E-Journal of Advanced Maintenance Vol.9-2 (2017) 72-77
Japan Society of Maintenology

Evaluation of Hardening under Ion Irradiation in Reactor Pressure Vessel
Model Alloys by Nano-indentation Techniques
Liang CHEN1,2, Kenji NISHIDA3, Kenta MURAKAMI4, Tomohiro KOBAYASHI2,
Zhengcao LI5 and Naoto SEKIMURA1,
1

Department of Nuclear Engineering and Management, School of Engineering, The University ofTokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
2
Central Research Institute of Electric Power Industry, 2-11-1 Iwadokita, Komae-shi, Tokyo
201-8511, Japan
3
Central Research Institute of Electric Power Industry, 2-6-1 Nagasaka, Yokosuka-shi, Kanagawa
240-0196, Japan
4
Nuclear Professional School, School of Engineering, The University of Tokyo, 2-22 Shirakata,
Tokai-mura, Ibaraki 319-1188, Japan
5
School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
ABSTRACTThe neutron irradiation embrittlement of reactor pressure vessel (RPV) steels needs to be properly predicted andmanaged for the safe long-term operation of light water reactors. To investigate the effects of the solute
elements Ni, Mn and Si on the irradiation hardening at high dose levels, pertinent to long-term operation, in
RPV materials without Cu, ion irradiation for Fe-1.0Ni-0.2Si, Fe-1.0Ni-1.4Mn and Fe-1.0Ni-1.4Mn-0.2Si alloys
was carried out at 290 oC up to 5.0 dpa by Fe-ions with the energy of 2.8 MeV. Nano-indentation techniques
were applied to evaluate the ion irradiation hardening. The results show that Mn significantly accelerates theirradiation hardening and addition of Si reduces the irradiation hardening. The depth dependence of irradiation
hardening was discussed, and the irradiation hardening was the most significant at the indentation depth of about
120 nm in all alloys. The hardening behavior in deeper region was slightly different from that observed at about
120 nm depth, which may be caused by the interactions between solutes and defects.
KEYWORDS

ARTICLE INFORMATION

reactor pressure vessel, long-term operation, irradiation embrittlement,
irradiation hardening, ion irradiation, nano-indentation

Article history:
Received 7 November 2016
Accepted 27 April 2017

1.

Introduction

Long-term operation, or life extension of light water reactors is being considered by manyutilities in their plant life management programs. The reactor pressure vessel (RPV) is the key
safety-related component to confine radioactive materials, and is considered irreplaceable.
Nano-features caused by neutron irradiation, including solute clusters and matrix damage, obstruct thedislocation movement, leading to hardening and accompanying embrittlement in RPV steels [1].
Long-term operation requires demonstration that RPVs operate within safety margins, including
ensuring that the irradiation embrittlement of RPV steels during life extension period can be properly
predicted and managed.
The response of RPV materials to neutron irradiation has been improved, especially by reducing
the amount of Cu content since the 1970s when the role of Cu was identified [2]. It is recognized that
Cu atoms cluster rapidly even at low neutron fluences due to its low solubility in Fe matrix and Ni,
Mn and Si commonly segregate to the cluster-matrix interface [3]. But even for the low Cu steels (<0.07%Cu), there is still a knowledge gap on how the microstructure and resultant hardening develop
at high neutron fluences, pertinent to long-term operation, since the formation conditions of
Ni-Mn-Si-rich clusters and their contribution to the hardening have not been well understood.
*

Corresponding author.
E-mail: chen@safety.n.t.u-tokyo.ac.jp (L. CHEN), sekimura@n.t.u-tokyo.ac.jp (N. SEKIMURA)
ISSN-1883-9894/10 © 2010 – JSM and the authors. All rights reserved.

