Effect of severe plastic deformation on radiation hardening of T91 ferritic-martensitic steel
Effect of thermomechanical treatment on radiation hardening behavior in T91 ferritic-martensitic steel was evaluated. An applying of severe plastic deformation (SPD) by the “upsetting-extrusion” method and subsequent heat treatment led to a considerable grain refinement, crushing of martensite lamel...
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Національний науковий центр «Харківський фізико-технічний інститут» НАН України
2021
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| Цитувати: | Effect of severe plastic deformation on radiation hardening of T91 ferritic-martensitic steel / V.N. Voyevodin, G.D. Tolstolutskaya, S.A. Karpov, A.N. Velikodnyi, M.A. Tikhonovsky, A.S. Kalchenko, G.N. Tolmachova, R.L. Vasilenko, I.E. Kopanets // Problems of Atomic Science and Technology. — 2021. — № 2. — С. 35-42. — Бібліогр.: 34 назв. — англ. |
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Voyevodin, V.N. Tolstolutskaya, G.D. Karpov, S.A. Velikodnyi, A.N. Tikhonovsky, M.A. Kalchenko, A.S. Tolmachova, G.N. Vasilenko, R.L. Kopanets, I.E. 2023-11-28T18:12:21Z 2023-11-28T18:12:21Z 2021 Effect of severe plastic deformation on radiation hardening of T91 ferritic-martensitic steel / V.N. Voyevodin, G.D. Tolstolutskaya, S.A. Karpov, A.N. Velikodnyi, M.A. Tikhonovsky, A.S. Kalchenko, G.N. Tolmachova, R.L. Vasilenko, I.E. Kopanets // Problems of Atomic Science and Technology. — 2021. — № 2. — С. 35-42. — Бібліогр.: 34 назв. — англ. 1562-6016 https://nasplib.isofts.kiev.ua/handle/123456789/194682 669.017:539.16 Effect of thermomechanical treatment on radiation hardening behavior in T91 ferritic-martensitic steel was evaluated. An applying of severe plastic deformation (SPD) by the “upsetting-extrusion” method and subsequent heat treatment led to a considerable grain refinement, crushing of martensite lamellas, reduction of MX carbides size and their more uniform distribution. Nanoindentation measurements of SPD-modified steel revealed a 1.4-fold increase in the hardness relative to the initial steel. Irradiation response of modified steel was examined after 1.4 MeV Ar+ ion irradiations in the dose range of 10…45 displacements per atom (dpa) at room temperature and 460 °C. Microstructure characterization was performed by means of transmission electron microscopy (TEM). It was found that dislocation loops and nano-sized argon bubbles dominated the damage microstructure after ion irradiation. The effects of SPD-induced transformations as well as nano-bubbles formation are discussed regarding to the hardening phenomenon observed in irradiated steel. Вивчено вплив термомеханічної обробки на радіаційне зміцнення феритно-мартенситної сталі Т91. Використання інтенсивної пластичної деформації (ІПД) методом «осаджування-видавлювання» з подальшою термообробкою призвело до значного подрібнення зерна, дроблення ламелей розподіленого мартенситу, зменшення розмірів карбідів типу МХ і більш рівномірному їх розподілу. Вимірювання нанотвердості ІПД модифікованої сталі показало збільшення твердості в 1,4 рази в порівнянні з вихідною сталлю. Радіаційну стійкість модифікованої сталі досліджували після опромінення іонами Ar+ з енергією 1,4 МеВ у діапазоні доз 10…45 зсувів на атом (зна) при кімнатній температурі і 460 °C. Вивчення мікроструктури виконувалося за допомогою просвічувальної електронної мікроскопії (ПЕМ). Виявлено, що дислокаційні петлі і нанорозмірні бульбашки аргону переважають у мікроструктурі пошкоджень після іонного опромінення. Обговорюються впливи індукованих ІПД-перетворень, а також нанобульбашок аргону на зміцнення, що спостерігається в опроміненій сталі. Изучено влияние термомеханической обработки на радиационное упрочнение ферритно-мартенситной стали Т91. Использование интенсивной пластической деформации (ИПД) методом «осадки-экструзии» с последующей термообработкой привело к значительному измельчению зерна, дроблению ламелей распределенного мартенсита, уменьшению размеров карбидов типа МХ и более равномерному их распределению. Измерение нанотвердости ИПД модифицированной стали показало увеличение твердости в 1,4 раза по сравнению с исходной сталью. Радиационную стойкость модифицированной стали исследовали после облучения ионами Ar+ с энергией 1,4 МэВ в диапазоне доз 10…45 смещений на атом (сна) при комнатной температуре и 460 °C. Изучение микроструктуры выполнялось с помощью просвечивающей электронной микроскопии (ПЭМ). Обнаружено, что дислокационные петли и наноразмерные пузырьки аргона преобладают в микроструктуре повреждений после ионного облучения. Обсуждаются влияния индуцированных ИПД-превращений, а также нанопузырьков аргона на упрочнение, наблюдаемое в облученной стали. The authors very much appreciated assistance of Dr Marta Serrano (Structural Material Division CIEMAT, Spain) in providing the T91-M material and great interest to this work. The work was financially supported by the National Academy of Science of Ukraine (program “Support of the development of main lines of scientific investigations” (KPKVK 6541230)). en Національний науковий центр «Харківський фізико-технічний інститут» НАН України Вопросы атомной науки и техники Physics of radiation damages and effects in solids Effect of severe plastic deformation on radiation hardening of T91 ferritic-martensitic steel Вплив інтенсивної пластичної деформації на радіаційне зміцнення феритно-мартенситної сталі T91 Влияние интенсивной пластической деформации на радиационное упрочнение ферритно-мартенситной стали Т91 Article published earlier |
| institution |
Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| collection |
DSpace DC |
| title |
Effect of severe plastic deformation on radiation hardening of T91 ferritic-martensitic steel |
| spellingShingle |
Effect of severe plastic deformation on radiation hardening of T91 ferritic-martensitic steel Voyevodin, V.N. Tolstolutskaya, G.D. Karpov, S.A. Velikodnyi, A.N. Tikhonovsky, M.A. Kalchenko, A.S. Tolmachova, G.N. Vasilenko, R.L. Kopanets, I.E. Physics of radiation damages and effects in solids |
| title_short |
Effect of severe plastic deformation on radiation hardening of T91 ferritic-martensitic steel |
| title_full |
Effect of severe plastic deformation on radiation hardening of T91 ferritic-martensitic steel |
| title_fullStr |
Effect of severe plastic deformation on radiation hardening of T91 ferritic-martensitic steel |
| title_full_unstemmed |
Effect of severe plastic deformation on radiation hardening of T91 ferritic-martensitic steel |
| title_sort |
effect of severe plastic deformation on radiation hardening of t91 ferritic-martensitic steel |
| author |
Voyevodin, V.N. Tolstolutskaya, G.D. Karpov, S.A. Velikodnyi, A.N. Tikhonovsky, M.A. Kalchenko, A.S. Tolmachova, G.N. Vasilenko, R.L. Kopanets, I.E. |
| author_facet |
Voyevodin, V.N. Tolstolutskaya, G.D. Karpov, S.A. Velikodnyi, A.N. Tikhonovsky, M.A. Kalchenko, A.S. Tolmachova, G.N. Vasilenko, R.L. Kopanets, I.E. |
| topic |
Physics of radiation damages and effects in solids |
| topic_facet |
Physics of radiation damages and effects in solids |
| publishDate |
2021 |
| language |
English |
| container_title |
Вопросы атомной науки и техники |
| publisher |
Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
| format |
Article |
| title_alt |
Вплив інтенсивної пластичної деформації на радіаційне зміцнення феритно-мартенситної сталі T91 Влияние интенсивной пластической деформации на радиационное упрочнение ферритно-мартенситной стали Т91 |
| description |
Effect of thermomechanical treatment on radiation hardening behavior in T91 ferritic-martensitic steel was evaluated. An applying of severe plastic deformation (SPD) by the “upsetting-extrusion” method and subsequent heat treatment led to a considerable grain refinement, crushing of martensite lamellas, reduction of MX carbides size and their more uniform distribution. Nanoindentation measurements of SPD-modified steel revealed a 1.4-fold increase in the hardness relative to the initial steel. Irradiation response of modified steel was examined after 1.4 MeV Ar+ ion irradiations in the dose range of 10…45 displacements per atom (dpa) at room temperature and 460 °C. Microstructure characterization was performed by means of transmission electron microscopy (TEM). It was found that dislocation loops and nano-sized argon bubbles dominated the damage microstructure after ion irradiation. The effects of SPD-induced transformations as well as nano-bubbles formation are discussed regarding to the hardening phenomenon observed in irradiated steel.
Вивчено вплив термомеханічної обробки на радіаційне зміцнення феритно-мартенситної сталі Т91. Використання інтенсивної пластичної деформації (ІПД) методом «осаджування-видавлювання» з подальшою термообробкою призвело до значного подрібнення зерна, дроблення ламелей розподіленого мартенситу, зменшення розмірів карбідів типу МХ і більш рівномірному їх розподілу. Вимірювання нанотвердості ІПД модифікованої сталі показало збільшення твердості в 1,4 рази в порівнянні з вихідною сталлю. Радіаційну стійкість модифікованої сталі досліджували після опромінення іонами Ar+ з енергією 1,4 МеВ у діапазоні доз 10…45 зсувів на атом (зна) при кімнатній температурі і 460 °C. Вивчення мікроструктури виконувалося за допомогою просвічувальної електронної мікроскопії (ПЕМ). Виявлено, що дислокаційні петлі і нанорозмірні бульбашки аргону переважають у мікроструктурі пошкоджень після іонного опромінення. Обговорюються впливи індукованих ІПД-перетворень, а також нанобульбашок аргону на зміцнення, що спостерігається в опроміненій сталі.
