B. Bal - On themicro-deformation mechanisms active in high-manganese austenitic steels under impact loading.pdf

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Materials Science & Engineering A 632 (2015) 29–34
Contents lists available at
ScienceDirect
Materials Science & Engineering A
journal homepage:
www.elsevier.com/locate/msea
On the micro-deformation mechanisms active in high-manganese
austenitic steels under impact loading
B. Bal
a
, B. Gumus
a
, G. Gerstein
b
, D. Canadinc
a,
n
, H.J. Maier
b
a
b
Koç University, Advanced Materials Group (AMG), Department of Mechanical Engineering, Sarıyer, 34450
İstanbul,
Turkey
Leibniz Universität Hannover, Institut für Werkstoffkunde (Materials Science), An der Universität 2, 30823 Garbsen, Germany
art ic l e i nf o
Article history:
Received 10 December 2014
Received in revised form
16 February 2015
Accepted 19 February 2015
Available online 28 February 2015
Keywords:
TWIP steel
Impact
Slip
Twinning
Microstructure
a b s t r a c t
The composition and temperature dependencies of deformation response of TWIP and XIP steels were
investigated under high-velocity impact loading with a focus on micro-scale deformation mechanisms.
The promotion of twinning deformation under high-velocity loading over the slip–twin interactions
usually observed in low-velocity loading conditions was comprehensively examined with scanning
electron microscopy and transmission electron microscopy. In addition, thermal analyses of plastic
deformation were carried out by in situ thermal imaging. The current
findings
demonstrate that the
deformation of TWIP steel is dictated by two major twin systems at elevated temperatures, while nano-
twin formation within one primary twin system dominates at subzero temperatures. The XIP steel, on
the other hand, deforms mainly by slip at elevated temperatures, while competing slip and twin
activities, and eventually nano-twin formation within primary twins dominates as the temperature
decreases. Overall, the current
findings
shed light on the complicated work hardening mechanisms
prevalent in high-manganese austenitic steels utilizing high-velocity deformation experiments.
&
2015 Elsevier B.V. All rights reserved.
1. Introduction
A significant fraction of research efforts aiming at improving
the mechanical performance of light-weight metallic materials
has focused on magnesium (Mg) and aluminum (Al) alloys
[1–5].
However, recent developments in the
field
of advanced high-
strength steels have opened the venue for utilizing steel compo-
nents with reduced weight without sacrificing from strength and
ductility. Consequently, various new generation steels, especially
high-manganese (Mn) austenitic steels, have become an alter-
native to Mg- and Al-alloys, and are now utilized in different
types of modern engineering applications, such as in the auto-
motive, railroad and mining industries
[6–9].
However, this class
of steels features complex microstructural interactions during
deformation, which makes the mechanical properties and defor-
mation behavior difficult to predict for any given initial texture
and microstructure. Hence, high-Mn austenitic steels have been
subject to numerous works.
Earlier studies on high-Mn austenitic steels focused mainly on
Hadfield steel, a type of austenitic high-Mn steel with 1–1.4 wt% C and
10–14 wt% Mn, which possesses a stable austenitic microstructure
with a face-centered cubic (fcc) lattice, offering a good combination of
high strength and significant ductility, in addition to high wear and
n
Corresponding author. Tel.:
þ
90 212 338 1891; fax:
þ90
212 338 1548.
E-mail address:
dcanadinc@ku.edu.tr
(D. Canadinc).
abrasion resistance
[10–17].
One of the important conclusions drawn
from these previous works was that low stacking fault energy (SFE)
resulting from the high-Mn content promotes formation of twins.
Specifically, as the SFE decreases, the level of energy required for
the abrupt lattice rotation associated with mechanical twinning also
decreases, eventually enhancing twin formation, which provides
effective barriers against dislocation glide, and leads to significant
strain hardening
[18].
On the other hand, the SFE increases upon
alloying with elements, such as Al and Si, favoring slip-dominated
plastic deformation while decreasing the contribution of twins and
stacking faults to the overall hardening
[12,19].
However, even in
the case of high SFE, interaction between dislocations and different
features of the microstructure, such as dense dislocation walls,
promotes rapid strain hardening in this class of steels
[12,20].
In addition to SFE, deformation temperature and dynamic strain
aging (DSA) also have significant influence on the work hardening
behavior of high-Mn steels. First of all, deformation becomes
slip-dominant with the increase in temperature
[21],
and thus,
due to hindered interaction with twins and twin boundaries, work
hardening capacity of the material decreases. Furthermore, interac-
tions between C atoms in the C–Mn clusters and dislocations, and
reorientation of C atoms in dislocation cores lead to DSA, and
improve the work hardening capacity of Hadfield steel by hindering
the dislocation motion
[22,23].
