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

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

[email protected]

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


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


AlMg4.7Si8

13

• The distribution extends towards the positive-positive quadrant

• Two peaks can be identified


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

AlMg7.3Si3.5

15

Spheroidisation of Mg2Si


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


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


18

• The distribution extends towards the positive-positive quadrant

• Two peaks can be identified in the solution treated condition

AlMg7.3Si3.5


19

• Decreasing strength with the solution heat treatment time.

• In as-cast condition softening can be observed.

100 µm

Elevated temperature compression


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


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.



24

‐0.005 µm‐2 Gauss curvature 0.005 µm‐2



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


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


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


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.


Acknowledgements

31

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

Thank you for the attention!

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

Documents

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