- Email: [email protected]
M steels under in situ ion irradiation
Journal of Nuclear Materials 448 (2014) 233–238
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Self-ordered defect structures in two model F/M steels under in situ ion irradiation D. Kaoumi ⇑, J. Adamson University of South Carolina, 300 Main St., SC 29208, USA
a r t i c l e
i n f o
Article history: Received 10 October 2013 Accepted 31 January 2014 Available online 7 February 2014
a b s t r a c t Two model F/M steels, 9Cr-model and 12Cr-model, were irradiated with 1 MeV Kr ions in situ in a TEM at temperatures between 20 K and 573 K to doses as high as 15 dpa. During the early stages of irradiation of the two F/M steels, defect clusters were rather uniformly distributed within grains, and a saturation density was quickly reached. However, at higher doses, self-ordering alignments of defect clusters were found in some grains. The regularly ordered arrays of small loops were observed in the two F/M steels along h1 1 0i directions with spacing about 30–50 nm. Once the aligned structure was created, it was stable under further irradiation. The possible mechanisms for the ‘‘self-organization’’/‘‘ordering’’ of the clusters were investigated. This paper describes the process and its temperature dependence, and the possible mechanisms are discussed. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction To help the development and optimization of advanced F/M steels of interest for structural and cladding applications in GenIV reactors, a fundamental understanding of radiation damage accumulation in such alloys is required, especially at high doses. It is essential to understand the basic mechanisms of radiation damage formation, build-up, and interactions. Dislocation structure development is one of the most important microstructure evolutions in metals under irradiation. In order to better understand the development of dislocation structure under irradiation one has to better understand the process of formation, growth and accumulation of dislocation loops. One of the difficulties of studying irradiation induced microstructure evolution is the lack of kinetics information, as most of the time studies are made ex situ i.e. after the material has been removed from the irradiation facility. For that matter, In Situ ion-irradiation in a TEM is a very useful technique as it allows for direct observation of the formation and evolution of irradiation-induced damage and the spatial correlation of the defect structures as a function of dose, temperature, and ion type/energy. Thus, in this work which was done under the Consortium on Cladding and Structural Materials (CCSM) initiative, two model F/M steels, ‘‘9Cr-model’’ and ‘‘12Cr-model’’, were custommade with the intent of reproducing the type of starting microstructure of F/M steels, but without the complications of additional
⇑ Corresponding author. E-mail address: [email protected] (D. Kaoumi). http://dx.doi.org/10.1016/j.jnucmat.2014.01.048 0022-3115/Ó 2014 Elsevier B.V. All rights reserved.
alloying elements. They were irradiated with 1 MeV Kr ions at 20 K, 50 K, 180 K, 298 K, 473 K and 573 K to doses up to 10 dpa in situ in a TEM. The microstructure evolution under irradiation was followed and characterized at successive doses using weak-beam dark-field imaging. Defect cluster formation and development was followed in the thin foils and the relationship between irradiation-induced microstructure and crystallographic orientation of the thin foils was investigated. The kinetics of the damage formation and evolution at the early doses (below 2 dpa) were reported in [1]; this paper thus reports observations done at doses higher than 2 dpa and focuses more particularly on the self-ordering of defect structures, a phenomenon observed systematically in some grains during the in situ experiments mainly at the lower temperatures. The temperature dependence and mechanisms are particularly discussed.
2. Materials and methods 2.1. Materials The ‘‘9Cr-model’’ and ‘‘12Cr-model’’ steels were fabricated at Ames Laboratory by arc melting of 99.98% purity Fe, 99.995% purity Cr, and with 99.9995% purity graphite. Their complete chemical composition is shown in Table 1. SEM micrographs of their microstructure are shown in Fig. 1 revealing a typical martensitic microstructure made of laths for the 9Cr-model steel with carbide precipitation along lath boundaries and along prior austenite grain boundaries (PAGB) and a ferritic/martensitic microstructure for
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D. Kaoumi, J. Adamson / Journal of Nuclear Materials 448 (2014) 233–238
Table 1 Material composition in wt.%. Alloy
Fe
Cr
C
O
N
P
S
9C model steel 12Cr model steel
Bal. Bal.
