J. Plasma Fusion Res. SERIES, Vol. 11 (2015)

Helium irradiation hardening in Fe, F82H-IEA and F82H-ODS steel
Siwei CHEN, Yongming WANG, Naoyuki HASHIMOTO, Somei OHNUKI
Graduate School of Engineering, Hokkaido University, N-13, W-8, Sapporo 060-8278, Japan(Received: 21 October 2014 / Accepted: 19 January 2015)

Helium implantation at room temperature was performed to investigate irradiation hardening offerritic/martensitic steels. Bulk hardness was extracted from Nix-Gao model using a Berkovich nano-indentor. The
correlation between irradiation hardening and helium concentration ranging 500 - 2000 appm was examined.
Nano-hardness increases as a function of helium concentration. The hardening of Fe is saturated when theconcentration of helium exceeds 1000 appm. No saturation tendency is observed for F82H-IEA and F82H-ODS.
F82H-ODS, having a higher hardness, provides a lower irradiation hardening than F82H-IEA. Transmission
electron microscopy (TEM) reveals that cavities with a uniform distribution are formed after helium implantation to2000 appm, showing a mean size of 1.4 nm with an average number density of 5.8 × 1023 m-3 in Fe, 1.1 nm with 4.9× 1023 m-3 in F82H-IEA and 1.3 nm with 7.4 × 1023 m-3 in F82H-ODS. The results reveal that the formation ofdefects by helium irradiation at room temperature is independent from the as-received microstructure, while thechange of mechanical property is dependent on the microstructure of the as-received materials. This suggests thesame type of irradiation induced defect may have a quite different hardening effect on materials with different
microstructure.
Keywords: helium, irradiation hardening, cavity, oxide particle, nanoindentation

Johnson Matthey Chem. Ltd.), F82H-IEA and F82H-ODSsteels (both provided by Japan Atom Energy Agency) werepunched into 3 mm diameter disks. The punched out
specimens of Fe were placed in a vacuum in a quartz tube
(1×10-4 Pa) and then annealed at 873 K for 1 h. Then thespecimens were removed from the tube, andelectrochemically polished with Acetic acid: Perchloric
acid = 19:1 to reduce stress concentrations on the surfaceand obtain a smooth surface for helium implantation. 70 250 kV He ion implantation (in Hokkaido University) wasperformed at room temperature. Chemical compositions
and heat treatment conditions are shown in Table 1[7㸪8].
The average helium concentration was 500 - 2000 appm.
Fig. 1 shows Stopping and Range of Ions in Matter (SRIM)
calculations of helium and displacement profile, showing
an approximately constant He level of 0.2 at.% and thedisplacement damage of 0.1 dpa. The displacement energy
used in the SRIM calculation is 25 eV. The implantedregion reaches to 800 nm from the surface with a relatively
homogeneous distribution of helium in the range of 200 650 nm.
Nano-hardness test was applied using an Elionix
ENT-1100a (Elionix Inc. Japan) nano-indenter withBerkovich-type tip. 5×6 array indents with 30 μm interval
and the hardness was calculated using Oliver and Pharr
method[9].
A JEOL JIB-4601F focused ion-beam (FIB)
micro-processing device, operated at 30 keV, was used toprepare XTEM foils. Then the foils were polished using a1. Introduction

Neutron with 14 MeV energy can produce transmuted
helium by reacting with constituent atoms in structural
materials. The effect of transmuted helium, which willcause the mechanical property degradation, is a criticalissue for the application of structural materials in fusion
reactors. An increasing attention has been paid to theincrease of strength – irradiation hardening.
F82H-IEA and F82H-ODS steels are candidate
materials for structure applications in fusion reactors due totheir high radiation resistance. Oxide dispersion
strengthened (ODS) reduced activation ferritic/martensitic(RAFM) steels attract an increasing attention because of its
advantage over high temperature strength[1-3]. In addition,
it is suggested that high density of nano-scale oxide
particles are effective trapping sites for helium atoms, aswell as vacancies, to resultantly suppress helium
effect[4-6]. However, further investigation is required forunderstanding the suppression effect of the oxide particles.
In this work, we report helium irradiation hardeningof Fe, F82H-IEA and F82H-ODS based on Nix-Gao model.
The micostructure with cavities was observed for
as-irradiated samples. This work investigates themechanical property change and microstructure evolution
with helium irradiation at room temperature.

2. Experimental procedure

Pure iron (purity better than 99.95%, produced byauthor’s e-mail: chensiwei@eng.hokudai.ac.jp

