B. MA[EK et al.: BEHAVIOUR OF NEW ODS ALLOYS UNDER SINGLE AND MULTIPLE DEFORMATION
891–898
BEHAVIOUR OF NEW ODS ALLOYS UNDER SINGLE AND
MULTIPLE DEFORMATION
OBNA[ANJE NOVIH ODS ZLITIN PRI ENOJNI IN VE^KRATNI
DEFORMACIJI
Bohuslav Ma{ek1, Omid Khalaj1, Zby{ek Nový2, Tomá{ Kubina2, Hana Jirkova1,
Jiøí Svoboda3, Ctibor [tádler1
1The Research Centre of Forming Technology, University of West Bohemia, Univerzitní 22, 306 14, Pilsen, Czech Republic
2COMTES FHT a.s., Prùmyslová 995, 334 41 Dobøany, Czech Republic
3Institute of Physics of Materials, Academy of Sciences Czech Republic, @i`kova 22, 616 62, Brno, Czech Republic
khalaj@vctt.zcu.cz
Prejem rokopisa – received: 2015-07-01; sprejem za objavo – accepted for publication: 2015-11-13
doi:10.17222/mit.2015.156
The application of innovative processing techniques to conventional raw materials can lead to new structural materials with
specific mechanical and physical properties, which open up new possibilities of use in some areas of industry. The processing is
enabled by powder metallurgy, which utilizes powders consisting of a metal matrix with dispersed stable particles achieved by
mechanical alloying and their hot consolidation by rolling. New oxide dispersion strengthened (ODS) Fe–Al-based alloys are
tested under different single and multiple thermomechanical treatments at different temperatures. The results show that new
ODS alloys are significantly affected by the thermo-mechanical treatment, leading to microstructural changes. Their analysis is
performed using different analytical methods such as optical microscopy, scanning electron microscopy and X-ray diffraction
analysis.
Keywords: ODS alloys, composite, steel, Fe-Al
Uporaba inovativnih tehnik preoblikovanja na obi~ajnih materialih lahko privede do novih konstrukcijskih materialov s
specifi~nimi mehanskimi in fizikalnimi lastnostmi, ki odpirajo nove mo`nosti uporabe v industriji. Metalurgija prahov omogo~a
uporabo prahov s kovinsko osnovo z dispergiranimi stabilnimi delci, ki jih dobimo pri mehanskem legiranju in vro~i
konsolidaciji z valjanjem. Nove zlitine Fe-Al, disperzijsko utrjene z oksidi (ODS), so bile preizku{ene pri razli~ni, eno- ali
ve~stopenjski obdelavi pri razli~nih temperaturah. Rezultati ka`ejo, da ima termomehanska obdelava novih ODS zlitin mo~an
vpliv, ki se vidi v spremembah mikrostrukture. Analiza je bila izvedena s pomo~jo razli~nih analitskih metod, kot so: svetlobna
mikroskopija, vrsti~na elektronska mikroskopija in rentgenska difrakcijska analiza.
Klju~ne besede: ODS zlitine, kompozit, jeklo, Fe-Al
1 INTRODUCTION
The demand to increase the efficiency of processes in
most industrial applications requires, in many cases,
metallic materials that can operate at high temperatures,
and often at high stresses, in corrosive environments.
The presently used high-temperature Ni-, Co- and Fe-
based alloys are strengthened by a combination of solid-
solution and precipitation hardening, the effectiveness of
which strongly decreases with increasing temperature.
