High Entropy Alloys and deign of structural materials

Tao Yong and C. T. LiuCenter for Advanced Structural Materials

Department of Materials Science and EngineeringCity University of Hong Kong

IAS Material Summit at CityU June 27, 2017

Three critical issues for the design of structural materials

1. The strength-ductility trade-off 2. Deformation Instability (early necking with low uniform

elongation)3. Embrittlement at low temperatures

• High entropy alloys (HEAs) are the potential material for studying and resolving these critical material issues

High Entropy Alloys (HEAs) open a new alloy design strategy

Y. F. Ye, Y. Yang, Mater. Today, (2015).

• A radical departure from the conventional alloy design

• Multiple-principal elements with a concentration range between 5 and 35 at.%

• Opens up a huge alloy design space, due to “the cocktail effect “

3

Ni

Co Fe

Single –phasedfcc NiCoFe SS

R. O. Ritchie, Nat. comm. 7 (2016).4

• The fcc CoCrFeNiMn HEA exhibits an extensive ductility and better strength at the cryogenic temperature of 77K.

FCC HEAs show no embrittlement at low temperatures

• Recent studies indicate that fccHEAs show excellent ductility even at close to 0°K

• Both calculations and experiments indicate the formation of stacking faults & micro-twins, which increase the strain induced hardening & the ductility at low temperatures

• Compressive stress–strain curves of (a) FeCoNi-Alx and (b) FeCoNi-Six alloys at RT.• On the other hand, HEAs can be effectively hardened by precipitation of intermetallic

particles.

In general, solid solutions provide limited strengthening in HEAs

T.T. Zuo, et al. J. Magnetism and Magnetic Materials, 2014

(a) (b)

SS alloy SS alloy

4

FeCoNi

Al addition Si addition

Dense precipitation of nanoscale L12 precipitates can be readily formed in NiCoFeAlxTiy HEAs

2. High volume fraction (~60%)

3. Very uniform distribution

4. Coherent with the matrix with a small misfit

1. Nano-meter size (~30-50 nm)

9

• 1150℃-solid solution/780℃-aging

• These particles with a complex alloy composition (NiCoFe)3(TiAlFe) are more ductile and harder than simple Ni3Al particles

• The particle-strengthened HEA#1 alloy has a superb combination of both strength and ductility, which outperform Ni-based superalloys

Ductile fracture without necking

0 10 20 30 40 500

200

400

600

800

1000

1200

1400

1600

HEA#1

HEA#2

Eng

inee

ring

str

ess,

σΕ (M

Pa)

Engineering strain, εΕ (%)

Base alloy

Dense precipitation of nanoscale complex L12 particles provides both strength and ductility for HEA#1 alloy ( no embrittlement at all)

a b

NiCoFe

Ni-base superalloys

Comparison of the work hardening behavior of HEA#1 & #2(both strengthened by 60% nanoscale L12 particles)

HEA#1

HEA#2

• HEA#1 shows a less decrease in the work hardening rate in the stage A and B.• Only HEA# 1 shows an increase, rather than a decrease, in the work-hardening

stage C, due to a quick formation of microbands

HEA#1

HEA#2

13

θ=dσ/dε

0 10 20 30 40 500

200

400

600

800

1000

1200

1400

1600

HEA#1

HEA#2

Engi

neer

ing

stre

ss, σ

Ε (M

Pa)

Engineering strain, εΕ (%)

Base alloy

No necking

TEM studies reveal a quick formation of micro-bands in the strain stage C in Alloy HEA#1

• The quick formation of micro-bands is the key for an effective increase in the work hardening in HEA#1 and the elimination of its strength-ductility trade-off effect 14

Dislocations-dominated Microbands-dominatedin the stage C

Conclusion

• The study of the HEA alloys with a dense precipitation of nanoscale particles has demonstrated the feasibility for the elimination of all critical issues for the design of advanced structural materials

Strain-induced hardening plays a key role in minimizing or eliminating the strength-ductility trade-off effect in fcc HEAs

• Strain-induced mechanisms: -Dislocation induced hardening mechanisms: dislocation density,

cell structure formation, micro-bend formation, etc-Defect formation mechanisms: stacking faults, micro-twinings-Hierarchical structures and back-stress effects

• Analyses of the work hardening behaviour would provide the key information about how to eliminate the strength-ductility trade-off effect in HEA#1 alloy