Effects of electron and ion irradiation on the dislocation structure of [001] twist boundaries in FeS alloys

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Acta metd/I. mater. Vol. 38, No.4, pp. 619-624, 1990 Printed in Great Britain. All rights reserved

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EFFECTS OF ELECTRON AND ION IRRADIATION ON THE DISLOCATION STRUCTURE OF [001] TWIST BOUNDARIES IN Fe-S ALLOYS C.-H. LIN and S. L. SASS Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853, U.S.A.

and C. W. ALLEN and L. E. REHN Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, U.S.A. (Received 4 August 1989) Abstract-Previous work has demonstrated that the dislocation structures of small and large angle [100] twist boundaries in Fe-S alloys are changed as S is implanted. It was not clear from that work, however, to what extent the radiation damage accompanying the implantation played a role in the change of boundary structure. In the present work the influence of irradiation on the dislocation structures of small angle and large angle (near L = 5) twist boundaries in the Fe~S system was studied using 80 keV S+ ions, 80 keV Ar+ ions, 1.5 and 0.8 MeV Ne+ ions and 800 keV electrons. Electron irradiation produced no change in structure of the boundary in the irradiated region. However, a neighboring region showed a change of structure which is due possibly to diffusion of S related to the high concentration of point defects produced by the electron irradiation (radiation-induced segregation). Ne+ ion irradiation also had no effect on the dislocation structure of the boundaries. With a similar mass as S, Ar+ ions create a defect structure very similar to that of implanted S+. Ar+ ion implantation caused no change in the structure of the large angle boundary, but did transform small areas of the small angle grain boundary. This is substantially different behavior from that caused by S+ ion implantation. These results lead to the conclusion that the observed change of structure as a result of implantation of S in the vicinity of [001] twist boundaries in Fe-S alloys is due primarily to the presence of the S, and that the point defects which are a product of the irradition may affect only the kinetics of the structural transformation. Resume-Des travaux anterieurs ont montre que les structures de dislocations de joints de torsion [001] a faible et grande desorientation dans les alliages Fe-S sont modifiees si I'on implante du S. Dans ces travaux, cependant, il n'etait pas precise jusqu'a quel point les degats d'irradiation accompagnant l'implantation jouaient un role dans la modification de la structure du joint. Dans cet article, on etudie l'influence de l'irradiation sur les structures de dislocations des joints de torsion (voisins de 1: = 5) de faible et de grande desorientation dans Ie systeme Fe-S en utilisant des ions S+ a 80 KeV, des ions Ar+ a 80 KeV, des ions Ne+ a 0,8 et 1,5 MeV et des electrons a 800 KeV. L'irradiation electronique ne produit pas de modification de la structure des joints dans la region irradiee. Cependant, une region voisine montre une changement de structure qui est peut etre dfl a la diffusion de S liee a la forte concentration de defauts ponctuels produite par l'irradiation electronique (segregation induite par irradiation). L'irradiation par les ions Ne+ n'a aucun effet non plus sur la structure de dislocations des joints. Avec une masse similaire :i celie de S, les ions Ar+ creent un deraut de structure tres semblable au deraut dii :i S+ implante. L'implantation par des ions Ar+ ne provoque pas de changement de la structure des joints de grande desorientation, mais transforme de petites surfaces de joints defaible desorientation. Ce comportement est tres different de celui que provoque I'implantation d'ions S+. Ces resultats permettent de conclure que la modification de structure observee par suite de l'implantation de S au voisinage de joints de torsion [001] dans les aliiages Fe-S est due principalement a la presence du S et que les derauts ponctuels qui sont Ie produit de I'irradiation peuvent affecter seulement la cinetique de la transformation structurale. Zusammenfassung-Vorausgehende Arbeiten haben gezeigt, daB sich die Versetzungstruktur von Kleinund GroBwinkel-[OOl]-Drillkorngrenzen in Fe-S-Legierungen veriindert, wenn S implantiert wird. Allerdings ist der Anteil der Implantation-Strahlenschiidigung an diesen Veriinderungen nicht klar. In dieser Arbeit wird der Einfluft der Bestrahlung auf die Versetzungsstruktur von Klein- und GroBwinkel-Drillkorngrenzen (in der Niihe von L = 5) untersucht; benutzt werden IoninS+ 80 keV, Ar+ 80 keV, Ne+ 1,5 MeV und 0,8 MeV, dazu 800 keV-Elektronen. Die Schiidigung durch elektronenbestrahlung fiihrt zu keiner Veriinderung in der Korngrenzstruktur im bestrahlten Bereich. Allerdings fand sich im benachbarten Bereich eine Strukturiinderung, die miiglicherweise auf S-Diffusion beruht, induziert durch die hohe, von der Elektronenbestrahlung erzeugte Konzentration an Pubktfehlern (bestrahlungsinduzierte Segregation). Ne+-Bestrahlung beeinflusste die Korngrenzstruktur auch nicht. Ar+ -ionen, die eine Masse iihnlich der von S haben, erzeugen eine Struktur, die der von S+ -Bestrahlung sehr iihnlich ist. Ar+ -Bestrahlung iinderte die Struktukr der GroBwinkelkorngrenze nicht, aber kleine Bereiche der Kleinwinkelkorngrenze. Dieses Verhalten unterschjeidet sich betriichtlich von demjenigen, welches durch S+-ionenimplantation ausgeliist wird. Diese Ergebnisse fiihren zu dem SchluB, daB die beobachteten Anderungen der Struktur nach Implantation von S in der Niiche von [OOl)-Drillkorngrenzen in Fe-S-Legierungen primiir von der Anwesenheit von S harriihren, und daB die Punktkfehler, produziert wiihrend der Bestrahlung, nur die Kinetik der strukturellen Umwandlung beeinflussen. AMM 38/4-F

