Materials Science and Engineering A283 (2000) 65 – 69

Mo¨ssbauer study of the retained austenitic phase in

multiphase steels

A. Mijovilovich

a,

*, A. Gonc¸alves Vieira

b

, R. Paniago

a

, H.D. Pfannes

a

,

B. Mendonc¸a Gonzalez

b

a

Departamento de Fı´sica, Uni

6ersidade Federal de Minas Gerais, C.P.

702

,

30123

-

970

Belo Horizonte, Brazil

b

Escola de Engenharia, Uni

6ersidade Federal de Minas Gerais, Rua Espı´rito Santo

35

,

30160

-

030

Belo Horizonte, Brazil

Received 31 May 1999; received in revised form 21 December 1999

Abstract

Samples of steels with composition 0.30%C-1.5%Mn-1.5%Si-0.5%Al-0.5%Mo (wt.%) were subjected to different thermomechan-

ical treatments to produce ferrite/pearlite/bainite (FPB), spheroidized (ESF) and martensite (MAR) microstructures. Subsequently
they underwent a two stage annealing to obtain a final structure comprising of ferrite, bainite, martensite and austenite. The
samples were studied by means of Mo¨ssbauer spectroscopy (transmission and conversion electron Mo¨ssbauer spectrocopy
(CEMS)), X-ray diffraction (XRD), and metallographic analysis. Austenite contents were found to be the same for all samples
except for the spheroidized sample annealed at 750°C that showed an increase of the austenite with increasing temperature of the
treatment. Mo¨ssbauer spectroscopy and quantitative XRD analysis exhibited significant discrepancies ascribed to texture effects.
It is shown that the thermal treatment was successful in retaining significant quantities of the austenite phase for steels of this
composition. © 2000 Elsevier Science S.A. All rights reserved.

Keywords

:

Multiphase steel; Mo¨ssbauer; Austenite; Martensite; Bainite

www.elsevier.com/locate/msea

1. Introduction

In order to enhance the ductility in high-strength

steels it was shown that it is necessary to increase the
content of their retained austenite. Alloys with high
ductility and excellent levels of mechanical strength can
be obtained by the transformation of austenite to
martensite during plastic deformation (i.e. trip: trans-
formation induced plasticity effect) [1]. Matsumura et
al. [2] increased the content of retained austenite in an
alloy of C – Mn – Si by a two stage thermal treatment:
an annealing followed by a quick quenching to the
range of temperatures for the bainitic transformation.
The amount of retained austenite increased with in-
creasing content of Mn and Si in the alloy [3].

It is usual to determine the phases present by metal-

lographic analysis as well as X-ray diffraction. The last
method is sometimes used for quantitative analysis in

spite that the result may be strongly influenced by
texture effects. Mo¨ssbauer spectroscopy is a well-known
technique used in the study of Fe-containing alloys [4].
The different phases can be distinguished from their
different signals, and different magnetic behaviors re-
gardless of the state of aggregation of the phases.
Martensite and austenite are easily distinguished from
their different hyperfine patterns in the Mo¨ssbauer
spectra with better accuracy than by other techniques.
Due to the low solubility of carbon in

a-Fe in equi-

librium, the interstitial solute C can not be detected by
Mo¨ssbauer spectroscopy. In the transmission made sig-
nals from all the

57

Fe atoms in the sample are obtained

regardless of the state of the aggregation or crystallinity
in the material. In the case of conversion electron
Mo¨ssbauer spectrocopy (CEMS) the spectrum stems
from a region of

10–100 nm below the surface of the

sample, and thus becomes an efficient tool to analyse
the surface. With Mo¨ssbauer spectroscopy the texture
effect does not affect the total area of the subspectra
corresponding to the different phases.

* Corresponding author. Present address: EMBL c/o DESY,

Notkestrasse 85, Geb. 25A, Notkestrasse 85, 2603, Hamburg, Ger-
many. Tel.: + 49-40-89902120; fax: + 49-40-89902149.

0921-5093/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S 0 9 2 1 - 5 0 9 3 ( 0 0 ) 0 0 6 2 0 - 1

A. Mijo

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/

Materials Science and Engineering A

283 (2000) 65 – 69

66

Fig. 1. Schematic diagram of two stage annealing. T

1

= intercritical

temperature; t

1

= annealing time; T

2

= bainitic temperature (425°C),

t

2

= bainitic transformation time.

We used transmission Mo¨ssbauer spectroscopy to

determine the amount of austenitic retained phase after
the thermal treatments. By CEMS we were able to
study the mechanical stability and transformation to
martensite during the laminating process. Quantitative
X-ray diffraction analyses were employed to determine
the relative amounts of retained austenite, and ferritic
phases present.

