P. PODANY et al.: RECRYSTALLIZATION BEHAVIOUR OF A NICKEL-BASED SUPERALLOY
199–205
RECRYSTALLIZATION BEHAVIOUR OF A NICKEL-BASED
SUPERALLOY
OBNA[ANJE SUPERZLITINE NA OSNOVI NIKLJA PRI
REKRISTALIZACIJI
Pavel Podany, Zbysek Novy, Jaromir Dlouhy
COMTES FHT, Prumyslova 995, 334 41 Dobrany, Czech Republic
pavel.podany@comtesfht.cz
Prejem rokopisa – received: 2014-08-01; sprejem za objavo – accepted for publication: 2015-04-16
doi:10.17222/mit.2014.163
The thermomechanical processing of a nickel-based superalloy is the way to considerably influence the grain size. A uniform
coarse-grain size increases the creep strength and the crack-growth resistance. In this work, the processing for achieving a
uniform recrystallized-grain structure with a variation in the thermomechanical parameters is investigated.
The MoNiCr alloy is intended for modern designs of nuclear reactors, in which molten fluoride salts are used as the coolants in
the primary and/or secondary circuit. It represents a material alternative with a high corrosion resistance in the area of fluoride
salts and it has very good creep properties in the temperature range of 650–750 °C as well.
The manufacture of vessels and fittings from the MoNiCr alloy requires the mastering of the technology for forming this
high-alloyed material. The key step seems to be the transition of the cast state of the material to the state of a cast recrystallized
microstructure with homogenous fine grains. A particular stress condition is, besides the temperature, very important during hot
forming. Nickel alloys are able to accept a significantly higher deformation level if the compressive stress prevails.
A forming process involving the compression state of stress increases the probability that the material will reach, without
failure, a level of deformation that allows recovery and recrystallization processes. A particular deformation process was carried
out on a physical simulator. The preceding cold deformation essentially accelerates the recrystallization process of a deformed
cast structure.
Keywords: recrystallization, nickel alloys, physical simulation
Termomehanska obdelava superzlitine na osnovi niklja je na~in, s katerim lahko mo~no vplivamo na velikost kristalnih zrn.
Enakomerna velikost velikih zrn pove~uje odpornost na lezenje in ovira rast razpok. V tem delu je s spreminjanjem parametrov
termomehanske predelave preiskovana predelava za doseganje enakomerne rekristalizirane mikrostrukture.
Zlitina MoNiCr je namenjena modernemu konceptu nuklearnih reaktorjev, v katerih se uporablja staljene fluoridne soli, kot
sredstvo za hlajenje v primarnem in/ali sekundarnem tokokrogu. Predstavlja alternativni material z visoko korozijsko odpor-
nostjo v podro~ju fluoridnih soli in ima hkrati zelo dobro odpornost na lezenje v temperaturnem podro~ju 650–750 °C.
Izdelava posod in prirobnic iz zlitine MoNiCr zahteva obvladovanje tehnologije preoblikovanja tega visoko legiranega
materiala. Klju~ni moment izgleda je preoblikovanje litega stanja materiala do stanja rekristalizirane mikrostrukture s homo-
genimi, drobnimi zrni. Pri vro~em preoblikovanju je poleg temperature pomembno tudi stanje napetosti. Nikljeve zlitine so
sposobne veliko ve~je deformacije, ~e prevladujejo tla~ne napetosti.
Preoblikovanje s tla~nimi napetostmi pove~uje verjetnost, da bo material brez poru{itve dosegel tak nivo deformacije, ki omo-
go~a procese poprave in rekristalizacije. Proces deformacije je bil izvr{en na fizikalnem simulatorju. Nadaljnja hladna deforma-
cija ob~utno pospe{i proces rekristalizacije deformirane lite strukture.
