yong du , w.w. zhang, w. xiong state key lab of powder metallurgy, central south university, china

Yong Du, W.W. Zhang, W. Xiong State Key Lab of Powder Metallurgy, Central South University, China

R.X. Hu, P. NashThermal Processing Technology Center, Illinois Institute of Technology, USA

A Novel Approach for Acquiring Thermodynamic Database of Al Alloys and Investigation of

Microstructure during Solidification of Al Alloys

The 3rd International Symposium Light Metals and Composite Materials,

Belgrade, Serbia, September 12-14, 2008.

Celebration of the 200th Anniversary of University of Belgrade

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Contents1. Motivation

2. Experimental and computational approaches

3. Results and discussion

3.1 Thermodynamic data of Al alloys: case study for the Al-Ni-Si system

3.2 Solidification behaviors of Al356.1 alloy

Equilibrium solidification

Scheil model

Mircomodel

4. Summary

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Al-based alloys are widely used as aeronautic and civil materials

Among many commercial alloys (Al-, Fe-, Ni-, Mg-based etc. alloys), only Fe-based thermodynamic database is well established by Thermo-calc company, Sweden.

Currently: Lack of reliable thermodynamic and kinetic databases for Al alloys!

Our work: (I) to establish thermodynamic and kinetic databases for multi- component Al alloys via a hybrid approach of experiment, CALPHAD and first-principles methods

(II) to describe the microstructure and micro-segregation during solidifications of Al alloys using thermodynamic and kinetic databases.

Al-based alloys

1. Motivation1. Motivation

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Scheil model

Mircomodel

4. Summary

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2.1 Experimental approach Phase diagram measurement:

Equilibrated alloys, diffusion couple, XRD, EPMA, DTA, DSC, SEM

Measurement of enthalpy of formation and heat capacity

Directional solidification:

Temperature gradient: 45K/cm; Growth rate: 0.04445cm/s XRD, EPMA

2.2 Computational approach CALPHAD method (thermodynamic modeling)

First-principles method (Enthalpy of formation computation)

Molecular dynamics (Diffusion coefficient caculation)

Point analysis for phase compositions

Area scan for solute redistribution in (Al)

SEM/EDX (Image) Fraction of phase

2. Experimental and computational approaches2. Experimental and computational approaches

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Fig. 1. Phase equilibria of the Al-Ni-Zn system at 1100 ℃determined by diffusion couple technique and equilibrated alloys


Diffusion couple technique + equilibrated alloys

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Al SiNi

phase transition temperatures Composition range of the primary phases

Crystal structure isothermal section

Liquidus surface and reaction scheme for the ternary system

XRD, EPMA, DTA

annealed at 550oC for 1 month

Arc-melting

MetallographyEDX,EPMA

30 ternary alloys

3. Experimental procedure


As-cast

Fig. 2. Experimental procedure to establish reaction scheme of the Al-Ni-Si system

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Calorimetry: Measurement of enthalpy of formationKleppa high temperature calorimeter

aAl (298K) + bNi (298K) + cX (298K)= AlaAl (298K) + bNi (298K) + cX (298K)= AlaaNiNibbXXcc (1473 K) (1473 K)

Hreaction (1)

AlAlaaNiNibbXXc c (298 K) = Al(298 K) = AlaaNiNibbXXc c (1473 K)(1473 K)

Hheat content (2)

(1) (1) -- (2) get: (2) get:

aAl (298K) + bNi (298K) + cX (298K) = AlaAl (298K) + bNi (298K) + cX (298K) = AlaaNiNibbXXcc (298 K) (298 K) (3)

298Kf reaction heat content

Samples preparation

Procedure (two steps)

Al

X

Ni Mixing Pressing

Elemental powder Sample pellets

Deoxidization


Fig. 3. Procedure to measure the enthalpy of formation via calorimetry

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CALPHAD Method

0

1 1

lnn n

E magm i i i i m m

i i

G x G RT x x G G

Gibbs energy at reference states

Ideal entropy of mixing

Excess Gibbs energy

Magnetic contributions to the Gibbs energy

EmG

Em A BG x x

0 0[a b T

1 1( )( )A Bx x a b T

......]