All these
findings
on Hadfield steel and further research led to the
introduction of relatively higher Mn containing (13–30 wt% Mn)
steels, and eventually the development of TWinning Induced Plasticity
http://dx.doi.org/10.1016/j.msea.2015.02.054
0921-5093/& 2015 Elsevier B.V. All rights reserved.
30
B. Bal et al. / Materials Science
&
Engineering A 632 (2015) 29–34
(TWIP) steels
[24–29].
This class of steels possesses an excellent
combination of high strength and ductility: typically, strengths up to
1500 MPa, elongations up to 90%, and outstanding impact toughness
ranging from 90 to 120 J/cm
2
at high strain rates have been reported
for TWIP steels at various temperatures
[24–29].
In addition to these
superior mechanical properties, TWIP steels possess relatively lower
density (
ρ
%
7.3 g/cm
3
) and much higher energy-absorption capacity
(0.5 J/mm
3
) than conventional steels, with typical density and energy
absorption values of 7.8 g/cm
3
and 0.25 J/mm
3
, respectively
[27–31].
Many of the previous investigations reported that twinning
accompanied by additional micro-mechanisms, such as stacking
faults, DSA and slip–twin interactions, dominates the deformation
response of TWIP steels, where the role of dislocation glide
remains rather limited in comparison
[32–35].
In particular, at
the onset of plastic deformation, twins start to nucleate and the
amount of nano- and micro-scale twins increases during plastic
deformation, forming twin boundaries of different magnitudes. In
materials that are only deformed by dislocation glide, the mean
free path of dislocations is a function of grain boundary (GB)
density and dislocation–dislocation interactions. However, in the
case of TWIP steels the twin boundaries formed within the
microstructure also play a crucial role. Specifically, the mean free
path of dislocations is reduced by the increasing twin boundaries,
which act as strong obstacles against dislocation glide and bring
about the noticeable strain hardening observed in TWIP steels
[19,32,33,36].
This is known as the TWIP effect, which is triggered
at low or medium SFE values (12–35 mJ/m
2
)
[37].
Both, the TWIP
effect and microstructural interactions suspend the onset of
necking since they are obstacles for dislocation glide, and as a
result, high ultimate tensile strength values with extraordinary
ductility is achieved before necking in high-manganese austenitic
TWIP steels
[27,29,38–40].
However, if the mobile dislocation
density is not large enough or the mean free path of dislocations
is not short enough to provide for the critical twin nucleation
stress, growth of existing twins is favored over the nucleation of
new ones. As a result, the volume for microstructural interactions
becomes smaller, and therefore, easier dislocation glide takes
place, eventually leading to softening of the material
[41].
The aforementioned superior mechanical performance of TWIP
steel has been observed in various investigations, which focused on
tensile properties
[8,38,39,42,43],
deformation behavior and the
corresponding micro-mechanisms
[29,40,44–46],
fatigue
[27,47–52],
effects of alloying elements
[31,53–56]
and fracture
[56,57]
of high-
Mn TWIP steels. Consequently, it was reported that TWIP steels
have high strain hardening capacity due to twin–slip interactions
under tensile or compressive loading. However, there is only a limited
number of studies investigating the microstructure evolution of high-
Mn steels under high-strain rate deformations, such as impact
loading, which induces the micro-deformation mechanism interac-
tions during deformation due to the complexity of the applied loading
[6,58].
Even though Wen et al.
[6]
and Toker et al.
[58]
reported the
deformation behavior of a new generation high-Mn austenitic steel
under impact loading, to the best of the authors' knowledge, a
detailed analysis of microstructure evolution of high-Mn steels under
impact loading that establishes both the temperature and chemical
composition dependencies has not been forwarded yet.
As opposed to static, quasi-static or cyclic tensile/compressive
loading, high strain rate impact loading enhances only one defor-
mation mechanism rather than their complex interactions, which
enables a more comprehensive and straightforward understanding
of the microstructural evolution of high-Mn steels
[58].
In the
present study, microstructural aspects of the high work hardening
capacity of two high-Mn steels with different chemical composi-
tions were investigated with the aid of Charpy impact experiments.