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INTRODUCTION

Sickafus and Sass [1] showed that solute segregation to small angle [001] twist boundaries in Fe-Au alloys causes the structure of the boundary to change from a square (110) dislocation network to a square (100) dislocation network with a different Burgers vector. Lin and Sass [2] recently demonstrated that the dislocation structure of small angle and large angle (near 1: = 5) [001] twist boundaries in Fe-S alloys can be changed by implantation of S in the vicinity of the interface. The purpose of the present study is to determine whether this structural transformation is due to the presence of the implanted solute per se or to the large concentration of defects introduced during the implantation process. There are two aspects to the latter possibility: the collapse of the irradiation-induced defects to form dislocations which might be incorporated into the boundary and radiation-induced segregation of solute to boundaries. In order to separate the chemical effects from those associated with the dynamics of the irradiationinduced defects, small and large angle [001] twist boundaries were irradiated with high energy electrons and several ion species at several energies. The irradiations were carried out in the HVEMTandem Facility at Argonne National Laboratory, in which a 1.2 MV AEI high voltage electron microscope (HVEM) is interfaced to a 2 MV tandem accelerator and a 650 kV ion implanter [3]. The incorporation of the ion beam in the microscope allows examination of the evolution of boundary structures during ion irradiation. Because of the high voltage of the microscope, it is also possible to study the effects of defect-production associated with electron irradiation. EXPERIMENTAL

The bicrystals containing [001] twist boundaries were produced by the method of Sickafus and Sass [1]. Approximately 30 nm thick (DOl) single crystal films were grown by electron beam evaporation from an Fe-D.2 at. % St alloy onto cleaved single crystals of rock salt which had been covered with an - 50 nm thick epitaxial layer of NaP. The bicrystal containing the [001] twist boundary was produced by sintering together in hydrogen two (DOl) single crystal films which had been misoriented by the desired angle of twist and placed face-to-face. During sintering the specimens were held successively for 24 h at each of three temperatures (450, 400 and 350°C). The estimated S diffusion distance for this treatment is > 1 Jlm, based on diffusion measurements made at 800°C and above [4-6] and extrapolated to 450°C. It is expected, therefore, that a near equilibrium tThis will be considered the nominal composition of the bicrystal. In general the actual bicrystal composition will be different from the composition of the alloy used for the deposition.

