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Nanoindentation hardness and its extrapolation to bulk-equivalent hardness of F82H steels after single- and dual-ion beam irradiation
Journal of Nuclear Materials xxx (2013) xxx–xxx
Contents lists available at SciVerse ScienceDirect
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Nanoindentation hardness and its extrapolation to bulk-equivalent hardness of F82H steels after single- and dual-ion beam irradiation Y. Takayama a, R. Kasada b,⇑, Y. Sakamoto a, K. Yabuuchi a, A. Kimura b, M. Ando c, D. Hamaguchi c, H. Tanigawa c a b c
Graduate School of Energy Science, Kyoto University, Japan Institute of Advanced Energy, Kyoto University, Kyoto, Japan IFERC, JAEA, Rokkasho, Aomori, Japan
a r t i c l e
i n f o
Article history: Available online xxxx
a b s t r a c t The irradiation hardening behavior of reduced-activation ferritic steels after single Fe-ion beam irradiation and dual-ion (Fe ion and He ion) beam irradiation experiments was investigated with nanoindentation tests. The ion-irradiation experiments were conducted at 563 K with 6.4 MeV Fe3+ ions up to 3 dpa at a 600 nm depth from the irradiated surface. Furthermore, these experiments were conducted with and without simultaneous energy-degraded 1 MeV He+ ions up to 300 appm. The materials used were F82H, F82H + 1Ni, and F82H + 2Ni to investigate the effect of Ni addition on the irradiation hardening behavior. The measured nanoindentation hardness was converted to the bulk-equivalent hardness based on a combination of the Nix–Gao model to explain the indentation size effect and the composite hardness model to explain the softer substrate effect of the nonirradiated region beyond the irradiated depth range. It is clearly shown that the Ni addition enhances the irradiation hardening of F82H. The bulkequivalent hardness is compared with the experimentally obtained Vickers hardness of F82H steels after neutron irradiation. The effect of simultaneously implanted helium on the irradiation hardening is negligible in the investigated irradiation conditions. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction To simulate fusion neutron irradiation damage of the structural materials, heavy ion irradiation experiments have been used because of the simplicity of use, easier control of irradiation parameters, reduction of cost, rapid damage production, the absence of induced radioactivity, and the occurrence of the co-implantation of helium/hydrogen [1–3]. However, heavy ion irradiations also have problems, such as a different recoil spectrum compared to neutrons, inhomogeneous and shallow damage profiles, and additional atoms implanted as ions. Nanoindentation is essential to evaluate the mechanical properties in the shallow depth area [4,5]. However, the correlation between nanoindentation hardness and other ‘‘bulk’’ mechanical properties should be established for further applications. There are few reports on the extrapolation of the nanoindentation hardness of the ion-irradiated materials to their bulk mechanical properties [6–8]. Ando et al. suggested an empirical relationship between nanoindentation hardness and the Vickers hardness of the neutron-irradiated steels to assess ion-irradiated materials [6]. Shin et al. calculated a dosedependent yield stress from the load/depth profiles for an ion⇑ Corresponding author. Tel.: +81 774 38 3483. E-mail address: [email protected] (R. Kasada).
irradiated Fe–9Cr alloy using a finite element model [7]. More recently, Kasada et al. have suggested a new model to extrapolate the experimentally obtained nanoindentation hardness to the bulk-equivalent hardness of ion-irradiated Fe-based binary model alloys [8]. This model is based on a combination of the Nix–Gao model [9] for the indentation size effect (ISE) and a composite hardness model for the softer substrate effect (SSE) of the nonirradiated region beyond the irradiation range. The preliminary results for the reduced-activation ferritic F82H steels have also indicated the existence of ISE and SSE after ion-irradiation [10]. In the present paper, depth profiles of the nanoindentation hardness are precisely measured for F82H and its variations containing 1% or 2% Ni after single Fe-ion beam irradiation and dual-ion (Fe ion and He ion) beam irradiation experiments. The objective is to determine whether there is increased hardening in the steels implanted with helium compared to the steels without helium. In addition, the effect of Ni addition on the irradiation hardening is examined to compare the extrapolated Vickers hardness with the Vickers hardness of neutron-irradiated steels. The reduced-activation ferritic steels are basically intended to be high purity steel as it has to achieve reduction of radiation activation, but the increase of impurity is inevitable when the steel is going to be made at the industrial level. P, S, Cu and Ni are the typical impurities which have to be expected to be increased [11]. Among
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Y. Takayama et al. / Journal of Nuclear Materials xxx (2013) xxx–xxx
these elements, Ni is one of the elements which could increase the as-prepared properties but could induce extra irradiation hardening.