8.68 12.46
0.720 0.776
0.0145 0.0098
0.0023 0.0015
<0.005 <0.005
0.0012 0.0011
12Cr-model with large grains of ferrite alternating with martensitic regions, and carbides decorating the grain boundaries, especially the PAGB grain boundaries. 2.2. Irradiation experiments TEM samples were prepared by electropolishing 3 mm discs using an electrolyte of 5% perchloric acid and 95% methanol. The irradiation experiments were done at the IVEM at Argonne National Laboratory which consists of an Hitachi H-9000NAR transmission electron microscope coupled with a TANDEM accelerator. The two model alloy steels (9Cr model and 12Cr model) were then irradiated with 1 MeV Kr ions with a flux of about 9.4 1015 ion/m2 s (2.72 103 dpa/s) at 20 K, 50 K, 180 K, 298 K, 473 K, and 573 K up to doses of 10–15 dpa. For such ion flux, the temperature rise of a thin foil due to the ion beam is estimated to be far below 100 K. The specimen were tilted so that they were inclined about 15° with respect to the electron beam and 15° to the ion beam. The ion energy was selected so that implantation of Kr ions is kept at a minimum while still depositing enough energy to induce displacements. For that matter, prior to irradiation, the SRIM2010 program was run to simulate irradiations of the 9Cr model steel with Kr ions. The calculation was performed for a 1000 A thick slab irradiated with 1 MeV Kr ions at an incidence angle of 15° to reflect the geometry of the experimental set up; the run was done for 10,000 ions in full cascade calculation mode using a displacement energy threshold of 40 eV for Fe and Cr, and 28 eV for C. As a result, an average value of 2.468 1010 disp/ion m is taken for estimating doses in dpa. The microstructure evolution under irradiation was followed and characterized at successive doses in terms of defect formation and evolution using weak-beam dark-field imaging. Dark-field
micrographs were taken in 3 g (or sometimes higher order) weak beam conditions for optimum defect contrast. In addition, videos were recorded during the experiments at a frame rate of 15 fps, and the video analysis was performed using adobe premiere. Frames were extracted at an extraction frame rate of 15 fps and frames of interest were selected and organized into sequences that show the development of the defect structures as presented in Figs. 6 and 7. All other TEM pictures presented in this paper were taken after the beam was shut off for a brief moment to optimize TEM diffracting conditions again. Finally, although void/bubble formation was not expected at the temperatures and doses of interest in this paper, after each irradiation was completed, the samples were checked using through-focus TEM method and no bubble formation was detected. 3. Results During the early stages of irradiation of the two F/M steels, defect clusters appeared to be rather uniformly distributed within grains, and a saturation density is quickly reached as reported in [1]. However, with further irradiation, self-ordering alignments of defect clusters were found in some grains (as shown in Fig. 2) at doses as low as 3 dpa whereas in other grains the defect spatial distribution remained uniform. In these latter grains, as the irradiation proceeded further, the loops grew and resulted in entanglements. Indeed, Fig. 3 shows adjacent grains with different irradiation microstructures: some grains show a random distribution of loops which grew and resulted in entanglements whereas in some other grains the alignment of the loops into arrays regularly spaced is observed. All the grains in this area have similar thicknesses, which indicates that totally different microstructures develop under the
Fig. 1. SEM micrographs of 9Cr and 12Cr model F/M alloys.
Fig. 2. Defect alignment in 9Cr-model alloy irradiated at 298 K to a dose of 10 dpa imaged with a g = [1 1 0] weak-beam dark-field mode DF1 and g = [1 1 0] in DF2. Each arrow point to the same segment in the BF and DF images respectively.
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Fig. 3. Adjacent ferrite grains in 12Cr-model alloy irradiated at 298 K to a dose of 15 dpa showing different irradiation induced microstructure development.
same irradiation conditions and specimen geometry; this also shows that the crystallographic orientation of the grain plays a significant role in the development of these arrays. In the grains where the defect cluster distribution was not found to be uniform, self-organization of the defects resulted in ‘‘rows’’ or walls of defects, and at the highest doses what resembled segments of dislocations as seen in Fig. 4. These structures were not confined to the thinnest areas near the TEM whole edge as they could also be found in regions thicker areas away from the edge. Also, noteworthy is the fact that, in the F/M 12Cr-model steel, these structures developed both in the ferritic grains and the martensitic regions as shown in Fig. 5. The regularly ordered arrays of small loops typically had a spacing of about 30–50 nm and they did not necessarily form a straight line as some segment show some curvature (as seen on Fig. 4 but
Fig. 4. Dark field TEM micrograph of self-ordered irradiation induced defect structures in 12Cr-model steel irradiated at 298 K to 15 dpa (z = [0 0 1] g = [1 1 0]).
an overall direction of alignment could be determined as shown in Fig. 5 and was consistently found to be along h1 1 0i crystallographic directions. Once the aligned structure was created, it was stable under further irradiation to 15 dpa. 4. Discussion This type of microstructure development has been reported in other bcc materials [2] and more so in fcc materials [3–8]. Alignment of irradiation-induced dislocation loops was found in tungsten, in (1 0 0) oriented grains irradiated; the alignment was also parallel to the h1 1 0i direction [2]. In their Ti-modified 316 stainless steel irradiated with 400 keV Ar+ ions at 300 K and 573 K, Sekimura et al. observed defect arrays with 30–50 nm spacing for doses as low as 0.1 dpa in some grains along the foil edge. They reported a defect alignment almost along h0 0 1i directions in grains of foil normal direction around h1 1 0i [6]. Jager et al. reported the formation of periodic arrays of planar walls of defects consisting of high local concentrations of dislocations, dislocation loops and stacking-fault tetrahedra in Cu and Ni; the observed ordered structures were in the form of periodic {0 0 1} walls with a typical periodicity length of 60 nm for all equivalent {0 0 1} planes [8]. Whitley et al. observed loop patterning with spacing of 40 nm in Ni irradiated with 14 MeV Cu ions at 473 K to 7 dpa [7]; they reported that the same loop patterning with same spacing could be observed from the surface down to 3 lm in depth within the grain, showing thus that the phenomenon extended to regions beyond the damage peak and did not vary with the local dose rate. These periodic arrays of defect clusters which are stable up to high doses could affect the mechanical properties of materials under irradiation at low to intermediate temperatures. The phenomenon reported here calls for comparison with the self-organisation of dissipative non-equilibrium systems such as the formation of void lattices or the evolution of dislocation cell structures during cyclic deformation of metals. Also, although this study reports
Fig. 5. TEM micrographs of 12Cr-model F/M steel irradiated at 20 K to 11.5 dpa (z = [0 0 1]): ferrite grain (left) vs. martensitic region (right) in the same sample.
