# Investigation of theα−γ−α Phase Transformation in Steel:  High-Temperature In Situ

Authors: I. Lischewski; D. M. Kirch; A. Ziemons; G. Gottstein
Journal: Texture, Stress, and Microstructure
 (2008-02-04)

---

## Abstract

A newly developed laser powered heating stage for commercial SEMs in combination with automated established electron backscatter diffraction (EBSD) data acquisition is presented. This novel experimental setup can be used to achieve more information about microstructure and orientation changes during grain growth, recrystallization, recovery, and phase transformations. First results on theα−γ−α
phase transformation in steel within 886∘C–900∘C are presented.


---

## Body

## 1. Introduction

Most investigations into
microstructure and texture evolution are limited
to postmortem analysis. Due to this limitation, there is a need for experimental setups permitting the in situ observation of microstructural
changes of polycrystalline materials. Besides in situ deformation, the main focus
lies on the in situ observation of temperature-influenced processes like grain growth, recrystallization, recovery, and
phase transformations. An
example is theα-γ-α phase transformation in low-carbon steels [1–3]. The
observation of this phase transformation by using
the EBSD technique is limited
to the low-temperature
regime, since it is a reversible process and
cooling down instantaneously leads to a back transformation to the low-temperature
phase.Due to this fact, a new laser powered heating stage was developed for commercial SEMs with the capacity to heat specimens up to a temperature of
1000°C. Data acquisition utilizes a
combination of the well-established electron back-scatter diffraction (EBSD) technique and orientation contrast (OC) imaging. By using a combination of a heating
stage with the automated EBSD technique, we
achieve a powerful tool for in situ observations of microstructural
changes at elevated temperatures. Previous studies on annealing and
recrystallization experiments utilizing a combination of a hot stage and
automated EBSD technique were limited to temperatures of about 400°C [4]. Seward et al. performed in situ SEM observations of the hcp-to-bcc-phase transformations in titanium at
900°C in a special SEM dedicated to high-temperature
EBSD measurements [5].In this study, a novel concept of a high vacuum heating stage utilizing the energy of an infrared diode laser as heating source
is introduced.
A great advantage of this approach is the very small size,
because only the sample holder is situated inside the SEM chamber and therefore
no influence on the electron beam
occurs, which would lead to image drift or distortion.


## 2. Experimental Setup

### 2.1. SEM-EBSD System

In Figure1, the whole SEM-EBSD system for
the high-temperature investigations is
presented. A Jeol JSM-6100 SEM equipped with a standard eucentric goniometer
stage and a scintillator-photomultiplier secondary electron-detector were used.
The electron gun consists of a tungsten hairpin filament. The SEM possesses a
standard EBSD detector system Nordlys
by HKL with a 70° tilted specimen stage for EBSD analyses. The analog detector signal is then digitized by a NORDIF
EBSP image processor supplied by the Hamamatsu Photonics company
and finally processed by the software Channel 5 supplied by HKL
technology.  The EBSD detector contains a four-quadrant
forward scatter detector mounted around the phosphor-screen, which provides
the OC images for enhancement of the microstructure investigation by EBSD.Figure 1
Sketch of the experimental setup during in situ EBSD measurement.


### 2.2. Laser-Powered Heating Stage

The heating stage was developed at
the Institute of Physical Metallurgy and Metal Physics of RWTH, Aachen
University. The general construction is shown schematically in Figures2(a), 2(b)
and contains three basic components. The main body consists of a small
SiC sample holder functioning as the actual sample heater.
The laser light source is a commercial diode infrared laser.
It is a continuous wave laser of 810 nm wavelength and maximum power
output of 100 W with a stability of ±2%. The incident infrared laser
light which is emitted from an optical fiber (Figure
3) is heating the SiC
sample holder only by absorption.
The specimen is finally mounted onto the SiC sample holder by a tungsten
clamp
(Figures 2(a), 2(b)(4)).Sketch of the heating stage. (a) Isometric front view depicting the basic outer parts. (b) Cross-section of the stage interior: (1) rear outer copper heating shield (active water cooled);
(2) internal tantalum heating shield (not cooled);
(3) SiC-sample holder—absorber platelet;
(4) specimen mount—tungsten clamp;
(5) heating stage copper base (active water cooled);
(6) optical fiber guidance;
(7) goniometer stage adapter. Overall dimensions are approximately8.2×3.5×3.6cm3.
(a)(b)Figure 3
Basic principle of the laser heating stage: SiC-sample holder is heated up by absorbing the infrared
laser light of wavelength 810 nm.The second component
is the inner heating shield (Figure2(b)(2)) which
is not cooled and
made of tantalum sheets, for high-temperature stability. The final component
consists of the outer heating shield
(Figures 2(a),
2(b)(3)) which is made of copper.
The rear part of the outer heating shield and also the copper base of the heating
stage are directly cooled by an integrated water cooling circuit
(Figure 2(a)).The temperatures of various attachments (EBSD-Detector, secondary electron
detector, and heating stage) in the SEM
chamber are monitored by thermocouples for safety reasons. Three
thermocouples are used to monitor the heating stage temperature
(heating shield, sample holder)
and to control the desired temperature and the heating rate.  Adjustment of the
laser power output proceeds by an external computer connected via an RS232
interface using Labview 8.0
(Figure1). The heating stage can be operated at
any tilting angle and working distance
WD down to 15 mm.