Изучено влияние термомеханической обработки на радиационное упрочнение ферритно-мартенситной стали Т91. Использование интенсивной пластической деформации (ИПД) методом «осадки-экструзии» с последующей термообработкой привело к значительному измельчению зерна, дроблению ламелей распределенного мартенсита, уменьшению размеров карбидов типа МХ и более равномерному их распределению. Измерение нанотвердости ИПД модифицированной стали показало увеличение твердости в 1,4 раза по сравнению с исходной сталью. Радиационную стойкость модифицированной стали исследовали после облучения ионами Ar+ с энергией 1,4 МэВ в диапазоне доз 10…45 смещений на атом (сна) при комнатной температуре и 460 °C. Изучение микроструктуры выполнялось с помощью просвечивающей электронной микроскопии (ПЭМ). Обнаружено, что дислокационные петли и наноразмерные пузырьки аргона преобладают в микроструктуре повреждений после ионного облучения. Обсуждаются влияния индуцированных ИПД-превращений, а также нанопузырьков аргона на упрочнение, наблюдаемое в облученной стали.
|
| issn |
1562-6016 |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/194682 |
| citation_txt |
Effect of severe plastic deformation on radiation hardening of T91 ferritic-martensitic steel / V.N. Voyevodin, G.D. Tolstolutskaya, S.A. Karpov, A.N. Velikodnyi, M.A. Tikhonovsky, A.S. Kalchenko, G.N. Tolmachova, R.L. Vasilenko, I.E. Kopanets // Problems of Atomic Science and Technology. — 2021. — № 2. — С. 35-42. — Бібліогр.: 34 назв. — англ. |
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ISSN 1562-6016. ВАНТ. 2021. №2(132) 35
https://doi.org/10.46813/2021-132-035
UDС 669.017:539.16
EFFECT OF SEVERE PLASTIC DEFORMATION ON RADIATION
HARDENING OF T91 FERRITIC-MARTENSITIC STEEL
V.N. Voyevodin
1,2
, G.D. Tolstolutskaya
1
, S.A. Karpov
1
, A.N. Velikodnyi
1
, M.A. Tikhonovsky
1
,
A.S. Kalchenko
1
, G.N. Tolmachova
1
, R.L. Vasilenko
1
, I.E. Kopanets
1
1
National Science Center “Kharkov Institute of Physics and Technology”, Kharkiv, Ukraine;
2
V.N. Karazin Kharkiv National University, Kharkiv, Ukraine
E-mail: voyev@kipt.kharkov.ua
Effect of thermomechanical treatment on radiation hardening behavior in T91 ferritic-martensitic steel was
evaluated. An applying of severe plastic deformation (SPD) by the “upsetting-extrusion” method and subsequent
heat treatment led to a considerable grain refinement, crushing of martensite lamellas, reduction of MX carbides size
and their more uniform distribution. Nanoindentation measurements of SPD-modified steel revealed a 1.4-fold
increase in the hardness relative to the initial steel. Irradiation response of modified steel was examined after
1.4 MeV Ar
+
ion irradiations in the dose range of 10…45 displacements per atom (dpa) at room temperature and
460 °C. Microstructure characterization was performed by means of transmission electron microscopy (TEM). It
was found that dislocation loops and nano-sized argon bubbles dominated the damage microstructure after ion
irradiation. The effects of SPD-induced transformations as well as nano-bubbles formation are discussed regarding
to the hardening phenomenon observed in irradiated steel.
INTRODUCTION
Ferritic-martensitic steels are prime candidate
materials for structural components in high-dose nuclear
power applications. Compared to austenitic stainless
steels, F-M alloys are considered to be more resistant to
neutron irradiation, from the viewpoint of radiation
swelling. In addition, these steels also have higher
thermal conductivity, lower coefficients of thermal
expansion, good creep characteristics, high strength at
elevated temperatures, and lower cost [1]. For F-M
steels with a high chromium content, the most
significant radiation effect is the degradation of
mechanical properties due to radiation hardening and
embrittlement. Since noticeable hardening is observed
at the lowest radiation dose of about 0.01 dpa [2],
understanding of the microstructural evolution and its
influence on the reduction of mechanical properties is of
high importance.
