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Au and Ag ion irradiation effects on the carbide precipitation and Ar bubble formation in solubilized AISI 316L alloys
Nuclear Inst. and Methods in Physics Research B xxx (xxxx) xxx–xxx
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Au and Ag ion irradiation effects on the carbide precipitation and Ar bubble formation in solubilized AISI 316L alloys Mariana M. Timma, , Ítalo M. Oyarzabala, Francine Tatscha, Lívio Amarala, Paulo F.P. Fichtnera,b ⁎
a b
Instituto de Física, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS, Brazil Escola de Engenharia, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS, Brazil
ARTICLE INFO
ABSTRACT
Keywords: Ion irradiation Bubble growth Stainless steel Collision cascade Carbide precipitation
This contribution reports on transmission electron microscopy observations of Au and Ag ion irradiated complex metal alloys obtained by solution annealing treatments in AISI 316L foils. Two sets of samples are used: i) pristine solution annealed ones, and ii) solution annealed samples implanted with Ar to form a dense array of small bubbles located in a 250 nm thick surface layer. The irradiations were done in samples kept at 550 °C using 5 MeV Au and 3.5 MeV Ag accelerated ions and fluences calculated to produce damage levels equivalent to 20 and 40 dpa. The results obtained demonstrate that the irradiations cause the formation of 7 times larger cavities in the pristine samples and bubble enlargement 30% superior in Ar contained samples for Au ions as compared to the Ag case. This means that Au irradiations produce a larger number of excess vacancies than Ag at the same dpa values. This phenomenon is discussed in terms of the point defect densities produced by individual ion induced displacement cascades. In addition, for Au irradiations, we observe the formation of MC phase carbide precipitation only in samples containing Ar bubbles. This effect is discussed considering that the nucleation kinetic of the MC precipitates can be controlled by vacancy supersaturation.
1. Introduction
previously implanted with Ar, as well as control ones without Ar. During the irradiations, the targets are held at 550 °C. The ion beam energies and fluences were calculated to produce similar damage content scaled to achieve 20 and 40 displacements per atom (dpa) values. The microstructure of the samples is characterized by transmission electron microscopy (TEM) observations. The results obtained show that the microstructure evolution observed for Ag ion irradiations are also affected by the presence of Ar bubbles as previously observed for Au irradiation experiments [5]. However, even for irradiations scaled to promote the same damage level in terms of dpa values, the effects promoted by the Ag ions are distinct from those observed in similar samples irradiated with Au ions. This is discussed considering the displacement cascade density as a relevant parameter to explain the distinct microstructure routes promoted by the irradiation of each ion specie.
The comprehension of microstructural changes in materials submitted to harsh radiation environments is of increasing interest to extend the lifetime and security of the existing nuclear reactors or to develop more advanced ones [1]. Ion irradiation is an alternative tool to introduce point defects and reproduce the generation of fission products in the reactor materials [2,3]. By varying the irradiation parameters like the ion species, energy, irradiation flux and fluence, as well as the target temperature and the initial target microstructure configuration, it is possible to analyze the atomic mechanisms governing the formation and evolution of irradiation-induced extended defects and second phase precipitation at distinct irradiation conditions. The results obtained by ion irradiation experiments cannot be directly used to predict the irradiation effects in a reactor environment, but the knowledge about the atomic mechanisms governing the formation of the irradiation-induced defects is important to generalize model calculations predicting the material reliability and its lifecycle for distinct nuclear environments [4]. In the present contribution we report on the effects caused by Au and Ag ion irradiations into solution annealed AISI 316L stainless steel targets as a model case material for fuel cladding structures. Our investigations comprise samples ⁎
2. Experimental procedures In the present experiments, disks with a diameter of 3 mm were mechanically punched from 250 μm thick 316L austenitic stainless steel foils. The foils were finely polished at one side, thermally treated at 1050 °C for 2 h in high vacuum (∼2 × 10−6 mbar) and then quenched
Corresponding author. E-mail address: [email protected] (M.M. Timm).
https://doi.org/10.1016/j.nimb.2018.12.031 Received 18 July 2018; Received in revised form 5 November 2018; Accepted 12 December 2018 0168-583X/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Timm, M.M., Nuclear Inst. and Methods in Physics Research B, https://doi.org/10.1016/j.nimb.2018.12.031
Nuclear Inst. and Methods in Physics Research B xxx (xxxx) xxx–xxx
M.M. Timm et al.