72

L. Chen, et al./Evaluation of Hardening under Ion Irradiation in
Reactor Pressure Vessel Model Alloys by Nano-indentation TechniquesThis work is to investigate the effects of the elements Ni, Mn and Si, which are the main constituentsof Ni-Mn-Si-rich clusters, on the irradiation hardening at high dose levels in RPV materials without
Cu. This will also provide the knowledge base for our study of the microstructures using the same setof materials, which will not be described in this paper. For this purpose, high-dose ion irradiation
experiments up to 5.0 dpa were carried out using three kinds of alloys prepared by removing Mn and
Si from the alloy Fe-1.0wt.%Ni-1.4wt.%Mn-0.2wt.%Si. Hardness measurements by nano-indentation
techniques are then presented.

2.

Experimental

2.1. Materials and irradiation conditions

1.2

Displacement damage
Implanted ion concentration (2.5 dpa)

1

0.12
0.1

0.8

0.08

0.6

0.06

0.4

0.04

0.2

0.02

0
0

500
1000
Depth from surface (nm)Implanted ion concentration (%)

Displacement damage (10/nm/ions)Three kinds of RPV model alloys (Fe-1.0Ni-0.2Si, Fe-1.0Ni-1.4Mn and Fe-1.0Ni-1.4Mn-0.2Si)
were used. The sample surface was treated by mechanical polishing. This was then followed byelectrochemical polishing to remove the work hardened layer. Ion irradiation with 2.8 MeV Fe-ions
was carried out using a tandem-type accelerator installed at the High Fluence Irradiation Facility, The
University of Tokyo (HIT) [4]. The irradiation temperature was 290 oC and the maximum ion fluence
was 4.65×1015 ions/cm2. The damage distribution associated with the ion irradiation was determinedby using SRIM-2008 [5], as shown in Fig. 1. The calculation was conducted with a displacement
energy of 40 eV and to be most consistent with the standard NRT dpa [6]. The displacement damage
peak appeared at 800 nm and the calculated doses (at the depth of damage peak) in this work were0.25 dpa, 2.5 dpa and 5.0 dpa. In this study, the dose rates of all the irradiation were well controlled at
the same level in order to facilitate studying the dose dependent behavior in each alloy and comparing
the results in different alloys. This is because the dose rate is recognized to affect the hardening [7] aswell as the formation of Ni-Mn-Si-rich clusters [8] in RPV materials.

01500

Fig. 1. Depth distribution of displacement damage and implanted ion concentration by the SRIM code

2.2. Nano-indentation tests
Nano-indentation tests were performed using two methods: the constant depth method by
DUH-211S (Shimazu) and the constant stiffness measurement (CSM) method by G200 (Agilent), bothwith a Berkovich type indentation tip. Calibration and confirmation for the roundness of theindentation tip is based on the Oliver-Pharr method [9].
An indentation depth of 200 nm was set for the constant depth method. In order to calculate thehardness, we must know the values of contact stiffness. The constant depth method estimates thecontact stiffness from the slope of fitted unloading curve. Such a calculation enables to determine73

L. Chen, et al./Evaluation of Hardening under Ion Irradiation in
Reactor Pressure Vessel Model Alloys by Nano-indentation Techniquescontact stiffness, and thus hardness at the maximum indentation depth. In this work, the hardness wasmeasured at approximately 400 indents per specimen and the distance between two indents was atleast 20 μm [10]. The scattering of the hardness obtained by approximately 400 indents was less than
9% in unirradiated alloys.
The database provided by the constant depth method helped design the strategy for CSM tests inthis work, including the shape of indentation area, indent distance and number of indents per
specimen. Accordingly, the CSM tests were performed with 60 indents per specimen and an indent
distance of at least 50 μm. Different from the constant depth method calculating the contact stiffness
only at the maximum indentation depth, the CSM method is able to estimate the contact stiffness
continuously during loading and then to give the depth profile of the hardness. This is accomplishedby superimposing a small oscillation on the primary loading signal and analyzing the response of thesystem based on harmonic oscillator modeling [11].