Significant improvement of mechanical properties is
anticipated through the formation of an ultra-fine-
grained or nanostructured state in the steel. The
formation of such a microstructure is possible by using
mechanical-heat treatment utilizing severe plastic
deformation, which can be carried out by various
methods 3, 4. It is expected that the radiation tolerance
of the material after severe plastic deformation will be
enhanced.
Irradiation of materials with accelerated ions to
simulate neutron reactor irradiation is a common
practice due to the short timescales for achieving
relatively high damage levels and the absence of
induced radioactivity.
However, ion irradiation has a significant drawback
a small depth of the damaged layer, which
complicates the study of mechanical properties. The
solution of this problem is possible by using of
nanoindentation, transmission and scanning electron
microscopy – small-scale methods that can be used to
study changes in the microstructure and mechanical
properties under irradiation in small volumes.
Recently, the effect of irradiation on the swelling
behavior in the promising T91-M ferritic-martensitic
steel after severe plastic deformation (SPD) and heat
treatment at 550 °C for 25 h was investigated [5].
The aim of the present work is to determine the dose
dependent hardness and evolution of microstructure of
T91 steel after SPD by the multiple “upsetting-
extrusion” method followed by heat treatment at 600 °С
and irradiation with accelerated argon ions up to 45 dpa
at temperatures 20 and 460 °С.
MATERIAL AND EXPERIMENTAL
DETAILS
T91-M steel manufactured by INDUSTELL,
Belgium (melting: 504/3, heat: 82566-4) was supplied
in the form of a 40 mm thick plate obtained by hot
rolling followed by heat treatment. The heat treatment
includes normalization at 1040 °C for 30 min, followed
by air cooling and then tempering at 730 °C for 60 min
with air cooling to room temperature (the so-called
N&T state). The chemical composition of T91-M steel
is shown in Table.
The chemical composition of Т91-M (wt.%)
Fe (bulk) Cr Mo Mn Si V Ni Nb Cu
Т91 8.76 0.862 0.597 0.317 0.186 0.099 0.073 0.054
Al C N P S Sn B Co As
0.021 0.088 0.003 0.019 0.0006 0.005 0.0001 0.019 0.007
mailto:voyev@kipt.kharkov.ua
36 ISSN 1562-6016. ВАНТ. 2021. №2(132)
In the present study the multi-cycle “upsetting-
extrusion” method was chosen to carry out the SPD.
The method was developed in NSC KIPT and it has
proved itself both in laboratory research and in the
industrial production of a number of materials with high
characteristics [3, 6]. This method consists in multiple
reiterations of operations of upsetting and extrusion on a
hydraulic press DB 2432 with a force of 160 tf. The
method is described in more detail in [4].
The deformation was carried out in the two-phase
region of stability of austenite and ferrite at a
temperature of 875 °C, which is below the Ac3 point by
30…35 °C. Samples with a 20 mm diameter and a
60 mm height were subjected to upsetting to a diameter
30 mm and then again extruded to a diameter of 20 mm.
The true deformation for 1 cycle of upsetting-
extrusion was e = 1.8. Three upsetting-extrusion cycles
were carried out (total true strain was e = 5.4). To
stabilize the microstructure formed as a result of SPD,
heat treatments were performed at 600 °C for 25 h.
Samples were irradiated with 1.4 MeV argon ions in
a dose range of 10…45 dpa. All irradiations were
carried out with accelerating-measuring system
“ESU-2” [7], which contain Van de Graaf accelerator.
The irradiations were performed at temperatures 20 and
460 °С.
Nanohardness was measured by Nanoindenter G200
with a Berkovich type indentation tip. Each sample was
applied at least 20 prints at a distance of 35 µm from
each other. Nominal maximum displacement of
2000 nm was used for all measurements on unirradiated
and ion-irradiated steel [8]. The methodology of Oliver
and Pharr was used to find the hardness [9].
For transmission studies, plates of 0.3 mm thick
were cut out using the electrospark method, which were
mechanically thinned to a thickness of 0.22 mm, and
then electropolished. To obtain a hole the thickness of
the samples was reduced by standard jet
electropolishing in a Tenupol installation in an
electrolyte of 80% C2H5OH, 10% HClO4, 10% C3H8O3
at a voltage of 70 V at room temperature. TEM in
kinematic bright-field mode was primarily used to
characterize radiation-induced structures. To remove a
specified depth layer of material from irradiated side of
the sample the electro-pulse technique was used and
then TEM studies were carried out [10]. Secondary
electron images produced in SEM were used for
investigations of as-received and irradiated specimens in
regions surrounding indents.