Fig. 1. Ion concentration and damage profiles for (a) Ag and (b) Au ion irradiations. The hatched area extending to a depth around 250 nm represents the area of interest for TEM analysis. It is also the area within which the Ar ions were implanted.
to room temperature. This treatment removes the plastic deformations introduced by the polishing process and produces a solution annealed alloy with rather large grain sizes. A set of these samples was implanted with Ar ions using a 500 kV HVEE ion implanter (Ion Implantation Laboratory, Physics Institute, Federal University of Rio Grande do Sul – UFRGS). The implantations were performed at room temperature with energies E = 100, 200 and 430 keV and fluences Ф = 4.25 × 1014, 9.5 × 1014 and 2.5 × 1015 cm−2, respectively. This combination of energies and fluences was chosen to produce a concentration-depth plateau extending from the sample surface to a depth of ≈250 nm with an Ar content of ≈0.25 at.%. The implanted samples were submitted to a thermal annealing at 550 °C for 2 h in high vacuum to trigger the nucleation of Ar bubbles. Along with the Ar-implanted samples, solution annealed ones (without Ar) were simultaneously irradiated with Au or Ag ions. The Au irradiations were done with beam energy of 5 MeV to fluences of 5 × 1015 and 1 × 1016 cm−2 calculated to produce a damage content equivalent to ≈20 and ≈40 dpa, respectively, at the center of the Ar layer. The dpa values were calculated following the procedure from Ref. [6], applying the Kinchin-Pease approach in the SRIM code running with a displacement energy Ed = 40 eV. A second set of samples, either with or without Ar content, was irradiated with 3.5 MeV accelerated Ag ions to fluences of 1.2 × 1016 and 2.4 × 1016 cm−2, which also provide similar damage levels obtained by the Au irradiation. Both irradiation experiments were done with the samples held at a temperature of 550 °C using a 3 MeV HVEE Tandem accelerator from the same laboratory. The damage level and Ar concentration profiles for the larger irradiation fluences of Au and Ag are shown in Fig. 1. The hatched area superposed with the Ar plateau profile represents the region of interest to investigate the microstructure evolution of the samples. This is done via transmission electron microscopy (TEM) observations using plan view specimens prepared by mechanical polishing and ion milling from the unirradiated side. The TEM observations were performed using a JEM 2010
microscope operated at 200 kV with a LaB6 filament (Center for Microscopy and Microanalysis, CMM-UFRGS). Table 1 summarizes all the implantation and irradiations parameters used in this work. 3. Results Fig. 2 shows the TEM micrographs from samples irradiated with Ag ions to produce a damage level of 20 dpa and 40 dpa. All micrographs present an array of small white disks surrounded by a dark ring obtained using bright field underfocus phase contrast conditions. Using overfocus conditions the contrast is reversed, as expected for bubble or cavity structures. The bubble size evaluation was done at larger magnifications with an underfocus about 600–800 nm (insets in Fig. 2). The micrographs also show larger and darker structures with oblate or more elongated shapes. These structures correspond to second phase precipitate images obtained under bright field diffraction contrast conditions. All these microstructure features are not present in the non-irradiated samples. For both irradiation cases of 20 and 40 dpa, the samples without the Ar content (Fig. 2(a) and (b)) present a diluted distribution of white disks with a mean diameter dm ≈ 3.5 nm. These structures represent cavities resulting from the agglomeration of vacancies produced during the irradiation process. In contrast, the Ag irradiation in samples containing an initial array of Ar bubbles present a more condensed system of smaller white disks, characterized by a size distribution with dm ≈ 1.59 nm and a standard deviation σ ≈ 0.3 nm for the 20 dpa case, and by dm ≈ 1.87 nm and σ ≈ 0.62 nm for the 40 dpa case. These features result from the pre-existing Ar bubble array (mean diameter ≈ 1.3 nm) produced by the thermal annealing treatment at 550 °C for 2 h done after the Ar implantation. Hence, with respect to the bubble size distributions, the irradiation effects can be interpreted as a result from the incorporation of irradiation produced vacancies into the pre-existing bubbles, relaxing their gas pressure by slightly enlarging their sizes. Fig. 3a shows a micrograph presenting the Au irradiation effect to a damage level of ≈36 dpa into a sample without Ar content. It shows the formation of a network of extended defects (dislocations, dislocation loops, and interstitial defect clusters) and a rather diluted system of large cavities (diameters from 20 to 30 nm). Fig. 3b shows a micrograph depicting the effects of the 36 dpa Au irradiation in a sample containing the Ar plateau. It shows the presence of a condensed bubble system characterized by dm ≈ 2.23 nm and σ ≈ 0.51 nm, and by the presence of large precipitates. Au irradiations to a damage level of 20 dpa in samples containing Ar produces smaller bubbles (dm ≈ 2.17 nm) as well as smaller precipitates. The 20 dpa irradiation case in samples without Ar have not been evaluated so far.