3.

Results and discussion

3.1. Dose dependence of hardness increase by constant depth method
Fig. 2 shows the dose dependence of hardness increase in these alloys under ion irradiation,
given by the constant depth method. The hardness increase of Fe-Ni-Mn and Fe-Ni-Mn-Si is similar,
and between them Fe-Ni-Mn shows higher irradiation hardening. Fe-Ni-Si shows the lowest
irradiation hardening among the alloys, and its hardening difference with Fe-Ni-Mn-Si is the largest at
2.5 dpa. These facts indicate that Mn significantly accelerates the irradiation hardening.
Phenomenologically, the addition of Si to Fe-Ni-Mn reduces the irradiation hardening.
Our results for the Mn and Si are consistent with the hardness data at lower doses obtained by
Fujii et al. [12]. Their transmission electron microscope (TEM) observations show that the dislocation
loops in Fe-Ni-Mn-Si are more developed (larger size and higher number density) than those in
Fe-Ni-Si. The effect of Mn in increasing the number density of dislocation loops is also reported by
Yabuuchi et al. [13]. This Mn effect can contribute to the high irradiation hardening of the alloys.
Besides, the formation of solute clusters is also expected under such high dose irradiation in this work.
Previous low temperature irradiation studies [14,15] conclude that the addition of Mn to pure Fe
obstructs the recovery of radiation defects. It is interesting that the addition of Si to the Fe-Mn alloy isreported to mitigate this effect of Mn [16], which appears to suggest an interaction between Mn and Si.
These differences in irradiation-induced defects may cause a difference in cluster formation.

Hardness increase (GPa)

2.5
2
1.5
1
Fe-1.0Ni-0.2Si
Fe-1.0Ni-1.4Mn

0.5

Fe-1.0Ni-1.4Mn-0.2Si
002
4
Irradiation dose (dpa)6Fig. 2. Dose dependence of hardness increase in all alloys
Error bars refer to the 95% confidence interval.

74

L. Chen, et al./Evaluation of Hardening under Ion Irradiation in
Reactor Pressure Vessel Model Alloys by Nano-indentation Techniques

3.2. Depth dependence of hardness increase by CSM method
Since ion irradiation has a damage distribution along with depth direction, several methods havebeen applied to evaluate the depth dependence of hardness increase in previous studies [17,18]. The
purpose of the CSM test in this work is to help correlate the hardening behavior of alloys to
microstructure changes under ion irradiation. The data in the indentation depth less than 50 nm wereconsidered less reliable and were ignored. Fig. 3 gives the indentation depth profile of averagehardness of all indents with error bars in the alloys. It is seen that the measured value of hardness
decreases with the increase of indentation depth in unirradiated alloys. This can be attributed to theindentation size effect introduced by Nix-Gao model [19].
6Fe-1.0Ni-0.2Si

Hardness (GPa)Fe-1.0Ni-1.4Mn
5Fe-1.0Ni-1.4Mn-0.2Si

4unirr.
0.25 dpa
2.5 dpa

3

2

1

0
0

100

200
300
Indentation depth (nm)

400

500

Fig. 3. Indentation depth dependence of hardness in alloys
Error bars refer to the 95% confidence interval.

In order to evaluate the ion irradiation hardening, one method is to assume simply that theindentation size effect is the same for the hardness before and after ion irradiation, and then to
estimate the hardness increase ∆HCSM by subtraction (see Eq. (1)), although this assumption is notbased on theoretical model.

