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Eutectic HighEntropy Alloys (EHEAs) Sheng Guo Materials and Manufacturing Technology Department Chalmers University of Technology, Gothenburg, Sweden Email: [email protected] CMAC Days 2014, Zagreb
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May 06, 2018

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Page 1: Eutectic High Entropy Alloys (EHEAs)shengguo.weebly.com/uploads/1/6/9/3/16931630/c-mac_days_2014_sh… · better compositional homogeneity (less segregation) Inspirations: Eutectics

Eutectic High‐Entropy Alloys (EHEAs)

Sheng Guo

Materials and Manufacturing Technology DepartmentChalmers University of Technology, Gothenburg, Sweden

E‐mail: [email protected]

C‐MAC Days 2014, Zagreb

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Outline

A brief introduction to HEAs Phase selection in cast HEAs Some issues with cast HEAs Eutectic HEAs: An example Conclusions

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Introduction: High‐Entropy Alloys

(Adv.Eng.Mater, 2004)

(Yeh, et al., Mater Chem Phys, 2007)

N=1

N=2

N=3

N=4

N=5

N=6

N=7

Highly concentrated solid solutions

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Potential of HEAs as structural materials

Very high hardness can be achieved

(after 1000 oc/12h)

(Yeh, et al., Adv Eng Mater, 2004)

AlCoCrFeNiTi0.5

y=2.26GPa

f=3.14GPa

p=23.3%

(Zhou et al., APL, 2007)

Disordered bcc solid solution was reserved after annealing at 1400 oc for 19h

(Senkov, et al., Intermetallics, 2011)

460 MPa@1600 oCbetter than superalloys

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High‐entropy effect enhances the formation of solution phases

Possible competing states(elemental phases, compounds, solid solutions)

△Gmix =△Hmix ‐T△Smix

Solid solution phases having the highest mixing entropy

thus become highly competitive and more stable especially at high T

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(Nature, 1993)1

lnN

mix i ii

S R c c

Based on the confusion principle and high entropy points of view, we can easily understand that  random solid solutions tend to be stable in HEAs.

But why not form a glassy (amorphous) phase then?

when N elements are mixing in equiatomic ratio (c1=c2=…=cN), the mixing entropy reaches the maximum:

lnmixS R N

Q1:Solid solution or amorphous phase? 

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(Gao et al., J Non-Crys. Solids, 2011)

High‐entropy bulk metallic glasses (Ma et al., Mater Trans, 2002)

(Takeuchi et al., Intermetallics, 2011)

(1.5mm)

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Intermetallic compounds can certainly form in equiatomic multi‐component alloys

For example:

XRD patterns of the CoCrCuFeNiTixsamples (x = 0, 0.5, 0.8, and 1)

(Wang et al., Intermetallics, 2007) (Yang et al., Mater Chem Phys, 2007)So, can we predict the phase selection (solid solution, amorphous phase and intermetallic compound) in equiatomic multi‐component alloys?

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(Guo et al., Prog Nat Sci: Mater Int, 2011;Guo et al., Intermetallics, 2013)

A1: Statistical analyses of phase selection in HEAs

2-parametermap

Solid solution phases form when  is small, and △Hmix is either slightly positive or insignificantly negative; Amorphous phases form when  is large, and △Hmix is noticeably negative; In the intermediate conditions (in terms of  and △Hmix ) , intermetallic compounds compete with tboth amorphous phases & solid solution phases. 

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(Tong et al., Metall Mater A, 2005)

fccfccfcc

bccbcc

AlxCoCrCuFeNi

fcc+bccfcc+bccfcc+bccfcc+bccfcc+bccfcc+bccfcc+bccfcc+bcc

(Yeh, et al., Mater Chem Phys, 2007)

N=1

N=2

N=3

N=4

N=5

N=6

N=7

x=0

x=3

Q2: fcc or bcc solid solution?

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Why is that?!

Q2: fcc or bcc solid solution?