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experiments done on thin foils, defect self-organization/patterning under irradiation has been reported in bulk materials such as void patterning which has for instance thoroughly been reported and the mechanisms of which have been discussed [16]. In this work, the development of the self-ordered structures in the F/M steels was caught in situ, which allowed to follow the process as it proceeded in the foil, bringing thus further insight into the mechanisms and kinetics of this irradiation microstructure development. 4.1. In situ observations of irradiation-induced defect formation and evolution 4.1.1. On the defect cluster formation The in situ experiments allowed for dynamic observations defect cluster appearance (i.e. appearance of a white dot in dark-field mode) and sometimes disappearance during irradiation. This dynamic appearance and disappearance of defect clusters continued after a defect saturation was visibly reached for each temperature. Defect clusters were seen to jump small distances under irradiation, even at cryogenic temperatures down to 20 K but the motion would stop as soon as the ion beam was turned off. The dynamic observations of defect cluster appearance and disappearance suggests that the formation of visible defect clusters (i.e. of size larger than the experimental resolution of the technique which was between 1 and 2 nm) is caused by a cascade-driven process, in which defect clusters can be formed within cascades or by cascade overlap. This is further confirmed by the fact that at the cryogenic temperatures, the irradiation induced defect clusters did not become visible until higher doses (0.3 dpa at the earliest), compared to higher temperature irradiations where detectable defects were visible at doses as low as 0.07 dpa. Indeed, this suggests that at cryogenic temperatures a large density of small defects under the resolution limit are formed at low doses and become visible/detectable only when high-enough doses are reached allowing for the clusters to grow to a size larger than the TEM resolution limit, by cascade overlap interaction, which does not require long range diffusion. 4.1.2. On the defect motion/jumps The jump motion of clusters was observed in both alloys. However, ‘‘to-and-from hops’’ of clusters (i.e. back and forth small quick (1D) movements about the same position) often observed in ultrahigh purity iron systems [9] were more rarely seen in these model alloys and the defect jumps had a smaller rattling frequency; the jump distances in the current model alloys were also less than the jumps described in pure iron [9]. When such cluster hops occurred during irradiation, the cluster could spend a few seconds before jumping back to the previous position, suggesting that impurity trapping slows down the motion. Experimental studies [9–14] and molecular dynamics (MD) simulations [15] have confirmed that SIA clusters are able to move one-dimensionally, along some particular crystallographic orientation. The 1D migration behavior of interstitial clusters has been reported for pure metals and alloys under electron irradiation, ion irradiation, and annealing after electron irradiation. In bcc metals, this 1D migration is usually along the close packing directions of h1 1 1i, consistently with results of the MD simulations. Irradiation runs were conducted at 50 K (in both alloys) and even at 20 K (in the 12Cr-model alloy) which is below stage II for Fe–Cr systems, so that thermal diffusion of clusters should not be possible. During the experiments, cluster jumps were observed at both cryogenic temperatures but the motion would stop as soon as the ion beam was turned off. The fact that these jumps only happened when the ion beam was on suggests that the jumps are not driven by diffusion but may rather be due to cascade interactions
i.e. the overlapping of defect cascade created under the ion impact. Two theoretical mechanisms could explain this phenomenon: (i) Solute and impurity atoms pin down SIA clusters to prevent 1D motion and during irradiation if a cascade displaces the solute atom then the SIA cluster can migrate until it is pinned again by another solute or impurity atom [14]; or (ii) the cascade may induce a shock wave that changes the strain fields around the pinned SIA cluster causing it to migrate until the energy is dissipated or it is pinned again by another solute atom [15]. Such cluster motion seems essential to the development of the aligned structures reported in this study as evidenced in Fig. 6 (for a run at 298 K).
4.2. Self-ordering of defects During the early stages of irradiation of the two F/M steels, defect clusters appear to be rather uniformly distributed within grains, and the defect density quickly approached saturation as reported in [1]. As the matrix was saturated with defects, further irradiation could then result in the self-ordering of defect clusters in some grains of specific crystallographic orientation. The fact that the defect alignment described in this paper was observed only at the lower temperatures investigated (20 K, 50 K, 180 K, 273 K) in these two F/M alloys; it was not observed at 473 K and higher, which indicates that the cut-off temperature if any would be between 298 K and 473 K since the phenomenon was observed at 298 K and not 473 K (in between temperatures have not been investigated). The fact that the arrangement of defects into such structures only occurs at low temperatures suggests that a high saturation density of defects is required for such structures to form since the lower the temperature, the higher the density of cluster defects (i.e. visible defects in TEM) as reported in [1]. For the higher the temperatures, the density of defects was lower and the growth of individual loops into extended loops which ultimately show entanglement was more probable. This suggests that loop accumulation at lower temperatures and their interaction are at the origin of the defect cluster arrays. The defects tend to get together in alignment along a row but the defects from the next row exert a repulsive force, which overall results in the regular spacing observed throughout the experiments. Typically the array spacing is between 30 and 50 nm and this is also the typical spacing reported in all other studies cited in this paper regardless of the material structure (fcc or bcc). Dislocation loop segments seem to extend by addition of clusters moving in their direction. Fig. 6 shows a sequence of still pictures taken from a video recorded in situ during the 1 MeV Kr ionirradiation of 12Cr-model alloy at room temperature. The rattling motion (moving down then up) of a cluster (yellow arrow) is observed; the cluster moves down (a ? b), then up to the initial position (c ? d), then it moves up (e ? f), then up again (g ? h), and (i ? j). Then it gets close enough to the previously aligned segment and they interact attractively: the cluster now moves from right to left towards the segment (k ? l; m ? n) and finally the cluster is sucked into the segment (o ? p) contributing to its reinforcement and development. The recorded movie shows how this mechanism allows the aligned structure to develop. Fig. 7 shows a sequence of dark-field still images taken from a video of the 12Cr-model steel irradiated at 20 K recorded in situ and evidences growth, change in curvature, and merger of segments. The yellow circle shows the growth and merger of two neighbouring segments; the red circle1 highlights the gradual change of curvature of two segments enabling their merger. Such 1 For interpretation of color in Fig. 7, the reader is referred to the web version of this article.
