Kinetics of Phase Transformations

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Kinetics of Phase Transformations in Steel and
Aluminium studied by 3DXRD
Niels van Dijk (FAME/TNW/TU Delft)
N.H.vanDijk_at_TNW.TUDelft.NL
In collaboration with
S.E. Offerman, J. Sietsma, S. van der Zwaag, N.
Iqbal, L. Katgerman, G.J. Kearley TU Delft L.
Margulies, S. Grigull, M.P. Moret ESRF H.F.
Poulsen, E.M. Lauridsen RISØ
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Why use synchrotron radiation for research on
structural materials ?
1. High energy large penetrating power in-situ
experiments 10 transmission thickness _at_80 keV
40 mm for Al, 5 mm for Fe _at_50 keV 20 mm for Al,
2 mm for Fe _at_ 8 keV 2 mm for Al, 10 ?m for Fe
(Cu-K?) 2. Small beam size high spatial
resolution individual grains beam sizes 5 500
?m (single crystal analysis of polycrystalline
samples) 3. High intensity high sampling rate
kinetic studies exposure times 1 s sampling
rate 5 s
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X-ray techniques
Tomography
3DXRD Microscopy
Imaging
Recrystallisation of Al S. Schmidt et
al. Science 305 (2004) 229.
Solidification of Sn-Pb R.H. Mathiessen et
al. Phys. Rev. Lett. 83 (1999) 5062.
Granular materials compactation S.F. Nielsen et
al. Acta Materialia 51 (2003) 2407.
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Evolution of microstructure during phase
transformations in structural materials

• Grain nucleation formation of new phase grains
• - nm size clusters
• - occurs on short time scales
• - positioned within bulk materials
• - strongly dependent on interface properties
• Grain growth increase in size of nucleated grain
• - often controlled by diffusion of alloying
  elements and/or heat
• - interaction between neighboring growing grains
• dependent on microstructure of the parent phase

Nucleus hard-sphere colloid S. Auer D.
Frenkel Nature 409 (2001) 1020.
Need for time-dependent in-situ probe of
individual grains
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Steel Austenite to Ferrite Transformation
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Phase Transformations in Steel
?-Fe ? ?-Fe Fe3C
Austenite (fcc)
Ferrite (bcc)
Cementite (orthorombic)
Ferrite (light) Pearlite (dark)
Pearlite ?-Fe Fe3C
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3D X-Ray Diffraction Microscope
2D detector
Furnace
Sample

w
slits
Bent crystal
Synchrotron
Beam line ID11 _at_ European Synchrotron Radiation
Facility
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Diffraction pattern austenite phase
T 900 oC Before transformation Beam
size 94 ? 97 ?m2 Energy 80 keV
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Austenite ferrite phase
Continuous cooling T 763 oC (-5 oC/min)
Half way transformation Ferrite (red)
Austenite
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S.E. Offerman et al., Science 298 (2002) 1003.
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Phase fractions of austenite ferrite
Cooling rate 5 oC/min
Austenite ferrite
Transformations Austenite Ferrite A3 822
oC Austenite Pearlite A1 709 oC
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Number of ferrite nuclei

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Ferrite nucleation rate
Activation energy for nucleation is orders of
magnitude smaller than predicted by theory!
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Ferrite grain growth

• Growth types
• Zener
• Continued into
• Pearlite
• C) Retarded
• D) Oscillatory

A
B
C
D
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Austenite decomposition
Carbon exchange between austenite grains
One ferrite grain per austenite No
oscillatory decomposition
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Grain growth and decomposition model

Fit-parameter Local density of potential nuclei
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S.E. Offerman et al., Acta Materialia 52 (2004)
4757.
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Conclusions
3DXRD gives in-situ kinetic information during
transformation on phase fraction, grain
density grain volume of individual grains
All nucleation sites are equal (in theory), but
some are more equal than others (in practice)
4 types of ferrite grain growth 3 types of
austenite grain decomposition Ferrite growth
Transition from non-overlapping to overlapping
diffusion fields
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Aluminium Liquid to Solid Transformation
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Grain refinement of Al-Ti-B alloys
pure Al
pure Al 0.03 wt. TiB2
pure Al 0.03 wt. TiB2 0.01 wt. solute Ti
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M. Easton D. StJohn, Metall. Mater. Trans. A 30
(1999) 1629.
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Al grain refiners
Solid
Liquid
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3D X-Ray Diffraction Microscope
2D detector
E 70 keV
Furnace
Sample

w
slits
Synchrotron
Sample size 5 mm diameter, 10 mm
height Rotation angle 1 degree Beam size
200x200 mm Exposure time 1 s
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Liquid to Solid Phase Transformation

liquid
solid
liquid solid
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Iqbal et al., Acta Materialia 53 (2005) 2875.
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Transformation kinetics during solidification
Sample high purity Al 0.1 wt. Ti 0.1 wt.
TiB2
Cooling rate 1 K/min (from 973 K)
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Nucleation during solidification
Al Ti TiB2
Al Ti
Al TiB2
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Growth of individual aluminum grains
Diffusion controlled growth
cooling rate 1 K/min
Zener theory
Zener theory
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Al
Al
TiAl3
TiAl3
TiB2
TiB2
Metastable TiAl3 grains are formed before the
solidification of Al starts

• TiAl3 nucleates on TiB2
• Al nucleates on TiAl3
• When the Al is formed
• then TiAl3 dissolves

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Conclusions
3DXRD gives in-situ kinetic information during
transformation on phase fraction, grain
density grain volume of individual grains
The increased nucleation during solidification of
aluminum alloys is due to the metastable TiAl3
phase formed on the surface of TiB2 particles.
Grain growth in Al-Ti-B system is controlled
by titanium diffusion in the beginning, by
latent heat in the middle and the free growth at
the end of solidification.
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