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Anja Weidnera, Alexander Glagea, Stefan Martinb, Jir
ˇíMan
c, Volker Klemmb, Ulrich Martinb,
Jaroslav Polákc, David Rafajab, Horst Biermanna
Microstructure of austenitic stainless steels
of various phase stabilities after cyclic
and tensile deformation
The microstructures of two metastable high-alloyed
CrMnNi cast TRIP steels and a stable AISI 316L austenitic
stainless steel were studied in detail after tensile and cyclic
deformation. Electron backscattered diffraction was em-
ployed to localize the martensitic phase transformation and
electron channelling contrast imaging to describe the typi-
cal dislocation arrangements. These were complemented
by transmission electron microscopy and by scanning trans-
mission electron microscopy performed in a scanning elec-
tron microscope. The TRIP steel with the lowest austenite
stability shows a more pronounced martensitic phase trans-
formation realized from the austenite via the intermediate
formation of e-martensite. Martensitic phase transforma-
tion also occurred in the stable 316L austenitic stainless
steel with a small volume fraction of a’-martensite, but only
with cyclic deformation at low temperatures and/or at very
high plastic strain amplitudes.
Keywords: Austenite stability; Martensitic phase transfor-
mation; Electron channelling contrast
1. Introduction
Newly developed types of TRIP (transformation induced
plasticity) steels are materials with exceptional mechanical
properties such as high strength in combination with high
ductility and high energy absorption capacity [1–3]. The
probability of the martensitic phase transformation from
the face-centred cubic (fcc) austenite into the body-centred
cubic (bcc) a’-martensite depends on the austenite stability
that is commonly characterized by the martensite start tem-
perature (Ms) and by the Md30 temperature, which is the low-
est temperature where 50 % of deformation-induced a’-mar-
tensite is formed with a true strain of 30 % [4]. Low Msand
Md30 temperatures indicate high austenite stability. The de-
formation mechanisms occurring are determined by the
stacking fault energy (SFE), which governs the transition
of the deformation behaviour from stacking fault formation
(at low SFE) via twinning to the dislocation glide dominated
processes (at high SFE) [5, 6]. Both the stability of the aus-
tenite and the SFE depend on the chemical composition,
where elements like Ni and Mn stabilize the austenite [7].
The aim of the present study is the microstructural inves-
tigation of three types of steels with different stability of
austenite during plastic deformation. Electron channelling
contrast imaging (ECCI) with electron backscatter diffrac-
tion (EBSD) measurements were used to analyze the evolu-
tion of stacking faults in combination with the deformation-
induced martensitic phase transformation in two metastable
high-alloyed CrMnNi cast TRIP steels at room temperature
and in a stable austenitic stainless steel (AISI 316L) at low
temperature.
2. Experimental details
The materials investigated were two metastable high-al-
loyed TRIP steels with different Ni content and a stable aus-
tenitic stainless steel (AISI 316L), see Table 1. The stack-
ing fault energy and the temperatures Msand Md30
calculated for the given chemical compositions using sev-
eral empirical equations [5, 8–10] are also given in Table 1.
The value of SFE represents an average since the real SFE
varies with the local chemical composition of the cast mate-
rial and depends also on the equation used for its calcula-
tion.
A. Weidner et al.: Microstructure of austenitic stainless steels of various phase stabilities after deformation
1374 Int. J. Mat. Res. (formerly Z. Metallkd.) 102 (2011) 11
Table 1. Chemical composition, stacking fault energy (SFE) and calculated temperatures Msand Md30 characterizing the stability of
austenite; the symbol ()* at the temperature values denotes their measured values [7].
CCrMnNiSiAlNSFE
(mJ m–2)
Ms
(8C)
Md30
(8C)
16Cr7Mn3Ni 0.03 16.5 6.8 2.9 1.0 0.09 0.06 9 95 (61)* 53
16Cr7Mn6Ni 0.03 16.0 6.9 6.0 1.0 0.09 0.06 17 –93 (1)* 30
AISI 316L 0.02 17.6 1.7 13.8 0.4 – – 37 < –196 –18
aInstitute of Materials Engineering, Technische Universität Bergakademie Freiberg, Freiberg, Germany
bInstitute of Materials Science, Technische Universität Bergakademie Freiberg, Freiberg, Germany
cInstitute of Physics of Materials, Academy of Sciences of the Czech Republic, Brno, Czech Republic
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The TRIP steel specimens were cut from cast plates hav-
ing the size of 200 ·200 ·16 mm
3
manufactured by
ACTech (Freiberg, Germany). The plates were solution an-
nealed (0.5 h at 1 0508C in vacuum) and gas quenched. The
material showed a dendritic cast microstructure with large
grain size up to 1 mm. Nevertheless, the mechanical prop-
erties as well as the martensitic phase transformation were
not significantly influenced by the cast structure of the ma-
terials as it was shown in [2] by a comparison of cast and
wrought states of the material.