D. Kaoumi, J. Adamson / Journal of Nuclear Materials 448 (2014) 233–238
237
Fig. 6. Sequence of still pictures taken from the video of the irradiation of 12-Cr model alloy at 298 K showing the motion of a cluster (yellow arrow) until it ultimately joins the aligned segment. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 7. Sequence of Dark Field still images taken from a video of the 12Cr-model steel irradiated at 20 K showing growth, change in curvature, and merger of segments.
processes are responsible for the final configuration of regularly spaced arrays. The corresponding video available online was accelerated 20 times. Mechanism and driving force: This structure is thought to result from elastic interactions between defect clusters in a thin foil. Indeed, the loop accumulation and their interactions may form the defect arrays. The fact that such defect alignment was not observed at higher temperatures suggests that the relatively high density of defect clusters (at lower temperatures) and the resultant internal strains may be the main reason for the development of the aligned
structure. Indeed, the arrangement of defects clusters in regular arrays could serve to minimize the stress caused by the high density of defects, which could be a driving force for the process. The defects may organize themselves along specific crystallographic orientations in order to minimize elastic interaction energy between defect clusters. Also, it is worth noting that although this study reports experiments done on thin foils, defect self-organization/patterning under irradiation has been reported in bulk materials such as void patterning which has for instance thoroughly been reported and the
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mechanisms of which have been discussed [16]. Moreover, the phenomenon reported here may be a case of Self-Organized Dislocation Structures (‘SODS’) as reported in dissipative non-equilibrium systems such as bulk metals under cyclic deformation (i.e. fatigued) [17,18]. These cases may be considered as a part of the general problem of nonlinear dynamical systems functioning in far-from-equilibrium, under which systems under irradiation should fall as they are connected to the dissipation of large quantities of energy. 5. Conclusions Two model F/M steels, 9Cr-model and 12Cr-model, were irradiated with 1 MeV Kr ions in situ in a TEM at temperatures between 20 K and 573 K to doses as high as 15 dpa. During the early stages of irradiation of the two F/M steels, defect clusters were rather uniformly distributed within grains, and the defect density quickly approached saturation. However, self-ordering alignments of defect clusters were found in some grains at higher doses. The regularly ordered arrays of small loops were observed in the two F/M steels along h1 1 0i directions with spacing about 30–50 nm. Once the aligned structure was created, it was stable under further irradiation. This structure is thought to result from elastic interactions between defect clusters in the foil. The fact that such defect alignment is not observed at higher temperatures suggests that the relatively high density of defect clusters at lower temperatures and the resultant internal strains may be the main reason for the development of the aligned structure i.e. The stress caused by a high density of loops would be minimized by the regular arrangement of defects clusters. The preferred crystallographic orientation of defect arrays may be driven by the minimization of elastic interaction energy between defect clusters. Cluster jump motion was similar to that of pure iron and other iron alloys, although the jump frequency was much lower and the jump distance much shorter in the current alloys than in pure iron. ‘‘Rattling’’ of clusters (i.e. back and forth motion along one crystallographic direction) often observed in ultra-high purity iron in specific directions was also seen in the model alloys but more rarely and with a more sluggish frequency.
Acknowledgments This work was performed under DOE-NEUP funding. The authors would like to thank Mark Kirk, Pete Baldo and Ed Ryan at ANL, and Bo-Shiuan Li at USC for assistance during the irradiation experiments and video data processing. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jnucmat.2014.0 1.048. References [1] [2] [3] [4] [5] [6] [7]
[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
D. Kaoumi, J. Adamson, M. Kirk, J. Nucl. Mater. 445 (1–3) (2014) 12–19. B. Navinsek, V. Marinkovic, M. Osredkar, G. Carter, Radiat. Eff. 3 (1970) 115. D.J. Mazey, R.S. Nelson, P.A. Thackery, J. Mater. Sci. 1 (1968) 26. K. Fukuya, H. Kawanishi, S. Ishinom, in: Proceedings of the Second Topical Meeting on Fusion Reactor Materials, 1982. J.O. Stiegler, K. Farrell, Scripta Metall. 8 (1974) 651. N. Sekimura, M. Taguchi, S. Ishino, in: Third International Conference on Fusion Reactor Materials – ICFRM-3, 4–8 October 1987, 1988, Netherlands. W. Jager, P. Ehrhart, W. Schilling, F. Dworschak, A.A. Gadalla, N. Tsukuda, in: International Conference on Vacancies and Interstitials in Metals and Alloys, 1987. J.B. Whitley, G.L. Kulcinski, P. Wilkes, H.V. Smith Jr., J. Nucl. Mater. 79 (1979) 159. M. Hernandez-Mayoral, Z. Yao, M.L. Jenkins, M.A. Kirk, Philos. Mag. 88 (2008) 2881. M. Kiritani, J. Nucl. Mater. 251 (1997) 237. H. Abe, N. Sekimura, Y. Yang, in: Proceedings of the Second IEA Fusion Materials Agreement Works, 2003. K. Arakawa, M. Hatanaka, H. Mori, K. Ono, J. Nucl. Mater. Part B 1194 (2004) 329–333. K. Arakawa, M. Hatanaka, E. Kuramoto, K. Ono, H. Mori, Phys. Rev. Lett. 96 (12) (2006) 1. Y. Satoh, H. Matsui, Philos. Mag. 89 (18) (2009) 1489. B.D. Wirth, G.R. Odette, D. Maroudas, G.E. Lucas, J. Nucl. Mater. 244 (3) (1997) 185. N.M. Ghoniem, D. Walgraef, S.J. Zinkle, J. Comp. – Aided Mater. Design 8 (2001) 1. M. Glazov, L.M. Llanes, C. Laird, Phys. Status Solidi (a) 149 (1995) 297. Z. Wang, Scripta Mater. 39 (4–5) (1998) 493.