Klju~ne besede: rekristalizacija, nikljeve zlitine, fizikalna simulacija
1 INTRODUCTION
Superalloys are nickel-, iron-nickel-, and cobalt-
based alloys generally used at temperatures above about
540 °C. The alloys that can be used even at higher tem-
peratures are continuously investigated thanks to their
excellent mechanical and corrosion properties.1 The
iron-nickel-based superalloys form an extension of the
stainless-steel technology and are generally wrought. A
large number of alloys were invented and studied; many
were patented. However, many alloys were winnowed
down over the years; only a few are extensively used.2
Due to their excellent creep resistance, the nickel-based
superalloys are used in the power industry, especially for
power engineering. The current nuclear-power engineer-
ing is mostly represented by the light water nuclear
reactor that uses low-enriched uranium in the form of
uranium dioxide as its fuel. However, the current use of
the uranium raw material is low, in most cases it ranges
between 3 % and 5 %. A limited number of reactors use
mixed uranium-plutonium fuel that increases the primary
use of the uranium raw material. The research of the
fourth-generation reactor system deals with the way of
how to prevent a non-economical use of uranium.3
The fourth generation of reactors is generally divided
into two types: thermal and fast reactors. The thermal
reactors use a moderator to slow down the neutrons
emitted by fission, making them more likely to be cap-
tured by the fuel. A fast reactor directly uses the fast
neutrons emitted by fission, without a moderation. These
new reactors are designed with the following objectives
in mind:3,4
• Enhanced nuclear-reactor safety
• Increase in the nuclear-reactor energy efficiency
Materiali in tehnologije / Materials and technology 50 (2016) 2, 199–205 199
UDK 669.245:669.018.254:66.017 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 50(2)199(2016)
• Closure of the nuclear-reactor fuel cycle
• Reduction in the fuel-radioactivity level
• Disposal of the used fuel
It appears that the waste management is the major
concern regarding the existing once-through cycle
because of the limited availability of the repository space
worldwide. Closed fuel cycles of recycling reactors
allow some of the fuel to be reused. An improvement in
the reactor performance can be achieved if the thermal
and fast reactors are operated in a coupled mode. An
increase in the fuel burnup of the thermal reactors can
improve the management of the produced actinides by
burning them in situ. With a transmutation, the relative
toxicity of the radioactive waste decreases considerably
(Figure 1).4
An increase in the nuclear-power-plant energy
efficiency is possible by increasing the output coolant
temperature in the reactor. This can be achieved if a
coolant other than water is used. Sodium, lead, gases,
supercritical water and other media are under considera-
tion. One of the concepts of the fourth-generation reac-
tors is a molten-salt reactor where molten fluoride salt is
considered for the primary and also for the secondary
circuit. The main advantage of this reactor is a high
efficiency (a high level of the nuclear-fuel usage), but the
main disadvantage is the environment of the molten fluo-
ride salt with a temperature of 700–800 °C. A suitable
material for fittings and vessels must have an extreme
corrosive and creep resistance in this environment. It is,
nevertheless, necessary to make moulded plates and
pipes that must be bent and hermetically joined together
– they must be welded to the shape of the final fittings.
The formation of semi-finished products from cast ingot
is one of the technologically most complicated opera-
tions during the manufacture of vessels and fittings.
Many studies stated that nickel-based superalloys
with high Mo and low Cr contents are generally superior
with respect to the high-temperature corrosion resistance
in fluorine environments.5
The main microstructure parameter that affects the
mechanical properties is the grain size. It greatly influ-
ences the strength, the creep, the fatigue-crack initiation
and the growth rate. A uniform coarse-grain size in-
creases the creep strength, the crack-growth resistance
and the ductility. On the contrary, a uniform fine-grain
size provides a high low-cycle fatigue life and high
tensile and yield strengths.6
Usually the microstructure of a nickel superalloy
after hot working consists of large and fine grains and is
not uniform. Irrespective of whether the microstructure
is coarse grained or fine grained it should be uniform for
further annealing processing. Previous studies found that
hot working followed by cold working and then by the
annealing process is the best way to achieve a uniform
recrystallized microstructure.7
The inability of the nickel-based superalloys to dyna-
mically recrystallize during forging leads to difficulties
during the forging at the highest temperatures close to
the temperature of solidus. Even more, such high tem-
peratures can lead to hot cracking on grain boundaries.