The 316L steel was purchased from Uddeholm (Sweden)
in a solution annealed state. Additional heat treatment (1 h
at 600 8C invacuum, furnace cooling) was applied after pre-
paration of specimens by spark erosion. The material had a
mean grain size of 39 lm found by linear intercept method
without counting twin boundaries.
Regardless the different materials’ conditions of the stud-
ied materials (cast vs. wrought structure, grain size), the
focus of the present paper is a basic microstructural investi-
gation on steels with different austenite stability.
Symmetrical (R= –1) push-pull tests of 16Cr7Mn3Ni
steel were performed at room temperature under total strain
control with a constant total strain amplitude of ea,t = 2.5 ·
10–3 up to fracture. Tensile tests of 16Cr7Mn6Ni steel were
carried out at room temperature to strains of 8% and 15 %.
The 316L steel specimen was cyclically deformed at low
temperature (–163 8C) in a multiple step test with increasing
plastic strain amplitudes ranging from ea,p = 2.1 ·10–4 to
9.5 ·10–3. Details on the mechanical behaviour of the two
cast TRIP steel specimens were published in [7, 11, 12]
and for the 316L steel in [13].
The microstructure of the deformed specimens was in-
vestigated using a scanning electron microscope (SEM).
The EBSD and ECCI investigations were performed using
a high-resolution FE-SEM (LEO 1530, ZEISS, Germany)
equipped with an EBSD system and a retractable standard
four-quadrant backscattered electron (BSE) detector. Re-
sults from ECCI studies for the 16Cr7Mn6Ni TRIP steel
were verified with transmission electron microscopy
(TEM) studies using a JEM 2010FEF (JEOL) with an elec-
tron energy in-column filter operating at 200 kV. The TEM
foils were prepared by ion milling. In addition, the TEM
foils were also investigated in a co-operation with the Insti-
tute of Semiconductor Physics at TU Dresden in an SEM
(ZEISS Ultra 55) using a retractable scanning transmission
electron microscopy (STEM) detector at 30 kV.
3. Results and discussion
The stability of austenite and the stacking fault energy have
a strong effect on the deformation mechanisms. According
to the Msand Md30 values given in Table 1, the
16Cr7Mn3Ni steel is the most unstable steel with a high
tendency to the martensitic phase transformation. The
316L steel is the most stable one with no martensitic phase
transformation at room temperature. According to the SFE
values, the stacking faults formation leading to the develop-
ment of hexagonal closed packed (hcp) e-martensite is ex-
pected to be the dominant deformation mechanism in
16Cr7Mn3Ni and 16Cr7Mn6Ni at room temperature. In
316L steel, dislocation glide will dominate the process of
the plastic deformation. In the following, the results of mi-
crostructural investigations will be shown in detail.
3.1. Metastable steel 16Cr7Mn6Ni
Figure 1 shows results of the EBSD and ECCI investiga-
tions of the microstructure in austenitic grains after tensile
deformation up to 8%. Figure 1a illustrates the formation
of pronounced deformation bands according to the acti-
vated slip system, which are closely packed by stacking
faults (SF) leading to the hexagonal lattice structure (yel-
low) [14]. A minor fraction of these bands was still indexed
as fcc austenite – see grey area within deformation bands in
Fig. 1a.
The bands of stacking faults are preferred sites for the
formation of a’-martensite nuclei (blue). Individual stack-
ing faults were observed mainly for slip systems with low
A. Weidner et al.: Microstructure of austenitic stainless steels of various phase stabilities after deformation
Int. J. Mat. Res. (formerly Z. Metallkd.) 102 (2011) 11 1375
Fig. 1. Microstructure of 16Cr7Mn6Ni TRIP
steel after 8% tensile deformation. (a) EBSD-
phase map with the c-austenite (fcc) in band
contrast (grey), e-martensite (hcp) in yellow
and a’-martensite (bcc) in blue. (b), (c) In-
verted ECCI micrographs showing deforma-
tion bands and individual stacking faults ac-
cording to different activated slip systems.