2. Experimental procedure
Fig. 1. Depth profiles of the displacement damage and the implanted helium calculated for the investigated irradiation conditions.
The materials used in the present study were F82H (nominal compositions: Fe–8Cr–2W–0.2V–0.05Ta–0.1C) and its variations with 1 or 2 wt.% Ni addition (F82H + 1Ni, 2Ni). Both Ni-free and Ni-added steels were normalized at 1313 K for 30 min followed by air cooling, and tempered at 1013 K for 60 min followed by air cooling. The microstructures of the steels are a tempered martensite of which prior-austenite grain size is seen to be about 100 lm. These steels were simultaneously irradiated with a single-ion beam of 6.4 MeV Fe3+ from a 1.7 MV tandem accelerator or a dual-ion beam of 6.4 MeV Fe3+ with a simultaneous implantation of energy-degraded 1 MeV He+ ions from a 1 MV Singletron accelerator in the DuET (a dual-ion beam accelerator facility) [2]. The irradiation temperature was well controlled at 563 K within ±10 K by monitoring with infrared thermal vision. Depth profiles of the displacement damage and the implanted helium were calculated by the TRIM-98 code [12], as shown in Fig. 1. The nominal displacement damage rate, nominal displacement damage, and
Fig. 2. Indentation-depth dependence of the nanoindentation hardness of F82H (a) nonirradiated, (b) single-ion beam irradiated, and (c) dual-ion beam irradiated.
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Fig. 3. Indentation-depth dependence of the averaged nanoindentation hardness for ten indents on each specimen of (a) F82H, (b) F82H + 1Ni, and (c) F82H + 2Ni.
nominal concentration of helium were 3 104 dpa/s, 3 dpa, and 300 appm, respectively. The nominal values stand for the ones at approximately 600 nm below the irradiated surface; the nominal displacement damage is also close to the averaged value for the entire damaged depth. The ratio of 100 appm He/dpa is relatively high for the fusion reactor blanket condition but is close to the spallation condition. The nanoindentation hardness H was measured by using a Nano Indenter G200 (Agilent Technologies) with a Berkovichtype indentation tip. The continuous stiffness measurement (CSM) was used to continuously obtain the hardness (H) vs. depth (h) profile up to a depth of about 1000 nm. In comparison with the loading-unloading method used in the previous studies [10], the CSM has a great advantage to obtain the depth profile only with a single indent. The hardness at a specific depth was also calculated by averaging the hardness obtained from ten indents using the Analyst™ software. The distance between indentations was about 50 lm. The calibration of the bluntness of the indentation tip is based on the Oliver–Pharr method [13]. The Vickers hardness Hv0.1 of nonirradiated steels was measured with a load of 0.1 kgf.
3. Results and discussion 3.1. ISE and SSE on nanoindentation hardness Fig. 2 shows typical indentation-depth profiles of nanoindentation hardness of F82H before and after the single-ion irradiation. A particular plot symbol represents data of a single indent. Even for the nonirradiated F82H steel, the depth dependence of the hardness is relatively complicated and deviates in comparison with the previous model alloys [8]; this deviation is likely due to many kinds of inhomogeneous microstructural features, such as laths, packets, dislocations, and carbides, in the tempered martensitic structure of F82H. Beyond the complexity, however, the increase in hardness with decreasing indent size or depth (i.e., an ISE) is clearly observed for the nonirradiated F82H. After the ionirradiations, the nanoindentation hardness increases at the depth region measured. To emphasize the depth dependence of hardness, Fig. 3 gives the average of all indents with error bars. To explain the ISE, Nix and Gao developed a model based on the concept of geometrically necessary dislocation [9]. The Nix–Gao model predicts the hardness depth profile as follows:
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Y. Takayama et al. / Journal of Nuclear Materials xxx (2013) xxx–xxx
Fig. 4. Plots of H2 vs. 1/h for the averaged nanoindentation hardness of (a) F82H, (b) F82H + 1Ni, and (c) F82H + 2Ni.