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Self-ordered defect structures in two model F/M steels under in situ ion irradiation D. Kaoumi ⇑, J. Adamson University of South Carolina, 300 Main St., SC 29208, USA
a r t i c l e
i n f o
Article history: Received 10 October 2013 Accepted 31 January 2014 Available online 7 February 2014
a b s t r a c t Two model F/M steels, 9Cr-model and 12Cr-model, were irradiated with 1 MeV Kr ions in situ in a TEM at temperatures between 20 K and 573 K to doses as high as 15 dpa. During the early stages of irradiation of the two F/M steels, defect clusters were rather uniformly distributed within grains, and a saturation density was quickly reached. However, at higher doses, self-ordering alignments of defect clusters were found in some grains. The regularly ordered arrays of small loops were observed in the two F/M steels along h1 1 0i directions with spacing about 30–50 nm. Once the aligned structure was created, it was stable under further irradiation. The possible mechanisms for the ‘‘self-organization’’/‘‘ordering’’ of the clusters were investigated. This paper describes the process and its temperature dependence, and the possible mechanisms are discussed. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction To help the development and optimization of advanced F/M steels of interest for structural and cladding applications in GenIV reactors, a fundamental understanding of radiation damage accumulation in such alloys is required, especially at high doses. It is essential to understand the basic mechanisms of radiation damage formation, build-up, and interactions. Dislocation structure development is one of the most important microstructure evolutions in metals under irradiation. In order to better understand the development of dislocation structure under irradiation one has to better understand the process of formation, growth and accumulation of dislocation loops. One of the difficulties of studying irradiation induced microstructure evolution is the lack of kinetics information, as most of the time studies are made ex situ i.e. after the material has been removed from the irradiation facility. For that matter, In Situ ion-irradiation in a TEM is a very useful technique as it allows for direct observation of the formation and evolution of irradiation-induced damage and the spatial correlation of the defect structures as a function of dose, temperature, and ion type/energy. Thus, in this work which was done under the Consortium on Cladding and Structural Materials (CCSM) initiative, two model F/M steels, ‘‘9Cr-model’’ and ‘‘12Cr-model’’, were custommade with the intent of reproducing the type of starting microstructure of F/M steels, but without the complications of additional
⇑ Corresponding author. E-mail address: [email protected] (D. Kaoumi). http://dx.doi.org/10.1016/j.jnucmat.2014.01.048 0022-3115/Ó 2014 Elsevier B.V. All rights reserved.
alloying elements. They were irradiated with 1 MeV Kr ions at 20 K, 50 K, 180 K, 298 K, 473 K and 573 K to doses up to 10 dpa in situ in a TEM. The microstructure evolution under irradiation was followed and characterized at successive doses using weak-beam dark-field imaging. Defect cluster formation and development was followed in the thin foils and the relationship between irradiation-induced microstructure and crystallographic orientation of the thin foils was investigated. The kinetics of the damage formation and evolution at the early doses (below 2 dpa) were reported in [1]; this paper thus reports observations done at doses higher than 2 dpa and focuses more particularly on the self-ordering of defect structures, a phenomenon observed systematically in some grains during the in situ experiments mainly at the lower temperatures. The temperature dependence and mechanisms are particularly discussed.
2. Materials and methods 2.1. Materials The ‘‘9Cr-model’’ and ‘‘12Cr-model’’ steels were fabricated at Ames Laboratory by arc melting of 99.98% purity Fe, 99.995% purity Cr, and with 99.9995% purity graphite. Their complete chemical composition is shown in Table 1. SEM micrographs of their microstructure are shown in Fig. 1 revealing a typical martensitic microstructure made of laths for the 9Cr-model steel with carbide precipitation along lath boundaries and along prior austenite grain boundaries (PAGB) and a ferritic/martensitic microstructure for
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D. Kaoumi, J. Adamson / Journal of Nuclear Materials 448 (2014) 233–238
Table 1 Material composition in wt.%. Alloy
Fe
Cr
C
O
N
P
S
9C model steel 12Cr model steel
Bal. Bal.