## 2.1. SEM-EBSD System

In Figure1, the whole SEM-EBSD system for
the high-temperature investigations is
presented. A Jeol JSM-6100 SEM equipped with a standard eucentric goniometer
stage and a scintillator-photomultiplier secondary electron-detector were used.
The electron gun consists of a tungsten hairpin filament. The SEM possesses a
standard EBSD detector system Nordlys
by HKL with a 70° tilted specimen stage for EBSD analyses. The analog detector signal is then digitized by a NORDIF
EBSP image processor supplied by the Hamamatsu Photonics company
and finally processed by the software Channel 5 supplied by HKL
technology.  The EBSD detector contains a four-quadrant
forward scatter detector mounted around the phosphor-screen, which provides
the OC images for enhancement of the microstructure investigation by EBSD.Figure 1
Sketch of the experimental setup during in situ EBSD measurement.


## 2.2. Laser-Powered Heating Stage

The heating stage was developed at
the Institute of Physical Metallurgy and Metal Physics of RWTH, Aachen
University. The general construction is shown schematically in Figures2(a), 2(b)
and contains three basic components. The main body consists of a small
SiC sample holder functioning as the actual sample heater.
The laser light source is a commercial diode infrared laser.
It is a continuous wave laser of 810 nm wavelength and maximum power
output of 100 W with a stability of ±2%. The incident infrared laser
light which is emitted from an optical fiber (Figure
3) is heating the SiC
sample holder only by absorption.
The specimen is finally mounted onto the SiC sample holder by a tungsten
clamp
(Figures 2(a), 2(b)(4)).Sketch of the heating stage. (a) Isometric front view depicting the basic outer parts. (b) Cross-section of the stage interior: (1) rear outer copper heating shield (active water cooled);
(2) internal tantalum heating shield (not cooled);
(3) SiC-sample holder—absorber platelet;
(4) specimen mount—tungsten clamp;
(5) heating stage copper base (active water cooled);
(6) optical fiber guidance;
(7) goniometer stage adapter. Overall dimensions are approximately8.2×3.5×3.6cm3.
(a)(b)Figure 3
Basic principle of the laser heating stage: SiC-sample holder is heated up by absorbing the infrared
laser light of wavelength 810 nm.The second component
is the inner heating shield (Figure2(b)(2)) which
is not cooled and
made of tantalum sheets, for high-temperature stability. The final component
consists of the outer heating shield
(Figures 2(a),
2(b)(3)) which is made of copper.
The rear part of the outer heating shield and also the copper base of the heating
stage are directly cooled by an integrated water cooling circuit
(Figure 2(a)).The temperatures of various attachments (EBSD-Detector, secondary electron
detector, and heating stage) in the SEM
chamber are monitored by thermocouples for safety reasons. Three
thermocouples are used to monitor the heating stage temperature
(heating shield, sample holder)
and to control the desired temperature and the heating rate.  Adjustment of the
laser power output proceeds by an external computer connected via an RS232
interface using Labview 8.0
(Figure1). The heating stage can be operated at
any tilting angle and working distance
WD down to 15 mm.


