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
Outline
A brief introduction to HEAs Phase selection in cast HEAs Some issues with cast HEAs Eutectic HEAs: An example Conclusions
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
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
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
(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?
(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)
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?
(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.
(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?
Why is that?!
Q2: fcc or bcc solid solution?
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
Some issue with cast HEAs Porosity, particular for large ingots Inhomogeneity/Segregation Conflict between strength/ductility
(Tong et al., Metall Mater Trans A, 2005)
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)
Eutectic High‐Entropy AlloysAn example: AlCoCrFeNi2.1
~ 2.5 kg of homogenous and almost casting defects free large ingots
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)
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
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
Eutectic High‐Entropy Alloys
after 8% cold rolling
mechanical properties can be further tuned by thermomechanical treatments
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.
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