H ¼ H0
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 þ ðh =hÞ
ð1Þ
where H0 is the hardness at infinite depth (i.e., bulk-equivalent hardness), and h is a characteristic length that depends on the material and the shape of the indenter tip. The averaged nanoindentation hardness data is plotted as H2 vs. 1/h in Fig. 4. The nonirradiated F82H steels have a good linearity in the range of h > 100 nm. However, the irradiated steels appear to have a bilinearity with a shoulder at around 300–500 nm. As reported in the previous study [8], the bilinear behavior is due to the SSE of the nonirradiated depth area beneath the irradiated depth area. Here it is assumed that the bulk-equivalent hardness of ion-irradiated materials can be obtained from the shallower depth region before the shoulder based on the composite hardness model [8]. Table 1 includes H0 extrapolated from the least squares fitting of hardness data to Eq. (1) in the range of 100 nm < h < 500 nm for non-irradiated samples and in the range of 100 nm < h < 200 nm for the irradiated samples. It should be pointed out that Pharr et al. have recently reported an artificial problem when the CSM technique is used to measure the hardness of high modulus/hardness materials at small depths
Table 1 The H0 and h, and experimentally obtained Hv0.1exp for F82H steels. Materials
H0 (GPa)
2 Hvexp 0:1 (kgf/mm )
Hvconv 0:1 (GPa)
F82H Unirr. 3 dpa 3 dpa + 300 appm He
2.45 ± 0.03 2.66 ± 0.07 3.14 ± 0.06
236 ± 2 – –
2.49 ± 0.02 – –
F82H + 1Ni Unirr. 3 dpa 3 dpa + 300 appm He
2.26 ± 0.06 4.18 ± 0.05 4.32 ± 0.06
241 ± 7 – –
2.55 ± 0.07 – –
F82H + 2Ni Unirr. 3 dpa 3 dpa + 300 appm He
2.60 ± 0.04 5.17 ± 0.08 4.48 ± 0.13
257 ± 7 – –
2.72 ± 0.07 – –
[14,15]. Despite the fact that homogeneous materials were used, they showed a bilinear-like behavior when H2 vs. 1/h was plotted. Such behavior was also found in nonirradiated tungsten by the CSM [16]. Therefore, it is questionable whether the bilinear behavior in the ion-irradiated F82H steels causes the SSE and/or the
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300 appm with 3 dpa at 563 K. Kimura et al. suggested that the highly resistant properties of the steels against helium embrittlement are due to high trapping capacity for helium atoms in the martensitic structure [22]. As summarized by Yamamoto et al. [23], however, extensively high levels of helium may contribute extra irradiation hardening beyond that due to displacement damage alone. Ando et al. indicated that extra irradiation hardening due to co-implanted helium exists above approximately 600 appm using dual-ion beam irradiation [24]. Higher irradiation-dose experiments with co-helium implantation will be conducted in the future. 4. Conclusion
Fig. 5. Hardness vs. Ni-content before and after ion-irradiations or neutron irradiation.
artificial effect in the CSM. Fortunately, the nonirradiated Fe-based materials show linear behaviors. The ratio of elastic modulus to hardness for Fe is probably suitable for CSM. It is also noted that the non-CSM method, which uses a repeated loading–unloading sequence, shows bilinear behavior in the ion-irradiated F82H steels [10]. Table 1 also includes the Hvexp 0:1 which is measured Vickers hardexp ness and the Hvconv 0:1 which is converted from the Hv0:1 using a simple correlation for the ratio of projected area (H) to surface area (Hv) with an exchange of unit (kgf/mm2 to GPa) in the following equation [4]: exp Hvconv 0:1 ¼ 0:01058 Hv0:1
Nanoindentation was used to investigate the irradiation hardening behavior of F82H with and without nickel addition after single- or dual-ion beam irradiation experiments at 563 K. An extrapolation of the experimentally obtained nanoindentation hardness to the Vickers hardness was also examined in comparison with the results of neutron irradiation. The increase of the extrapolated Vickers hardness clearly indicated that the Ni addition enhanced the irradiation hardening of F82H as shown in the neutron-irradiated F82H. The co-implanted 300 appm helium caused no significant effect on the irradiation hardening of F82H under the investigated irradiation conditions. Acknowledgments This work was partially supported by the Japan Atomic Energy Agency under the Joint Work, as a part of Broader Approach activities. The authors also would like to thank Dr. Sosuke Kondo and Mr. Okinobu Hashitomi for their help in the ion-irradiation experiments.