This can be caused partially by the presence of low-melt-
ing impurities on the grain boundaries and also by the
oxidation, which occurs much faster through the grain
boundaries than within the grains. High-temperature oxi-
dation is much promoted by the main solid-solution
strengthening element – molybdenum. Therefore, the
forging temperature has to be kept in the interval below
the hot cracking and above the point that ensures
dynamic recrystallization.
The strain rate is the second important parameter of
forming. A high strain rate does not bring enough time
for "self-healing" processes in the crystal lattice and
usually strengthening takes place in the microstructure.
This also leads to cracking during forming.
2 EXPERIMENT DESCRIPTION
An experiment was carried on the MoNiCr nickel
superalloy, developed by COMTES FHT Inc. regarding
its application in molten-salt reactor circuits. The
chemical composition of this alloy is shown in Table 1.
Table 1: Chemical composition of the experimental alloy in mass
fractions (w/%)
Tabela 1: Kemijska sestava preizkusnih zlitin v masnih dele`ih (w/%)
Ele-
ment Mo Cr Ti Fe Mn Nb Al W Ni
w/% 15.81 6.82 0.03 2.32 0.04 0.01 0.26 0.06 base
The experiment was focused on an evaluation of the
influences of various thermomechanical processes on the
behaviour of the processed samples and their microstruc-
tures. At first, an evaluation of the strain-deformation
characteristics of the samples made from the material in
the as-cast state was performed. A similar evaluation of
the strain-deformation characteristics was also made on
the samples in the as-cast state, with a preceding hot
deformation (while alternating the strain) (Figure 2).
Further experiment consisted of the following steps:
P. PODANY et al.: RECRYSTALLIZATION BEHAVIOUR OF A NICKEL-BASED SUPERALLOY
200 Materiali in tehnologije / Materials and technology 50 (2016) 2, 199–205
Figure 1: Relative toxicity of the nuclear waste in repository4
Slika 1: Relativna toksi~nost nuklearnih odpadkov na odlagali{~u4
• Evaluation of the influence of thermomechanical
processing (heat deformation) on recrystallization
• Evaluation of the influence of cold deformation on
recrystallization.
A strain-deformation characterisation and thermome-
chanical processing were carried out on the samples
whose shape is represented on Figure 3. These samples
were tested on a simulator of heat-deformation cycles
(Figure 3). This simulator was used in the previous
studies for similar purposes.8 In the device, the body of a
sample is heated by resistance heating to the controlled
temperature. In the case of the experimental focus on the
evaluation of the thermomechanical processing, a sample
is loaded with cycles of tensile and compression defor-
mation. Each partial compressive deformation was
higher than the preceding tensile deformation. The total
logarithmic degree of deformation for a sample was
approximately 0.5–2.5.
In the case of the experimental focus on the evalua-
tion of cold forming, cold deformation preceded high-
temperature deformation – upsetting. Table 2 shows an
overview of the performed ways of the treatment.
After the thermomechanical processing, all the sam-
ples were cut in the longitudinal direction and they
underwent the standard metallographic preparation
(grinding and subsequent polishing). The microstructure
was revealed by etching in Marble’s reagent and docu-
mented on a Nikon light microscope. An image analysis
of the recrystallized-grain fraction was performed by
means of the NIS-Elements software.
Table 2: Processing of the samples with cold deformation and the
subsequent cyclic hot deformation
Tabela 2: Obdelava vzorcev s hladno deformacijo, ki ji je sledila
vro~a cikli~na deformacija
Embedded deformation
Hot Deformation at 1200 °C
0.3 0.7 1.1
Cold Deformation
(at room
temperature)
0.05 CD0.05-HD0.3
CD0.05-
HD0.7
CD0.05-
HD1.1
0.25 CD0.25-HD0.3
CD0.25-
HD0.7
CD0.25-
HD1.1
3 RESULTS AND DISCUSSION
3.1 Strain-deformation characteristics of the as-cast
state
The tensile test was made on the samples in a
temperature range of 1050–1250 °C. As they were in the
cast state, the ductility was very low up to the breaking
limit of Ag (Figure 4). Even with the cyclic deformation
prior to the tensile test, it did not reach the value suffi-
cient for the development of a massive recrystallization.
