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Секция 2. Радиационные эффекты в твердом теле Section 2.
Radiation effects in solids
13-я Международная конференция «Взаимодействие излучений с
твердым телом», 30 сентября - 3 октября 2019 г., Минск, Беларусь
13th International Conference “Interaction of Radiation with
Solids”, September 30 - October 3, 2019, Minsk, Belarus
103
IRRADIATION-INDUCED HARDENING OF STRUCTURAL MATERIALS AFTER LOW
TEMPERATURE IRRADIATION
S.A. Karpov, G.D. Tolstolutskaya
National Science Center “Kharkov Institute of Physics and
Technology”, 1 Academicheskaya Str., 61108 Kharkov, Ukraine,
[email protected], [email protected]
Irradiation-induced hardening has been investigated in relation
to austenitic SS316 steel, ferritic-martensitic T91 steel and
20Cr-
40Fe-20Mn-20Ni high-entropy alloy (HEA). Specimens were
irradiated with 1.4 MeV/Ar ions to doses of 0.5 - 10 displacements
per atom (dpa) at room temperature. Hardening of the irradiated
layer was examined with nanoindentation technique. The behavior of
the hardness-depth curves was analyzed with respect to the ion
irradiation dose. Regression analysis performed for high dose
regime of irradiation by using a power-law function of the form ∆Н
∝ (dpa)n gives good agreement with the experimental data at n ~
0.1-0.16 for structural alloys having different composition and
different structural state.
Keywords: irradiation; nanoindentation; hardness; steels;
high-entropy alloy.
Introduction Metals exposed to irradiation are known to
harden
due to the generation of Frenkel pair defect clusters that act
as obstacles to dislocation motion under an applied stress. This
hardening increases the yield strength, σy, of the material but
reduces the ductility and causes embrittlement. Irradiation
hardening in metallic materials is strong after irradiation at low
temperatures (usually below 300 °C) because significant quantities
of radiation-induced defect clusters are retained, and they impede
the generation and glide of dislocations during deformation
[1].
To simulate 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. The solution of problem ion irradiation – shallow
depth of damage layer that making it difficult to investigate the
mechanical properties – is possible by using nanoindentation
method. However, for the successful implementation of this
methodology, it is necessary to resolve such issues as the
correlation the change in strength with the plastic deformation,
the dose dependent of defect-cluster accumulation.
The purpose of the present work is the investigation of the
irradiation hardening behavior in different types of structural
materials with irradiation dose.
Main part
The specimens of SS316 and T91 steels and high entropy alloy
having the dimensions of 10×7×0.3 mm were used for investigations.
The specimens of SS316 steel before experiments were annealed at
1340 K for one hour in a vacuum ~10-4 Pa. The T91
ferritic-martensitic steel of composition 9Cr–1Mo with minor
alloying elements of Ni, Nb, V, and C was supplied by INDUSTELL,
Belgium (melting: 504/3, heat: 82566-4). The material was delivered
as hot rolled and heat treated plates with a thickness of 40 mm.
The heat treatment consisted of a normalization treatment at 1040
°C for 30 min followed by air cooling and then tempered at 730 °C
for 60 min followed by air cooling to room temperature.
High entropy alloy with the compositions (in at.%) of
20Cr-40Fe-20Mn-20Ni were produced by arc
melting in a high-purity argon in a water-cooled copper mould.
The purities of the alloying elements were above 99.9%. To ensure
chemical homogeneity, the ingots were flipped over and re-melted a
least 5 times. The produced ingots had dimensions of about 6×15×60
mm. The alloy was studied after homoge-nization annealing.
Homogenization was carried out at 1050 °C and lasted for 24 hours.
Prior to homoge-nization samples were sealed in vacuumed (10-2
Torr) quartz tubes filled with titanium chips to prevent
oxidation.
TEM observation of unirradiated samples (Fig. 1) showed
single-phase FCC crystal lattice for SS316 and 20Cr-40Fe-20Mn-20Ni
high-entropy alloy and dual phase morphology containing ferrite and
martensite phases for T91 steel.
a
b
Ferrite Martensite
c
Fig. 1. The initial microstructure of SS316 steel (a),
20Cr-40Fe-20Mn-20Ni high-entropy alloy (b) and T91 steel (c)
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Секция 2. Радиационные эффекты в твердом теле Section 2.
Radiation effects in solids
13-я Международная конференция «Взаимодействие излучений с
твердым телом», 30 сентября - 3 октября 2019 г., Минск, Беларусь
13th International Conference “Interaction of Radiation with
Solids”, September 30 - October 3, 2019, Minsk, Belarus
104
Samples were irradiated with 1.4 MeV argon ions in a dose range
of 0.5-10 dpa. All irradiations were carried out with
accelerating-measuring system “ESU-2” [2] at room temperature (RT).