RESULTS AND DISCUSSIONS
The microstructure of as-received T91 steel
(hereinafter sample code T91-M) represents the
tempered martensite structure with the occurrence of
boundaries of former austenite grains and subgrains
(Fig. 1).
These boundaries are decorated with precipitates of
М23С6 carbides. The average size of the former austenite
grains is ~ 20 μm, and the average size of subgrains is
≈ 8 μm. Martensite lamellas with a transverse size of
0.25…0.5 μm are observed inside the former austenite
grains. In additions, a significant number of small
precipitates of ≤ 50 nm in size, which are usually
referred to as MX-type phases is also observed.
Fig. 1. Microstructure of as-received T91-M steel
An applying of SPD by the “upsetting-extrusion”
method led to the development of a broken martensite
type structure and to an effective decrease of the
average grain size from 8 μm in the as-received state to
100 nm after SPD. After heat treatment at 600 °C for
25 h (hereinafter, sample code T91-MSPD), sufficiently
large grains of typical “ferritic” morphology have
formed (Fig. 2), which appear to be the result of
recrystallization of fine grains of final austenite or fine-
grained areas of “broken” martensite. “Broken”
martensite in some zones is transformed into a more
“classic” one as a result of the enlargement of
martensite lamellas with the average grain size
increasing up to 210 nm. The average size and density
of M23C6 precipitates is 120 nm, and 4.2∙10
19
m
–3
,
respectively. Carbides of the MX type having the size of
≤ 24 nm and density of 2.6∙10
20
m
-3
are more evenly
distributed in the grains. TEM observation of
unirradiated T91-MSPD sample is represented in Fig. 2.
Fig. 2. Microstructure of T91-MSPD
Samples of T91-MSPD were irradiated with
1.4 MeV argon ions to doses of (0.3…1)10
17
cm
–2
. The
average damage level of 10…45 dpa over the whole
projective ranges of Ar ions instead of the peak damage
level was used. The depth distributions of Ar atoms
concentration and produced by irradiation damage are
shown in Fig. 3.
ISSN 1562-6016. ВАНТ. 2021. №2(132) 37
Fig. 3. Calculated profiles of damage and concentration
of 1.4 MeV Ar ions implanted in T91-M steel to a dose
of 1∙10
17
cm
–2
Charged particle implantations in this study were
simulated using SRIM. Following the recommendations
of Stoller et al. [11], the damage energy (the kinetic
energy available for creating atomic displacements [12])
was calculated, and subsequently the dpa values via the
NRT equation were obtained [13].
Fig. 4 shows the nanohardness (H) as a function of
the indenter displacement (h) for unirradiated and
irradiated to 10 dpa sample of T91-MSPD. Irradiation
with Ar ions up to a dose of 10 dpa at 460 °C leads to an
increase in nanohardness.
0 500 1000 1500 2000
3.5
4.0
4.5
5.0
5.5
6.0
irradiated
H
a
rd
n
e
s
s
,
G
P
a
Indentation depth, nm
unirradiated
Fig. 4. Average nanohardness values as a function of
indentation depth for unirradiated and irradiated to
10 dpa at 460 °C sample of T91-MSPD
For all samples in the first 150 nm, there is a
significant scatter in the data through indenter tip
artifacts and surface preparation effects. Therefore, for
all samples, data for the first 150 nm will be ignored in
the rest of the analysis.
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 has
been evaluated as 4.75 GPa for irradiation fluence of
10 dpa (Fig. 5). For irradiation doses of 25 and 45 dpa
the nanohardness of T91-MSPD was evaluated to be
4.85 and 4.95 GPa, respectively.
0 1 2 3 4
15
20
25
30
Hunirr
0 = 3.9 GPa
S
q
u
a
re
d
h
a
rd
n
e
s
s
,
G
P
a
2
Reciprocal depth, mm-1
Hirr
0 = 4.75 GPa
Fig. 5. Nix-Gao plot for unirradiated and argon
irradiated T91-MSPD to a dose of 10 dpa at 460 °С
In the case of unirradiated T91-MSPD samples
H0
unirr
was estimated as 3.9 GPa (see Fig. 5), while for
the as-received T91-M samples, this value was
2.98 GPa. A significant increase in hardness of T91-
MSPD appears to be due to the grain refinement,
crushing of lamellas, a decrease in MX carbides size
and their more uniform distribution.
An important feature of indentation experiments is
that the material around the contact area tends to deform
upwards (pile-up) or downwards (sink-in) with respect
to the indented surface plane [14, 15]. Depending on the
strain-hardening characteristics of the examined
material, the area around indents tends to sink in, when
the sample is fully annealed and has a high strain-
hardening potential. On the contrary, the surface around
indents tends to pile up, when the indented sample is
heavily pre-strained or exhibits a low strain-hardening
potential.