8.68 12.46
0.720 0.776
0.0145 0.0098
0.0023 0.0015
<0.005 <0.005
0.0012 0.0011
12Cr-model with large grains of ferrite alternating with martensitic regions, and carbides decorating the grain boundaries, especially the PAGB grain boundaries. 2.2. Irradiation experiments TEM samples were prepared by electropolishing 3 mm discs using an electrolyte of 5% perchloric acid and 95% methanol. The irradiation experiments were done at the IVEM at Argonne National Laboratory which consists of an Hitachi H-9000NAR transmission electron microscope coupled with a TANDEM accelerator. The two model alloy steels (9Cr model and 12Cr model) were then irradiated with 1 MeV Kr ions with a flux of about 9.4 1015 ion/m2 s (2.72 103 dpa/s) at 20 K, 50 K, 180 K, 298 K, 473 K, and 573 K up to doses of 10–15 dpa. For such ion flux, the temperature rise of a thin foil due to the ion beam is estimated to be far below 100 K. The specimen were tilted so that they were inclined about 15° with respect to the electron beam and 15° to the ion beam. The ion energy was selected so that implantation of Kr ions is kept at a minimum while still depositing enough energy to induce displacements. For that matter, prior to irradiation, the SRIM2010 program was run to simulate irradiations of the 9Cr model steel with Kr ions. The calculation was performed for a 1000 A thick slab irradiated with 1 MeV Kr ions at an incidence angle of 15° to reflect the geometry of the experimental set up; the run was done for 10,000 ions in full cascade calculation mode using a displacement energy threshold of 40 eV for Fe and Cr, and 28 eV for C. As a result, an average value of 2.468 1010 disp/ion m is taken for estimating doses in dpa. The microstructure evolution under irradiation was followed and characterized at successive doses in terms of defect formation and evolution using weak-beam dark-field imaging. Dark-field
micrographs were taken in 3 g (or sometimes higher order) weak beam conditions for optimum defect contrast. In addition, videos were recorded during the experiments at a frame rate of 15 fps, and the video analysis was performed using adobe premiere. Frames were extracted at an extraction frame rate of 15 fps and frames of interest were selected and organized into sequences that show the development of the defect structures as presented in Figs. 6 and 7. All other TEM pictures presented in this paper were taken after the beam was shut off for a brief moment to optimize TEM diffracting conditions again. Finally, although void/bubble formation was not expected at the temperatures and doses of interest in this paper, after each irradiation was completed, the samples were checked using through-focus TEM method and no bubble formation was detected. 3. Results During the early stages of irradiation of the two F/M steels, defect clusters appeared to be rather uniformly distributed within grains, and a saturation density is quickly reached as reported in [1]. However, with further irradiation, self-ordering alignments of defect clusters were found in some grains (as shown in Fig. 2) at doses as low as 3 dpa whereas in other grains the defect spatial distribution remained uniform. In these latter grains, as the irradiation proceeded further, the loops grew and resulted in entanglements. Indeed, Fig. 3 shows adjacent grains with different irradiation microstructures: some grains show a random distribution of loops which grew and resulted in entanglements whereas in some other grains the alignment of the loops into arrays regularly spaced is observed. All the grains in this area have similar thicknesses, which indicates that totally different microstructures develop under the
Fig. 1. SEM micrographs of 9Cr and 12Cr model F/M alloys.
Fig. 2. Defect alignment in 9Cr-model alloy irradiated at 298 K to a dose of 10 dpa imaged with a g = [1 1 0] weak-beam dark-field mode DF1 and g = [1 1 0] in DF2. Each arrow point to the same segment in the BF and DF images respectively.
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Fig. 3. Adjacent ferrite grains in 12Cr-model alloy irradiated at 298 K to a dose of 15 dpa showing different irradiation induced microstructure development.
same irradiation conditions and specimen geometry; this also shows that the crystallographic orientation of the grain plays a significant role in the development of these arrays. In the grains where the defect cluster distribution was not found to be uniform, self-organization of the defects resulted in ‘‘rows’’ or walls of defects, and at the highest doses what resembled segments of dislocations as seen in Fig. 4. These structures were not confined to the thinnest areas near the TEM whole edge as they could also be found in regions thicker areas away from the edge. Also, noteworthy is the fact that, in the F/M 12Cr-model steel, these structures developed both in the ferritic grains and the martensitic regions as shown in Fig. 5. The regularly ordered arrays of small loops typically had a spacing of about 30–50 nm and they did not necessarily form a straight line as some segment show some curvature (as seen on Fig. 4 but
Fig. 4. Dark field TEM micrograph of self-ordered irradiation induced defect structures in 12Cr-model steel irradiated at 298 K to 15 dpa (z = [0 0 1] g = [1 1 0]).