ODS alloys contain small amounts (0.5–1 % of weight
fractions) of finely dispersed oxide phase (mostly
yttrium), which is thermodynamically much more stable
than other strengthening phases such as ’ or carbides,
present in conventional high-temperature alloys.1 There-
fore, the strengthening imparted by the oxide dispersions
is retained up to very high temperatures because only li-
mited coarsening or dissolution of the particles occurs.2,3
In addition, the presence of the fine dispersions com-
bined with a very coarse-grained microstructure that is
stable over long exposure times leads to excellent creep
resistance up to higher temperatures than those that can
be achieved with conventional wrought or cast alloys.4,5
The ODS alloys commercially produced at the end of the
20th century and the beginning of the 21st century are
represented by MA 956 or MA 9576, PM 2000 or PM
20106, ODM alloys7 and 1DK or 1DS8 with a ferritic
matrix by ODS Eurofer steels with a tempered ferritic-
martensitic matrix9 and by austenitic Ni-ODS PM 1000
or Ni-ODS PM 3030.10 ODS alloys are produced by
high-energy milling of powder mixtures consisting of the
alloying elements, master alloys and the oxide disper-
sion. The volume fraction of dispersed spherical oxides
(usually Y2O3) is typically below 1 % and the oxides are
typically of a mean size of 5–30 nm. The mechanically
alloyed powder is then consolidated at high temperatures
and pressures to produce the bulk material in the form of
bar or tube stock. Afterwards, different thermomecha-
nical treatments are applied to optimize its microstruc-
ture and mechanical properties. In the consolidation step
the processing temperatures are critical in order to retain
the nanocrystalline structure generated during the me-
chanical alloying and to impede particle coarsening and
grain growth.11–14 The Ni- and Fe-based ODS alloys rely
on the formation of slowly growing and strongly
Materiali in tehnologije / Materials and technology 50 (2016) 6, 891–898 891
UDK 67.017:669.018:537.533:621.763 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 50(6)891(2016)
adherent chromium and aluminium scales for their
high-temperature oxidation/corrosion resistance. Be-
cause of the lower diffusion coefficient, austenitic ODS
alloys show a better creep resistance for the same oxide
volume fraction and contain some minimum chromium/
aluminium content to guarantee sufficient oxidation
resistance. However, the resistance to the coarsening of
oxides is given by the product of the solubility of oxygen
in the matrix and its diffusion coefficient;16 this factor is
more advantageous for ferritic ODS alloys. Also, a
sufficient content of Al and/or Cr in the ODS alloy is
decisive for its oxidation resistance.17–19 This is probably
the reason why the application of ferritic ODS steels
dominates.15–23
The new ODS alloys consist of a ferritic Fe-Al matrix
strengthened with about 6 to 10 % volume fractions of
Al2O3 particles.24,25 In order to get a more detailed insight
into these new groups of materials, an experimental pro-
gramme was carried out to better understand their
processing behaviour and their operational properties.
2 EXPERIMENTAL PART
Mechanically alloyed (MA) powders were prepared
in a low-energy ball mill, developed by the authors (Fig-
ure 1), which enables evacuation and filling by oxygen.
It has two steel containers (each 24 L) and each con-
tainer is filled with 80 steel balls of diameter 40 mm.
The revolution speed is variable between 20 min–1 to
75 min–1.
The mechanically alloyed powders consisting of
Fe10wt%Al matrix and 6 % to 10 % volume fractions of
Al2O3 particles were deposited into a steel container of
diameter 70 mm, evacuated and sealed by welding (Fig-
ure 2). The steel container was heated up to a tempera-
ture of 800–900 °C and rolled by a hot-rolling mill
(Figure 3) to a thickness of 20–25 mm in the first rolling
step and then heated up to a temperature of 1100 °C and
rolled to a thickness of 9 mm in the second step. A
6-mm-thick sheet of the ODS alloy was produced in this
way. Afterwards, the specimens were cut by water jet.
In order to investigate the thermomechanical treat-
ment of specimens, a servohydraulic MTS thermomecha-
nical simulator (Figure 4) was used, which allows the
running of various temperature-deformation paths
necessary to find conditions leading to, e.g., the most
effective grain coarsening by recrystallization. Several
B. MA[EK et al.: BEHAVIOUR OF NEW ODS ALLOYS UNDER SINGLE AND MULTIPLE DEFORMATION
892 Materiali in tehnologije / Materials and technology 50 (2016) 6, 891–898
Figure 3: Rolling process: a) hot-rolling mill, b) steel container in
rolling process
Slika 3: Valjanje: a) ogrodje za vro~e valjanje, b) zbirnik med postop-
kom valjanja
Figure 1: Low-energy mill for mechanical alloying
Slika 1: Nizko energijski mlin za mehansko legiranje
Figure 2: Container for mechanical alloyed powder
Slika 2: Zbirnik za mehansko legiran prah
procedures of thermomechanical treatment were de-
signed and carried out, which differed in the number of
deformation steps characterized by different strains,
strain rates and temperatures. The thermomechanical
simulator also allows the combination of tensile and
compressive deformation, thus accumulating a high plas-
tic deformation (and a high dislocation density) in the
specimen.
