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TitlePhase Field Modelling of the Austenite to Ferrite Transformation in Steels
LanguageEnglish
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Document Text Contents
Page 1

Phase Field Modelling of the
Austenite to Ferrite

Transformation in Steels

Page 2

The research described in this thesis was performed in the department of Material Science
and Technology, the Delft University of Technology.








The research described in this thesis was carried out in the first two years under the
VESPISM project (contract number G5RD-CT-2000-00315), a Fifth Framework project
funded by the European Commission, and in the second two years under the project
number MC5.03172 in the framework of the Strategic Research Program of the
Netherlands Institute for Metal Research (NIMR) in the Netherlands (www.nimr.nl).

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Chapter 5

81

at lower temperatures is not shown. But it should be noted that the final ferrite fraction of
approximately 0.9 is the same in simulation and experiment. As indicated in Figure 5.2, the
simulated transformation rate is independent of the assumed initial austenite microstructure
in terms of grain morphology provided the ferrite nuclei density and spatial distribution are
approximately the same. The latter is achieved by taking similar initial austenite grain sizes
and aiming for similar resulting ferrite grain sizes.

1020 1040 1060 1080 1100
0.0

0.2

0.4

0.6

0.8

1.0

fe
rr

ite
fr

ac
tio

n

Temperature (K)

Experimental
Cube
Tetrakaidecahedron
Random
film



Figure 5.2 Effect of initial austenite microstructure on transformation behaviour at

0.4K/s.

The details of the microstructure evolution with ferrite shown in red are summarized in
Figure 5.3 as a function of temperature for the tetrakaidecahedron initial austenite
microstructure where the largest number of ferrite triple line nuclei, i.e. 15, is realized with
the present phase field simulation. At 1085 K, four newly formed ferrite grains are visible
and their nucleation positions are confirmed to be located at grain boundary triple lines.
The size of these four ferrite grains scales with their nucleation temperature. Even though
only four grains are visible, already six nuclei have formed when 1085 K is reached but the
last two nuclei are still too small to be visible in the representation scale of Figure 5.3. At
1080 K, the number of ferrite grains has increased to 10 and some of the new nuclei have
formed at positions that are located close to the domain boundaries. Most notably there is
one ferrite grain located near the corner point of the domain such that this grain is cut into

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82

8 pieces as a result of the periodic boundary conditions. Further, substantial growth of the
four initial nuclei located more in central positions of the domain is clearly identified and a
more or less spherical growth geometry applies to each individual ferrite grain under these
nucleation conditions. At 1075 K all but one of the 15 nuclei are present and the beginning
of impingement of the growing ferrite can be seen. The final figure in this series, i.e. at
1060 K when a ferrite fraction of approximately 0.75 has been formed, displays the
situation when the transformation approaches the saturation regime with substantial
impingement of ferrite grains.


1085 K 1080 K


1075 K 1060 K

Figure 5.3 Predicted 3D microstructure evolution for 0.4K/s using the initial

tetrakaidecahedron configuration, ferrite shown in red and interfaces in
green. For the colour version refer to the appendix.

Page 174

Appendix

7-A

900 950 1000 1050 1100
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0

Fe
rr

ite
fr

ac
tio

n

Temperature (K)

homogeneus nuclei distribution
nuclei clusters









. Figure 6.9



a

b

c

Page 175

Appendix

8-A










Figure 7.1



a


b




a

d

b

c

A
A

B B

Figure 7.5

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