Figure 5 shows the microstructure near the fracture of a
sample treated in this way at 1200 °C. Before the tensile
test, the sample was loaded with the preceding hot
deformation (by alternating the strain). The deformation
was not sufficient for the development of a massive
recrystallization. Recrystallization only takes place at the
boundaries of the large grains of the cast structure.
3.2 Thermomechanical processing – cyclic loading and
its influence on the microstructure
The samples were subjected to the loading with
cycles of tensile and compression deformation. The
amount of deformation was increased in steps. It is
P. PODANY et al.: RECRYSTALLIZATION BEHAVIOUR OF A NICKEL-BASED SUPERALLOY
Materiali in tehnologije / Materials and technology 50 (2016) 2, 199–205 201
Figure 2: Example of cyclic heat deformation prior to the heat-tensile
test – 10 °C/s at 1200 °C, 15 s, 10 mm × 0.5 mm, 1 mm/s
Slika 2: Primer cikli~ne toplotne deformacije pred vro~im nateznim
preizkusom – 10 °C/s pri 1200 °C, 15 s, 10 mm × 0,5 mm, 1 mm/s
Figure 4: Stress-strain curves of the samples heated with resistance
heating at 1050–1200 °C
Slika 4: Krivulje napetost-raztezek za vzorce, ogrevane z uporovnim
gretjem pri 1050–1200 °C
Figure 3: a) Shape of an experimental sample, b) simulator of
heat-deformation cycles – servo-hydraulic mechanical testing system
MTS complemented by resistance heating
Slika 3: a) Oblika preizku{anca, b) simulator toplotnih deformacijskih
ciklov – servo hidravli~ni mehanski preizkusni sistem MTS, dopolnjen
z uporovnim ogrevanjem
evident that the increase in deformation (see Figure 6 for
the applied deformation) led to an increase in the volume
of recrystallized grains. At lower deformation, recry-
stallization occurs only at the boundaries of the grains,
but with higher deformation, it extends into the grains
(Figures 7 to 11).
A modelling of the graduation of embedded deforma-
tion was performed by means of the DEFORM software
for a numerical simulation. Figure 12 shows the distri-
bution of the effective strain in a sample after the last
step.
P. PODANY et al.: RECRYSTALLIZATION BEHAVIOUR OF A NICKEL-BASED SUPERALLOY
202 Materiali in tehnologije / Materials and technology 50 (2016) 2, 199–205
Figure 10: Microstructure after deformation  	1.9
Slika 10: Mikrostruktura po deformaciji  	1,9
Figure 7: Microstructure after deformation  	0.5
Slika 7: Mikrostruktura po deformaciji  	0,5
Figure 6: Deformation of particular samples
Slika 6: Deformacija posameznih vzorcev
Figure 5: Microstructure of a sample near the fracture area after the
tensile test performed at 1200 °C
Slika 5: Mikrostruktura vzorca blizu preloma po nateznem preizkusu,
izvr{enem na 1200 °C
Figure 9: Microstructure after deformation  	1.4
Slika 9: Mikrostruktura po deformaciji  	1,4
Figure 8: Microstructure after deformation  	0.9
Slika 8: Mikrostruktura po deformaciji  	0,9
3.3 Cold working of the samples prior to hot deforma-
tion
It was stated in the introduction that cold deformation
of nickel superalloys promoted recrystallization during
the following hot processing. An acceleration of the
recrystallization process is obvious already in the case of
a relatively small plastic deformation.
The volume fraction of the recrystallized micro-
structure with the highest cold deformation of a sample
after the highest hot deformation is about 90 %.13
3.4 Experimental forging
Experimental forging was carried out on a rectangu-
lar-shaped piece made from ingot. The dimensions of the
sample were 45 mm × 45 mm × 125 mm (Figure 14a).
The sample was gradually heated (Figure 14b) to a
forging temperature of 1170 °C.