2.1 Preparation of specimens
One specimen (Figure 5) was selected from several
examples regarding their most homogeneous temperature
fields. The steel containers were removed from all the
specimens that were cut by water jet in a longitudinal
direction (Figure 6). The thickness of specimens was
approximately 6 mm after grinding.
Six types of material were used in this research, as
described in Table 1. All these materials are based on a
Fe10wt%Al ferritic matrix with different particle sizes
and volume fractions in % of Al2O3. Al2O3 powder was
added to prepare the composite, fine oxides in ODS
alloys were obtained by internal oxidation during mecha-
nical alloying and precipitated during hot consolidation.
The microscopic SEM observations indicated several
inhomogeneities due to sticking of the material during
mechanical alloying on the walls of the milling con-
tainer. These inhomogeneities can also influence the me-
chanical and fracture properties of the material, but the
mechanical alloying process is steadily optimized with
respect to the homogeneity of the materials.
Table 1: Material parameters
Tabela 1: Parametri materiala
Mate-
rial
No.
Material
type
Milling
time (h)
Ferritic
matrix
(% of mass
fractions)
% of
volume
fractions
of Al2O3
Typical
particle
size
(nm)
1 Composite – Fe10%Al 10 300
2 ODS Alloy 100 Fe10%Al 6 50–200
3 ODS Alloy 150 Fe10%Al 6 50–150
4 ODS Alloy 200 Fe10%Al 6 30–150
5 ODS Alloy 245 Fe10%Al 7 20–50
6* ODS Alloy 245 Fe10%Al 7 20–50
* Different rolling force
2.2 Testing programme
The test programme was divided into six different
series. The tests are summarized in Table 2.
Single deformation tests series were carried out to
investigate the thermomechanical behaviour of the diffe-
rent materials (1 to 4) at different temperatures regarding
single tensile loading with a constant strain rate of 1 s–1
(Figure 7). In order to give a clearer comparison of the
results, only the results at room temperature (RT),
800 °C and 1200 °C are presented.
Multiple deformation-test series were carried out to
investigate the thermomechanical behaviour of Materials
B. MA[EK et al.: BEHAVIOUR OF NEW ODS ALLOYS UNDER SINGLE AND MULTIPLE DEFORMATION
Materiali in tehnologije / Materials and technology 50 (2016) 6, 891–898 893
Figure 6: Position of specimens on rolled semi-product
Slika 6: Polo`aj vzorcev na valjanem polproizvodu
Figure 4: Treatment on thermomechanical simulator
Slika 4: Obdelava na termomehanskem simulatorju
Figure 5: Specimen dimensions
Slika 5: Dimenzije vzorca
Figure 7: Treatment no. 1
Slika 7: Obdelava {t. 1
5 and 6 at 1200 °C regarding multiple tensile loading
with a constant strain rate of 1 s–1 (Figure 8) followed by
two different holding times (10 s and 30 s).
3 RESULTS AND DISCUSSION
3.1 Single deformation-test series
Single deformation-test series were carried out in
order to investigate different materials under different
conditions. Figure 9 shows the stress-strain curves for
all the materials at different temperatures regarding the
5 % compression corresponding to treatment number 1.
Material 2 exhibits a better strength at 30 °C and 800 °C,
but at 1200 °C Material 1 shows a better strength. The
hot-working behaviour of alloys is generally reflected by
flow curves, which are a direct consequence of micro-
structural changes: the nucleation and growth of new
grains, dynamic recrystallization (DRX), the generation
of dislocations, work hardening (WH), the rearrange-
ment of dislocations and their dynamic recovery (DRV).
In the deformed materials, DRX seems to be the pro-
minent softening mechanism at high temperatures. DRX
occurs during the straining of metals at high temperature,
characterized by nucleation of low-dislocation-density
grains and their posterior growth to produce a homo-
geneous grain structure if a dynamic equilibrium is
reached.
Material 4 showed a strange curve shape at 800 °C.
The test was repeated several times and similar beha-
viour was observed. It could be concluded that it
happens because of the inhomogeneity of the microstruc-
ture of this material.