Fig. 5. Procedure of CALPHAD method

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VASP-Vienna VASP-Vienna Ab InitioAb Initio Simulation Package Simulation Package

Physical Fundamental: Density Function Theory

First principles calculation

theory ： DFT Base set ： Plane Waves Pseudopotential ： UltraSoft Pseudopotential

Projector Augmented Wave method Exange and correlation ： LDA, GGA, LDA + U

Total energy

T[n]: Kinetic Energy EH:Hartree Energy(e-e repulsion)Exc: Exchange and correlation energies V(r) :External potential

rdrnrV 3][nE ][nT ][nE H ][nE xc

Enthalpy of formation of AlNi2Si

2 2(AlNi Si) (AlNi Si) (Al) 2 (Ni) (Si)H E E E E


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Scheil model

Mircomodel

4. Summary

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Thermodynamic database for the Al-Fe-Mg-Mn-Si-Cu-Ni-Zn systemThermodynamic database for the Al-Fe-Mg-Mn-Si-Cu-Ni-Zn system

28 binary system 56 ternary systems

3.1 Thermodynamic data of Al alloys: case study for the Al-Ni-Si system3.1 Thermodynamic data of Al alloys: case study for the Al-Ni-Si system

Al-Fe, Al-Fe-Zn etc.: Literature Al-Mn, etc.: Present work (finished) Mn-Si-Cu, etc.: in progress

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The following phases are included in the modeling:Thermodynamic modeling

A symmetric model (Al,Ni,Si,Va)0.5(Al,Ni,Si,Va)0.5 for A2 and B2 and the one (Al,Ni,Si)0.75(Al,Ni,Si)0.25 for Fcc_A1 and Fcc_L12.

Fcc_A1Fcc_L12

Bcc_A2Bcc_B2


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Thermodynamic modeling• Thermo-calc software accepts 1000 experimental data;

• Measured sections at 550, 800 and 1000 oC plus 13 vertical sections with 22 invariant equilibria [2003Ric, 2004Ric, 2006Cha, this work]: 3000 experimental data;

• Only key experimental data are used: three isothermal sections and 22 invariant reactions

Key References:

[2003Ric] K.W. Richter, H. Ipser: Intermetallics 11 (2003) 101 – 109.

[2004Ric] K.W. Richter, K. Chandrasekaran, H. Ipser: Intermetallics 12 (2004) 545 – 554.

[2006Cha] K. Chandrasekaran, K.W. Richter, H. Ipser: Intermetallics 14 (2006) 491 – 497.

80

70

60

50

40

30

20

10

0

Ni

Al Si


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The Al-Ni-SiThe Al-Ni-Si ternary systemternary system

Wei Xiong, Yong Du et al., Int. J. Mater. Res. 99 (2008) 598-612.

(a) (b)

Fig. 6. Calculated isothermal sections with the experimental data at (a) 1000 and (b) 800 oC

Calculated isothermal sections

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(a)

Fig. 7. Calculated isothermal sections with the experimental data at (a) 750 and (b) 550 oC

(b)

Calculated isothermal sections


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Fig. 8. Model-predicted vertical sections with the experimental data. (a) 80 at.% Ni; (b) 75 at.% Ni


(a) (b)

Model predicted vertical sections


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Fig. 9. Model-predicted vertical sections with the experimental data. for 66.67 at.% Ni (a) CALPHAD predicted; (b) experimental constructed

(a) (b)



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(a) (b)



20



(a) (b)



21

(a)



(b)



22



(a) (b)



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Table 1. Calculated enthalpy of formation for AlNi2Si (kJ/mole-atoms)

Composition CALPHAD Experiment VASP

AlNi2Si –55.00 –56.43 –56.36

Enthalpy of melting CALPHAD Experiment*

L = Al3Ni + (Al) + (Si) –14.13 –12.22

* DSC measurement (N.M. Martynova et al., Russ. J. Phys. Chem. 58 (1984) 616 – 617.