Due to a rather quick natural cooling of the sample
during the forging, only three reduction blows were
applied in one forging step, immediately followed by
further reheating of the sample. The first forging ope-
ration before upsetting rounded the edges because sharp
corners are usually the source of cracking. After that, the
sample with dimensions of about 47 mm in diameter and
about 150 mm in length was subjected to the upsetting to
a height of about 90 mm and a diameter of about 60 mm.
The last forging operation was drawing. The final shape
of the forged sample was rectangular, with two diffe-
rently reduced parts (Figure 15). These two parts were
subjected to a microstructure examination.
The microstructure of both parts shows a uniform
distribution of the grains with no traces of the as-cast
structure (Figures 16 and 17). Sample A shows a pre-
sence of small recrystallized grains (5 μm – 10 μm)
around the boundaries of larger grains (Figure 17b).
P. PODANY et al.: RECRYSTALLIZATION BEHAVIOUR OF A NICKEL-BASED SUPERALLOY
Materiali in tehnologije / Materials and technology 50 (2016) 2, 199–205 203
Figure 14: a) Sample for open-die forging on a hydraulic press, b)
heating of the sample before forging
Slika 14: a) Vzorec za prosto kovanje na hidravli~ni pre{i, b) postopek
ogrevanja vzorca pred kovanjem
Figure 13: Influence of the size of cold and hot deformation on the
share of the recrystallized structure
Slika 13: Vpliv stopnje hladne in vro~e deformacije na dele` rekrista-
lizirane strukture
Figure 12: Effective-strain distribution in a sample after the last
tensile-compression cycle. The calculated value of deformation is 
	2.5.
Slika 12: Razporeditev efektivnega raztezka v vzorcu po zadnjem
natezno-tla~nem ciklu. Izra~unana stopnja deformacije je  	2,5.
Figure 11: Microstructure after deformation  	2.5
Slika 11: Mikrostruktura po deformaciji  	2,5
The microstructure was also subjected to a grain-size
measurement. The grain size was measured by means of
image-analysis software NIS-Elements – module D.
Table 3: Grain size according to ASTM E 112
Tabela 3: Velikost zrn po ASTM E 112
Sample Near the surface – G(ASTM E 112)
Centre – G
(ASTM E 112)
A 5.5 5.0
B 6.5 6.5
3.5 Further processing – annealing
The obtained uniform microstructure had to be
further processed with high-temperature annealing. It is
necessary to obtain a uniform coarse-grained microstruc-
ture for ensuring good creep strength and crack-growth
resistance. Figures 18a and 18b illustrate the grain
growth after the annealing at 1300 °C. Particular grain
size shows Table 3.
4 CONCLUSION
The following facts were found during the research
of the nickel-based-superalloy recrystallization beha-
viour:
• The warm strength of the monitored alloy is very
high. Even at 1200 °C it significantly exceeds 100
MPa. The ductility at the maximum stress is very
low. After reaching the maximum stress, the material
is further deformed but the first cracks start to arise.
When the deformation continues a neck is not
formed.
• It was found that the minimum deformation level for
the start of the recrystallization during the hot work-
ing is 	2.5. Only such a high strain level ensures
the recrystallization within the cast grains.
• The recrystallization process is significantly accele-
rated by the preceding cold forming. The structure
was perfectly recrystallized after 	0.25 cold defor-
mation followed by 	1.1 hot deformation.
• The microstructure examination after an intense
experimental forging showed a good distribution of
the recrystallized grains within the whole cross-sec-
tion with respect to the compliance of the forging
temperature.
• A coarse-grained microstructure for a good creep
resistance can be obtained with the annealing at 1300
°C for 12 h.
Acknowledgments
These results were created under the project entitled
Development of West-Bohemian Centre of Materials and
Metallurgy No.: LO1412, financed by the Ministry of
Education, Youth and Sports of the Czech Republic.
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P. PODANY et al.: RECRYSTALLIZATION BEHAVIOUR OF A NICKEL-BASED SUPERALLOY
204 Materiali in tehnologije / Materials and technology 50 (2016) 2, 199–205
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Slika 17: a) Mikrostruktura v delu A – blizu povr{ine, b) mikrostruk-
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Figure 15: Forged sample after the final forging operation
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