ð2Þ Hvconv 0:1
It should be pointed out that the from Vickers hardness tests and the H0 values from nanoindentation hardness tests are quite similar for the nonirradiated F82H. The relationship between H0 and the Vickers hardness is still under consideration. The H0 agrees well with Hvconv 0:1 for the nonirradiated F82H steels. 3.2. Ni and He effect The bulk-equivalent hardness H0 and the Hv0.1conv for the F82H steels are plotted against Ni content in Fig. 5. The left axis is H0 and Hv0.1conv in units of GPa. The right axis is the experimentally obtained Hv0.1exp or Hv1exp in units of kgf/mm2 and the inversely converted Hv0.1exp from H0 by Eq. (2). The results for the irradiated F82H steels represent an increase of irradiation hardening with the amount of nickel. The Ni-doping techniques were used to study the effects of helium on the mechanical properties and void swelling of ferritic steels [17,18]. However, the technique is not suitable for investigating the effect of helium on the irradiation hardening of ferritic steel irradiated at temperatures below approximately 573 K due to the enhancement of hardening [19,20]. This result supports the theory that the enhancement of irradiation hardening occurs due to the nickel addition. Moreover, the obtained H0 of ionirradiated F82H + 1Ni is comparable with that of Hv1conv converted from Hv1exp for similar steels irradiated at 563 or 590 K up to 7.7 dpa in the HFIR JP26 [21] in spite of the different irradiation conditions. It is expected that the present method will give precise hardness data for ion-irradiated steels containing lower Ni contents to determine the acceptable limit of Ni vs. irradiation hardening/embrittlement. The effect of co-implanted helium on the irradiation hardening of F82H steels is negligible for the investigated conditions up to
References [1] C. Abromeit, J. Nucl. Mater. 216 (1994) 78–96. [2] A. Kohyama, Fusion Eng. Des. 51–52 (2000) 789–795. [3] Y. Serruys, M.-O. Ruault, P. Trocellier, S. Miro, A. Barbu, L. Boulanger, O. Kaïtasov, S. Henry, O. Leseigneur, P. Trouslard, S. Pellegrino, S. Vaubaillon, C. R. Phys. 9 (2008) 437–444. [4] ISO14577-1: 2002 Metallic Materials-Instrumented Indentation Test for Hardness and Materials Parameters – Part 1: Test method. [5] A.C. Fischer-Cripps, Nanoindentation, Springer, 2002. [6] M. Ando, E. Wakai, H. Tanigawa, Y. Kawasaki, Nip. Kinzoku Gakkaishi/J. Jpn. Inst. Met. 72 (2008) 785–788. [7] C. Shin, H. Jin, M.-W. Kim, J. Nucl. Mater. 392 (2009) 476–481. [8] R. Kasada, Y. Takayama, K. Yabuuchi, A. Kimura, A new approach to evaluate irradiation hardening of ion-irradiated ferritic alloys by nano-indentation techniques, Fusion Eng. Des. 86 (2011) 2658–2661. [9] W.D. Nix, H. Gao, J. Mech. Phys. Solids 46 (1998) 411. [10] Y. Takayama, R. Kasada, K. Yabuuchi, A. Kimura, D. Hamaguchi, M. Ando, H. Tanigawa, Mater. Sci. For. 654–656 (2010) 2915–2918. [11] R.L. Klueh, E.T. Cheng, M.L. Grossbeck, E.E. Bloom, J. Nucl. Mater. 280 (2000) 353–359. [12] http://www.srim.org/. [13] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. [14] G.M. Pharr, E.G. Herbert, Y. Gao, Annu. Rev. Mater. Res. 40 (2010) 271–292. [15] G.M. Pharr, J.H. Strader, W.C. Oliver, J. Mater. Res. 24 (2009) 653–666. [16] Y. Himei, Master’s Thesis, Kyoto University, 2011. [17] R.L. Klueh, N. Hashimoto, M.A. Sokolov, K. Shiba, S. Jitsukawa, J. Nucl. Mater. 357 (2006) 156–168. [18] R.L. Klueh, N. Hashimoto, M.A. Sokolov, P.J. Maziasz, K. Shiba, S. Jitsukawa, J. Nucl. Mater. 357 (2006) 169–182. [19] R. Kasada, A. Kimura, Mater. Trans. 46 (2005) 475. [20] R. Kasada, A. Kimura, H. Matsui, M. Narui, J. Nucl. Mater. 258–263 (1998) 1199–1203. [21] M. Ando, K. Shiba, H. Tanigawa, private communications. [22] A. Kimura, R. Kasada, K. Morishita, R. Sugano, A. Hasegawa, K. Abe, T. Yamamoto, H. Matsui, N. Yoshida, B.D. Wirth, T.D. de la Rubia, J. Nucl. Mater. 307–311 (2002) 521–526. [23] T. Yamamoto, G.R. Odette, H. Kishimoto, J.-W. Rensman, P. Miao, J. Nucl. Mater. 356 (2006) 27–49. [24] M. Ando, E. Wakai, T. Sawai, H. Tanigawa, K. Furuya, S. Jitsukawa, H. Takeuchi, K. Oka, S. Ohnuki, A. Kohyama, J. Nucl. Mater. 329–333 (2004) 1137–1141.