Table 1 Implantation and irradiation parameters.
Energy (MeV) fluence (1015 at/cm2) Damage dose (dpa) Damage flux (dpa/s) Temperature (°C)
Implantation
Irradiation
Ar
Au
Ag
0.1; 0.2; 0.43 0.425; 0.95; 0.25 – – Room
5.0 5 and 10 18 and 36 1.28 × 10−3 550
3.5 12 and 24 20 and 40 0.53 × 10−3 550
2
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Fig. 2. TEM micrographs of the as-prepared AISI 316L samples irradiated with 3.5 MeV Ag ions to fluences of (a) 1.2 × 1016 at/cm2 (20 dpa) and (b) 2.4 × 1016 at/ cm2 (40 dpa). (c) TEM micrographs of the Ar-implanted AISI 316L samples irradiated with 3.5 MeV Ag ions to fluences of 1.2 × 1016 at/cm2 (20 dpa) and (d) 2.4 × 1016 at/cm2 (40 dpa). The insets show enlarged images from the bubble arrays for each irradiation case.
4. Discussions
same amount of vacancies (hence the same dpa level), the results demonstrate that the number of excess vacancies resulting from the Au irradiations must be larger than for the Ag case. A similar effect is also observed in the samples without Ar. For example, the cavity sizes resulting from the Au irradiations at 40 dpa is ≈7 times larger than the ones produced by the Ag irradiation (see Figs. 2b and 3a). The collision cascade density produced when an ion hits the target is a function of its mass [7,8]. Since Au ions have approximately twice the mass of Ag ions, the collision cascades (and sub-cascades) produced by individual Au ions are larger and denser than those produced by Ag ions. According to SRIM [9] estimations based on the Kinchin-Pease approach, individual Au ions accelerated at 5 MeV produce ≈45% more vacancies than Ag ions accelerated at 3.5 MeV. To correlate this larger number of vacancies per ion with the enlargement of the cavities or bubble sizes it is necessary to consider how the vacancies recombine with their interstitials counterparts and how the non-recombined vacancies and interstitials agglomerate forming extended interstitial defects and cavities. In ref. [10], it is suggested that the cascade collapse process, leading to the agglomeration of vacancies and interstitials, is more efficient for denser cascades, i.e., where the vacancy and interstitial supersaturation is higher. This concept can indeed explain the differences observed in the irradiation experiments using Au and Ag ions. If the cascade collapse process is more efficient for the Au ions as compared to the Ag case, the total number of excess vacancies within the irradiation process must be also larger for the Au case, thus leading to larger cavity and bubble sizes. This is observed either for the samples without Ar (Fig. 2) as well as for the samples containing the Ar bubbles (Figs. 3 and
The results comparing the Au irradiation effects in samples with and without the Ar content have been discussed in a previous contribution [5]. In this previous work the precipitate phases were characterized by Selected Area Diffraction (SAD) and Energy Dispersive X-ray Spectroscopy (EDS) measurements and also by high resolution lattice images interpretation. The results obtained demonstrate the predominance of a metal-carbon (MC) phase with a cubic structure (SG # 225) with a lattice parameter a ≈ 0.43 nm and with an atomic density ρ ≈ 5.67 g cm−3. The same precipitate phases are also produced by the Ag irradiation. Therefore, in the present contribution we will focus on the discussion of the Ag irradiation effects as compared with the corresponding Au case with respect to the bubble or cavity size evolutions (Section 4.1) and to the Ar influence on the formation of the precipitates (Section 4.2). 4.1. Size evolution of the bubble and cavity systems Fig. 4 shows how the mean bubble sizes evolve. It compares the bubbles obtained after the 20 and 40 dpa irradiation cases for Ag and Au ions. The error bars correspond to the standard error of the mean, defined as σdm = σ Ν−1/2, being σ the standard deviation from the bubble size distribution and N the number of bubbles considered in the size distribution ensemble. For the same dpa values, the bubble system resulting from the Au irradiations presents diameters ≈30% larger than the Ag irradiations. Since both irradiation experiments produce the 3