Table 2. Enthalpy of melting for the invariant eutectic L = Al3Ni + (Al) + (Si) (kJ/mole-atoms)

The Al-Ni-SiThe Al-Ni-Si ternary systemternary system Model predicted thermodynamic properties


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Scheil model

Mircomodel

4. Summary

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Thermodynamic database

Kinetic databaseReal solidification condition

Kinetic database input Impurity diffusivity of Ni, Mg, Mn, Si in liquid Al and solid (Al)

Energy of solid/liquid interface

Specific latent heat of solidification

Geometric factor for coarsening

t3 32 2,0 f0

λ (t) λ G Mdt

Liquid

Solid

3.2 Solidification behaviors of Al356.1 alloy 3.2 Solidification behaviors of Al356.1 alloy

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L L (Al) : (Al) :

Calculated- 615Calculated- 615ooC, Measured-616 C, Measured-616 ooCC

L L (Al)+(Si)+ (Al)+(Si)+αα-AlMnSi :-AlMnSi :

CalculatedCalculated- 573- 573ooC, C, MeasuredMeasured-575 -575 ooCC

Fig. 14. The DSC curve of equilibrium solidification of multi-component Al 356.1 alloy (∆T=3 oC)

EquilibriumEquilibrium Solidification: Solidification: Complete diffusion in both liquid and solid phases

Al356.1 is annealed at 550oC for 45 days


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Table 3. The comparison between the non-equilibrium calculation and the experimental solidification of multi-component Al 356.1 alloy (∆T=6oC)

Scheil model calculationScheil model calculation Measured [1990Bae]Measured [1990Bae]

L L (Al) at (Al) at 615 615 ooCC L L (Al) at (Al) at 614 614 ooCC

L L (Al) + (Al) + -AlMnSi at -AlMnSi at 588 588 ooCC L L (Al) + (Al) + -AlMnSi at -AlMnSi at 594 594 ooCC

L L (Al) + (Si) + (Al) + (Si) + -AlNiSi -AlNiSi+ + -AlMnSi at -AlMnSi at 572 572 ooCC

L L (Al) + (Si) + (Al) + (Si) + -AlNiSi -AlNiSiat at 575 575 ooCC

L L (Al) + (Si) + Mg (Al) + (Si) + Mg22SiSi

+Al+Al88NiMgNiMg33SiSi66++-AlMnSi at -AlMnSi at 556 556 ooCC

L L (Al) + (Si) + Mg (Al) + (Si) + Mg22SiSi

+ Al+ Al88NiMgNiMg33SiSi6 6 at at 554 554 ooCC

Scheil model: Scheil model: No diffusion in solid phase, complete diffusion in liquid

1990Bac: L. Bäckerud et al., Solidification Characteristics of Aluminum Alloys, Vol. 2, Foundry Alloys, AFS/Skanaluminium, Sweden (1990).


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Fig. 15. Microstructure (directional solidification with a cooling rate of

2K/S)

Micromodel:Micromodel:

3.2 Solidification behaviors of Al356.1 alloy 3.2 Solidification behaviors of Al356.1 alloy • complete diffusion in liquid• back diffusion in solid phases• undercooling

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Cylindrical model

Scheil model

ExperimentSphere model

Secondary dendrite: sphere, cylinder

(I) Diffusion in solid phase;

(II) The growth of dendirte;

(III) Solute super-cooling, temperature gradient

super-coolingYong Du et al., Z. Metallkd., 96, 1351-1362 (2005)

Fig. 16. Si distribution in the primary (Al) during the directional solidification of

multi-component Al 356.1 alloy (Cooling rate: 2K/S)

3.2 Solidification behaviors of Al356.1 alloy 3.2 Solidification behaviors of Al356.1 alloy Micromodel:Micromodel:

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Scheil model

Mircomodel

4. Summary

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A thermodynamic database of Al-Fe-Mg-Mn-Si-Cu-Ni-Zn-

(+more elements) system is being constructed;

A kinetic database of Al-Fe-Mg-Mn-Si-Cu-Ni-Zn system is

being constructed;

Hybrid approach: Key experiment + CALPHAD + First-

principles method;

The thermodynamic and kinetic database are used to

describe the solidification behaviors of Al alloys.

4. Summary4. Summary

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Prof. Dr. Yong DuState Key Lab of Powder Metallurgy

Central South UniversityChangsha, Hunan, 410083, P.R. China

E-mail: [email protected]: +86-731-8710855http://www.imdpm.net

Thank you for your attention!Welcome to China!

Thank you for your attention!Welcome to China!

yong du , w.w. zhang, w. xiong state key lab of powder metallurgy, central south university, china

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