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5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5

8.0

bcc+fcc fcc

AlCo0.5CrCuFeNi; AlCoCr0.5CuFeNi AlCoCrCu0.5FeNi; AlCoCrCuFe0.5Ni AlCoCrCuFeNi0.5; AlCoxCrCu0.5FeNi AlCoxCrCu0.5FeNi; AlCoxCrCu0.5FeNi AlCoCrxCu0.5FeNi; AlCoCrCu0.5FexNi AlCoCrCu0.5FeNix; AlCoCrCu0.5FeNix

CrCuFeMnNi; CoCrFeMnNi AlxCrCuFeMnNi; AlxCrCuFeMnNi Al0.8CrCu1.5FeMnNi; Al0.8CrCuFe1.5MnNi Al0.8CrCuFeMn1.5Ni; MoNbTaW MoNbTaVW; AlBxMnNiTi AlxC0.2CuFeMnNi

Valence electron concentration

bcc

6.87

(Guo et al., JAP, 2011)

A2: Valence Electron Concentration is the key

A higher VEC favors the formation of fcc solid solutions, while a smaller VEC tends to stabilize the bcc solid solutions A mixture of fcc and bcc solid solutions forms at intermediate VEC

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Some issue with cast HEAs Porosity, particular for large ingots Inhomogeneity/Segregation Conflict between strength/ductility

(Tong et al., Metall Mater Trans A, 2005)

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Why Eutectic Alloys? highly stable microstructures that do not revert, or coarsen, easily at elevated temperatures;

high thermodynamic stability and kinetic resistance to thermaldegradation;

development of low‐energy lamellar and rod‐form boundary structures;

high strengths and creep resistance because their microstructures act as natural  ‘in situ’ composite materials; 

better castability (less porosity) better compositional homogeneity(less segregation)

Inspirations: Eutectics with high‐melting points have formed the basis for a number of interesting candidate high‐temperature alloys for application to the high temperature components of gas turbine engines. 

(Glicksman, Principle of Solidification, 2011)

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Eutectic High‐Entropy AlloysAn example: AlCoCrFeNi2.1

~ 2.5 kg of homogenous and almost casting defects free large ingots 

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Eutectic High‐Entropy Alloys

soft fcc/ hard NiAl‐like B2 eutetic microstructuremelting temperature ~ 1350 oC (NiAl: 1674 oC) density of ~ 7.4 g/cm3 (NiAl: 6 g/cm3)

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0 5 10 15 20 25 300

200

400

600

800

1000

1200

Stre

ss/M

Pa

Strain/%

Engineering stress-strain True stress-strain

0 5 10 15 20 25 30 350

200

400

600

800

Tru

e st

ess/

MPa

True strain/%

600 oC 700 oC

a b

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.80

20

40

60

80

100

Eutectic High‐Entropy Alloys

balanced tensile fracture strength and ductility, for large ingots the decent mechanical properties can be maintained to 700 oC strong work hardening behavior

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0 2 4 6 8 10 12 14 16 18 20 22 240

200

400

600

800

1000

1200

1400

0.2, non-EHEAs UTS, non-EHEAs 0.2, EHEA UTS, EHEA

Tru

e st

ress

/MPa

Elogation to failure/%

NiAl <001>a b

0 50 600 700 8000

200

400

600

800

1000

1200

1400

Tru

e st

ress

/MPa

Temperature/oC

Eutectic High‐Entropy Alloys

overall fracture strength/tensile ductility better than NiAl/Cr(Co)eutectic alloys a large space to improve at higher temperatures though, with a compromise with density

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Eutectic High‐Entropy Alloys

after 8% cold rolling

mechanical properties can be further tuned by thermomechanical treatments

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Conclusions

Entropy alone can not stabilize the solid solutions in multi‐principal‐element alloys;

By using empirical physical metallurgy principles, formation and even type of solid solutions can be reasonably controlled;

Eutectic high‐entropy alloys might be a promising alloying strategy to develop new class of high‐temperature alloys.