The loading conditions, such as deformation temperature and
velocity, and the material's chemical composition, play a crucial
role for the determination of the dominant deformation mechanism
and the corresponding impact response. In order to explore the role
of temperature, which dictates the deformation mechanism para-
meters, such as the twin volume fraction, twin thickness and glide
dislocation density, three different testing temperatures were
chosen to represent a wide temperature range:
À
170
1C
to 200
1C.
The microstructure and deformation mechanisms of the as-is
and deformed materials were investigated by optical microscopy,
stereographic microscopy, scanning electron microscopy (SEM) and
transmission electron microscopy (TEM).
The current set of experiments have opened a venue to investigate
the complicated deformation response of high-Mn austenitic steels by
promoting twinning over the various mechanisms with the applica-
tion of high velocity impact loading within a wide temperature range.
Specifically, it has been demonstrated that twinning significantly
contributes to the overall hardening of the high-Mn austenitic steels
owing to the progressive nature of twin–slip interaction since twins
form at the earlier stages of deformation and start to interact with
glide dislocations as plastic deformation proceeds
[58].
In addition to
these interactions, nucleation of new twins and a larger volume
fraction of slip–twin interactions are promoted by slip that further
distorts the lattice
[41].
The investigation of two high-Mn austenitic
steels with different chemical compositions, on the other hand,
allowed for the investigation of the role of alloying elements on the
aforementioned interactions. Overall, the current
findings
demon-
strate that the deformation of TWIP steel is dictated by two major
twin systems at elevated temperatures, while nano-twin formation
within one primary twin system dominates at low temperatures. The
XIP steel, also commercially named as X-IP™ 1000 (provided by
Arcelor Mittal), on the other hand, deforms mainly by slip at elevated
temperatures, while competing slip and twin activities, and even-
tually nano-twin formation within primary twins prevails as the
temperature decreases. Furthermore, the current results clearly lay
out the effects of temperature and chemical composition on the
twinning response of high-Mn austenitic steels under impact loading
and open a venue for the utility of this class of alloys in applications
bearing high-velocity loading.
2. Experimental details
Two different grades of high-manganese austenitic steels, namely
a TWIP steel and an XIP steel, were studied.
Table 1
presents the
chemical compositions of these two steels. Surfaces of the TWIP and
XIP steel samples were prepared by standard grinding and polishing
metallographic equipment, and etched with a solution of 5% nitric
acid in methanol for approximately 1 min, and cleaned with ethanol.
Following the metallographic preparations, microhardness values
(HV0.1) of all TWIP and XIP samples around the notch were
examined using a micro-hardness tester. Line measurements with a
spacing of 0.09 mm between the indents were conducted on both
sides of the notch as this is the critical region for impact deformation.
The average Vickers hardness values for the undeformed TWIP and
XIP steels were found as 280 and 330 HV, respectively, and the
hardness values were observed to be near-uniform around the notch,
which demonstrated the near-uniform microstructure of the as-is
samples.
Table 1
Chemical compositions of TWIP and XIP steels (wt%).
Steel
TWIP
XIP
C
0.628
0.600
Si
2.601
0.190
Mn
14.843
20.990
Ni
0.065
0.018
Cr
0.043
0.118
V
0.023
0.129
Co
0.012
0.011
Ti
0.006
0.004
Fe
Balance
Balance
B. Bal et al. / Materials Science
&
Engineering A 632 (2015) 29–34
31
Fig. 1.
Impact energy vs. temperature plot for the XIP and TWIP steels.
Table 2
Deformation mechanisms observed at all temperatures for the TWIP and XIP steels.
200
1C
TWIP
XIP
2T, S, V
S
RT
TT, S, V
2T, S, V
À
170
1C
TT, S, V
TT, S
S: slip, T: twin, TT: twin-inside-twin, V: void formation.
For monitoring the impact response of the materials, Charpy-V
shaped TWIP samples with dimensions of 55 mm
Â
10 mm
Â
2 mm,
and XIP samples with dimensions of 50 mm
Â
10 mm
Â
2 mm were
cut utilizing electro-discharge machining
1
. All impact samples fea-
tured a 451 notch with a radius of 0.25 mm and a depth of 2 mm, and
were subjected to an impact energy of 150 J, which corresponds to an
impact velocity of 5.5 m/s. The experiments were conducted at
À170
1C,
room temperature (RT) and 200
1C,
where liquid nitrogen
and furnace heating were utilized for cooling down and heating up of
the samples, respectively.