Please cite this article in press as: Y. Takayama et al., J. Nucl. Mater. (2013), http://dx.doi.org/10.1016/j.jnucmat.2012.12.033
Contents lists available at SciVerse ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Nanoindentation hardness and its extrapolation to bulk-equivalent hardness of F82H steels after single- and dual-ion beam irradiation Y. Takayama a, R. Kasada b,⇑, Y. Sakamoto a, K. Yabuuchi a, A. Kimura b, M. Ando c, D. Hamaguchi c, H. Tanigawa c a b c
Graduate School of Energy Science, Kyoto University, Japan Institute of Advanced Energy, Kyoto University, Kyoto, Japan IFERC, JAEA, Rokkasho, Aomori, Japan
a r t i c l e
i n f o
Article history: Available online xxxx
a b s t r a c t The irradiation hardening behavior of reduced-activation ferritic steels after single Fe-ion beam irradiation and dual-ion (Fe ion and He ion) beam irradiation experiments was investigated with nanoindentation tests. The ion-irradiation experiments were conducted at 563 K with 6.4 MeV Fe3+ ions up to 3 dpa at a 600 nm depth from the irradiated surface. Furthermore, these experiments were conducted with and without simultaneous energy-degraded 1 MeV He+ ions up to 300 appm. The materials used were F82H, F82H + 1Ni, and F82H + 2Ni to investigate the effect of Ni addition on the irradiation hardening behavior. The measured nanoindentation hardness was converted to the bulk-equivalent hardness based on a combination of the Nix–Gao model to explain the indentation size effect and the composite hardness model to explain the softer substrate effect of the nonirradiated region beyond the irradiated depth range. It is clearly shown that the Ni addition enhances the irradiation hardening of F82H. The bulkequivalent hardness is compared with the experimentally obtained Vickers hardness of F82H steels after neutron irradiation. The effect of simultaneously implanted helium on the irradiation hardening is negligible in the investigated irradiation conditions. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction To simulate fusion neutron irradiation damage of the structural materials, heavy ion irradiation experiments have been used because of the simplicity of use, easier control of irradiation parameters, reduction of cost, rapid damage production, the absence of induced radioactivity, and the occurrence of the co-implantation of helium/hydrogen [1–3]. However, heavy ion irradiations also have problems, such as a different recoil spectrum compared to neutrons, inhomogeneous and shallow damage profiles, and additional atoms implanted as ions. Nanoindentation is essential to evaluate the mechanical properties in the shallow depth area [4,5]. However, the correlation between nanoindentation hardness and other ‘‘bulk’’ mechanical properties should be established for further applications. There are few reports on the extrapolation of the nanoindentation hardness of the ion-irradiated materials to their bulk mechanical properties [6–8]. Ando et al. suggested an empirical relationship between nanoindentation hardness and the Vickers hardness of the neutron-irradiated steels to assess ion-irradiated materials [6]. Shin et al. calculated a dosedependent yield stress from the load/depth profiles for an ion⇑ Corresponding author. Tel.: +81 774 38 3483. E-mail address: [email protected] (R. Kasada).
