Phase formation rules for high entropy alloys

1.

HIT 2008
Phase Formation Rules for
High Entropy Alloys
Yong Zhang
University of Science and Technology Beijing
ICAMP5

2.

Acknowledgements
Prof. GuoLiang Chen;
Prof. Hywel A Davies;
Prof. Peter K Liaw;
Prof. George Smith;
Prof. Zhaoping Lu;
XueFei Wang; YunJun Zhou;
FangJun Wang.

3.

Outlines
I. Background & Motivations
II. Results & Discussions
III. Summaries

4.

I. Background & Motivations
1.1 Alloys Design Strategy
(1) Conventional alloys
Alloy=A+ B+ C+;
A>50%; …
Steel, A=Fe,
B=Carbon, B<2%;
Cast Iron, A=Fe,
B=Carbon, B<6.5%

5.

(2) High Entropy Alloys
HEAs=A+B+C+D+E; 50%<A\B\C\D\E>15%
FCC type HEA Solid Solution
CoCrCuFeNi=HEA,
Yeh, MMTA, 2004;
BCC type HEA Solid Solution
AlCoCrFeNi=HEA ,
Zhou, APL, 2007
Al20[TiVMnHEA]80,
Zhou, MSEA, 2007

6.

1.2 Thermodynamically
S X A S A X B S B S mix
Entropy
Solid Solution
For the regular solution:
S mix R( X A LnX A X B LnX B )
Mecanical Mixture
0.00
A
0.25
0.50
Molar Fraction
0.75
S X AS A X B SB
1.00
B
Solid solution has higher entropy than the mechanical
mixture does.

7.

GA
Gibbs Free Energy
GB
Gmix G AB ( X AG A X B GB )
Gmix = Hmix-T Smix
mix
GA B
Gmix = mix -T Smix
Hmix
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Composition
,
Gmix LEA
,
Smix
Smix HEA
,
Smix LEA
,
Gmix HEA
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Composition
0.8
0.9
1.0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Composition
0.7
0.8
0.9
1.0

8.

1.3 Properties and Applications
Properties
1. High Strength;
Zhou, APL, 2007;
2. High wear resistance; Lin, Surface
Coating technology, 2008.
3. High corrosion resistance; Lee, Thin
Solid Films, 2008;
4. High thermo-stability; Tsai, APL, 2008.

9.

Potential Applications
1 Coatings, Barriers, etc.
Diffusion barriers for Cu interconnections; Tsai, APL, 2008
2 Structural Materials
3 Energy Storage Materials,
Raju, Journal of power Sources, 2008;
4 Molds

10.

1.4 Motivations
To understand what is the dominant factors
for the phase formation of the HEAs
1 Atomic radius, or atomic volume;
The contents of Al, Ti, Cu, Co in
the HEAs were changed
N
c (1 r / r )
i 1
i
Atomic Radius
2
i
N
r Ci ri
i 1
Kittel, Introduction to Solid State Physics

11.

2 Enthalpy of Mixing;
H mix
N
c c
i 1,i j
ij i
j
3 Entropy of Mixing
N
S mix R Ci LnCi
i 1

12.

4 Cooling Rate
Critical cooling rate? Like the BMG?
5 Tensile and compressive properties
Tensile elongation=0? Like BMG?

13.

II. Results & Discussions
2.1. Alloying with different atomic size, Al, Cu, Co, Ti
Al=1.438A
(y=0, 0.25, 0.5, 0.75)
3.579A CoCrFeNiCu1-yAly
FCC
2.913A,2.872A
Ti0.5CoCrFeNiCu1-yAly
BCC, High APE to Lower APE, with larger atoms Al

14.

Cu=1.278A
( y=0, 0.25, 0.5)
CoCrFeNiAlCuy
Ti0.5CoCrFeNiAlCuy
No PHASE TRANSITION

15.

Co=1.251A
Biger BCC1phase:2.913A;
Smaller BCC2phase:2.872A
The smaller BCC transit to FCC firstly after adding Co

16.

Ti=1.448A
Intensity (a.u.)
Ti1.5
(200)
(201)
2
(211)
(220)
BCC
Laves phase
1
(110)
Double BCC+
Laves
(110)
[Al1Co1Cr1Fe1Ni1]Tix alloys
1
1
2
2
1
Big BCC
2
1
1 2
Ti1
2
12
Double BCC
Ti0.5
1 2
12
Single BCC
Ti0
20
30
40
50
60
70
80
90
2 (Degree)
BCC+Ti
BCC+BCC
16

17.

After adding Ti, Laves phase forms

18.

The transition is mainly lattice distortion induced and APE related
Zhou, APL, 2008

19.

Al
Ti
FCC
A schematic showing the additional effects
FCC
Laves
BCC
FCC
BCC
Ti
BCC
Cu
BCC
Co
Cu

20.

2.2. Considering of the enthalpy of mixing Hmix
Mg based BMG
Zr based BMG
20
Zhang, AEM, 2008

21.

2.3. Considering of the entropy of mixing Smix
High Entropy is not good for the formation of BMG
21

22.

2.4 Cooling Rate
AlCoCrFeNi

23.

5mm
2mm
8mm
AlCoCrFeNi
10mm

24.

AlCoCrFeNi

25.

2.5 Tensile and Compressive properties
XRD pattern for the CoCrCuFeNiAl0.5 alloy.

26.

5 10
Table Room temperature mechanical test results for the CoCrCuFeNiAl0.5 alloy
This alloy
P (%)
0.2 (MPa)
max (MPa)
Compressive
>51.5
460
>1380
Tensile
19.1
360
707
P: plastic strain; 0.2 : yield strength; max: compressive/tensile strength

27.

III. Summaries
1 Atomic size mismatch is the dominant factor for the phase
formation of the high entropy alloys;
2 The formation of solid solution for the HEAs intends to have
enthalpy of mixing close to zero;
3 High entropy of mixing facilitates the formation of the solid
solution rather than the BMGs;
4 Cooling rate plays rather important role for the homogeneous
microstructure than for the phase formation;
5 HEA can have tensile elongations as high as 19%.

28.

Thanks for your
attention
28