The depth distribution of gas atoms concentration and damage for
ion irradiation with argon ions shown in Fig. 2.
Fig. 2. Damage and concentration profiles as a function of depth
calculated using SRIM [3] for 1.4 MeV Ar+ irradiation of Fe to a
dose of 4·1019 m-2
Fig. 3 shows nanoindentation hardness as a function of indenter
displacement of the unirradiated and irradiated 316 steel. The
irradiation of SS316 with Ar ions at RT leads to an increase of
nanohardness.
0 200 400 600 800 1000
2
3
4
5 0 dpa 0,5 dpa 0,7 dpa 1,0 dpa 2,0 dpa 5,0 dpa 10 dpa
Indentation depth, nm
Hard
ness
, GPa
Irradiation dose
Fig. 3. Nanoindentation hardness vs. indentation depth measured
for the unirradiated and irradiated SS316 steel
In all samples, the first 100 nm of displacement shows a
considerable increase in the scatter of the data due to
tip-rounding artifacts and surface preparation effects. Therefore,
for all samples the first 100 nm of data will be ignored for the
remainder of the analysis.
Generally, indentation hardness of ion irradiated materials
represents the superposition of the bulk hardness, indentation size
effect (ISE) and the irradiation induced hardening [4]. The
analysis method by nanoindentation measurement is based on the
Nix-Gao model [5] that describes the concept of geometrically
necessary dislocations required to accommodate the indenter as well
as Kasada et al. method [6] that extended model [5] by a
film-substrate system based on so-called the soft substrate effect.
The ion irradiated materials, according to [6], can be considered
as “hardened layer–substrate” systems. By
redrawing the hardness profile in terms of Nix-Gao plot (squared
hardness vs. reciprocal depth), the bulk-equivalent hardness of the
ion-irradiated region, H0irr, has been evaluated (Fig. 4).
2 4 6 8
5
10
15
20 1 dpa un-irradiated
H0=2.16 GPa
Squa
red
hard
ness
, GPa
2
Reciprocal depth, µm-1
Hirr0 =3.76 GPa
2 4 6 8
5
10
15
20 1 dpa un-irradiated
H0=2.16 GPa
Squa
red
hard
ness
, GPa
2
Reciprocal depth, µm-1
Hirr0 =3.76 GPa
Fig. 4. Nix-Gao plot for unirradiated and argon irradiated (1
dpa) SS316 steel
For the unirradiated SS316, an approximately linear relationship
is observed for data with depths greater than 100 nm. The
irradiated material represents a bilinear relationship with a
shoulder around 160 nm. Such bilinear behavior is associated with
the plastic zone extending into the unirradiated region of
material, beneath the irradiated layer.
In the same way, the bulk-equivalent hardness of the
ion-irradiated region was determined for all irradiation doses of
SS316, T91 and HEA.
In this study hardness profiles that have been analyzed for
determining the bulk-equivalent hardness of as-received, H0, and
irradiated alloys, H0irr, were obtained from load-displacement data
using the method of Oliver and Pharr [7]. This method is adopted as
the standard method for the analysis of nanoindentation results.
One significant limitation of this method is that it does not
account for pile-up or sink-in of testing material around the
indent. In the case of pile-up, the contact area is greater than
predicted by the method that can lead to an overestimation of the
indentation hardness.
Thereby, after indentation the hardness impres-sions were imaged
using SEM (Fig. 5) to measure contact areas and examine the extent
of pile-up, in a similar method to that proposed in [8].
The effect of pile-up was not observed in SS316 and HEA alloy
and the effect of sink-in around the in-dents appears to be
insignificant for these two mate-rials and not taken into account
at processing data on nanoindentation (Fig. 5, a, b, d, e).
However, for the T91 steel, the pile-up effect is quite pronounced
(Fig. 5 c, f).
In the case of T91, two different contact areas were determined
from the SEM images. According to Fig. 5 the pile-up unaffected
corner-to-corner area, Acc, represents the area of the triangle
defined by the corners of the hardness impression (see Fig. 5 c,
f). The second measure of the contact area was the actual contact
area, Aact, which includes the extra area contained in the pile-up
(Fig. 5 c, f). These specified areas were determined by a digital
image processing, and their ratio, Aact/Acc, provided an estimation
of the pile-up extent.
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Секция 2. Радиационные эффекты в твердом теле Section 2.
Radiation effects in solids
13-я Международная конференция «Взаимодействие излучений с
твердым телом», 30 сентября - 3 октября 2019 г., Минск, Беларусь
13th International Conference “Interaction of Radiation with
Solids”, September 30 - October 3, 2019, Minsk, Belarus
105
2 µm a b c
d e f
Fig. 5. SEM images showing deformed regions surrounding indents
in un-irradiated (a, b, c) and irradiated (d, e, f) regions of HEA
alloy (a, d), SS316 (b, e) and T91 steel (c, f). White lines -
corner-to-corner (Acc) of contact area (c, f)
The values of correction factor, Aact/Acc, were found in the
range of 1.10-1.18 and had a statistical scatter for as-received
and argon irradiated specimens. The average value of 1.14 ± 0.04
was accepted as a correction factor for T91 steel.
