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3D characterization of microstructure
evolution of cast AlMgSi alloys by
synchrotron tomography

D. Tolnai1,2, G. Requena2, L. Salvo3, P. Cloetens4
1) Magnesium Innovation Centre, Helmholtz-Zentrum Geesthacht
2) Institute of Materials Science and Technology, Vienna University of Technology
3) Université de Grenoble, SIMaP/GPM2
4) European Synchrotron Radiation Facility

domonkos.tolnai@hzg.de

Bordeaux, 1st June 2012

AlMgSi alloys

AlMgSi alloys are potential candidates for automotive industry
α-Al, Mg2Si, Fe and Mn based aluminides

Primary Mg2Si Eutectic Mg2Si
Octahedron or truncated Highly interconnected
octahedron shape seaweed-like structure

Li et al. Acta Materialia, 2011; 59:1058-1067.
Introduction 2

Motivation

Effect of microstructure on mechanical properties (Al-Si alloys)
Size, shape, connectivity, contiguity

Casting, heat treatment

Investigate the evolution of the microstructure during
solidification and solution heat treatment

3D non destructive imaging

Introduction 3

Materials

AlMg4.7Si8
Mg:Si ratio: 0.58:1
Expected phases: α-Al dendrites, α-Al/Mg2Si
eutectic, Fe-alumninides, α-Al/Mg2Si/Si ternary
eutectic
Conditions: As-cast, 1h/540°C, 25h/540°C

Mg2Si stoichiometric
composition: 1.74:1

AlMg7.3Si3.5
Mg:Si ratio: 2.1:1
Expected phases: α-Al dendrites, α-Al/Mg2Si
eutectic, Fe-alumninides
Conditions: Strip-cast, 30 min/540°C

Methodology 4

Synchrotron tomography (ID19)

• Beam energy: 29 keV
• Voxelsize: (0.28 μm)3
• Sample-to-detector distance: 39 mm
• No. of proj.: 1500

Methodology 5

Zoom tomography (ID22NI)

Sample-to-focal-point distance (mm) Voxel size

29.68 (60 nm)3
30.6 (62 nm)3
34.6 (70.1nm)3
44.6 (90.4 nm)3

• Energy: 17.5 keV
• 360° rotation

M=(zs+zd)/zs

Ø��0.4��mm

Methodology 6

In situ solidification tests (ID15A)

• Voxel size: (1.4 μm)3
• Acqusition time: 18 ms
• Cooling rate: 5K/min

Methodology 7

AlMg4.7Si8
Back Scattered Electron image Secondary Electron image

• Mg2Si presents a high interconnectivity
• AlFeSi is platelet like
• Si ternary eutectic is highly interconnected
• Contiguity between Mg2Si and Si

Materials in as-cast condition 8

AlMg4.7Si8

1h/540°C 25h/540°C

Spheroidisation of the eutectic phases
The contiguity between Mg2Si and Si remains

Microstructure evolution during solution treatment 9

AlMg4.7Si8

As-cast 1h/540 °C 25h/540 °C

100��µm 100��µm 100��µm

Mg2Si AlFeSi


AlMg4.7Si8

• The number of particles increases (5x),
while the mean volume decreases
• Disintegration of Mg2Si starts
immediately

0h: 87% of
1h: 57% 25h: 4%
Mg2Si connected


AlMg4.7Si8

• The probability of spherical particles
increases
• Shape of Mg2Si changes after long
exposure

• Disintegration of the large particles and
spheroidisation of the smaller ones


AlMg4.7Si8

• The distribution extends towards
the positive-positive quadrant
• Two peaks can be identified


AlMg7.3Si3.5
Secondary Electron image

• Fine microstructure resulted from the strip cast process
• Mg2Si presents a high interconnectivity
• AlFeSi is platelet-, particle-like


AlMg7.3Si3.5

Spheroidisation of Mg2Si


AlMg7.3Si3.5

As-cast 30 min/540°C

As-cast 30 min/540°C

Number of 530 x5
particles
Vf of the largest 9100 x 0.65
particle
Rel. Vf of the 91% 73%
largest particle

60��µm 60��µm

D. Tolnai et al. Materials Science and Engineering A, In Press.


AlMg7.3Si3.5

Slight spheroidisation of
the particles.

The disintegrating smaller
particles spheroidise

17

AlMg7.3Si3.5

• The distribution extends towards the
positive-positive quadrant
• Two peaks can be identified in the
solution treated condition


Elevated temperature compression

• Decreasing strength with the solution
heat treatment time.
• In as-cast condition softening can be
observed.