2. Experimental

By a specific thermal treatment of an alloy of compo-

sition 0.30%C-1.5%Mn-1.5%Si-0.5%Al-0.5%Mo three
initial structures were obtained: ferrite/pearlite/bainite
(FPB), spheroidized (ESF) and martensite (MAR).
Subsequently they underwent a two stage annealing
(Fig. 1) to obtain a final structure of ferrite, bainite,
martensite and austenite. We will keep the acronyms of
the initial phases when we refer to the samples after the
two-stage annealing. For the metallographic analysis a
selective etching with Nital 2%, Picral 5% and Na-thio-
sulfate [5] was used.

Fig. 2. Photographs of the microstructures: (a), FPB780; (b), MAR780; (c), ESF750 and (d), ESF840. Ferrite is gray color, bainite in dark gray
and austenite-martensite in light gray.

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Materials Science and Engineering A

283 (2000) 65 – 69

67

Table 1
Hyperfine parameters and relative areas (%) for the identified phases
from transmission Mo¨ssbauer spectra

a

Sample

d (mm s

−1

)

H(T)

D (mm s

−1

)

Relative area
(%)

FPB

780

Ferrite 1

0.047

33.0

–

45.42

Ferrite 2

0.067

30.6

–

28.02

0.072

–

27.9

6.84

Ferrite 3

–

Austenite 1

−0.023

–

13.67

Austenite 2

0.032

–

0.6

6.08

MAR

780

Ferrite 1

0.045

33.0

–

45.70

0.064

–

26.07

Ferrite 2

30.7

0.073

–

28.4

8.34

Ferrite 3

–

Austenite 1

−0.027

–

13.51

0.033

0.6

6.38

Austenite 2

–

Mar

810

0.008

–

43.80

Ferrite 1

33.0

0.03

–

31.0

22.83

Ferrite 2

29.4

Ferrite 3

0.033

–

12.46

−0.059

–

–

13.72

Austenite 1

–

Austenite 2

0.005

0.6

7.20

Esf

750

0.008

–

33.0

48.2

Ferrite 1

31.0

Ferrite 2

0.031

–

28.4

Ferrite 3

0.025

29.2

–

13.0

−0.053

–

–

7.1

Austenite 1

0.051

0.6

3.2

Austenite 2

–

Esf

840

Ferrite 1

0.005

33.0

–

40.2

0.028

Ferrite 2

–

31.3

22.8

0.041

–

29.5

17.4

Ferrite 3

−0.062

–

13.6

Austenite 1

–

−0.004

0.6

6.0

–

Austenite 2

a

H(T) is the hyperfine magnetic field in Tesla,

d (mm s

−1

) is the

isomer shift refered to

a-Fe in mm s

−1

, and

D (mm s

−1

) is the

quadrupole splitting.

were able to take into account the thickness of the
samples. The sample FPB780 was investigated also by
CEMS using the same source as above. In this case, as
the effective thickness is small, we used a least
squared fit with simple Lorentzian lines [11]. The values
of

x

2

to measure the quality of the fit ranged from 1.7

to 3.4.

Fig. 3. Mo¨ssbauer spectra for: (a), MAR780 and (b), FP780 samples.

Integrated intensities of X-ray diffraction peaks were

used to determine the content of retained austenite [6].
The samples were grounded, embedded in epoxy, pol-
ished with 1

mm diamond paste and measured in a

Philips diffractometer (K

a

-Cu radiation, 0.01°-steps of

2

u). The reflections used for the quantitative analysis

were: (200), (220) and (311) for the

g-phase; (200) and

(211) for the

a-phase.

We measured transmission Mo¨ssbauer spectra of all

samples. The measuring temperature was room temper-
ature and the source was

57

Co in Rh matrix. Two

samples (FPB780 and MAR780) were also measured at
77 K. Since the steels were 50 nm thick foils the lines
were broadened because of thickness effect. It is com-
mon to use fits with hyperfine field distributions or
Voigtian line profiles [7] to take this effect into account.
In fitting these spectra we used the program WOTAN
[8] which is based on the integral form of the absorp-
tion line calculated by Margulies [9,10]. In this way we

Table 2
Hyperfine parameters and percentages of phases from CEMS spectra
(surfaces)

a

H(T)

d (mm s

−1

)

Relative area

D (mm s

−1

)

(%)

FPB780

33.0

0.00

Ferrite 1

–

48.12

Ferrite 2

31.0

0.04

–

39.36

27.0

3.37

–

−0.08

Ferrite 3

–

−0.18

Austenite 1

–

5.38

–

0.00

Austenite 2

0.6

3.79

a

Symbols as in Table 1.