distribution of S is established in the vicinity of the grain boundary. The NaCI and NaF were dissolved in H 2 0 leaving -60 nm thick bicrystals. In addition to the in situ observations, the bicrystal films were examined in a JEOL 1200EX electron microscope before and after irradiation. The specimen was mounted in a double-tilt heating stage and maintained at 270°C during the irradiation. An 800 keY electron beam was employed for the electron irradiation. The beam was focussed to approximately 2 Jlm diameter, and the total current of the incident beam was measured by a Faraday cup located just above the objective lens of the HVEM; the profile of the electron beam was measured (without a sample or objective aperture in place) by a second Faraday cup located in the viewing chamber. The dose rate for the electron irradiation was -5 x 10 19 cm- 2 s- 1, which corresponds to 3-4 displacement per atom (dpa) per hour. For the ion implantations, specimens were subjected to irradiation with different ion species with the microscope operated at 400 kV, below the threshold for significant electron-induced damage. The ion fluxes were maintained between 2 and 4 x 1011 cm- 2 S-I. Calculations by TRIM 89 Version 5.2 [7] show that this rate corresponds to calculated displacement rates of 2-4 dpa per hour for 80 keY Ar and S, for a displacement threshold of 17 eV. The average projected range for these ions is 35 nm. EXPERIMENTAL RESULTS

(A) Electron irradiation

Figure l(a,b) shows the dislocation structure of a small angle [001] twist boundary in an Fe-D.2 at. % S bicrystal, after 800 keY electron irradiation at 270°C to a dose of -6 x 1022 cm- 2• The dislocations are in a square network aligned along (lID); this is the same boundary structure that is present before irradiation of the alloy as well as in pure Fe [1,2]. Figure 2 shows the secondary dislocation structure in a large angle [001] twist boundary with a misorientation in the vicinity of 1: = 5 (0 = 36.9°), after irradiation up to - 1 dpa. The original dislocation structure, consisting of a square (310) network remains unchanged. In addition irradiation-induced dislocation loops are observed in the bulk of the single crystals that sandwich both small and large angle twist boundaries, and in this orientation are elongated along (100) with a length of -100 nm. The loops are expected to be in the perfect edge configuration and to be interstitial in nature [8]. The formation of the dislocation loops results from aggregation of selfinterstitials on {11O} planes to form faulted a/2(11O) loops followed by shear in the fault plane to form a /2(111) and a (100) loops [9]. The growth rate of the loops increases rapidly at first and then decreases at a later stage. The nonlinearity in the growth rate is a characteristic of the interstitial dislocation loop [8]. The electron beam generally produces only

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Fig. 1. (a,b) Fe-{).2 at.% S small angle [001] twist boundary after 800 keY electron irradiation at 270°C up to - I dpa. The g vector used is indicated on each micrograph. Projected images of dislocation loops along (100) can be seen in both welded and unwelded areas.

Frenkel pairs, whereas ion irradiation generates both Frenkel pairs and highly energetic displacement cascades. For electron irradiation the defects produced are roughly uniformly distributed throughout the thickness of the 60 nm specimens. Figure 3 shows the region near the edge of the electron-irradiated area in a small angle twist boundary, where a small region of square dislocation

network aligned along (100) is observed. It is wellknown that interstitials are generally much more mobile than are vacancies. Solutes may interact with the interstitials to form interstitial-solute complexes which then migrate to sinks, such as dislocations and grain boundaries. The change in microstructure seen in Fig. 3 may be related to S atoms being driven outward from the center of the irradiated area due to

Fig. 2. Fe---{).2 at.% S large angle near L = 5 [001] twist boundary under 800 keY electron irradiation up to -I dpa at 270°C. Images at different stages are shown after (a) 3 min; (b) 10 min; (c) 17 min; (d) 30 min.

