preinjected gases on radiation voids in aluminium. Radiation Effects, 8:107-114. FISHER, S. B., 1970. On the temperature rise in electron irradiated foils. Radiation Eff'ects, 5 : 239-243. IPOHORSKI,M. and SPRING, M. S., 1969. Electron radiation damage in a high voltage electron microscope. Phil. Mag., 20: 937-941. IPOHORSKI, M. and SPRING, M. S., 1970. Vacancy tetrahedra in copper due to electron irradiation in the high-voltage microscope. Phil. Mag., 22 : 1279-1283. ISELER, G. W., DAWSON,H. I., MEHNER, A. S. and KAUFFMAN,J. W., 1966. Production rates of electrical resistivity in copper and aluminum induced by electron irradiation. Phy.~ Rev., 146: 468-471.
ELECTRON IRRADIATION DAMAGEIN ALUMINIUM
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MAKIN,M.J., 1968. Electron displacement damage in copper and aluminium in a high voltage electron microscope. Phil. Mag., 18 : 637-653. NORRIS, D. I. R., 1970a. Voids in nickel electron irradiated after previous argon ion bombardment. Nature, Lond., 227: 830-831. NORRIS, D. I. R., 1970b. Dislocation loop growth in an electron irradiated thin foil. Phil. Mag., 22: 1273-1278. NORRIS,D. I. R., 1971a. The growth of voids in nickel in a high voltage electron microscope. Phil. Mag., 23: 135-152. NORRIS, D. I. R., 1971b. The dose dependence of swelling in electron irradiated nickel. Phys. Star. Sol. a, 4: KS-K8. SHIMOMUr~, Y., 1965. Annealing of secondary defects in quenched aluminum. 07. Phys. Soc. japan, 20: 965-979. SnoAm, K. A. and S~GALL,R. L., 1970. Dependence of radiation damage on deviation from the Bragg angle. Phil. Mag., 22: 1269-1272. SIMPSON, H. M. and CHAPLIN,R. L., 1969. Damage and recovery of aluminum for low-energy electron irradiations. Phys. Rev., 185: 958-961. URBAN, K., 1970. Voids in nickel after electron irradiation. Phys. Stat. Sol. a, 3: K167-K168. URBAN, K., 1971a. Growth of defect clusters in thin nickel foils during electron irradiation (I). Phys. Stat. Sol. a, 4: (in press). URBAN, K., 1971b. Growth of interstitial and vacancy agglomerates in nickel during electron irradiation. In : Proceedings of the European Conference on Voids Formed by Irradiation of Reactor Materials, University of Reading, England, British Nuclear Energy Society, (in press). URBAN, K. and WIL~ENS, M., 1970. Radiation damage in nickel in a high voltage electron microscope. In: Proceedings of the 7th International Congress of Electron Microscopy, Grenoble, France, P. Favard (ed.), Soci6t6 Fran~ais de Microscopie Electronique, Paris, 2: 217-218. URBAN, K. and WILKENS, M., 1971. Growth of defect clusters in thin nickel foils during electron irradiation (II). Phys. Stat. sol. a, (to be published). WOLFENDEN,A., 1970. Effects ofpreinjected helium on void formation in quenched aluminum. 07. Nucl. Mater., 116: 218-222. WOLFENDF.N,A., 1971. Damage in aluminum by 200kV electrons. 07. Nucl. Mater., 118: 114-115.
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WOI,FENI)EN
FIGURES 1 and 2
Electron damage in unquenched and quenched high-purity aluminium Figure I. Electron damage in unquenched high-purity aluminium after about 20min exposure to 200kV electrons. The damage appears in the form of dislocation loops distributed randomly. (a) Electron irradiation time, 0rain. ×55,000. (b) Electron irradiation time, 22min. x 55,000.
Figure 2. Interaction of electron beam-induced displacement defects with previously quenchedin vacancy loops in quenched high-purity aluminium. (a) Electron irradiation time, 0rain. x50,000. (b) Electron irradiation time, 60rain. x 50,000.
E L E C T R O N I R R A D I A T I O N DAMAGE IN A L U M I N I U M
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W()I,FEN1)EN
F I G U R E S 3-5
Effects of electron irradiation on faulted vacancy loops in quenched high-purity aluminium Figure 3. An effect of electron irradiation on pre-existing vacancy loops. The hexagonal loops are 'eaten-away' by the beam-induced interstitials. × 50,000.
Figure 4. Quenched-in vacancy loops sometimes have a change in shape from hexagonal to star-shaped during electron bombardment, x 175,000.
Figure 5. Removal of contrast fringes denoting removal of the stacking fault during electron irradiation of quenched-in faulted loops. After about 40min zones denuded of visible electron displacement damage surround the loops. (a) Electron irradiation time, 8min. × 112,500. (b) Electron irradiation time, 38min. × 112,500.
ELECTRON IRRADIATION DAMAGE IN ALUMINIUM
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WOI,FEN DEN
F I G U R E S 6 and 7
Electron irradiation of 99% aIuminium after irradiation with neutrons Figure 6. After neutron irradiation, the electron b o m b a r d m e n t causes dislocation loops to develop at preferred sites including grain boundaries. × 56,500.
Figure 7. After neutron and electron irradiation, additional preferred sites where dislocation loops develop are at precipitates and grown-in dislocations. (a) Electron irradiation time, 0rain. × 56,500. (b) Electron irradiation time, 32min. x 56,500.
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