The document summarizes research on characterizing the microstructure evolution of cast AlMgSi alloys using synchrotron tomography. Key findings include: (1) Synchrotron tomography was used to investigate microstructure evolution during solidification and heat treatment, (2) Primary α-Al dendrites and eutectic α-Al/Mg2Si formed with a highly interconnected seaweed-like morphology, (3) During heat treatment, the eutectic phases spheroidized and the contiguity between Mg2Si and Si remained.

1. 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

2. 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

3. 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

4. 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

5. Synchrotron tomography (ID19)
• Beam energy: 29 keV
• Voxelsize: (0.28 μm)3
• Sample-to-detector distance: 39 mm
• No. of proj.: 1500
Methodology 5

6. 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
• No. of proj.: 1500
• 360° rotation
M=(zs+zd)/zs
Ø 0.4 mm
Methodology 6

7. In situ solidification tests (ID15A)
• Voxel size: (1.4 μm)3
• No. of proj.: 800
• Acqusition time: 18 ms
• Cooling rate: 5K/min
Methodology 7

8. 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

9. 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

10. AlMg4.7Si8
As-cast 1h/540 °C 25h/540 °C
100 µm 100 µm 100 µm
Mg2Si AlFeSi
Microstructure evolution during solution treatment 10

11. 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
Microstructure evolution during solution treatment 11

12. 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
Microstructure evolution during solution treatment 12

13. AlMg4.7Si8
• The distribution extends towards
the positive-positive quadrant
• Two peaks can be identified
Microstructure evolution during solution treatment 13

14. AlMg7.3Si3.5
Secondary Electron image
• Fine microstructure resulted from the strip cast process
• Mg2Si presents a high interconnectivity
• AlFeSi is platelet-, particle-like
Materials in as-cast condition 14

15. AlMg7.3Si3.5
Spheroidisation of Mg2Si
Microstructure evolution during solution treatment 15

16. 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.
Microstructure evolution during solution treatment 16

17. AlMg7.3Si3.5
Slight spheroidisation of
the particles.
The disintegrating smaller
particles spheroidise
Microstructure evolution during solution treatment 17
17

18. AlMg7.3Si3.5
• The distribution extends towards the
positive-positive quadrant
• Two peaks can be identified in the
solution treated condition
Microstructure evolution during solution treatment 18

19. Elevated temperature compression
• Decreasing strength with the solution
heat treatment time.
• In as-cast condition softening can be
observed.
100 µm
Microstructure evolution during solution treatment 19

20. Elevated temperature strength and microstructure
Microstructure evolution during solution treatment 20

21. 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

22. 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.
Microstructure evolution during solidification 22

23. Dendritic solidification AlMg4.7Si8
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
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
Gaussian curvature /m
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
0.03250 -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
Microstructure evolution during solidification 23

24. Dendritic solidification AlMg4.7Si8
‐0.005 µm‐2 Gauss curvature 0.005 µm‐2
Microstructure evolution during solidification 24

25. 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
Microstructure evolution during solidification 25

26. 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
Microstructure evolution during solidification 26

27. 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 -
Microstructure evolution during solidification 27

28. 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
Materials in as-cast condition 28

29. 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.
Microstructure evolution during solution treatment 29

30. 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.
Microstructure evolution during solidification 30

31. Acknowledgements
• Peter Degischer, János Lendvai
• Marco DiMichiel, ESRF
• Peter Townsend, University of Cambridge
• IMST, TU-Wien
• DMP, ELTE
Thank you for the attention!
31