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Fig. 3. The same specimen as in Fig. I showing a square (100) dislocation network (marked A) in a region near the edge of an electron irradiated area in Fe-Q.2 at. % S small angle [001) twist boundary. The center of the irradiated region is located towards the bottom left portion of the micrograph, but out of the field of view. a net interstitial flux resulting from the presence of a high concentration of defects in the irradiated area [10]. The alloy would then become enriched in S at the periphery of the irradiated region, favoring the formation of the (100) dislocation network relative to the (110) network that is present in low solute Fe-S alloys. Further study is needed to clarify this point.

(B) Neon irradiation Figures 4 and 5(a,b) show the dislocation structures of large angle [001] twist boundaries (near 1: = 5) after irradiation with Ne+ ions at 270°C to a

Fig. 5. (a,b) Fe-Q.2 at.% S large angle near E = 5 [001) twist boundary imaged with different g vectors after 1.5 MeV Ne+ implantation at 270°C to a dose = I x 10 15 Ne+ cm- 2. final dose of 1 x 10 15 cm -2, with accelerating voltages of 0.8 and 1.5 MV, respectively. For these energies essentially all the Ne ions penetrate the entire specimen thickness. Therefore, this type of irradiation minimizes any possible chemical effects related to implanted ions. It is seen from both Figs 4 and 5 that the original dislocation structure, consisting of a square (310) network, remains unchanged after irradiation. The number of displaced atoms produced in this manner « 0.1 dpa) is significantly less than with any of the other irradiations employed in this study due to the small displacement cross-sections for the lighter mass Ne+ ions at higher energies.

(C) Argon irradiation

Fig. 4. Fe-{).2 at. % S large angle near E = 5 [001) twist boundary after 0.8 MeV Ne+ implantation at 270°C to a dose = I x 10 15 Ne+ cm- 2.

Figure 6 shows the dislocation structure of a small angle twist boundary after being subjected to 80 keY Ar+ ion irradiation at 270°C to a final dose of 1.5 x 10 15 cm -2. More than 90% of the Ar ions are stopped within the specimen, producing an average Ar concentration of 0.2-D.3 at. % in the film. The majority of the grain boundary area remains unchanged after irradiation, but there are small regions containing a new ODD) dislocation network (see area B). Figure 7(a,b) shows the secondary dislocation structure of a near 1: = 5 boundary after it was

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Fig. 6. Fe-o.2 at.% S small angle [001] twist boundary after 80 keY Ar+ implantation at 270°C to a dose = 1.5 x 10 15 Ar+ cm- 2. Region B with (100) square dislocation network can be seen.

subjected to 80 keY Ar irradiation at 270 e to a dose of I x 10 15 Ar+ cm -2. The original dislocation structure, consisting of a square (310) network, is unchanged by the ion irradiation. Black-white contrast characteristic of small irradiation-induced defect clusters is also present. 0

Fig. 8. (a) Fe-o.2 at. % S small angle [001] twist boundary after 80 keY S+ implantation at 270°C to a dose = 1.5 x 10 15 S+ cm -2. (b) The same specimen in (a) showing the original dislocation structure in a region near the specimen grid bar, after S+ implantation.