The impact energies of XIP and TWIP steels at each tempera-
ture were measured as provided in
Fig. 1,
indicating that the
ductile-to-brittle transition temperatures were below RT. More-
over, there is a slight decrease in impact energy values when the
temperature changes from RT to 200
1C,
such that the increased
number of active micro-deformation mechanisms at RT provides
more means of dissipating energy, increasing the impact energy
absorption capacity of both the TWIP and XIP steels, as will be
detailed in the following section. The observed ductile-to-brittle
transition behavior enables the investigation of micro-deformation
mechanisms leading to both ductile and brittle fracture since the
test temperatures (
À
170
1C,
RT, 200
1C)
were chosen with the
purpose of covering both the low temperature brittle response and
the high temperature ductile deformation, as well as the transition
range. One can argue that, based on the data presented in
Fig. 1,
RT
does not fully cover the transition range; however, the changes
in micro-deformation mechanisms demonstrate the beginning
of transition, as evidenced by the results of the thorough TEM
analysis (Table
2).
The temperature changes at the instant of impact and during
fracture were detected by a thermal camera operated in-situ.
Optical microscopy and stereographic microscopy were carried
out to examine the surface microstructure prior to and following
the experiments, where the latter provides a more detailed three-
dimensional image of the sample surface. SEM and TEM observa-
tions were performed in order to obtain more information about
the TWIP effect and the states of micro-deformation mechanisms
under impact loading. For TEM work, thin foils in the form of
flat
Fig. 2.
(a) The two twin variants integrated with dense dislocations and (b) voids
formed at GBs for TWIP steel deformed at 200
1C.
disks were extracted from the notch tip of each tested sample, such
that the surface normal of a disk foil coincided with the impact
direction and the corresponding disk thickness was approximately
250
mm.
These disks were then mechanically thinned down to
80
mm,
and in order to obtain large electron-transparent areas,
electropolished on both sides by conventional twin-jet electropol-
ishing with an electrolyte consisting of 100 ml 40% perchloric acid,
500 ml butoxyethanol and 400 ml acetic acid. The electropolishing
was carried out at
À
20
1C
under an acceleration voltage of 20.5 V.
3. Results and discussion
3.1. Microstructural investigation
The active micro-deformation mechanisms observed in both
materials at each test temperature are tabulated in
Table 2.
The
investigations on the lower Mn-containing sample, i.e. the TWIP
steel, have demonstrated the coexistence of slip, twin, and voids
at 200
1C
as illustrated in
Fig. 2.
The two twin variants observed
throughout the microstructure are highlighted in
Fig. 2(a).
In
addition to twinning, dense dislocations (Fig.
2(a))
and voids
(Fig.
2(b))
are visible at elevated temperatures. This deformation
response dictated by two active twin systems was also previously
observed, where high velocity ballistic impact tests were carried
out at RT
[44].
The significant increase of temperature stemming
from the high-speed impact load during ballistic penetration
causes the steel to deform via the same mechanisms, i.e. twinning
with two variants and dense dislocations interacting with these
twins
[44],
as in conventional impact test at elevated temperatures
which was utilized in this study. However, in addition to void
This slight difference in sample dimensions stems from the geometry of the
XIP steel initially available.
1
32
B. Bal et al. / Materials Science
&
Engineering A 632 (2015) 29–34
formation (Fig.
3)
as temperature decreased down to RT (Fig.
4(a)),
and then to
À
170
1C
(Fig.
4(b)),
secondary nano-twins were
observed to form within the primary twins instead of different
twin variants. The nano-twin formation in TWIP steel at RT was not
reported in studies where conventional low velocity uniaxial
tension
[34]
and high-pressure torsion experiments
[59]
were
performed. In the former, only a secondary twin system in addition
to the primary twins was activated with increasing amount of strain
[34].
For the latter, instead of nano-twin formation, secondary and
tertiary twin systems were shown to be triggered for two and three
torsional cycles, respectively
[59].
The change in twinning activity
observed in the current TWIP steel samples when the deformation
temperature changed from 200
1C
to RT, i.e. the enhanced nano-
twin formation, not only improves the work hardening capacity, but
also increases the impact energy absorption capacity of TWIP steel
at RT (Fig.
1).
In addition to the change in twinning behavior, as
deformation temperature decreased down to RT (Fig.
4(a))
then to
À170
1C
(Fig.
4(b)),
dislocation activity also decreased as only a low
density of dislocations was observed and dislocation–twin interac-
tions were scarce.
For the XIP steel, on the other hand, the dominant deformation
mechanism dictating the deformation was slip at elevated tem-
peratures (Fig.
5)
due to the higher content of Mn, which increases
the SFE: this increase in SFE increases the threshold energy required
for abrupt orientation evolution during twin nucleation, such
that the resulting twinning can only form at low temperatures
(Figs.