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Materials Science and Engineering A

283 (2000) 65 – 69

68

Fig. 4. CEMS spectra for sample FP780.

bainite shows dark gray, and both, the martensite and
the austenite become light gray. The microstructure of
the FPB sample is heterogeneous with fine and coarser
regions (Fig. 2a). The final structure for the sample
obtained from an initial martensite is finer (Fig. 2b).
For the samples obtained from the spheroidized initial
structure an increase of the amount of bainite and
martensite-austenite with increasing intercritical tem-
perature is found (Fig. 2c, d).

The main contributions to the Mo¨ssbauer spectra

arise from the ferrite and martensite phases of the steel,
which lead to magnetically split spectra. We used the
designations Ferrite 1, 2 and 3 to denote subspectra
corresponding to Fe atoms in the ferritic/bainitic/
martensitic matrix with three different environments,
following the nomenclature of Uwakweh et al. [12]. But
due to the different nomenclatures used in the literature
it is not possible to give unique assignments for the
different Fe – C configurations [13]. The austenitic para-
magnetic phase is clearly distinguishable with two con-
tributions: austenite 1 and 2 for the singlet and the
doublet, respectively. It amounts

B21% in all samples.

Since there is insignificant contribution of cementite or
other carbide precipitates seen in the spectra, they have
been neglected in the fitting. The hyperfine parameters
and the percentages of each phase as determined from
the Mo¨ssbauer spectra are listed in Table 1. Typical
Mo¨ssbauer spectra are shown in Fig. 3.

The Mo¨ssbauer results indicate similar austenite con-

tents for the MAR and FPB samples. For the ESF
samples it is clearly observed that by the treatment at
higher temperature more austenite is retained. By com-
paring the room temperature and liquid nitrogen spec-
tra of FPB780 (ferrite-perlite system) and MAR780
(martensite rich sample), we conclude that both exhibit
similar austenite contents.

We measured the FPB780 sample also with CEMS

and deduced a decrease of the austenitic phase, indicat-
ing a transformation from austenite to martensite in the
surface during the polishing process (see Table 2 and
the corresponding spectrum in Fig. 4). This was simi-
larly observed by [14].

The quantitative determination of phases by XRD is

given in Table 3 and a characteristic pattern is shown in
Fig. 5. The Mo¨ssbauer results for the austenite content
differs significantly from the XRD results, except for
the ESF samples where both techniques indicate the
same trend. This is ascribed to a cristallographic texture
effect that strongly influences the XRD measurements.
Mo¨ssbauer results concerning the spin texture in these
steels will be published elsewhere [15].

4. Conclusions

Metallographic analysis showed the presence of

bainite, ferrite and martensite-austenite in all samples.

Table 3
XRD results for retained austenite

g phase (volume%) for different

two-stage thermal treatments

Volume (%) of

g

Thermal treatment

Sample

phase

780°C 20 min−425°C 10 min

FPB780

13.2

91.5

7.1

91.5

750°C 20 min−425°C 10 min

ESF750

840°C 20 min−425°C 10 min

ESF840

16.1

91.5

MAR780

780°C 20 min−425°C 10 min

17.8

91.5

MAR810

810°C 20 min−425°C 10 min

17.2

91.5

Fig. 5. X-ray diffraction pattern of a multiphased structure obtained
from martensitic initial structure-MAR-780 (intercritically annealed
at 780°C for 20 min and at 425°C for 10 min).

3. Results and discussion

Photographs of optical microscopy are shown in Fig.

1. Three phases can be distinguished in the photo-
graphs, namely martensite plus austenite, bainite and
ferrite. With the used etching, ferrite becomes gray,

A. Mijo

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Materials Science and Engineering A

283 (2000) 65 – 69

69

The finer microstructure is present in the samples ob-
tained from the martensitic phase due to its acicular
morphology.

From the micrographs it is seen that the amount of

carbide precipitates is negligible which is in concordance
with the Mo¨ssbauer results, which do not indicate any
significant contribution from carbides. For the evalua-
tion of the retained austenite the Mo¨ssbauer spectra
indicate that the annealing at higher temperature is
effective in stabilizing the austenite phase in the
spheroidized sample. Results for other samples under
different thermal treatments are similar, in disagreement
with XRD measurements. The difference was attributed
to a texture effect.

From CEMS results a significant decrease of the

austenite content in the surface due to the mechanical
polishing is deduced.

Acknowledgements

The support of the Brazilian research agencies

Fapemig, CAPES and CNPq is greatfully acknowl-
edged.

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