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4). In the samples implanted with Ar, however, the vacancies are trapped in the small bubbles produced before the irradiations and therefore promote their growth. This occurs because the incorporation of vacancies into bubbles is an energetically favorable situation [11–13]. 4.2. Influence of the Ar bubbles on the formation of precipitates According to our previous investigations [5], the preferential formation of MC precipitates observed in 5 MeV Au ion irradiations at 40 dpa only took place in samples containing Ar bubbles (Fig. 3). This effect was explained considering that the MC phase is atomically denser that the matrix, which implies that the nucleation and growth processes of MC precipitates requires the emission of vacancies or the absorption of interstitial atoms to adjust the volume mismatch, thus reducing the strain energy of the precipitate-matrix system. In this sense, the formation of a supersaturation of vacancies as caused by the Au irradiation increases the nucleation barrier for the precipitates, reducing their nucleation probability. In a system containing a high density of Ar bubbles, however, the vacancy supersaturation is quickly reduced because the vacancies are efficiently absorbed by bubbles, thus allowing precipitate nucleation. Furthermore, since the precipitate growth also requires vacancy production or interstitial absorption, the presence of bubbles may also facilitate the precipitate growth process. As a consequence, the formation of MC precipitates can also explain why a large density of extended interstitial defects is not observed in samples containing the irradiation induced precipitates. The same principle applies for the formation of precipitates under Ag irradiation. In this case, however, the collision cascades produced by the Ag ions result in a smaller number of excess vacancies as compared to the Au case. This is demonstrated by the observed smaller sizes of cavities and bubbles. As a consequence, we infer that a sufficiently large vacancy supersaturation to inhibit precipitate nucleation is not achieved by the Ag irradiation process. 5. Conclusions Our results demonstrate that heavy ion irradiation experiments (performed with Au and Ag ions) may lead to distinct microstructure evolution routes even when the irradiation parameters are scaled to produce the same damage levels in terms of the dpa value. The experiments are performed in two set of samples: i) solution annealed AISI 316L and ii) solution annealed AISI 316L containing a dense array of Ar bubbles produced by Ar implantation at room temperature followed by thermal annealing at 550 °C. In the samples with Ar bubbles, the irradiations with Au ions causes a ≈30% more pronounced growth of the bubbles as compared with the Ag case. In the samples without Ar bubbles, both ions species causes the formation of irradiation-induced cavities, but cavities produced by the Au ions are significantly larger. These observations demonstrate that irradiations with heavier ions produce a larger number of excess vacancies. This phenomenon is discussed considering that the point defect collapse within a displacement cascade is more effective for denser cascades as produced by heavier ions. We also investigate the formation of second phase precipitates predominantly of the MC phase. These precipitates have a negative volume mismatch and therefore their nucleation and growth processes can be affected by the excess of point defects produced during the irradiations. Consistently with the arguments based on the point defect density within the collision cascades, the experimental results show that precipitate formation only occurs in samples containing a low concentration of excess vacancies.
Fig. 3. TEM micrographs of the (a) as-prepared AISI 316L samples irradiated with 5 MeV Au ions to a fluence of 1 × 1016 at/cm2 (40 dpa) and (b) of the Arimplanted AISI 316L samples irradiated with 5 MeV Au ions to a fluence of 1 × 1016 at/cm2 (40 dpa).
Acknowledgements Fig. 4. Comparison between Ar-bubble medium sizes for Au and Ag irradiations at 20 and 40 dpa.
The authors would like acknowledge the use of the facilities from the Ion Implantation Laboratory and from the Center for Microscopy 4
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and Microanalysis – UFRGS. This study was financed by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brazil (CAPES) - Finance Code 001 and by the Conselho Nacional de Desenvolvimento Científico e Tecnológico – Brazil (CNPq).
[6] [7]
References [8]
[1] B.D. Wirth, K. Nordlund, D.G. Whyte, D. Xu, Fusion materials modeling: challenges and opportunities, MRS Bull. 36 (2011) 216–222, https://doi.org/10.1557/mrs. 2011.37. [2] O.V. Ogorodnikova, V. Gann, Simulation of neutron-induced damage in tungsten by irradiation with energetic self-ions, J. Nucl. Mater. 460 (2015) 60–71, https://doi. org/10.1016/j.jnucmat.2015.02.004. [3] L.K. Mansur, Correlation of neutron and heavy-ion damage. II. The predicted temperature shift if swelling with changes in radiation dose rate, J. Nucl. Mater. 78 (1978) 156–160, https://doi.org/10.1016/0022-3115(78)90514-7. [4] G.S. Was, Z. Jiao, E. Getto, K. Sun, A.M. Monterrosa, S.A. Maloy, O. Anderoglu, B.H. Sencer, M. Hackett, Emulation of reactor irradiation damage using ion beams, Scr. Mater. 88 (2014) 33–36, https://doi.org/10.1016/j.scriptamat.2014.06.003. [5] Í.M. Oyarzabal, M.M. Timm, W.M. Pasini, F.S.M. Oliveira, F. Tatsch, L. Amaral, P.F.P. Fichtner, Influence of Ar implantation on the precipitation in Au ion