B. MA[EK et al.: BEHAVIOUR OF NEW ODS ALLOYS UNDER SINGLE AND MULTIPLE DEFORMATION
894 Materiali in tehnologije / Materials and technology 50 (2016) 6, 891–898
Figure 8: Treatment no. 2
Slika 8: Obdelava {t. 2
Table 2: Parameters of test programme
Tabela 2: Parametri programa preizkusa
Test series Materialno. Treatment no. Treatment type
Maximum
temperature (°C) Number of tests Purpose of tests
A 1 1 Single
1200, 1100, 1000,
900, 800, RT
6
Single deformation
thermomechanical
behaviour
B 2 1 Single 6
C 3 1 Single 6
D 4 1 Single 6
E 5 2 Multiple
1200
2 Multiple deformation
thermomechanical
behaviourF 6 2 Multiple 2
Figure 9: Stress-strain curves (5 % compression) for: a) RT, b) 800 °C,
c) 1200 °C
Slika 9: Krivulje napetost-raztezek (5 % stiskanje) za: a) RT, b) 800 °C,
c) 1200 °C
Figure 10 shows the stress-strain curves for Mate-
rials 1 to 4 at different temperatures corresponding to the
3 % tension of treatment number 1 (Figure 7). As can be
seen in Figure 10, Material 2 shows a higher strength at
30 °C and 800 °C, but at 1200 °C, again Material 1
shows a better strength. All four materials have almost
the same elastic modulus and none of them failed during
3 % deformation. The yield stress as well as the shape of
the flow curves is sensitive to temperature. Comparing
all these curves, it is found that decreasing the deforma-
tion temperature increases the yield stress level, in other
words, it prevents the occurrence of softening due to
dynamic recrystallization (DRX) and dynamic recovery
(DRV) and allows the deformed metals to exhibit work
hardening (WH). For every curve, after a rapid increase
in the stress to a peak value, the flow stress decreases
monotonically towards a steady-state regime with a
varying softening rate, which typically indicates the
onset of DRX (Figure 9c).
Figure 11 shows the stress-strain curves for Mate-
rials 1 to 4 at different temperatures corresponding to the
50 % tension of treatment number 1 (Figure 7). All four
B. MA[EK et al.: BEHAVIOUR OF NEW ODS ALLOYS UNDER SINGLE AND MULTIPLE DEFORMATION
Materiali in tehnologije / Materials and technology 50 (2016) 6, 891–898 895
Figure 11: Stress-strain curves (50 % tension) for: a) RT, b) 800 °C,
c) 1200 °C
Slika 11: Krivulja napetost-raztezek (50 % natezna obremenitev) za:
a) RT, b) 800 °C, c) 1200 °C
Figure 10: Stress-strain curves (3 % tension) for: a) RT, b) 800 °C,
c) 1200 °C
Slika 10: Krivulja napetost-raztezek (3 % natezna obremenitev) za:
a) RT, b) 800 °C, c) 1200 °C
materials failed at RT, but only two materials failed
below 50 % tension at higher temperatures. Material 2
failed at 34 % strain and Material 4 failed at 44 % strain
at 800 °C. At 1200 °C, only Material 1 failed at 41 %
and Material 2 failed at 45 % strain. From these curves,
it can also be seen that the stress evolution with strain
exhibits three distinct stages.
In the first stage work hardening (WH) predominates
and causes dislocations to polygonize into stable sub-
grains. Flow stress exhibits a rapid increase with in-
creasing strain up to a critical value. Then DRX occurs
due to a large difference in dislocation density within the
subgrains or grains. When the critical driving force of
DRX is attained, new grains are nucleated along the
grain boundaries, deformation bands and dislocations,
resulting in the formation of equiaxed DRX grains.
In the second stage, flow stress exhibits a smaller and
smaller increase until a peak value or an inflection of the
work-hardening rate is reached. This shows that the ther-
mal softening due to DRX and dynamic recovery (DRV)
becomes more and more important and it exceeds WH.
In the third stage, three types of curves can be re-
cognized:
• Decreasing gradually to a steady state with DRX
softening (Material 3 & 4 in Figure 11c),
• Increasing continuously with significant work-hard-
ening (Material 1 & 2 in Figure 11b),
• Decreasing continuously with significant DRX soft-
ening.