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Schmid factors, but also between pronounced deformation
bands [15]. Figure 1b and c shows inverted ECC images
from different grains where the typical fringe contrast of
stacking faults known from TEM observations is clearly
visible. Additionally, investigations on thin TEM foils from
a specimen strained in tension up to 15 % elongation were
performed in a TEM and also in an SEM applying a retract-
able STEM detector. Figure 2a shows a TEM bright field
micrograph of SFs appearing in the characteristic fringe
contrast. Intersecting and/or overlapping stacking faults ac-
cording to different slip systems occur, exhibiting various
widths and different diffraction conditions. Partial disloca-
tions bounding the stacking faults can be clearly seen
(white arrows in Fig. 2a).
Two extended and crossing stacking faults in the middle
part of Fig. 2a (marked by white dot-and-dash lines) may
be the beginning of pronounced deformation bands consist-
ing of many densely packed parallel SFs. Images taken
from different grains in a TEM foil with a STEM detector
in an SEM at 30 kV and a working distance of 3 mm
(Fig. 2b– d) confirm this information. Deformation bands
consisting of overlapping parallel SFs as well as individual
SFs on different activated slip systems can be clearly distin-
guished (Fig. 2b). Furthermore, a’-martensite nuclei inside
the stacking fault bands were observed (Fig. 2c and d). Un-
fortunately, no crystallographic information about the mu-
tual orientation of the neighbouring grains can be obtained
applying the STEM detector.
3.2. Metastable steel 16Cr7Mn3Ni
Figure 3 shows results of the microstructural investigations
on a 16Cr7Mn3Ni TRIP steel specimen after cyclic defor-
mation at R= –1 with a total strain amplitude of ea,t =
2.5 ·10–3 at room temperature up to failure. This TRIP
steel shows a more pronounced martensitic phase transfor-
mation during cyclic deformation than the 16Cr7Mn6Ni
steel due to lower austenite stability and lower stacking
fault energy as expected.
Majority of former austenitic grains are completely
transformed into a’-martensite as evidenced by the upper
part of Fig. 3a and b. The deformation bands consisting of
stacking faults (e-martensite, yellow) and a’-martensite nu-
clei (blue) were observed only in a small number of austeni-
tic grains (e.g. lower part of Fig. 3a and b). However, the
formation of deformation bands at earlier stages of fatigue
life is anticipated in areas which are nearly almost comple-
tely transformed into a’-martensite. The formation of stack-
ing faults is a dominating deformation mechanism as it is
demonstrated by ECCI (Fig. 3c) showing several individual
stacking faults, cf. [16].
3.3. AISI 316L steel
The stable austenitic stainless steel 316L was cyclically de-
formed at a low temperature (–1638C) in a multiple step
test with increasing plastic strain amplitudes. After finish-
ing the test at ea,p =9.5·10–3, the microstructural study re-
vealed the formation of deformation bands as well as pre-
sence of martensite areas as shown in Fig. 4.
Two different types of deformation bands are shown in
Fig. 4a. There are deformation bands (Fig. 4b) which show
a similar structure to the deformation bands that develop in
metastable TRIP steels. However, in the stable 316L steel
these bands consist only of a few stacking faults (marked
by arrows). Nevertheless, these areas still have an fcc crys-
tal structure which is an indication for a low density of
stacking faults [15]. In the present case, the structure inside
the band between the few individual stacking faults is cov-
ered by a dislocation cell structure. The other type of the de-
formation bands (shown in Fig. 4d) is the classical one,
consisting of planar dislocation arrangements occurring in
two activated slip systems. However, areas of martensitic
phase transformation were found in both types at the inter-
section of the deformation bands (Fig. 4c and d).