(D) Sulfur irradiation

Figure 8 shows a region in a small angle twist boundary which was irradiated with 80 keY S ions at 270°C to a final dose of 1.5 x 10 15 cm- 2, as well as a

neighboring region, which was shadowed from the ion beam by a specimen mesh grid bar. More than 90% of the S ions were implanted within the sample, thus the increase in S content is - 0.2 at. %. The effect of S ion implantation is to change the square <1I0) dislocation network [in Fig. 8(b)] to a square (00) network [in Fig. 8(a)], as observed previously by Lin and Sass [2]. Figure 9 shows a near J; = 5 twist boundary after 80 keY S ion irradiation at 270 e to a final dose of I x 10 15 cm- 2 • The square (310) network has partially transformed to a square (210) network, again in agreement with previous observations [2]. After irradiation, specimens containing both small angle and large angle boundaries were annealed for 24 h at the irradiation temperature. No additional change in microstructure was observed. The changes 0

Fig. 7. (a,b) Fe-{).2 at.% S large angle near 1: = 5 [001] twist boundary after 80 keY Ar+ implantation at 270°C to a dose = 1 x 10 15 Ar+ cm -2. The g vector used is indicated on each micrograph.

in structure seen in Figs 8 and 9 are consistent with those observed in bicrystal specimens having higher

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EFFECTS OF IRRADIATION ON DISLOCATION STRUCTURE with S+ implantation for which large areas of grain boundary change structure during irradiation. The origin of the small change in structure of the small angle boundary as a result of the implantation of Ar+ is unknown at present and may be due to S enrichment which was produced by the formation and segregation of interstitial-S clusters. These results, together with the observation that bicrystals of higher initial S content exhibit the same structure as those of lower initial S content which have been implanted with S, demonstrate that the change in the grain boundary dislocation structure during implantation is due to the chemical effects of the presence of S, and not to the large concentrations of point defects introduced during implantation. CONCLUSIONS

Fig. 9. Fe--{).2 at. % S large angle near 1: = 5 [001) twist boundary after 80 keY S+ implantation at 270°C to a dose = I x 10 15 S+ cm- 2 • S content than the nominal 0.2 at. % S composition, and which were not ion implanted. DISCUSSION Irradiation experiments were carried out at

270°C using 80 keY S+ ions, 80 keY Ar+ ions, 1.5 and 0.8 MeV Ne+ ions, and 800 keY electrons. In the case of electron irradiation, isolated vacancyinterstitial pairs are generated. In the region undergoing irradiation, dislocation loops were generated in both the top and bottom crystals. However, no transformation of the grain boundary dislocation structure was observed in the electron-irradiated region. These observations demonstrate that the presence of a high point defect concentration alone will not cause a change in the boundary structure in the Fe-S alloys studied, though as seen in Fig. 3 solute transport due to a gradient in point defect concentration may play a role in the kinetics of the structural transformation. For the 1.5 and 0.8 MV Ne+ ion implantations which produce defects while leaving little, if any, Ne in the specimen, the boundary structures again show no indication of change. When irradiating with Ar+ ions, which have a mass similar to that of S, displacement damage resembling that of S+ ions occurs. The observations showed that the Ar+ implantation produces no change in the structure of the large angle boundary, but does transform very small regions in the small angle grain boundary. For equivalent doses, this behavior is quite different from that observed

The present work has shown that the structural transformation in [001] twist boundaries in Fe-S alloys observed by Lin and Sass [2] after S implantation, is due to the chemical effects of the implant. Non-equilibrium effects related to the transport of solute due to the presence of point defect fluxes in the vicinity of the implanted region may playa role in the kinetics of the transformation of the boundary structure. Acknowledgements-This research was supported by U.S. Department of Energy, Basic Energy Sciences-Materials Sciences, under grant DE-FG02-85ER45211 (Cornell University) and contract W-31-109-Eng-38 (Argonne National Laboratory). One of the authors (C.-H. L.) would like to thank Dr Paul Okamoto for his invaluable suggestions on the electron irradiation experiment and'is grateful to Argonne for a Thesis Parts Appointment. Discussions with Professor D. N. Seidman in the early part of this work were greatly appreciated. The authors are indebted to Loren Funk and Edward Ryan for assistance with the ion irradiations at the HVEM-Tandem Facility and to the Cornell Materials Science Center Electron Microscopy Facility.

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