In previous work [16], the plastically deformed
material has found to form a pile-up around the indent
in the unirradiated T91-M (see Fig. 6,a). In the argon-
irradiated samples (see Fig. 6,b) the pile-up lobes were
also formed. In addition, the presence of highly
localized slip steps that appear to be curved rather than
linear implies the extensive dislocation cross-slip
between the existing slip planes.
It should be noted that the pile-up behavior is not
only modified by ion implantation. For example,
residual stresses can also significantly alter the pile up
effect [17].
Indeed, the pile-up unaffected corner-to-corner area
which represents the area of the triangle defined by the
corners of the hardness impression (Fig. 7,a,c,e) has
been observed in unirradiated T91-MSPD samples after
applying of SPD by the “upsetting-extrusion” method
and subsequent heat treatment. There are no indications
of the formation of pile-up lobes or localized slip steps.
This fact confirms that the T91-MSPD is heavily pre-
strained in comparison with as-received T91-M.
The irradiated samples also showed virtual, if any,
pile-up effect (see Fig. 7,b,d,f). For this reason, a
contact area correction for the pile-up was not
attempted.
38 ISSN 1562-6016. ВАНТ. 2021. №2(132)
a b
Fig. 6. SEM images showing deformed regions surrounding indents in un-irradiated (a) and irradiated regions (b)
in the T91-M [16]
a b c
d e f
Fig. 7. SEM images showing areas that surround prints in unirradiated (a, b, c) and irradiated up to 10 (d), 25 (e),
and 45 dpa (f) T91-MSPD samples
Although the complexity of T91-MSPD micro-
structure impedes a detailed analysis of the underlying
microscopic mechanism, there are clear indications that
the irradiated samples showed considerable changes in
microstructure compared to the unirradiated steel
sample.
In the region of the first 200 nm, irradiation-induced
defects of very small-sized “black spots” and
dislocation loops were observed in the microstructure of
T91-MSPD samples irradiated at Troom. These defects
disappeared upon tilting of sample by small degrees,
which clearly indicates that these are not precipitates
(Fig. 8,a,b).
A high number density of nanoscale white dots,
distributed mainly in the depth range of 400…600 nm
from the sample surface, was observed in the
microstructure of T91-MSPD samples irradiated at Troom
(see Fig. 8,c) and 460 °C (see Fig. 8,d). The distribution
of these white dots approximately agrees with the argon
concentration profile shown in Fig. 3.
The image in the inset of Fig. 8,c was obtained
under overfocus conditions. For the underfocus
condition, defects appear as white dots surrounded by
black fringes, while for the overfocus condition, the
defects are black and the fringes are white, indicating
the formation of cavities.
Near spherical shape of nanocavities is characteristic
feature for gas bubbles when the surface energy and
internal pressure are balanced [18]. Based on these
observations, the cavities formed in the present
irradiation condition were considered to be Ar-filled
bubbles.
Bubble diameters were determined using ImageJ
software as the distance measured between two inside
edge of the first dark Fresnel fringe in underfocus
conditions. In order to assure the statistical significance
of the determined size distributions, a minimum number
of 200 bubbles was analyzed for each examined
micrograph. Errors in the estimates may occur due to
surface oxidation effects, surface roughness, bubble
ISSN 1562-6016. ВАНТ. 2021. №2(132) 39
placement within the TEM foil, and the Ar content
within individual bubbles.
For more accurate determination of cavity density,
the TEM sample thickness was determined using a
convergent beam electron diffraction (CBED) pattern
acquired in the two-beam approximation [19]. The
thickness estimation method is based on a comparison
of the measured and simulated intensity profiles across
the diffraction disc. The method has a fairly good
accuracy with a limited number of thickness fringes,
which is typical for crystals of small thickness.
An average bubble diameter was estimated to be
(0.8±0.25) nm with a density of 310
24
m
–3
for
irradiation T91-MSPD at Troom up to a dose of 10 dpa.
For irradiation at 460 °С up to the same dose the
average bubble diameter was (2.5±0.5) nm and a density
of 1.510
24
m
–3
in the depth range of 400…600 nm.
Today it is believed that defects that become
obstacles for moving dislocations at low temperatures
are dislocation loops and network dislocations. At
intermediate and high temperatures, voids and
precipitates become more important and loops less
important to strength and hence ductility changes.
a b
c d
Fig. 8. Bright-field (BF) images showing the irradiation induced microstructures T91-MSPD irradiated by Ar ions
up to 10 dpa at Troom (a–c) and at 460 °C (d). The image in Fig. 8,a,b was obtained upon tilting of sample by 0 and
2.0 degrees and shows some dislocation loops and precipitates were marked by dotted line circles and arrows,
respectively. 111 zone selected area electron diffraction pattern obtained from the region in Fig. 8,a.