an overall direction of alignment could be determined as shown in Fig. 5 and was consistently found to be along h1 1 0i crystallographic directions. Once the aligned structure was created, it was stable under further irradiation to 15 dpa. 4. Discussion This type of microstructure development has been reported in other bcc materials [2] and more so in fcc materials [3–8]. Alignment of irradiation-induced dislocation loops was found in tungsten, in (1 0 0) oriented grains irradiated; the alignment was also parallel to the h1 1 0i direction [2]. In their Ti-modified 316 stainless steel irradiated with 400 keV Ar+ ions at 300 K and 573 K, Sekimura et al. observed defect arrays with 30–50 nm spacing for doses as low as 0.1 dpa in some grains along the foil edge. They reported a defect alignment almost along h0 0 1i directions in grains of foil normal direction around h1 1 0i [6]. Jager et al. reported the formation of periodic arrays of planar walls of defects consisting of high local concentrations of dislocations, dislocation loops and stacking-fault tetrahedra in Cu and Ni; the observed ordered structures were in the form of periodic {0 0 1} walls with a typical periodicity length of 60 nm for all equivalent {0 0 1} planes [8]. Whitley et al. observed loop patterning with spacing of 40 nm in Ni irradiated with 14 MeV Cu ions at 473 K to 7 dpa [7]; they reported that the same loop patterning with same spacing could be observed from the surface down to 3 lm in depth within the grain, showing thus that the phenomenon extended to regions beyond the damage peak and did not vary with the local dose rate. These periodic arrays of defect clusters which are stable up to high doses could affect the mechanical properties of materials under irradiation at low to intermediate temperatures. The phenomenon reported here calls for comparison with the self-organisation of dissipative non-equilibrium systems such as the formation of void lattices or the evolution of dislocation cell structures during cyclic deformation of metals. Also, although this study reports
Fig. 5. TEM micrographs of 12Cr-model F/M steel irradiated at 20 K to 11.5 dpa (z = [0 0 1]): ferrite grain (left) vs. martensitic region (right) in the same sample.
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experiments done on thin foils, defect self-organization/patterning under irradiation has been reported in bulk materials such as void patterning which has for instance thoroughly been reported and the mechanisms of which have been discussed [16]. In this work, the development of the self-ordered structures in the F/M steels was caught in situ, which allowed to follow the process as it proceeded in the foil, bringing thus further insight into the mechanisms and kinetics of this irradiation microstructure development. 4.1. In situ observations of irradiation-induced defect formation and evolution 4.1.1. On the defect cluster formation The in situ experiments allowed for dynamic observations defect cluster appearance (i.e. appearance of a white dot in dark-field mode) and sometimes disappearance during irradiation. This dynamic appearance and disappearance of defect clusters continued after a defect saturation was visibly reached for each temperature. Defect clusters were seen to jump small distances under irradiation, even at cryogenic temperatures down to 20 K but the motion would stop as soon as the ion beam was turned off. The dynamic observations of defect cluster appearance and disappearance suggests that the formation of visible defect clusters (i.e. of size larger than the experimental resolution of the technique which was between 1 and 2 nm) is caused by a cascade-driven process, in which defect clusters can be formed within cascades or by cascade overlap. This is further confirmed by the fact that at the cryogenic temperatures, the irradiation induced defect clusters did not become visible until higher doses (0.3 dpa at the earliest), compared to higher temperature irradiations where detectable defects were visible at doses as low as 0.07 dpa. Indeed, this suggests that at cryogenic temperatures a large density of small defects under the resolution limit are formed at low doses and become visible/detectable only when high-enough doses are reached allowing for the clusters to grow to a size larger than the TEM resolution limit, by cascade overlap interaction, which does not require long range diffusion. 4.1.2. On the defect motion/jumps The jump motion of clusters was observed in both alloys. However, ‘‘to-and-from hops’’ of clusters (i.e. back and forth small quick (1D) movements about the same position) often observed in ultrahigh purity iron systems [9] were more rarely seen in these model alloys and the defect jumps had a smaller rattling frequency; the jump distances in the current model alloys were also less than the jumps described in pure iron [9]. When such cluster hops occurred during irradiation, the cluster could spend a few seconds before jumping back to the previous position, suggesting that impurity trapping slows down the motion. Experimental studies [9–14] and molecular dynamics (MD) simulations [15] have confirmed that SIA clusters are able to move one-dimensionally, along some particular crystallographic orientation. The 1D migration behavior of interstitial clusters has been reported for pure metals and alloys under electron irradiation, ion irradiation, and annealing after electron irradiation. In bcc metals, this 1D migration is usually along the close packing directions of h1 1 1i, consistently with results of the MD simulations. Irradiation runs were conducted at 50 K (in both alloys) and even at 20 K (in the 12Cr-model alloy) which is below stage II for Fe–Cr systems, so that thermal diffusion of clusters should not be possible. During the experiments, cluster jumps were observed at both cryogenic temperatures but the motion would stop as soon as the ion beam was turned off. The fact that these jumps only happened when the ion beam was on suggests that the jumps are not driven by diffusion but may rather be due to cascade interactions
i.e. the overlapping of defect cascade created under the ion impact. Two theoretical mechanisms could explain this phenomenon: (i) Solute and impurity atoms pin down SIA clusters to prevent 1D motion and during irradiation if a cascade displaces the solute atom then the SIA cluster can migrate until it is pinned again by another solute or impurity atom [14]; or (ii) the cascade may induce a shock wave that changes the strain fields around the pinned SIA cluster causing it to migrate until the energy is dissipated or it is pinned again by another solute atom [15]. Such cluster motion seems essential to the development of the aligned structures reported in this study as evidenced in Fig. 6 (for a run at 298 K).