3.2 Multiple deformation-test series
Multiple deformation-test series were carried out in
order to investigate the material behaviour under various
conditions. Figure 12 shows the stress-strain curves for
both materials at different holding times following the 5
% tension during treatment number 2 (Figure 8). Both
materials show approximately the same behaviour under
the multiple tensile loading. However, Material 6 exhi-
bits greater strength for both holding times (10 s and
30 s). It is supposed that the oxide particles prevent un-
desirable cyclic softening, which is observed in ferritic-
martensitic steels.22 Obviously, the oxide particles
strengthen the material substantially, nevertheless, cyclic
softening is observed at both holding times. The cyclic
softening rate depends on the applied loading. A higher
strain amplitude results in a higher softening rate. For
instance, the softening rate was about 23 % during the
second cycle in Material 5, while it decreased to 12 %
during the last cycles. Although the softening in ODS
steel is lower than in the ferritic-martensitic steel26, it
indicates that oxide dispersion itself does not guarantee a
stable cyclic behaviour and other microstructural aspects
have to be taken into account. It is obvious that the stress
amplitude decreased with an increasing number of
cycles, while the amplitude of the plastic strain in-
creased. The softening rate in Material 6 is lower than in
Material 5, as observed for both holding times. The
slight cyclic hardening is observed only during the first
cycle in Material 6 with 30 s holding time (Figure 12b),
while continuous softening behaviour is observed in the
remaining part of the curve.
4 CONCLUSIONS
This paper outlines the results of the characterization
of the single and multiple deformation thermomecha-
nical behaviour of a new generation of ODS alloys. Six
materials differing from each other in the amount and
size of the oxides embedded in the ferritic matrix were
tested under different conditions. The advantages of all
the materials are their low-cost and creep-corrosion and
oxidation-resistance due to the Fe–Al-based ferritic ma-
trix of the ODS alloy. It can be concluded that in general
the oxide dispersion significantly strengthens the mate-
rial. However, the typical form of the flow curve with
DRX softening, including a single peak followed by a
steady state flow as a plateau, is more recognizable at
high temperatures than at low temperatures. This is be-
cause at high temperatures the DRX softening compen-
sates the WH, and both the peak stress and the onset of
steady-state flow are therefore shifted to lower strain
levels. The characteristics of softening flow behaviour
coupled with DRX have been discussed for six materials
and can be summarized as follows:
B. MA[EK et al.: BEHAVIOUR OF NEW ODS ALLOYS UNDER SINGLE AND MULTIPLE DEFORMATION
896 Materiali in tehnologije / Materials and technology 50 (2016) 6, 891–898
Figure 12: Stress-strain curves (5 % tension) for: a) holding time 10 s,
b) holding time 30 s
Slika 12: Krivulja napetost-raztezek ( pri 5 % natezni obremenitvi) za:
a) ~as zadr`anja 10 s, b) ~as zadr`anja 30 s
1. Decreasing deformation temperature causes the flow
stress level to increase, in other words, it prevents the
occurrence of softening due to DRX and dynamic
recovery (DRV) and makes the deformed metals
exhibit work hardening (WH).
2. For every curve, after a rapid increase in the stress to
a peak value, the flow stress decreases monotonically
towards a steady-state regime (a steady-state flow as
a plateau due to DRX softening is more recognizable
at higher temperatures). A varying softening rate
typically indicates the onset of DRX, and the stress
evolution with strain exhibits three distinct stages.
3. At higher temperatures, a higher DRX softening
compensates the WH, and both the peak stress and
the onset of steady-state flow are therefore shifted to
lower strain levels.
4. The ODS alloy exhibits cyclic softening in most of
the tests and its rate decreases with increasing strain.
5. The elastic part of the total strain amplitude is always
higher than the plastic one in all the specimens
tested, even for the highest total strain amplitudes of
15 %. This is further confirmation of the strong
strengthening effect of oxide particles.
Acknowledgements
This paper includes results created within the projects
14-24252S Preparation and Optimization of Creep Resis-
tant Submicron-Structured Composite with Fe-Al Matrix
and Al2O3 Particles subsidised by the Czech Science
Foundation, and LO1412 Development of West-Bohe-
mian Centre of Materials and Metallurgy subsidised by
the Ministry of Education, Youth and Sports from spe-
cific resources of the state budget of the Czech Republic
for research and development.
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