[9] [10] [11] [12] [13]
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irradiated AISI 316L solution annealed alloy, MRS Adv. (2018), https://doi.org/10. 1557/adv.2018.414. R.E. Stoller, M.B. Toloczko, G.S. Was, A.G. Certain, S. Dwaraknath, F.A. Garner, On the use of SRIM for computing radiation damage exposure, Nucl. Instrum. Meth. Phys. Res. B 310 (2013). A.Y. Azarov, S.O. Kucheyev, A.I. Titov, P.A. Karaseov, Effect of the density of collision cascades on ion implantation damage in ZnO, J. Appl. Phys. 102 (2007), https://doi.org/10.1063/1.2801404. H.L. Heinisch, B.N. Singh, On the structure of irradiation- induced collision cascades in metals as a function of recoil energy and crystal structure, Philos. Mag. A 67 (1993) 407–424, https://doi.org/10.1080/01418619308207167. J.F. Ziegler, J.P. Biersack, Stopping and range of ions in matter: SRIM, 2003, http:// www.srim.org/. A.J.E. Foreman, B.N. Singh, The role of collision cascades and helium atoms in cavity nucleation, Radiat. Eff. Defects Solids 113 (1990) 175–194, https://doi.org/ 10.1080/10420159008213064. D.A. Porter, Phase Transform. Metals Alloys (2009), https://doi.org/10.1017/ CBO9781107415324.004. G.S. Was, Fundamentals of Radiation Materials Science, Springer, 2007. H. Schroeder, P.F.P. Fichtner, On the coarsening mechanisms of helium bubbles – Ostwald ripening versus migration and coalescence, J. Nucl. Mater. 179–181 (1991) 1007–1010, https://doi.org/10.1016/0022-3115(91)90261-5.
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Au and Ag ion irradiation effects on the carbide precipitation and Ar bubble formation in solubilized AISI 316L alloys Mariana M. Timma, , Ítalo M. Oyarzabala, Francine Tatscha, Lívio Amarala, Paulo F.P. Fichtnera,b ⁎
a b
Instituto de Física, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS, Brazil Escola de Engenharia, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS, Brazil
ARTICLE INFO
ABSTRACT
Keywords: Ion irradiation Bubble growth Stainless steel Collision cascade Carbide precipitation
This contribution reports on transmission electron microscopy observations of Au and Ag ion irradiated complex metal alloys obtained by solution annealing treatments in AISI 316L foils. Two sets of samples are used: i) pristine solution annealed ones, and ii) solution annealed samples implanted with Ar to form a dense array of small bubbles located in a 250 nm thick surface layer. The irradiations were done in samples kept at 550 °C using 5 MeV Au and 3.5 MeV Ag accelerated ions and fluences calculated to produce damage levels equivalent to 20 and 40 dpa. The results obtained demonstrate that the irradiations cause the formation of 7 times larger cavities in the pristine samples and bubble enlargement 30% superior in Ar contained samples for Au ions as compared to the Ag case. This means that Au irradiations produce a larger number of excess vacancies than Ag at the same dpa values. This phenomenon is discussed in terms of the point defect densities produced by individual ion induced displacement cascades. In addition, for Au irradiations, we observe the formation of MC phase carbide precipitation only in samples containing Ar bubbles. This effect is discussed considering that the nucleation kinetic of the MC precipitates can be controlled by vacancy supersaturation.
1. Introduction
previously implanted with Ar, as well as control ones without Ar. During the irradiations, the targets are held at 550 °C. The ion beam energies and fluences were calculated to produce similar damage content scaled to achieve 20 and 40 displacements per atom (dpa) values. The microstructure of the samples is characterized by transmission electron microscopy (TEM) observations. The results obtained show that the microstructure evolution observed for Ag ion irradiations are also affected by the presence of Ar bubbles as previously observed for Au irradiation experiments [5]. However, even for irradiations scaled to promote the same damage level in terms of dpa values, the effects promoted by the Ag ions are distinct from those observed in similar samples irradiated with Au ions. This is discussed considering the displacement cascade density as a relevant parameter to explain the distinct microstructure routes promoted by the irradiation of each ion specie.