irradiated Fe–9Cr alloy using a finite element model [7]. More recently, Kasada et al. have suggested a new model to extrapolate the experimentally obtained nanoindentation hardness to the bulk-equivalent hardness of ion-irradiated Fe-based binary model alloys [8]. This model is based on a combination of the Nix–Gao model [9] for the indentation size effect (ISE) and a composite hardness model for the softer substrate effect (SSE) of the nonirradiated region beyond the irradiation range. The preliminary results for the reduced-activation ferritic F82H steels have also indicated the existence of ISE and SSE after ion-irradiation [10]. In the present paper, depth profiles of the nanoindentation hardness are precisely measured for F82H and its variations containing 1% or 2% Ni after single Fe-ion beam irradiation and dual-ion (Fe ion and He ion) beam irradiation experiments. The objective is to determine whether there is increased hardening in the steels implanted with helium compared to the steels without helium. In addition, the effect of Ni addition on the irradiation hardening is examined to compare the extrapolated Vickers hardness with the Vickers hardness of neutron-irradiated steels. The reduced-activation ferritic steels are basically intended to be high purity steel as it has to achieve reduction of radiation activation, but the increase of impurity is inevitable when the steel is going to be made at the industrial level. P, S, Cu and Ni are the typical impurities which have to be expected to be increased [11]. Among
0022-3115/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2012.12.033
Please cite this article in press as: Y. Takayama et al., J. Nucl. Mater. (2013), http://dx.doi.org/10.1016/j.jnucmat.2012.12.033
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these elements, Ni is one of the elements which could increase the as-prepared properties but could induce extra irradiation hardening.
2. Experimental procedure
Fig. 1. Depth profiles of the displacement damage and the implanted helium calculated for the investigated irradiation conditions.
The materials used in the present study were F82H (nominal compositions: Fe–8Cr–2W–0.2V–0.05Ta–0.1C) and its variations with 1 or 2 wt.% Ni addition (F82H + 1Ni, 2Ni). Both Ni-free and Ni-added steels were normalized at 1313 K for 30 min followed by air cooling, and tempered at 1013 K for 60 min followed by air cooling. The microstructures of the steels are a tempered martensite of which prior-austenite grain size is seen to be about 100 lm. These steels were simultaneously irradiated with a single-ion beam of 6.4 MeV Fe3+ from a 1.7 MV tandem accelerator or a dual-ion beam of 6.4 MeV Fe3+ with a simultaneous implantation of energy-degraded 1 MeV He+ ions from a 1 MV Singletron accelerator in the DuET (a dual-ion beam accelerator facility) [2]. The irradiation temperature was well controlled at 563 K within ±10 K by monitoring with infrared thermal vision. Depth profiles of the displacement damage and the implanted helium were calculated by the TRIM-98 code [12], as shown in Fig. 1. The nominal displacement damage rate, nominal displacement damage, and
Fig. 2. Indentation-depth dependence of the nanoindentation hardness of F82H (a) nonirradiated, (b) single-ion beam irradiated, and (c) dual-ion beam irradiated.