H CSM  H CSM,irr  H CSM, unirr(1)This simplified subtraction analysis shows that the hardening differences among these three alloys
irradiated to 0.25 dpa can be observed up to 300 nm indentation depth, and these differences under 2.5
dpa can be observed until even deeper. The effects of Mn addition and Si addition on irradiation
hardening observed in the shallow region agree qualitatively with the results obtained by the constantdepth method. This analysis also indicates that there is a maximum value of hardness difference
between irradiated and unirradiated alloys with the increase of indentation depth. The maximum75

L. Chen, et al./Evaluation of Hardening under Ion Irradiation in
Reactor Pressure Vessel Model Alloys by Nano-indentation Techniques

difference appears at about 120 nm indentation depth for all alloys irradiated to 0.25 dpa and 2.5 dpa.
At this indentation depth, the radius of volume affected by the indent reaches much deeper and may
correspond to the depth of peak hardening region. The TEM studies [20,21] of deformation
microstructure under nano-indentation show that the radius of volume affected by the indent can reachup to 10 times as large as the indentation depth for unirradiated pure Fe and unirradiated steel, andthat this affected volume will be reduced by damage structure developed in ion irradiation.
As stated in the previous paragraph, irradiation hardening and their differences among these threealloys are still observed at deeper indentation depth, such as twice as deep as 120 nm. This is natural,
because the measured value of hardening is the average hardening from the surface to the depth whichcorresponds to the boundary of the volume affected by the indent. In addition to this, microstructure
may also form at deeper depth. While ion beam irradiation is performed, ions lose energy quickly
because of high electronic energy loss, giving rise to not only a spatially non-uniform damage profile,
but also a stopping of ions in the material itself. The peak of ion stopping range in this work is locatedat 1000 nm depth, deeper than the peak of damage profile (800 nm) , as shown in Fig. 1. As for 2.5
dpa irradiation, implanted Fe-ion concentration at this peak location reaches up to 0.06%. These
implanted Fe-ions may act as effective self-interstitial atoms (SIAs) since there is no corresponding
vacancy production for the recombination. Further modelling studies on migration of point defects
along ion implantation direction are required, since these defects are expected to interact with soluteatoms, causing different hardening behaviors along ion implantation direction in these alloys.

4.

Conclusion

The hardening of RPV model alloys without Cu under high dose ion irradiation by 2.8 MeV
Fe-ions at 290 oC was examined using nano-indentation techniques. The alloy without Mn showed
less irradiation hardening and the alloys without Si showed more irradiation hardening. These facts
indicate that Mn significantly accelerates the irradiation hardening, and addition of Si reduces theirradiation hardening. The depth dependence of irradiation hardening was evaluated, and a maximum
hardness difference was observed at about 120 nm indentation depth for all alloys irradiated to 0.25
dpa and 2.5 dpa. This indicates that the radius of the volume affected by the indent at about 120 nm
may correspond to the depth of peak hardening region. The hardening behavior in deeper region wasslightly different from that observed at about 120 nm indentation depth. This may be caused by theinteractions between solutes and defects.

References[1]

K. Fukuya: ''Current understanding of radiation-induced degradation in light water reactor structural
materials'', J. Nucl. Sci. Technol., Vol. 50, No. 3, pp.213-254 (2013).
[2] N. Yamashita, M. Iwasaki, K. Dozaki, N. Soneda: ''Industry Practice for the Neutron Irradiation
Embrittlement of Reactor Pressure Vessels in Japan'', J. Eng. Gas Turb. Power, Vol. 132, 102919 (2010).
[3] G.R. Odette, G.E. Lucas: ''Recent progress in understanding reactor pressure vessel steel embrittlement'',
Radiat. Eff. Defect. S., Vol. 144, pp.189-231 (1998).
[4] K. Murakami, T. Iwai, H. Abe, N. Sekimura: "Improvement of the High Fluence Irradiation Facility at the
University of Tokyo", Nucl. Inst. Meth. B, Vol. 381, pp.67-71 (2016).
[5] J.F. Ziegler, J. Biersack, U. Littmark: ''The Stopping and Range of Ions in Matter", Pergamon Press (1985).
[6] 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. Inst. Meth. B, Vol. 310, pp.75-80 (2013).
[7] A. Ballesteros, R. Ahlstrand, C. Bruynooghe, A. Chernobaeva, Y. Kevorkyan, D. Erak, D. Zurko:"Irradiation temperature, flux and spectrum effects", Prog. Nucl. Energy, Vol. 53, pp.756-759 (2011).
[8] G.R. Odette, T. Yamamoto, B.D. Wirth: "Late Blooming Phases and Dose Rate Effects in RPV Steels:
Integrated Experiments and Models", MMM-2 Proc., Los Angeles, California (2004).
[9] W.C. Oliver, G.M. Pharr: "An improved technique for determining hardness and elastic modulus using
load and displacement sensing indentation experiments", J. Mater. Res., Vol. 7, pp.1564- 1583 (1992).
[10] K. Murakami, N. Sekimura, T. Iwai, H. Abe: ''Heterogeneity of ion irradiation-induced hardening in
A533B reactor pressure vessel model alloys'', J. Nucl. Sci. Technol., Vol. 53, No. 7, pp.1061-1066 (2016).