The comprehension of microstructural changes in materials submitted to harsh radiation environments is of increasing interest to extend the lifetime and security of the existing nuclear reactors or to develop more advanced ones [1]. Ion irradiation is an alternative tool to introduce point defects and reproduce the generation of fission products in the reactor materials [2,3]. By varying the irradiation parameters like the ion species, energy, irradiation flux and fluence, as well as the target temperature and the initial target microstructure configuration, it is possible to analyze the atomic mechanisms governing the formation and evolution of irradiation-induced extended defects and second phase precipitation at distinct irradiation conditions. The results obtained by ion irradiation experiments cannot be directly used to predict the irradiation effects in a reactor environment, but the knowledge about the atomic mechanisms governing the formation of the irradiation-induced defects is important to generalize model calculations predicting the material reliability and its lifecycle for distinct nuclear environments [4]. In the present contribution we report on the effects caused by Au and Ag ion irradiations into solution annealed AISI 316L stainless steel targets as a model case material for fuel cladding structures. Our investigations comprise samples ⁎
2. Experimental procedures In the present experiments, disks with a diameter of 3 mm were mechanically punched from 250 μm thick 316L austenitic stainless steel foils. The foils were finely polished at one side, thermally treated at 1050 °C for 2 h in high vacuum (∼2 × 10−6 mbar) and then quenched
Corresponding author. E-mail address: [email protected] (M.M. Timm).
https://doi.org/10.1016/j.nimb.2018.12.031 Received 18 July 2018; Received in revised form 5 November 2018; Accepted 12 December 2018 0168-583X/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Timm, M.M., Nuclear Inst. and Methods in Physics Research B, https://doi.org/10.1016/j.nimb.2018.12.031
Nuclear Inst. and Methods in Physics Research B xxx (xxxx) xxx–xxx
M.M. Timm et al.
Fig. 1. Ion concentration and damage profiles for (a) Ag and (b) Au ion irradiations. The hatched area extending to a depth around 250 nm represents the area of interest for TEM analysis. It is also the area within which the Ar ions were implanted.
to room temperature. This treatment removes the plastic deformations introduced by the polishing process and produces a solution annealed alloy with rather large grain sizes. A set of these samples was implanted with Ar ions using a 500 kV HVEE ion implanter (Ion Implantation Laboratory, Physics Institute, Federal University of Rio Grande do Sul – UFRGS). The implantations were performed at room temperature with energies E = 100, 200 and 430 keV and fluences Ф = 4.25 × 1014, 9.5 × 1014 and 2.5 × 1015 cm−2, respectively. This combination of energies and fluences was chosen to produce a concentration-depth plateau extending from the sample surface to a depth of ≈250 nm with an Ar content of ≈0.25 at.%. The implanted samples were submitted to a thermal annealing at 550 °C for 2 h in high vacuum to trigger the nucleation of Ar bubbles. Along with the Ar-implanted samples, solution annealed ones (without Ar) were simultaneously irradiated with Au or Ag ions. The Au irradiations were done with beam energy of 5 MeV to fluences of 5 × 1015 and 1 × 1016 cm−2 calculated to produce a damage content equivalent to ≈20 and ≈40 dpa, respectively, at the center of the Ar layer. The dpa values were calculated following the procedure from Ref. [6], applying the Kinchin-Pease approach in the SRIM code running with a displacement energy Ed = 40 eV. A second set of samples, either with or without Ar content, was irradiated with 3.5 MeV accelerated Ag ions to fluences of 1.2 × 1016 and 2.4 × 1016 cm−2, which also provide similar damage levels obtained by the Au irradiation. Both irradiation experiments were done with the samples held at a temperature of 550 °C using a 3 MeV HVEE Tandem accelerator from the same laboratory. The damage level and Ar concentration profiles for the larger irradiation fluences of Au and Ag are shown in Fig. 1. The hatched area superposed with the Ar plateau profile represents the region of interest to investigate the microstructure evolution of the samples. This is done via transmission electron microscopy (TEM) observations using plan view specimens prepared by mechanical polishing and ion milling from the unirradiated side. The TEM observations were performed using a JEM 2010
microscope operated at 200 kV with a LaB6 filament (Center for Microscopy and Microanalysis, CMM-UFRGS). Table 1 summarizes all the implantation and irradiations parameters used in this work. 3. Results Fig. 2 shows the TEM micrographs from samples irradiated with Ag ions to produce a damage level of 20 dpa and 40 dpa. All micrographs present an array of small white disks surrounded by a dark ring obtained using bright field underfocus phase contrast conditions. Using overfocus conditions the contrast is reversed, as expected for bubble or cavity structures. The bubble size evaluation was done at larger magnifications with an underfocus about 600–800 nm (insets in Fig. 2). The micrographs also show larger and darker structures with oblate or more elongated shapes. These structures correspond to second phase