The enlarged micrographs of that in the box of figure (c) obtained under the over-focus condition
To evaluate the efficiency of different types of
cavities as obstacles, molecular dynamics simulation
was previously used by authors [20] to detail the
interaction of an edge dislocation with a 2-nm void or
He bubble in Fe single crystal. It was found that the
dislocation is strongly pinned by these obstacles. A
2-nm void is a stronger obstacle than a 2-nm He bubble
at low He contents, whereas at the highest He content,
five He atoms per vacancy, the He bubble becomes
stronger than the void.
It has been recently suggested that high-density
argon bubbles could act as obstacles to dislocation
motion leading to hardening [21]. Uniformly distributed
Ar bubbles can be considered as a stronger barrier to the
motion of dislocation resulting in the hardening in the
irradiated P92 steel. In addition, it can be inferred that
the hardness increase caused by the formation of Ar
bubbles is stronger than the hardness decrease resulting
from annihilation of other irradiation defects [22]. The
microstructure examination performed in this study
have confirmed that hardening of the irradiated T91-
MSPD steel gradually increases with an increase in the
irradiation dose up to 45 dpa due to the formation of
dislocation loops and nano-sized Ar-associeted bubbles.
In general, radiation strengthening is highly dose-
dependent at low fluences and saturates at doses
exceeding the critical value. The critical saturation dose
depends on the irradiation temperature, increasing with
40 ISSN 1562-6016. ВАНТ. 2021. №2(132)
increasing irradiation temperature up to 330 °C, and
then decreasing at higher irradiation temperatures [28].
G. Was et al. [2], analyzing data on radiation-
induced strengthening of the heats of 304SS and 316SS
steels irradiated with protons and neutrons, showed that
radiation hardening of austenitic steels reaches
saturation at about a few dpa. The same results were
revealed at irradiations of a number of austenitic alloys
with accelerated ions, and in particular, with argon ions
[23–25].
It appears that a similar tendency can be extended to
ferritic-martensitic steels. At least, for steels that are in
the focus of current study, i.e., T91-MSPD and T91-M,
the radiation hardening also tends to saturation at about
a few dpa. However, the magnitude of the hardening is
almost half compared to austenitic alloys, which
indicates a lower probability of embrittlement.
Comparison of the radiation-induced hardening (ΔН,
the difference of hardness values of irradiated and
unirradiated materials) of ferritic-martensitic steels
obtained by different authors [16, 26–33] (Fig. 9) shows
some differences in the data, which is probably due,
first of all, to the difference in the irradiation
temperature. Additionally, a noticeable difference in
data is observed for materials with different
microstructure. For instance, a clear discrepancy can be
seen by comparing alloys with ferrite (ΔН = 0.95 GPa)
and tempered martensite (ΔН = 0.58 GPa) structures at
low doses of 2…3 dpa. At a dose of 30 dpa these values
are estimated as 1.32 and 0.85 GPa, respectively,
[27, 28].
Fig. 9. Relative radiation-induced hardening of ferritic-martensitic steels.
Dotted and dash-dotted lines are drawn for eyes only
Clearly, the fine microstructure of tempered
martensite has a significant effect on the decrease in
hardening under irradiation. The ultra-fine-grained
structure of T91-MSPD steel, in turn, has a high density
of distribution boundaries, which act as absorbers of
radiation defects. For this reason, the formation of
obstacles to the movement of dislocations is expected to
be impeded in ferritic-martensitic steel with a high
density of initial traps/sinks for radiation defects. And,
thus, ferritic-martensitic steels are less susceptible to
radiation hardening/embrittlement compared to
materials with a purely ferritic microstructure (see
Fig. 9).
Due to the substantial difference in hardness values
of unirradiated samples, a comparison between T91-
MSPD and T91-M, in terms of irradiation-induced
hardening, appears to be the more correct considering
the corresponding nanohardness increments described
as the ratio of H and H0 [25, 34]. According to the
obtained data, the ratios of hardness are approximately
19 and 24% in case of irradiations performed at 20 °С
for T91-MSPD and T91-M, respectively, and 22% for
T91-MSPD irradiated at 460 °С. So, the hardening
degree of both alloys is nearly equivalent when the
irradiation dose is approximately 10 dpa. At this dose,
the hardening reaches almost saturation or
insignificantly increases with the radiation dose.
Although the initial strength and density of sinks are
different in T91-M and T91-MSPD steels, it appears
that the formation of Ar-associated nano-bubbles in a
similar manner influences the hardening behavior of
steels under argon-ion irradiation. In addition, the
processes of recombination of radiation defects become
comparable due to the large number of defects at doses
about 10 dpa that predetermines a comparable degree of
hardening of both alloys.