100��µm


Elevated temperature strength and microstructure


In situ solidification AlMg4.7Si8

• α-Al dendrites
590°C

• α-Al/Mg2Si
eutectic 575°C

• Fe aluminides 565°C

• α-Al/Mg2Si/Si
ternary eutectic
555°C

Microstructure evolution during solidification 21

Dendritic solidification AlMg4.7Si8

The structure
coarsens

The growth is
asymmetric

Small arms
dissapear, larger
ones tend to grow

DCP between
580°C and 575°C

D. Tolnai et al. Acta Materialia, 2012; 60:2568-2577.



0.025 0.025
Norm. Freq. Norm. Freq.
0.020
590C 0.020
585C

0.015 0.015 0

-2
0
-2

Gaussian curvature /m

0.010 0.010

0.005 0.005

0.000 0.000

-0.005 -0.005

-0.010 -0.010

-0.015 -0.015

-0.020 -0.020

-0.025
0.03250 -0.025
-0.15 -0.10 -0.05 0.00 0.05 0.10 0.15
0.03250
-0.15 -0.10 -0.05 0.00 0.05 0.10 0.15
-1
Mean Curvature /m
-1 Mean Curvature /m

0.025 0.025
580C Norm. Freq. Norm. Freq.
0.020 0.020
575C
0.015 0 0.015 0
-2

-2

0.010 0.010

0.005 0.005

0.000 0.000

-0.005 -0.005

-0.010 -0.010

-0.015 -0.015

-0.020 -0.020

-0.025 0.03250
0.03250 -0.025
-0.15 -0.10 -0.05 0.00 0.05 0.10 0.15 -0.15 -0.10 -0.05 0.00 0.05 0.10 0.15
-1
Mean Curvature /m Mean Curvature /m
-1



‐0.005��µm‐2 Gauss��curvature 0.005��µm‐2


Eutectic solidification in AlMg4.7Si8

575°C

490��µm

The initiation of the solidification
of Mg2Si is linked to the base of Primary Mg2Si Eutectic Mg2Si
the secondary dendritic arms


Interconnectivity of Mg2Si in AlMg4.7Si8

0.10
0.50
Interconnectivity
Interconnectivity in the phase

0.45 Volume fraction 0.08

0.40 • The interconnectivity of the

Volume fraction
0.35
0.06
phase is increasing at a higher
rate than the volume fraction
of the whole phase
0.30 0.04

0.25
0.02
• Increase of interconnectivity
0.20 with the ternary eutectic
0.15 0.00
575 570 565 560 555 550 545 540

Temperature / °C
Solidification


Correlation with simulation

Calorimetry In situ Thermocalc
tomography
AlMg4.7Si8 (°C)
α-Al 594 590 591
α-Al/Mg2Si 575 575 577.5
AlFeSi Overlap 565 -
α-Al/Mg2Si/Si 555 555 558
AlMg7.3Si3.5 (°C)
α-Al 610 605 606.5
α-Al/Mg2Si 595 590 593.5
AlFeSi Overlap 590 -


Conclusions

• α-Al dendrites , eutectic α-Al/Mg2Si,
(α-Al/Mg2Si/Si ternary eutectic)

• ~1 vol% of Fe-based aluminides

• The eutectic Mg2Si and the ternary
eutectic Si have highly
interconnected seaweed-like
morphology

• Contiguity between the eutectic
Mg2Si and the ternary eutectic Si


Conclusions

• Disintegration followed by
spheroidisation.

• Morphological change in ternary
eutectic Si is similar to the eutectic
Mg2Si.

• The contiguity between the Mg2Si
phase and the Si is observed after
the heat treatment.

• A partial loss of interconnectivity
causes decline in strength, while the
shape of the particles has less effect.


Conclusions

• AlMg4.7Si8: α-Al at 590°C, α -Al/Mg2Si eutectic at 577°C , Fe aluminides, α -
Al/Mg2Si/Si ternary eutectic at 558°C.
• AlMg7.3Si3.5: α -Al dendrites at 610°C, α -Al/Mg2Si eutecic at 595°C, Fe
aluminides.

• Dendritic structure coarsens, coalescence and growth of the secondary
dendrite arms. Asymmetric growth results in a droplet-like shape.

• Dendritic coherency temperature can be determined: AlMg4.7Si8: between
580°C and 575°C, AlMg7.3Si3.5: between 595°C and 590°C.

• The nucleation of the Mg2Si at the base of the secondary dendrite arms.

• Octahedral primary particles, followed by the eutectic solidification.

• Several nucleation sites can be observed. The initially separated Mg2Si
particles coalesce during cooling.


Acknowledgements

• Peter Degischer, János Lendvai
• Marco DiMichiel, ESRF
• Peter Townsend, University of Cambridge
• IMST, TU-Wien
• DMP, ELTE

Thank��you��for��the��attention!

31

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3D characterization of microstructure evolution of cast AlMgSi alloys by synchrotron tomography

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