precipitate images obtained under bright field diffraction contrast conditions. All these microstructure features are not present in the non-irradiated samples. For both irradiation cases of 20 and 40 dpa, the samples without the Ar content (Fig. 2(a) and (b)) present a diluted distribution of white disks with a mean diameter dm ≈ 3.5 nm. These structures represent cavities resulting from the agglomeration of vacancies produced during the irradiation process. In contrast, the Ag irradiation in samples containing an initial array of Ar bubbles present a more condensed system of smaller white disks, characterized by a size distribution with dm ≈ 1.59 nm and a standard deviation σ ≈ 0.3 nm for the 20 dpa case, and by dm ≈ 1.87 nm and σ ≈ 0.62 nm for the 40 dpa case. These features result from the pre-existing Ar bubble array (mean diameter ≈ 1.3 nm) produced by the thermal annealing treatment at 550 °C for 2 h done after the Ar implantation. Hence, with respect to the bubble size distributions, the irradiation effects can be interpreted as a result from the incorporation of irradiation produced vacancies into the pre-existing bubbles, relaxing their gas pressure by slightly enlarging their sizes. Fig. 3a shows a micrograph presenting the Au irradiation effect to a damage level of ≈36 dpa into a sample without Ar content. It shows the formation of a network of extended defects (dislocations, dislocation loops, and interstitial defect clusters) and a rather diluted system of large cavities (diameters from 20 to 30 nm). Fig. 3b shows a micrograph depicting the effects of the 36 dpa Au irradiation in a sample containing the Ar plateau. It shows the presence of a condensed bubble system characterized by dm ≈ 2.23 nm and σ ≈ 0.51 nm, and by the presence of large precipitates. Au irradiations to a damage level of 20 dpa in samples containing Ar produces smaller bubbles (dm ≈ 2.17 nm) as well as smaller precipitates. The 20 dpa irradiation case in samples without Ar have not been evaluated so far.
Table 1 Implantation and irradiation parameters.
Energy (MeV) fluence (1015 at/cm2) Damage dose (dpa) Damage flux (dpa/s) Temperature (°C)
Implantation
Irradiation
Ar
Au
Ag
0.1; 0.2; 0.43 0.425; 0.95; 0.25 – – Room
5.0 5 and 10 18 and 36 1.28 × 10−3 550
3.5 12 and 24 20 and 40 0.53 × 10−3 550
2
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Fig. 2. TEM micrographs of the as-prepared AISI 316L samples irradiated with 3.5 MeV Ag ions to fluences of (a) 1.2 × 1016 at/cm2 (20 dpa) and (b) 2.4 × 1016 at/ cm2 (40 dpa). (c) TEM micrographs of the Ar-implanted AISI 316L samples irradiated with 3.5 MeV Ag ions to fluences of 1.2 × 1016 at/cm2 (20 dpa) and (d) 2.4 × 1016 at/cm2 (40 dpa). The insets show enlarged images from the bubble arrays for each irradiation case.
4. Discussions
same amount of vacancies (hence the same dpa level), the results demonstrate that the number of excess vacancies resulting from the Au irradiations must be larger than for the Ag case. A similar effect is also observed in the samples without Ar. For example, the cavity sizes resulting from the Au irradiations at 40 dpa is ≈7 times larger than the ones produced by the Ag irradiation (see Figs. 2b and 3a). The collision cascade density produced when an ion hits the target is a function of its mass [7,8]. Since Au ions have approximately twice the mass of Ag ions, the collision cascades (and sub-cascades) produced by individual Au ions are larger and denser than those produced by Ag ions. According to SRIM [9] estimations based on the Kinchin-Pease approach, individual Au ions accelerated at 5 MeV produce ≈45% more vacancies than Ag ions accelerated at 3.5 MeV. To correlate this larger number of vacancies per ion with the enlargement of the cavities or bubble sizes it is necessary to consider how the vacancies recombine with their interstitials counterparts and how the non-recombined vacancies and interstitials agglomerate forming extended interstitial defects and cavities. In ref. [10], it is suggested that the cascade collapse process, leading to the agglomeration of vacancies and interstitials, is more efficient for denser cascades, i.e., where the vacancy and interstitial supersaturation is higher. This concept can indeed explain the differences observed in the irradiation experiments using Au and Ag ions. If the cascade collapse process is more efficient for the Au ions as compared to the Ag case, the total number of excess vacancies within the irradiation process must be also larger for the Au case, thus leading to larger cavity and bubble sizes. This is observed either for the samples without Ar (Fig. 2) as well as for the samples containing the Ar bubbles (Figs. 3 and