## 3. Experimental Results

All experiments were conducted using the following EBSD parameters:
tilting angle 70°,
WD = 15 mm, accelerating voltage 20 kV,
aperture size 110 μm,
6–8 bands, 0.7 μm step size,
and magnification x500. The measurement of
an area of 250×180μm takes 45 minutes. A ferritic hot band was cold rolled down to 80% thickness reduction
in reverse manner. After recrystallization, annealing the center part of the
sample was investigated subsequently to metallographic preparation.
The sample was heated up, respectively, cooled down in 1°C steps
in the temperature range between 880°C and
920°C. The temperature was directly measured by a thermocouple
placed inside the sample. The exact chemical composition of the investigated
material is given in Table 1.
All data were collected using beam-controlled measurements and the commercial
Channel 5 software by HKL technology.Table 1
Chemical composition in wt% of the investigated microalloyed ferritic
low-carbon steel.
CSiMnAlNCrVMoTiNb0.0650.050.990.0420.0030.0270.0060.0050.070.003Figures4(a)–4(d) shows acquired EBSD data maps of identical positions of the sample surface at four different temperatures between
897°C and 900°C
for the
α-γ phase
transformation. The thick black lines mark the grain boundaries of
misorientation angles larger
than 15°, the thin black lines denote misorientations
larger than 5°, and the red lines indicate Σ3-twin boundaries. The grey-shaded
area corresponds to the
low-temperature bcc α-phase and the colored grains
are the high-temperature fcc
γ-phase. The phase boundaries between the
α- and
γ-phases are colored in reference to the
Kurdjumov-Sachs (K-S) orientation relationship
[6]. Disorientations from K-S between
0° and 5° are marked with white lines,
larger than 5° yellow and larger than 10° green
lines. In the following, mainly a phenomenological view
on the achieved data is given to demonstrate what kind of information
we get from high-temperature in situ EBSD measurements.α-γ phase transformation
from 897°C to
900°C during heating up. Thick black lines mark
grain boundaries of misorientation angles larger than
15°; thin black lines larger than
5°; red lines mark Σ3-twin boundaries;
grey-shaded grains identify the
bcc
α-phase; colored grains are the fcc γ-phase. Disorientations from
K-S between 0° and 5° are marked with white
lines, larger than 5° yellow and larger
than 10° green lines.
(a)(b)(c)(d)Theα-γ phase transformation can
be seen to start with nucleation and growth in the temperature range of
897°C
(Figure 4(a)) (5%γ-phase). With increasing temperature further growth of
the already formed nuclei as well as the formation of new fcc γ-nuclei occur. Besides, those new γ-grains with twin-relationships to the already
formed neighboring fcc γ-grains appear. At
900°C,   already 50% of the area has turned
into γ-phase
(Figure 4(d)). As expected from former ex
situ experiments, the nucleation sites are predominantly at grain boundaries
and triple junctions (Figures 4(a)–4(d)) [1, 2]. Further, it can be seen that the growth of the new γ-grains is mainly homogeneous and
isotropic. Also, a grain refinement can be observed
(bcc 5.3 μm, fcc 3.1 μm).The K-S correspondence holds for the majority of the newly
formedγ-grains in relation to mostly one neighboring
bcc mother grain. Furthermore, the texture development of the γ-phase for different stages of transformation
was investigated. The result is shown in Figure
5 using the fcc α-fiber and β-fiber. The fcc texture
shows only slight changes in the range of 5 to 50% phase transformation except for
the copper component on theβ-fiber during the
first transformation step at 897°C (5% fcc).fccα-fiber and β-fiber orientations for different transformation steps
(897°C/5% fcc; 898°C/13.5% fcc;
899°C/27.5% fcc; and 900°C/49.4% fcc).
(a)
β-fibre (maximum density)(b)
β-fibre orientation(c)
α-fibreThe reverse process, theγ-α-phase transformation, is shown in Figures
6(a)–6(e). The EBSD maps reflect the
microstructural evolution of the surface area between 893°C and
886°C, respectively, after two
complete transformation cycles. The marking of the lines is the same as in
Figure 4 except for the colouring of the grains
(now: fcc grains are in grey, bcc grains are colored). A complete
back transformation into the low-temperature bcc-α phase was observed.γ-α phase transformation
from 893°C to 886°C during cooling down. The marking
of the lines is the same as in Figure
4. Grey grains are the fcc γ-phase; colored grains are the bcc α-phase. Some grains with less or without growth are marked with black circles. Some K-S phase boundaries are marked with black arrows.
(a)(b)(c)(d)(e)In comparison to the forward transformation, the back transformation of the
newly formedα-phase occurs also by nucleation mainly at
triple junctions and its growth. The γ-α phase transformation can be seen
to start at the temperature of
893°C (Figure 6(a)) (1.4%α-phase) and is completed at 886°C
(Figure 6(e)) (99.8%α-phase). With the decreasing
temperature, further growth of the already formed nuclei as well
as the formation of new α-bcc nuclei occur. but there
are obvious differences to the α-γ phase transformation. The main difference is the inhomogeneous growth of the bcc grains. The white and yellow lines indicate a good K-S relationship between the two phases; at these
K-S boundaries no further phase transformation was observed
(see the marked lines). The marked bcc grains which show no significant growth
during transformation are bordered by more K-S boundaries with less deviation from the perfect relationship. These bcc grains have a good K-S relationship with two or
three of the surrounding fcc grains, and no further growth of such grains
takes place. In contrast to the α-γ phase transformation no grain refinement
occurred, rather during the γ-α phase transformation a grain coarsening
(fcc 9.3 μm, bcc 24.2 μm) was observed.No textures for theγ-α phase  transformation could be calculated due to the poor
grain statistics.