6 and 7).
As temperature decreased, slip activity became less
significant, such that the material deformed by both twinning and
slip at lower temperatures. The introduction of twins into the
microstructure resulted in an increase in impact energy of XIP steel
Fig. 4.
Microstructure of TWIP steel following deformation at (a) RT and (b)
À
170
1C:
nano-twins inside primary twins forming ladder-like structures and dense dislocations
are visible.
Fig. 5.
Microstructure of XIP steel deformed at 200
1C:
dislocation slip dominates
the microstructure.
Fig. 3.
SEM images illustrating the void formation (within red circles) in TWIP steel
deformed at (a) RT and (b)
À
170
1C.
(For interpretation of the references to color in
this
figure
legend, the reader is referred to the web version of this article.)
at RT as compared to 200
1C
(Fig.
1),
as the additional twins act as
an effective mechanism for absorbing the energy imposed by the
impact. In addition to the change in deformation response with
decreasing temperature, the twinning scenario also changed from
two different twin variants forming at RT (Fig.
6(a))
to nano-twin
formation within primary twins at
À
170
1C
(Fig.
7).
The decrease
in diffusion was also realized by an increased deformation rate in
a study where tension tests were carried out at velocities of up to
20 m/s
[8].
The increased deformation speed also resulted in a
decrease in dislocation activity and enhanced twin formation
which increased the overall strength of the material
[8].
By a
decrease in temperature down to RT, two different twin variants
were observed in addition to slip lines (Fig.
6(a)).
The number of
active twin variants and their interactions with glide dislocations
B. Bal et al. / Materials Science
&
Engineering A 632 (2015) 29–34
33
deformation velocity during the impact test. However, the disloca-
tion activity was similar in the study in which high velocity SHB
tension testing was utilized
[28].
The decrease in dislocation activity
was also demonstrated in a study where isothermal compressive load
was applied to XIP steel
[62].
The deformation response was changed
from only slip to two different twin variants with integrated disloca-
tion substructures
[62].
This enhanced twinning due to a temperature
decrease was previously reported for tension tests conducted at RT
and in liquid nitrogen
[26],
where the twin density over dislocation
density was observed to increase with decreasing temperature. The
decrease in dislocation density observed in the current study is
attributed to the high-velocity deformation, which limits dislocation
motion within the microstructure. In addition to twins, voids were
detected only at RT (Fig.
6(b)).
Further decrease in temperature to
À
170
1C
caused nano-twins to form within previously formed twins
(Fig.
7).
This nano-twin formation was also observed in the work by
Timokhina et al.
[63],
where severe plastic deformation was applied
via equal angular channel pressing at high temperatures. The high
temperature during loading resulted in enhanced dislocation slip
[63],
which is not the case in this study.
3.2. Thermographical analyses
The deformation of both steels at 200
1C
and RT was recorded by
a thermal camera in order to detect sudden temperature increases at
the moment of impact. The sensitivity of the thermal camera utilized
in the current study was less than 0.02
1C,
such that the accuracy
of the measurements was within the
71
1C
for temperatures up to
150
1C
and
72
1C
for higher temperatures. The results of the
analyses at 200
1C
and RT are presented in
Figs. 8 and 9,
respectively.
The ductile and more brittle fracture behaviors of XIP and TWIP
steels were evident from the temperature distribution within the
fracture zone during impact. At 200
1C,
the temperature increase
in XIP was higher than that of TWIP, i.e. about 60
1C
vs. about 20
1C.
Fig. 6.
Microstructure of XIP steel deformed at RT: (a) two different twin variants
and dense dislocation structures within twins and (b) voids formed during
deformation.
Fig. 8.
In-situ thermal analyses of impact deformation of TWIP and XIP steels at
200
1C.
The inset indicates which location on the samples the thermographs
correspond to.
Fig. 7.
Dense dislocations and nano-twins that formed in XIP steel sample upon
deformation at
À
170
1C.
agree well with the
findings
of previous works, where room
temperature uniaxial tension tests
[47,60,61]
and split-Hopkinson
bar (SHB) tensile tests
[28]
were carried out on steels with a Mn
content close to that of the XIP steel. However, the twin activity
was detected to be increased in the current work with respect
to standard uniaxial tension tests
[47,60,61]
related to the high
Fig. 9.
In-situ thermal analyses of impact deformation of TWIP and XIP steels at RT.
The inset indicates which location on the samples the thermographs correspond to.

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