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3D characterization of microstructureevolution of cast AlMgSi alloys by
synchrotron tomography
Bordeaux, 1st June 2012
1) Magnesium Innovation Centre, Helmholtz-Zentrum Geesthacht2) Institute of Materials Science and Technology, Vienna University of Technology
3) Université de Grenoble, SIMaP/GPM24) European Synchrotron Radiation Facility
D. Tolnai1,2, G. Requena2, L. Salvo3, P. Cloetens4
AlMgSi alloys
2Introduction
AlMgSi alloys are potential candidates for automotive industryα-Al, Mg2Si, Fe and Mn based aluminides
Primary Mg2SiOctahedron or truncated
octahedron shape
Eutectic Mg2SiHighly interconnected seaweed-like structure
Li et al. Acta Materialia, 2011; 59:1058-1067.
Motivation
Effect of microstructure on mechanical properties (Al-Si alloys)Size, shape, connectivity, contiguity
Casting, heat treatment
3
Investigate the evolution of the microstructure during solidification and solution heat treatment
3D non destructive imaging
Introduction
Materials
Methodology 4
Mg2Si stoichiometric composition: 1.74:1
AlMg4.7Si8Mg:Si ratio: 0.58:1Expected phases: α-Al dendrites, α-Al/Mg2Si eutectic, Fe-alumninides, α-Al/Mg2Si/Si ternary eutecticConditions: As-cast, 1h/540°C, 25h/540°C
AlMg7.3Si3.5Mg:Si ratio: 2.1:1Expected phases: α-Al dendrites, α-Al/Mg2Si eutectic, Fe-alumninidesConditions: Strip-cast, 30 min/540°C
5
Synchrotron tomography (ID19)
• Beam energy: 29 keV• Voxelsize: (0.28 μm)3
• Sample-to-detector distance: 39 mm• No. of proj.: 1500
Methodology
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
Ø 0.4 mm
M=(zs+zd)/zs
Methodology
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
AlMg4.7Si8
8Materials in as-cast condition
• Mg2Si presents a high interconnectivity• AlFeSi is platelet like• Si ternary eutectic is highly interconnected• Contiguity between Mg2Si and Si
Back Scattered Electron image Secondary Electron image
AlMg4.7Si8
9Microstructure evolution during solution treatment
1h/540°C 25h/540°C
Spheroidisation of the eutectic phasesThe contiguity between Mg2Si and Si remains
AlMg4.7Si8
10
100 µm 100 µm 100 µm
25h/540 °C1h/540 °CAs-cast
Mg2Si AlFeSi
Microstructure evolution during solution treatment
AlMg4.7Si8
11
• The number of particles increases (5x), while the mean volume decreases• Disintegration of Mg2Si startsimmediately
1h: 57% 25h: 4%0h: 87% of Mg2Si connected
Microstructure evolution during solution treatment
AlMg4.7Si8
12
• 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
AlMg4.7Si8
13
• The distribution extends towards the positive-positive quadrant
• Two peaks can be identified
Microstructure evolution during solution treatment
AlMg7.3Si3.5
14
• Fine microstructure resulted from the strip cast process• Mg2Si presents a high interconnectivity• AlFeSi is platelet-, particle-like
Secondary Electron image
Materials in as-cast condition
16
As-cast 30 min/540°C
60 µm 60 µm
As-cast 30 min/540°C
Number ofparticles
530 x 5
Vf of the largest particle
9100 x 0.65
Rel. Vf of the largest particle
91% 73%
AlMg7.3Si3.5
Microstructure evolution during solution treatment
D. Tolnai et al. Materials Science and Engineering A, In Press.
17
Slight spheroidisation of the particles.
17
The disintegrating smaller particles spheroidise
AlMg7.3Si3.5
Microstructure evolution during solution treatment
18
• The distribution extends towards the positive-positive quadrant
• Two peaks can be identified in the solution treated condition
AlMg7.3Si3.5
Microstructure evolution during solution treatment
19
• Decreasing strength with the solution heat treatment time.
• In as-cast condition softening can be observed.
100 µm
Elevated temperature compression
Microstructure evolution during solution treatment
Elevated temperature strength and microstructure
20Microstructure evolution during solution treatment
In situ solidification AlMg4.7Si8
21
• 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
22
The structure coarsens
The growth is asymmetric
Small arms dissapear, larger ones tend to grow
DCP between 580°C and 575°C
Dendritic solidification AlMg4.7Si8
Microstructure evolution during solidification
D. Tolnai et al. Acta Materialia, 2012; 60:2568-2577.
-0.15 -0.10 -0.05 0.00 0.05 0.10 0.15-0.025
-0.020
-0.015
-0.010
-0.005
0.000
0.005
0.010
0.015
0.020
0.025
Mean Curvature /m-1
Gau
ssia
n cu
rvat
ure
/m
-2 0
0.03250
580C Norm. Freq.
23
-0.15 -0.10 -0.05 0.00 0.05 0.10 0.15-0.025
-0.020
-0.015
-0.010
-0.005
0.000
0.005
0.010
0.015
0.020
0.025
Mean Curvature /m-1
Gau
ssia
n cu
rvat
ure
/m
-2 0
0.03250
590C Norm. Freq.
-0.15 -0.10 -0.05 0.00 0.05 0.10 0.15-0.025
-0.020
-0.015
-0.010
-0.005
0.000
0.005
0.010
0.015
0.020
0.025
Mean Curvature /m-1
Gau
ssia
n cu
rvat
ure
/m
-2 0
0.03250
585C Norm. Freq.
-0.15 -0.10 -0.05 0.00 0.05 0.10 0.15-0.025
-0.020
-0.015
-0.010
-0.005
0.000
0.005
0.010
0.015
0.020
0.025
Mean Curvature /m-1
Gau
ssia
n cu
rvat
ure
/m
-2 0
0.03250
575C Norm. Freq.
Dendritic solidification AlMg4.7Si8
Microstructure evolution during solidification
24
‐0.005 µm‐2 Gauss curvature 0.005 µm‐2
Dendritic solidification AlMg4.7Si8
Microstructure evolution during solidification
Eutectic solidification in AlMg4.7Si8
25
490 µm
575°C
Primary Mg2Si Eutectic Mg2SiThe initiation of the solidification of Mg2Si is linked to the base of the secondary dendritic arms
Microstructure evolution during solidification
26
575 570 565 560 555 550 545 5400.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.00
0.02
0.04
0.06
0.08
0.10
Inte
rcon
nect
ivity
in th
e ph
ase
Temperature / °C
Interconnectivity
Volum
e fraction
Volume fraction
• The interconnectivity of the phase is increasing at a higher rate than the volume fraction of the whole phase• Increase of interconnectivitywith the ternary eutectic
Interconnectivity of Mg2Si in AlMg4.7Si8
Solidification
Microstructure evolution during solidification
Correlation with simulation
27Microstructure evolution during solidification
Calorimetry In situ tomography
Thermocalc
AlMg4.7Si8 (°C)α-Al 594 590 591α-Al/Mg2Si 575 575 577.5AlFeSi Overlap 565 -α-Al/Mg2Si/Si 555 555 558
AlMg7.3Si3.5 (°C)α-Al 610 605 606.5α-Al/Mg2Si 595 590 593.5AlFeSi Overlap 590 -
28
• 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
Materials in as-cast condition
29
• 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
Microstructure evolution during solution treatment
Conclusions
30
• 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