The dose dependence of measured and corrected for a pile-up
bulk-equivalent hardness in investigated materials is presented in
Fig. 6.
0 2 4 6 8 101.5
2.0
2.5
3.0
3.5
4.0
4.5
SS316 T91 HEA
Har
dnes
s, G
Pa
Damage, dpa Fig. 6. The dose dependence of bulk-equivalent
hardness of ion irradiated SS316, T91 and HEA alloy
The plots of hardness vs. dpa data showed two distinctive
regimes: a low-dose regime where a rapid hardening occurs and a
high-dose regime where the plot shows a considerably reduced slope.
Nano-indentation results showed that irradiation hardening in the
ion-irradiated SS316 and T91 approaches the quasisaturation mode at
doses ≥ 1 dpa. This is not as clear as in the case of HEA alloy due
to the limited data points, but the tendency appears to be the
same.
In the regression analysis [9], the radiation-induced increase
in yield stress, ∆σys (yield strength was calculated from hardness
measurement), was expressed in the form of a power law: ∆σys
=h·(dpa)n, where h and n are the regression coefficients and dpa is
displacements per atom. According to [10], n values were in the
range 0.31–0.4 in the low-dose regime for the fcc metals and
0.4–0.55 for bcc metals. In the high-dose regime n values for the
fcc metals varied more widely in the range 0.01–0.24.
Considering a simple barrier strengthening model with one
obstacle type with the same size and strength factor, a rapid
hardening can be explained if the number density of the obstacles
increasing linearly with displacement damage. The smaller n values
are probably due to effects of saturation at higher doses. Yamamoto
et al. [11] also suggested that the saturation can be physically
related to the depletion of solutes, in the case of a precipitation
hardening mechanism, or an excluded volume type effect in the case
of the accumulation of displacement damage-type defects.
Approximation of the irradiation hardening data, ∆H=H0irr–H0,
obtained in the present study for high-dose regime by a power
function of the form ∆Н ∝ (dpa)n indicates that n values are
changing in the range 0.10 – 0.16 for austenitic SS316 steel,
ferritic-martensitic T91 steel and 20Cr-40Fe-20Mn-20Ni high-entropy
alloy (Fig. 7).
1 10
0.5
1
2
3
4
n=0.16
n=0.10
SS316 T91 HEA n=0.11
∆H, G
Pa
Damage, dpa Fig. 7. Log-log plot for dose dependence of
irradiation hardening for SS316, T91 and HEA alloy
Defects that obviously cause hardening at low irradiation
temperature are dislocation-type defects. For all investigated here
materials, the irradiation-induced microstructure consists
predominantly of small (
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Секция 2. Радиационные эффекты в твердом теле Section 2.
Radiation effects in solids
13-я Международная конференция «Взаимодействие излучений с
твердым телом», 30 сентября - 3 октября 2019 г., Минск, Беларусь
13th International Conference “Interaction of Radiation with
Solids”, September 30 - October 3, 2019, Minsk, Belarus
106
The similar microstructural evolution and irradiation hardening
behavior between the HEA and SS316 indicate that at room
temperature the irradiation damage of fcc alloys is not sensitive
to compositional variation and configurational entropy.
Loop density is considerably smaller for T91 steel. It appears
that the ferritic-martensitic microstructure has a significant
effect in reducing hardening in T91. This can be explained by the
fine microstructure which contains carbides along boundaries, so a
high density of interfaces act as defect sinks for radiation
induced defects. This testifies that the materials containing a
number of defects or trapping sites suffer less irradiation
hardening or embrittlement.
Conclusions
Nanoindentation results showed that irradiation hardening in the
ion-irradiated austenitic SS316 steel, ferritic-martensitic T91
steel and 20Cr-40Fe-20Mn-20Ni high-entropy alloy approach of the
quasisaturation mode at doses ≥ 1 dpa.
Regression analysis performed using a power-law function of the
form ∆Н ∝ (dpa)n gives good agreement with the experimental data at
n ~ 0.10-0.16 for high-dose regime of irradiation.
The similarity of irradiation hardening behavior bet-ween SS316
and HEA as well as their similar microstructural evolution indicate
that the irradiation damage of fcc alloys is slightly sensitive to
compositional variation and configurational entropy at low
temperature irradiation.
The ferritic-martensitic microstructure with high density of
interfaces which act as defect sinks for radiation induced defects
has a significant effect in reducing hardening in T91. References
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