The results comparing the Au irradiation effects in samples with and without the Ar content have been discussed in a previous contribution [5]. In this previous work the precipitate phases were characterized by Selected Area Diffraction (SAD) and Energy Dispersive X-ray Spectroscopy (EDS) measurements and also by high resolution lattice images interpretation. The results obtained demonstrate the predominance of a metal-carbon (MC) phase with a cubic structure (SG # 225) with a lattice parameter a ≈ 0.43 nm and with an atomic density ρ ≈ 5.67 g cm−3. The same precipitate phases are also produced by the Ag irradiation. Therefore, in the present contribution we will focus on the discussion of the Ag irradiation effects as compared with the corresponding Au case with respect to the bubble or cavity size evolutions (Section 4.1) and to the Ar influence on the formation of the precipitates (Section 4.2). 4.1. Size evolution of the bubble and cavity systems Fig. 4 shows how the mean bubble sizes evolve. It compares the bubbles obtained after the 20 and 40 dpa irradiation cases for Ag and Au ions. The error bars correspond to the standard error of the mean, defined as σdm = σ Ν−1/2, being σ the standard deviation from the bubble size distribution and N the number of bubbles considered in the size distribution ensemble. For the same dpa values, the bubble system resulting from the Au irradiations presents diameters ≈30% larger than the Ag irradiations. Since both irradiation experiments produce the 3
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4). In the samples implanted with Ar, however, the vacancies are trapped in the small bubbles produced before the irradiations and therefore promote their growth. This occurs because the incorporation of vacancies into bubbles is an energetically favorable situation [11–13]. 4.2. Influence of the Ar bubbles on the formation of precipitates According to our previous investigations [5], the preferential formation of MC precipitates observed in 5 MeV Au ion irradiations at 40 dpa only took place in samples containing Ar bubbles (Fig. 3). This effect was explained considering that the MC phase is atomically denser that the matrix, which implies that the nucleation and growth processes of MC precipitates requires the emission of vacancies or the absorption of interstitial atoms to adjust the volume mismatch, thus reducing the strain energy of the precipitate-matrix system. In this sense, the formation of a supersaturation of vacancies as caused by the Au irradiation increases the nucleation barrier for the precipitates, reducing their nucleation probability. In a system containing a high density of Ar bubbles, however, the vacancy supersaturation is quickly reduced because the vacancies are efficiently absorbed by bubbles, thus allowing precipitate nucleation. Furthermore, since the precipitate growth also requires vacancy production or interstitial absorption, the presence of bubbles may also facilitate the precipitate growth process. As a consequence, the formation of MC precipitates can also explain why a large density of extended interstitial defects is not observed in samples containing the irradiation induced precipitates. The same principle applies for the formation of precipitates under Ag irradiation. In this case, however, the collision cascades produced by the Ag ions result in a smaller number of excess vacancies as compared to the Au case. This is demonstrated by the observed smaller sizes of cavities and bubbles. As a consequence, we infer that a sufficiently large vacancy supersaturation to inhibit precipitate nucleation is not achieved by the Ag irradiation process. 5. Conclusions Our results demonstrate that heavy ion irradiation experiments (performed with Au and Ag ions) may lead to distinct microstructure evolution routes even when the irradiation parameters are scaled to produce the same damage levels in terms of the dpa value. The experiments are performed in two set of samples: i) solution annealed AISI 316L and ii) solution annealed AISI 316L containing a dense array of Ar bubbles produced by Ar implantation at room temperature followed by thermal annealing at 550 °C. In the samples with Ar bubbles, the irradiations with Au ions causes a ≈30% more pronounced growth of the bubbles as compared with the Ag case. In the samples without Ar bubbles, both ions species causes the formation of irradiation-induced cavities, but cavities produced by the Au ions are significantly larger. These observations demonstrate that irradiations with heavier ions produce a larger number of excess vacancies. This phenomenon is discussed considering that the point defect collapse within a displacement cascade is more effective for denser cascades as produced by heavier ions. We also investigate the formation of second phase precipitates predominantly of the MC phase. These precipitates have a negative volume mismatch and therefore their nucleation and growth processes can be affected by the excess of point defects produced during the irradiations. Consistently with the arguments based on the point defect density within the collision cascades, the experimental results show that precipitate formation only occurs in samples containing a low concentration of excess vacancies.
Fig. 3. TEM micrographs of the (a) as-prepared AISI 316L samples irradiated with 5 MeV Au ions to a fluence of 1 × 1016 at/cm2 (40 dpa) and (b) of the Arimplanted AISI 316L samples irradiated with 5 MeV Au ions to a fluence of 1 × 1016 at/cm2 (40 dpa).
Acknowledgements Fig. 4. Comparison between Ar-bubble medium sizes for Au and Ag irradiations at 20 and 40 dpa.
The authors would like acknowledge the use of the facilities from the Ion Implantation Laboratory and from the Center for Microscopy 4
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and Microanalysis – UFRGS. This study was financed by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brazil (CAPES) - Finance Code 001 and by the Conselho Nacional de Desenvolvimento Científico e Tecnológico – Brazil (CNPq).
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