76

L. Chen, et al./Evaluation of Hardening under Ion Irradiation in
Reactor Pressure Vessel Model Alloys by Nano-indentation Techniques[11] X.D. Li, B. Bhushan: ''A review of nanoindentation continuous stiffness measurement technique and its
applications", Mater. Charact., Vol. 48, pp.11-36 (2002).
[12] K. Fujii, T. Ohkubo, K. Fukuya: '' Effects of solute elements on irradiation hardening and microstructural
evolution in low alloy steels", J. Nucl. Mater., Vol. 417, pp.949-952 (2011).
[13] K. Yabuuchi, M. Saito, R. Kasada, A. Kimura: ''Neutron irradiation hardening and microstructure changes
in Fe-Mn binary alloys", J. Nucl. Mater., Vol. 414, pp.498-502 (2011).
[14] F. Maury, A. Lucasson, P. Lucasson, P. Moser, F. Faudot: ''Interstitial migration in irradiated iron andiron-based dilute alloys. II. Interstitial migration and solute transport in FeNi, FeMn and FeCu dilutealloys", J. Phys.: Condens. Matter, Vol. 2, pp.9291 (1990).
[15] K. Murakami, T. Iwai, H. Abe, N. Sekimura, Y. Katano, T. Iwata, T. Onitsuka: ''Role of nickel andmanganese in recovery of resistivity in iron-based alloys after low-temperature proton irradiation", Phil.
Mag., Vol. 95, pp.1680-1695 (2015).
[16] K. Murakami, T. Iwai, H. Abe, N. Sekimura: ''Interaction between solute atoms and radiation defects in
Fe-Ni-Si and Fe-Mn-Si alloys under irradiation with proton ions at low-temperature", J. Nucl. Mater., Vol.
482, pp.47-52 (2016).
[17] N. Sekimura, T. Kamada, Y. Wakasugi, T. Okita, Y. Arai: ''Evaluation of radiation hardening in Fe alloys
under heavy ion irradiation by micro-indentation technique", J. Nucl. Mater., Vol. 307-311, pp.308-311(2002).
[18] 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., Vol. 86, pp.2658-2661(2011).
[19] W.D. Nix, H.J. Gao: ''Indentation size effects in crystalline materials: A law for strain gradient plasticity",
J. Mech. Phys. Solid, Vol. 46, pp.411-425(1998).
[20] H. Tanigawa, S. Jitsukawa, A. Hishinuma, M. Ando, Y. Katoh, A. Kohyama, T. Iwai: ''Effects of helium
implantation on hardness of pure iron and a reduced activation ferritic–martensitic steel", J. Nucl. Mater.,Vol. 283-287, pp.470-473 (2000).
[21] T. Miura, K. Fujii, K. Fukuya, K. Takashima: ''Influence of crystal orientation on hardness andnanoindentation deformation in ion-irradiated stainless steels", J. Nucl. Mater., Vol. 417, pp.984-987(2011).

77