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Fig. 3. Indentation-depth dependence of the averaged nanoindentation hardness for ten indents on each specimen of (a) F82H, (b) F82H + 1Ni, and (c) F82H + 2Ni.
nominal concentration of helium were 3 104 dpa/s, 3 dpa, and 300 appm, respectively. The nominal values stand for the ones at approximately 600 nm below the irradiated surface; the nominal displacement damage is also close to the averaged value for the entire damaged depth. The ratio of 100 appm He/dpa is relatively high for the fusion reactor blanket condition but is close to the spallation condition. The nanoindentation hardness H was measured by using a Nano Indenter G200 (Agilent Technologies) with a Berkovichtype indentation tip. The continuous stiffness measurement (CSM) was used to continuously obtain the hardness (H) vs. depth (h) profile up to a depth of about 1000 nm. In comparison with the loading-unloading method used in the previous studies [10], the CSM has a great advantage to obtain the depth profile only with a single indent. The hardness at a specific depth was also calculated by averaging the hardness obtained from ten indents using the Analyst™ software. The distance between indentations was about 50 lm. The calibration of the bluntness of the indentation tip is based on the Oliver–Pharr method [13]. The Vickers hardness Hv0.1 of nonirradiated steels was measured with a load of 0.1 kgf.
3. Results and discussion 3.1. ISE and SSE on nanoindentation hardness Fig. 2 shows typical indentation-depth profiles of nanoindentation hardness of F82H before and after the single-ion irradiation. A particular plot symbol represents data of a single indent. Even for the nonirradiated F82H steel, the depth dependence of the hardness is relatively complicated and deviates in comparison with the previous model alloys [8]; this deviation is likely due to many kinds of inhomogeneous microstructural features, such as laths, packets, dislocations, and carbides, in the tempered martensitic structure of F82H. Beyond the complexity, however, the increase in hardness with decreasing indent size or depth (i.e., an ISE) is clearly observed for the nonirradiated F82H. After the ionirradiations, the nanoindentation hardness increases at the depth region measured. To emphasize the depth dependence of hardness, Fig. 3 gives the average of all indents with error bars. To explain the ISE, Nix and Gao developed a model based on the concept of geometrically necessary dislocation [9]. The Nix–Gao model predicts the hardness depth profile as follows:
Please cite this article in press as: Y. Takayama et al., J. Nucl. Mater. (2013), http://dx.doi.org/10.1016/j.jnucmat.2012.12.033
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Fig. 4. Plots of H2 vs. 1/h for the averaged nanoindentation hardness of (a) F82H, (b) F82H + 1Ni, and (c) F82H + 2Ni.
H ¼ H0
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 þ ðh =hÞ
ð1Þ
where H0 is the hardness at infinite depth (i.e., bulk-equivalent hardness), and h is a characteristic length that depends on the material and the shape of the indenter tip. The averaged nanoindentation hardness data is plotted as H2 vs. 1/h in Fig. 4. The nonirradiated F82H steels have a good linearity in the range of h > 100 nm. However, the irradiated steels appear to have a bilinearity with a shoulder at around 300–500 nm. As reported in the previous study [8], the bilinear behavior is due to the SSE of the nonirradiated depth area beneath the irradiated depth area. Here it is assumed that the bulk-equivalent hardness of ion-irradiated materials can be obtained from the shallower depth region before the shoulder based on the composite hardness model [8]. Table 1 includes H0 extrapolated from the least squares fitting of hardness data to Eq. (1) in the range of 100 nm < h < 500 nm for non-irradiated samples and in the range of 100 nm < h < 200 nm for the irradiated samples. It should be pointed out that Pharr et al. have recently reported an artificial problem when the CSM technique is used to measure the hardness of high modulus/hardness materials at small depths
Table 1 The H0 and h, and experimentally obtained Hv0.1exp for F82H steels. Materials
H0 (GPa)
2 Hvexp 0:1 (kgf/mm )
Hvconv 0:1 (GPa)
F82H Unirr. 3 dpa 3 dpa + 300 appm He
2.45 ± 0.03 2.66 ± 0.07 3.14 ± 0.06
236 ± 2 – –
2.49 ± 0.02 – –
F82H + 1Ni Unirr. 3 dpa 3 dpa + 300 appm He
2.26 ± 0.06 4.18 ± 0.05 4.32 ± 0.06
241 ± 7 – –
2.55 ± 0.07 – –
F82H + 2Ni Unirr. 3 dpa 3 dpa + 300 appm He
2.60 ± 0.04 5.17 ± 0.08 4.48 ± 0.13
257 ± 7 – –
2.72 ± 0.07 – –
[14,15]. Despite the fact that homogeneous materials were used, they showed a bilinear-like behavior when H2 vs. 1/h was plotted. Such behavior was also found in nonirradiated tungsten by the CSM [16]. Therefore, it is questionable whether the bilinear behavior in the ion-irradiated F82H steels causes the SSE and/or the
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300 appm with 3 dpa at 563 K. Kimura et al. suggested that the highly resistant properties of the steels against helium embrittlement are due to high trapping capacity for helium atoms in the martensitic structure [22]. As summarized by Yamamoto et al. [23], however, extensively high levels of helium may contribute extra irradiation hardening beyond that due to displacement damage alone. Ando et al. indicated that extra irradiation hardening due to co-implanted helium exists above approximately 600 appm using dual-ion beam irradiation [24]. Higher irradiation-dose experiments with co-helium implantation will be conducted in the future. 4. Conclusion
Fig. 5. Hardness vs. Ni-content before and after ion-irradiations or neutron irradiation.