Additional neutron diffraction measurements performed
on this specimen revealed 9.9 vol.% of hcp e-martensite
and 1.9 vol.% of bcc a’-martensite. This indicates that a re-
markable part of the deformation bands consists of stacking
faults leading to the hcp lattice structure (e-martensite). The
present observations are consistent with previous TEM
A. Weidner et al.: Microstructure of austenitic stainless steels of various phase stabilities after deformation
1376 Int. J. Mat. Res. (formerly Z. Metallkd.) 102 (2011) 11
Fig. 2. Microstructure of 16Cr7Mn6Ni TRIP steel after 15 % tensile
deformation as seen by: (a) TEM bright field micrograph. (b, c) Micro-
graphs taken with a STEM detector in an SEM from two different indi-
vidual grains. (d) Area marked in (c) at higher magnification.
Fig. 3. Microstructure of 16Cr7Mn3Ni TRIP
steel cyclically deformed with ea,t = 2.5 ·10–3
for Nf= 103 385 cycles. (a) SEM micrograph
(BSE contrast) showing a grain with nearly
complete martensitic phase transformation
(upper part) and another grain with deformation
bands in the austenite (lower part). (b) EBSD
phase map with e-martensite (yellow) and
a’-martensite (blue) of the area marked in (a).
(c) Inverted ECCI micrograph showing defor-
mation bands and individual stacking faults.
W2011 CarlHanserVerlag, Munich, Germanywww.ijmr.de Notforuseininternetorintranetsites. Notforelectronic distribution.
studies performed on 316L and 316LN steel specimens de-
formed in the low cycle fatigue (LCF) regime at –1968C
[17, 18]. Finally, the experimental study showed that 316L
stainless steel is stable during room temperature fatigue at
low or medium strain amplitudes. The probability of the
martensitic phase transformation increases with decreasing
temperature, with increasing strain amplitude and with in-
creasing grain size [19].
4. Summary
The kind of deformation mechanism and the process of
martensitic phase transformation strongly depend on the
chemical composition and the temperature. Therefore, the
development of microstructure and the martensitic phase
transformation were investigated in austenitic cast TRIP
steels with various austenite stabilities during tensile and
cyclic deformation at room temperature. Analogous micro-
structure studies were performed on a stable austenitic
stainless steel (AISI 316L) after a low temperature cyclic
deformation. In the metastable austenitic stainless cast
TRIP steels (16Cr7Mn3Ni and 16Cr7Mn6Ni), deformation
bands consisting of overlapping stacking faults were found
that led to the formation of e-martensite. Areas of e-marten-
site are nucleation sites for the formation of a’-martensite.
Stacking fault formation was investigated using TEM,
STEM and ECCI. The TRIP steel with the lower austenite
stability (16Cr7Mn3Ni) showed a more pronounced mar-
tensitic phase transformation with higher volume fraction
of martensite. Even in the stable austenitic stainless AISI
316L steel a martensitic transformation was detected, but
only to a limited extent and after low temperature cyclic de-
formation with high plastic strain amplitude.
The authors thank Dr. E. Hieckmann from the Institute of Semiconduc-
tor Physics, Technische Universität Dresden, Germany, for the STEM
investigations on a TEM foil in a FE-SEM (ZEISS Ultra55). The
authors acknowledge gratefully the German Research Foundation for
the financial support of the Collaborative Research Centre \TRIP-Ma-
trix Composite"(SFB 799).
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Bibliography
DOI 10.3139/146.110604
Int. J. Mat. Res. (formerly Z. Metallkd.)
102 (2011) 11; page 1374– 1377
#Carl Hanser Verlag GmbH & Co. KG
ISSN 1862-5282
Correspondence address
Dr. Anja Weidner
Institute of Materials Engineering
Technische Universität Bergakademie Freiberg
Gustav-Zeuner-Str. 5, D-09596 Freiberg
Tel.: +49 3731 39 2124
Fax: +49 3731 39 3703
E-mail: weidner@ww.tu-freiberg.de
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ing the document number MK110604 on our website at
www.ijmr.de
A. Weidner et al.: Microstructure of austenitic stainless steels of various phase stabilities after deformation
Int. J. Mat. Res. (formerly Z. Metallkd.) 102 (2011) 11 1377
Fig. 4. Microstructure of a stable AISI 316L austenitic stainless steel
cyclically deformed in a multiple step test with increasing plastic strain
amplitude up to ea,p = 9.5 ·10–3 at –1638C. (a, b, d) Inverted ECCI mi-
crographs. (c) EBSD phase map (blue colour corresponds to the a’-
martensite).
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