Eutectic High‐Entropy Alloys (EHEAs)
Sheng Guo
Materials and Manufacturing Technology Department
Chalmers 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
(Yeh, et al., Mater Chem Phys, 2007)
Introduction: High‐Entropy Alloys
N=7
N=6
N=5
N=4
N=3
(Adv.Eng.Mater, 2004)
N=2
N=1
Highly concentrated solid solutions
AlCoCrFeNiTi0.5
Potential of HEAs as structural materials
y=2.26GPa
f=3.14GPa
p=23.3%
(Yeh, et al., Adv Eng Mater, 2004)
(after 1000 oc/12h)
460 MPa@1600 oC
better than superalloys
(Zhou et al., APL, 2007)
(Senkov, et al., Intermetallics, 2011)
Very high hardness can be achieved
Disordered bcc solid solution was reserved after annealing at 1400 oc for 19h
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
Q1:Solid solution or amorphous phase? N
Smix   R  ci ln ci
i 1
(Nature, 1993)
when N elements are mixing in equiatomic ratio (c1=c2=…=cN), the mixing entropy reaches the maximum:
Smix  R ln N
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?
High‐entropy bulk metallic glasses (Ma et al., Mater Trans, 2002)
(1.5mm)
(Takeuchi et al.,
Intermetallics, 2011)
(Gao et al., J Non-Crys.
Solids, 2011)
Intermetallic compounds can certainly form in equiatomic multi‐component alloys
For example:
XRD patterns of the CoCrCuFeNiTix
samples (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?
A1: Statistical analyses of phase selection in HEAs
2-parameter
map
 Solid solution phases form when  is small, and △Hmix is either slightly positive or insignificantly negative;
(Guo et al., Prog Nat Sci: Mater Int, 2011;
 Amorphous phases form when  is large, and △Hmix is noticeably negative;
Guo et al., Intermetallics, 2013)
 In the intermediate conditions (in terms of  and △Hmix ) , intermetallic compounds compete with tboth amorphous phases & solid solution phases. Q2: fcc or bcc solid solution?
N=7
N=6
N=5
N=4
N=3
N=2
N=1
bcc
bcc
fcc+bcc
fcc+bcc
fcc+bcc
fcc+bcc
fcc+bcc
fcc+bcc
fcc+bcc
fcc+bcc
fcc
fcc
fcc
(Yeh, et al., Mater Chem Phys, 2007)
AlxCoCrCuFeNi
(Tong et al., Metall Mater A, 2005)
x=3
x=0
Q2: fcc or bcc solid solution?
Why is that?!
A2: Valence Electron Concentration is the key
6.87
8.0
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;
AlxCrCuFeMnNi;
Al0.8CrCu1.5FeMnNi;
Al0.8CrCuFe1.5MnNi
Al0.8CrCuFeMn1.5Ni;
MoNbTaW
MoNbTaVW;
bcc
bcc+fcc
fcc
5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5
Valence electron concentration
CoCrFeMnNi
AlxCrCuFeMnNi
AlBxMnNiTi
AlxC0.2CuFeMnNi
(Guo et al., JAP, 2011)
 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 thermal
(Glicksman, Principle of degradation;
Solidification, 2011)
 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. Eutectic High‐Entropy Alloys
An 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 microstructure
 melting temperature ~ 1350 oC (NiAl: 1674 oC)
 density of ~ 7.4 g/cm3 (NiAl: 6 g/cm3)
Eutectic High‐Entropy Alloys
b
1200
Engineering stress-strain
True stress-strain
Stress/MPa
1000
o
600 C
o
700 C
800
800
100
600
80
60
400
40
200
True stess/MPa
a
600
400
200
20
0
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
0
0
5
10
15
Strain/%
20
25
30
0
0
5
10
15
20
True strain/%
25
 balanced tensile fracture strength and ductility, for large ingots
 the decent mechanical properties can be maintained to 700 oC
 strong work hardening behavior
30
35
Eutectic High‐Entropy Alloys
1200
True stress/MPa
b
1400
NiAl <001>
1400
1200
1000
800
0.2, non-EHEAs
600
UTS, non-EHEAs
400
0.2, EHEA
200
UTS, EHEA
True stress/MPa
a
1000
800
600
400
200
0
0
0
2
4
6
8 10 12 14 16 18 20 22 24
Elogation to failure/%
0
50
600
o
Temperature/ C
700
 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
800
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.