4.2. Self-ordering of defects During the early stages of irradiation of the two F/M steels, defect clusters appear to be rather uniformly distributed within grains, and the defect density quickly approached saturation as reported in [1]. As the matrix was saturated with defects, further irradiation could then result in the self-ordering of defect clusters in some grains of specific crystallographic orientation. The fact that the defect alignment described in this paper was observed only at the lower temperatures investigated (20 K, 50 K, 180 K, 273 K) in these two F/M alloys; it was not observed at 473 K and higher, which indicates that the cut-off temperature if any would be between 298 K and 473 K since the phenomenon was observed at 298 K and not 473 K (in between temperatures have not been investigated). The fact that the arrangement of defects into such structures only occurs at low temperatures suggests that a high saturation density of defects is required for such structures to form since the lower the temperature, the higher the density of cluster defects (i.e. visible defects in TEM) as reported in [1]. For the higher the temperatures, the density of defects was lower and the growth of individual loops into extended loops which ultimately show entanglement was more probable. This suggests that loop accumulation at lower temperatures and their interaction are at the origin of the defect cluster arrays. The defects tend to get together in alignment along a row but the defects from the next row exert a repulsive force, which overall results in the regular spacing observed throughout the experiments. Typically the array spacing is between 30 and 50 nm and this is also the typical spacing reported in all other studies cited in this paper regardless of the material structure (fcc or bcc). Dislocation loop segments seem to extend by addition of clusters moving in their direction. Fig. 6 shows a sequence of still pictures taken from a video recorded in situ during the 1 MeV Kr ionirradiation of 12Cr-model alloy at room temperature. The rattling motion (moving down then up) of a cluster (yellow arrow) is observed; the cluster moves down (a ? b), then up to the initial position (c ? d), then it moves up (e ? f), then up again (g ? h), and (i ? j). Then it gets close enough to the previously aligned segment and they interact attractively: the cluster now moves from right to left towards the segment (k ? l; m ? n) and finally the cluster is sucked into the segment (o ? p) contributing to its reinforcement and development. The recorded movie shows how this mechanism allows the aligned structure to develop. Fig. 7 shows a sequence of dark-field still images taken from a video of the 12Cr-model steel irradiated at 20 K recorded in situ and evidences growth, change in curvature, and merger of segments. The yellow circle shows the growth and merger of two neighbouring segments; the red circle1 highlights the gradual change of curvature of two segments enabling their merger. Such 1 For interpretation of color in Fig. 7, the reader is referred to the web version of this article.
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Fig. 6. Sequence of still pictures taken from the video of the irradiation of 12-Cr model alloy at 298 K showing the motion of a cluster (yellow arrow) until it ultimately joins the aligned segment. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 7. Sequence of Dark Field still images taken from a video of the 12Cr-model steel irradiated at 20 K showing growth, change in curvature, and merger of segments.
processes are responsible for the final configuration of regularly spaced arrays. The corresponding video available online was accelerated 20 times. Mechanism and driving force: This structure is thought to result from elastic interactions between defect clusters in a thin foil. Indeed, the loop accumulation and their interactions may form the defect arrays. The fact that such defect alignment was not observed at higher temperatures suggests that the relatively high density of defect clusters (at lower temperatures) and the resultant internal strains may be the main reason for the development of the aligned
structure. Indeed, the arrangement of defects clusters in regular arrays could serve to minimize the stress caused by the high density of defects, which could be a driving force for the process. The defects may organize themselves along specific crystallographic orientations in order to minimize elastic interaction energy between defect clusters. Also, it is worth noting that although this study reports experiments done on thin foils, defect self-organization/patterning under irradiation has been reported in bulk materials such as void patterning which has for instance thoroughly been reported and the
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mechanisms of which have been discussed [16]. Moreover, the phenomenon reported here may be a case of Self-Organized Dislocation Structures (‘SODS’) as reported in dissipative non-equilibrium systems such as bulk metals under cyclic deformation (i.e. fatigued) [17,18]. These cases may be considered as a part of the general problem of nonlinear dynamical systems functioning in far-from-equilibrium, under which systems under irradiation should fall as they are connected to the dissipation of large quantities of energy. 5. Conclusions Two model F/M steels, 9Cr-model and 12Cr-model, were irradiated with 1 MeV Kr ions in situ in a TEM at temperatures between 20 K and 573 K to doses as high as 15 dpa. During the early stages of irradiation of the two F/M steels, defect clusters were rather uniformly distributed within grains, and the defect density quickly approached saturation. However, self-ordering alignments of defect clusters were found in some grains at higher doses. The regularly ordered arrays of small loops were observed in the two F/M steels along h1 1 0i directions with spacing about 30–50 nm. Once the aligned structure was created, it was stable under further irradiation. This structure is thought to result from elastic interactions between defect clusters in the foil. The fact that such defect alignment is not observed at higher temperatures suggests that the relatively high density of defect clusters at lower temperatures and the resultant internal strains may be the main reason for the development of the aligned structure i.e. The stress caused by a high density of loops would be minimized by the regular arrangement of defects clusters. The preferred crystallographic orientation of defect arrays may be driven by the minimization of elastic interaction energy between defect clusters. Cluster jump motion was similar to that of pure iron and other iron alloys, although the jump frequency was much lower and the jump distance much shorter in the current alloys than in pure iron. ‘‘Rattling’’ of clusters (i.e. back and forth motion along one crystallographic direction) often observed in ultra-high purity iron in specific directions was also seen in the model alloys but more rarely and with a more sluggish frequency.
Acknowledgments This work was performed under DOE-NEUP funding. The authors would like to thank Mark Kirk, Pete Baldo and Ed Ryan at ANL, and Bo-Shiuan Li at USC for assistance during the irradiation experiments and video data processing. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jnucmat.2014.0 1.048. References [1] [2] [3] [4] [5] [6] [7]
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