CONCLUSIONS
Ferritic-martensitic steel T91-MSPD with an ultra-
fine-grained modified structure was irradiated with
1.4 MeV Ar
+
ions to doses from 10 to 45 dpa at room
temperature and 460 °C. The radiation microstructure
and hardening of steel were studied and the following
conclusions were drawn:
Observed significant increase (1.3 times) in the
hardness of unirradiated samples T91-MSPD is due to
grain refinement, crushing of lamellae, a decrease in the
ISSN 1562-6016. ВАНТ. 2021. №2(132) 41
size of carbides of the MX type and their more uniform
distribution.
The hardening of the irradiated steel gradually
increases with an increase in the irradiation dose up to
45 dpa due to the formation of dislocation loops and
nano-sized Ar-associeted bubbles.
The hardening degree of T91-MSPD and T91-M is
nearly equivalent when the irradiation dose is
approximately 10 dpa.
ACKNOWLEDGEMENTS
The authors very much appreciated assistance of Dr
Marta Serrano (Structural Material Division CIEMAT,
Spain) in providing the T91-M material and great
interest to this work.
The work was financially supported by the National
Academy of Science of Ukraine (program “Support of
the development of main lines of scientific
investigations” (KPKVK 6541230)).
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Статья поступила в редакцию 25.02.2021 г.
ВЛИЯНИЕ ИНТЕНСИВНОЙ ПЛАСТИЧЕСКОЙ ДЕФОРМАЦИИ
НА РАДИАЦИОННОЕ УПРОЧНЕНИЕ ФЕРРИТНО-МАРТЕНСИТНОЙ СТАЛИ Т91
В.Н. Воеводин, Г.Д. Толстолуцкая, С.А. Карпов, А.Н. Великодный, М.А. Тихоновский,
А.С. Кальченко, Г.Н. Толмачева, Р.Л. Василенко, И.Е. Копанец
Изучено влияние термомеханической обработки на радиационное упрочнение ферритно-мартенситной
стали Т91. Использование интенсивной пластической деформации (ИПД) методом «осадки-экструзии» с
последующей термообработкой привело к значительному измельчению зерна, дроблению ламелей
распределенного мартенсита, уменьшению размеров карбидов типа МХ и более равномерному их
распределению. Измерение нанотвердости ИПД модифицированной стали показало увеличение твердости в
1,4 раза по сравнению с исходной сталью. Радиационную стойкость модифицированной стали исследовали
после облучения ионами Ar
+
с энергией 1,4 МэВ в диапазоне доз 10…45 смещений на атом (сна) при
комнатной температуре и 460 °C. Изучение микроструктуры выполнялось с помощью просвечивающей
электронной микроскопии (ПЭМ). Обнаружено, что дислокационные петли и наноразмерные пузырьки
аргона преобладают в микроструктуре повреждений после ионного облучения. Обсуждаются влияния
индуцированных ИПД-превращений, а также нанопузырьков аргона на упрочнение, наблюдаемое в
облученной стали.
ВПЛИВ ІНТЕНСИВНОЇ ПЛАСТИЧНОЇ ДЕФОРМАЦІЇ НА РАДІАЦІЙНЕ
ЗМІЦНЕННЯ ФЕРИТНО-МАРТЕНСИТНОЇ СТАЛІ Т91
В.М. Воєводін, Г.Д. Толстолуцька, С.О. Карпов, О.М. Великодний, М.А. Тихоновський,
О.С. Кальченко, Г.М. Толмачова, Р.Л. Василенко, І.Є. Копанець
Вивчено вплив термомеханічної обробки на радіаційне зміцнення феритно-мартенситної сталі Т91.
Використання інтенсивної пластичної деформації (ІПД) методом «осаджування-видавлювання» з
подальшою термообробкою призвело до значного подрібнення зерна, дроблення ламелей розподіленого
мартенситу, зменшення розмірів карбідів типу МХ і більш рівномірному їх розподілу. Вимірювання
нанотвердості ІПД модифікованої сталі показало збільшення твердості в 1,4 рази в порівнянні з вихідною
сталлю. Радіаційну стійкість модифікованої сталі досліджували після опромінення іонами Ar
+
з енергією
1,4 МеВ у діапазоні доз 10…45 зсувів на атом (зна) при кімнатній температурі і 460 °C. Вивчення
мікроструктури виконувалося за допомогою просвічувальної електронної мікроскопії (ПЕМ). Виявлено, що
дислокаційні петлі і нанорозмірні бульбашки аргону переважають у мікроструктурі пошкоджень після
іонного опромінення. Обговорюються впливи індукованих ІПД-перетворень, а також нанобульбашок аргону
на зміцнення, що спостерігається в опроміненій сталі.
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