artificial effect in the CSM. Fortunately, the nonirradiated Fe-based materials show linear behaviors. The ratio of elastic modulus to hardness for Fe is probably suitable for CSM. It is also noted that the non-CSM method, which uses a repeated loading–unloading sequence, shows bilinear behavior in the ion-irradiated F82H steels [10]. Table 1 also includes the Hvexp 0:1 which is measured Vickers hardexp ness and the Hvconv 0:1 which is converted from the Hv0:1 using a simple correlation for the ratio of projected area (H) to surface area (Hv) with an exchange of unit (kgf/mm2 to GPa) in the following equation [4]: exp Hvconv 0:1 ¼ 0:01058 Hv0:1
Nanoindentation was used to investigate the irradiation hardening behavior of F82H with and without nickel addition after single- or dual-ion beam irradiation experiments at 563 K. An extrapolation of the experimentally obtained nanoindentation hardness to the Vickers hardness was also examined in comparison with the results of neutron irradiation. The increase of the extrapolated Vickers hardness clearly indicated that the Ni addition enhanced the irradiation hardening of F82H as shown in the neutron-irradiated F82H. The co-implanted 300 appm helium caused no significant effect on the irradiation hardening of F82H under the investigated irradiation conditions. Acknowledgments This work was partially supported by the Japan Atomic Energy Agency under the Joint Work, as a part of Broader Approach activities. The authors also would like to thank Dr. Sosuke Kondo and Mr. Okinobu Hashitomi for their help in the ion-irradiation experiments.
ð2Þ Hvconv 0:1
It should be pointed out that the from Vickers hardness tests and the H0 values from nanoindentation hardness tests are quite similar for the nonirradiated F82H. The relationship between H0 and the Vickers hardness is still under consideration. The H0 agrees well with Hvconv 0:1 for the nonirradiated F82H steels. 3.2. Ni and He effect The bulk-equivalent hardness H0 and the Hv0.1conv for the F82H steels are plotted against Ni content in Fig. 5. The left axis is H0 and Hv0.1conv in units of GPa. The right axis is the experimentally obtained Hv0.1exp or Hv1exp in units of kgf/mm2 and the inversely converted Hv0.1exp from H0 by Eq. (2). The results for the irradiated F82H steels represent an increase of irradiation hardening with the amount of nickel. The Ni-doping techniques were used to study the effects of helium on the mechanical properties and void swelling of ferritic steels [17,18]. However, the technique is not suitable for investigating the effect of helium on the irradiation hardening of ferritic steel irradiated at temperatures below approximately 573 K due to the enhancement of hardening [19,20]. This result supports the theory that the enhancement of irradiation hardening occurs due to the nickel addition. Moreover, the obtained H0 of ionirradiated F82H + 1Ni is comparable with that of Hv1conv converted from Hv1exp for similar steels irradiated at 563 or 590 K up to 7.7 dpa in the HFIR JP26 [21] in spite of the different irradiation conditions. It is expected that the present method will give precise hardness data for ion-irradiated steels containing lower Ni contents to determine the acceptable limit of Ni vs. irradiation hardening/embrittlement. The effect of co-implanted helium on the irradiation hardening of F82H steels is negligible for the investigated conditions up to
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