## 4. Discussion

An important aspect of theα-γ-α phase transformation in steel is the texture development and the occurrence of variant selection [3, 7, 9]. Variant selection means that during the
phase transformation not all possible crystallographic variants are
equally selected, for example, among the 24 variants of the K-S relationship.
The α-γ-α phase transformation proceeds by nucleation and growth.
It is, therefore, of particular interest which stage determines the final texture and when and how variant selection occurs.The results of the high-temperature in situ EBSD measurements for theα-γ phase transformation demonstrate
that the nucleation of the newly formed phase occurs mainly at triple junctions.
Also, a good K-S correspondence in relation to mostly one neighboring bcc
mother grain was found for the majority of the newly formed γ-grains. The fcc texture changes only slightly with
progressing phase transformation for a
transformation fraction of 5 to 50% except for the copper component on the β-fiber during the first transformation step at 897°C (5% fcc). The observed deviation of the fcc texture at low transformed
volume fraction (5% fcc) is likely due to insufficient statistics because in this
case only 123 grains could be investigated. Otherwise, the results show that the
fcc transformation texture does not significantly change with
progressing transformation. From this result we can conclude that the
developed austenite texture is essentially determined by nucleation, that is,
variant selection takes place at the nucleation stage.During theγ-α phase transformation,
the nucleation occurs also mainly at triple junctions and large ferrite
grains develop. In contrast to the α-γ phase transformation, several ferrite grains exhibit
a good K-S relationship not only to one but two or three of the surrounding
austenite grains, and virtually no growth of such grains takes place. Therefore,
the microstructure during the γ-α phase transformation consists of small bcc grains
surrounded by K-S boundaries and large bcc grains with less K-S
boundaries. Some of the small bcc grains develop an elongated shape during
the γ-α phase transformation.
This can be attributed to the fact that the phase transformation to ferrite is
only feasible in a direction free of K-S boundaries (see Grain A).This effect of the K-S phase boundaries during theγ-α phase transformation was noticed
also for the α-γ phase transformation,
but was not as obvious as during the back transformation. This is associated
with faster growth kinetics of the bcc grains during back transformation, by which
the difference between the K-S and non-K-S phase boundaries becomes more
pronounced and which also leads to a coarsening of the ferrite grains. Because of
the poor grain statistics no bcc texture could be calculated for the γ-α phase transformation.The circumstance that an fcc grain has mostly one mother grain (good K-S matching) and a newly nucleated bcc grain, very often more than one K-S neighbor, can be
associated with the so-called texture memory effect and the nucleation rate.
From previous investigations it is known that a texture memory effect occurs
during theα-γ-α phase transformation cycle
[3, 7–9]. This means that the
bcc textures prior and subsequent to this transformation are comparable,
apparently because a bcc grain transformed into an fcc grain by using one specific
out of 24 K-S variants. During back transformation of the fcc grain, the same
variant seems to be preferred which leads to the same bcc orientation as prior to transformation. If we assume that the nucleation rate during the γ-α phase transformation is
smaller than the α-γ phase transformation, the following (extreme) case
is possible. A bcc grain possesses three fcc grains with a good K-S
relationship during the α-γ phase transformation. During back transformation at one
of these three fcc grains which possibly share a triple junction, a new bcc grain nucleates with the same variant as before. If this bcc grain nucleates inside
the triple junction, it will be surrounded only by K-S boundaries. In case of a
small nucleation rate for the γ-α phase transformation, this bcc grain will be “isolated” in
a subsequent transformation cycle, and the new fcc grains which nucleate with a
K-S relationship at this bcc grain will have a K-S boundary only to this grain.


## 5. Summary

A new laser-induced heating stage for commercial SEMs is introduced with the
capacity to heat specimens to a temperature of 1000°Cwhilst
acquiring microstructural and crystallographic data by means of the
EBSD technique. The in situ investigations into the
high-temperature α-γ-α phase transformation in
a microalloyed low-carbon steel demonstrate the excellent performance of this
laser powered heating stage and its potential for other high-temperature
applications. The following first results are presented.(i)
Nucleation was observed mainly at triple junctions.(ii)
The austenite texture is determined by nucleation.(iii)
The microstructure development during theγ-α phase
transformation is inhomogeneous.(iv)
The growth kinetics during theγ-α phase transformation is faster than
the α-γ transformation.(v)
The mobility of the transformation front depends on its proximity to a
K-S relationship.



---

*Source: 294508-2008-03-12.xml*