79©2015 by The Japan Society of Plasma
Science and Nuclear Fusion Research

S. Chen et al., Helium Irradiation Hardening in Fe, F82H-IEA and F82H-ODS Steel

Table 1.
Chemical compositions and heat treatment conditions of Fe, F82H-IEA and F82H-ODS steel (mass%).
C and NSiCrWVTaTiY2O3Ex. OFeFe0.04––––––––bal.
F82H-IEA
0.10
0.11
7.71
1.95
0.16
0.02
–––bal.
F82H-ODS
0.17
0.013
7.85
1.9
0.18
0.10
0.19
0.368
0.092
bal.
Fe: tempered at 600 Υfor 1h
F82H-IEA: normalized at 1040 Υfor 38min and tempered at 750 Υfor 1h
F82H-ODS: normalized at 1050 Υfor 1h and tempered at 750 Υfor 1h
profile, where concentration of He is ~0.2 at.% anddisplacement damage is ~0.1 dpa.
precision ion polishing system (PIPS) at 1 keV to remove
the damage layer induced by FIB. The micro-structure wasobserved by using a 200 kV transmission electron
3. Results and discussion
microscope (TEM).
Fig. 2 shows typical microstructure of Fe, F82H-IEA
and F82H-ODS before He irradiation. Fe contains few
pre-existing defects. The number density of dislocation isless than 1012 m-2. F82H-IEA contains martensite laths andprecipitates along lath boundaries. The number density ofdislocation is (5s1)‫ݳ‬1014 m-2. Relative to F82H-IEA,
F82H-ODS contains a large number ((1.1 s 0.04)×1023m-3) of oxide particles with mean size of (3.8 s 1.3)
nm[10]. The number density of dislocations is (1.1s0.8)‫ݳ‬1014 m-2.
Fig. 3 shows helium concentration dependence ofbulk hardness extracted from Nix-Gao model andirradiation hardening for Fe, F82H-IEA and F82H-ODS.
Bulk hardness increases as a function of helium
concentration. The bulk hardness of Fe is saturated when
Fig. 1 SRIM calculations of He and displacement depth(a)

(b)

(c)

500 nm

200 nm50 nm(d)(e)

50 nm

50 nm

Fig.2

Bright field micrographs of Fe (a), F82H-IEA showing structure of matensite lath (b) and dislocation (c), andF82H-ODS showing dislocation (d) and weak-beam dark field image showing nano-scale oxide particles(e).

80S. Chen et al., Helium Irradiation Hardening in Fe, F82H-IEA and F82H-ODS Steel

number density of 5.8 × 1023 m-3 for Fe, 1.1 nm with 4.9 ×1023 m-3 for F82H-IEA and 1.3 nm with 7.4 × 1023 m-3 for
F82H-ODS, respectively. Black dots were also observed.

the He concentration is higher than 1000 appm.
F82H-ODS with a higher hardness shows a smaller
increase in bulk hardness than F82H-IEA. Irradiation
hardening is defined as the bulk hardness difference
between samples before and after helium irradiation.
Irradiation hardening (ǼH) is 3-5, 1-3, and 0.5-1 GPa withhelium concentration of 500-2000 appm for Fe, F82H-IEA
and F82H-ODS, respectively.
Fig. 4 shows under-focused (Ǽf = -500 nm) TEM
images of 0.2 at.% He implanted Fe, F82H-IEA andF82H-ODS. The images indicate that cavities uniformly
appear as white dots surrounded by a dark Fresnel fringe.
The mean size of the cavities is 1.4 nm with an average

(a)

(a)

(b)

(b)

(c)

(c)

Fig.4

Fig.3 Helium concentration dependence of bulk
hardness extracted from Nix-Gao model byusing
nanoindentation
andirradiation
hardening.

Cavities appearing in Fe (a), F82H-IEA (b), andF82H-ODS (c) after He irradiation with 2000
appm helium.

Although the microstructure of as-received Fe,
F82H-IEA and F82H-ODS are quite different, the mean
size and distribution of cavities induced by He irradiation

81S. Chen et al., Helium Irradiation Hardening in Fe, F82H-IEA and F82H-ODS Steelat room temperature are not largely different. While, theirradiation hardening differs significantly with materials.
Irradiation hardening is produced by helium irradiation. Fe
possesses the highest irradiation hardening, followed by
F82H-IEA and then F82H-ODS. Irradiation hardening ofFe is saturated when the He concentration is higher than
1000 appm. No saturation tendency was observed for
F82H-IEA and F82H-ODS. The results suggest that the
microstructure of the as-received materials does not havesignificant effects on the induced defects by He irradiation
at room temperature. However the contribution of theinduced defects to the strengthening depends on the
microstructure of as-received materials.

4. Conclusions

Helium irradiation induced hardening at room
temperature was examined for Fe, F82H-IEA andF82H-ODS. Irradiation hardening occurred by helium
implantation with concentration ranging 500-2000 appm,
resulting in 3-4, 1-3, and 0.5-1 GPa hardening in Fe,
F82H-IEA and F82H-ODS, respectively. The dependenceof He irradiation induced hardening with Heconcentration was investigated. F82H-ODS exhibited alower irradiation hardening than Fe and F82H-IEA.
The effect of He irradiation on microstructure
evolution in Fe, F82H and its ODS steel was examined by
TEM observation. Cavities with homogeneous
distribution were observed in all materials with Heconcentration of 2000 appm. The mean size and number
density of cavities are not largely different for Fe,
F82H-IEA and F82H-ODS.
By He irradiation at room temperature, themechanical property change is dependent on themicrostructure of as-received materials. A material withhigher hardness indicates a lower hardening. However,
the as-received microstructure does not affectsignificantly the He irradiation induced defects at room
temperature in this experiment.

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