Seoul National University High Entropy Alloy

Traditional alloy High entropy alloy Minor Major element 3 element …

Minor Major element 2 element 3 Major Major element element 1

Major Minor element 2 element 1

= ( ) 풏풏풏풏풏풏 풐풐 �풆𝒆𝒆풆풆 ↑ ↔. 풄𝒄�풄풄𝒄풄�풄𝒄풄� 𝒆풆𝒆풆풆 ↑ ∆푆푐푐푐푐푐푐 푅푅푅 �

Y.Zhang et al., Prog.Mat.Sci. 61, 1 (2014) (1) Thermodynamic : high entropy (3) Kinetics : sluggish diffusion (2) Structure : severe lattice distortion (4) Property : cocktail effect High entropy alloy : Potential candidate material for extreme environment applications

Seoul National University Mechanical properties of HEA

. Ashby map Bernd Gludovatz / SCIENCE VOL 345 ISSUE 6201

There are clearly stronger materials, which is understandable given that CrMnFeCoNi is a single-phase material, but the toughness of this high-entropy alloy exceeds that of virtually all pure metals and metallic alloys.

Seoul National University Fracture toughness of HEA

Bernd Gludovatz / SCIENCE VOL 345 ISSUE 6201

Although the toughness of the other materials decreases with decreasing temperature, the toughness of the high- entropy alloy remains unchanged.

mechanical properties actually improve at cryogenic temperatures

Seoul National University Fracture Toughness Testing

Compact tension test & Three point bending test

Pre-cracking

Limitation

 Destructive Fracturing  Complex testing procedure

 Cannot apply to in-service

Development of testing method to measure the fracture properties more easily and more economically

Seoul National University Indentation Fracture Toughness . Instrumented Indentation Technique (IIT)

 In-situ & In-field System, Non-destructive & Local test, Simple & fast . Indentation Cracking Method (cracking in indentation)

1  E  2  P  = α   K IC    3   H   c 2 

 Only for ceramic materials (very brittle) [Vickers indenter, Macroscale]

Seoul National University Indentation Fracture Toughness

. In case of metallic materials (no cracking in indentation)

Fracture test IIT

Indenter

Crack propagation and fracture No crack and no fracture

. How to correlate flat punch indentation with crack tip behavior

isotropic Virtual crack

Seoul National University How to correlate flat punch indentation with crack tip behavior

Contact mechanics Fracture mechanics of flat punch indentation of cracked round bar specimen

Direct calculation of J from indentation curve

Determination of Fracture toughness

Flat punch: regarded as a very deep cracked round bar specimen

Seoul National University How to correlate flat punch indentation with crack tip behavior

* Size effect * Load-depth curve * Normalization

* Crack initiation point

J. H. Giovanola et al., ASTM STP 1244 (1995) In flat-punch indentation test

Seoul National University How to correlate flat punch indentation with crack tip behavior

* Load criterion * Depth criterion

* Evaluation

= 푷 푲푰 ퟐ� 흅� ( ) = + = + ퟐ ퟐ ퟏ − 흂 푲푰 푨𝒑 푱 푱� 푱� 휼𝒑 ퟐ Seoul National University 푬 흅� Fracture toughness of cantor alloy

Reference alloy : Cantor alloy – Cr20Mn20Fe20Co20Ni20 Bernd Gludovatz / SCIENCE VOL 345 ISSUE 6201

Process : Arc melting, copper mold drop casting, cold forging and cross rolling at RT, recrystallization

KjIc Ave. (Mpa·m1/2) Grain size (μm)

Cantor @293K ~217 ~ 6

Cantor @200K ~221 ~ 6

Cantor3 @77K ~219 ~ 6

Seoul National University Fracture toughness of cantor alloy

Reference alloy : CrCoNi

Bernd Gludovatz/ NATURE COMMUNICATIONS | 7:10602 | DOI: 10.1038/ncomms10602 |

Process : Arc melting, copper mold drop casting, cold forging and cross rolling at RT, recrystallization

KjIc Ave. (Mpa·m1/2) Grain size (μm)

@293K ~205 ~ 5

@200K ~265 ~ 5

@77K ~273 ~ 5

Seoul National University XRD data of cantor alloy

FCC(111) FCC(200) FCC(220)

Intensity(a.u.)

Cantor1 (As-cast)

Cantor2 (24h homogenization)

Cantor3 (After recrystallization)

20 30 40 50 60 70 80 2 theta (deg.)

XRD data shows that all specimen have FCC crystal structure

Seoul National University Microstructure of Cantor alloy

Cantor 1 Cantor 2 Cantor 3 (as-cast) (24h homogenization) ( + cold rolling + recrystallization)

Dendritic Annealing growth twin

100 m 100 m 5 m

μ μ μ

Grain size : ~ 97 m ~ 107 m ~ 4 m

μ μ μ

Seoul National University Indentation test for toughness of cantor alloy

. Test condition Loading rate : 0.3mm/min, Depth control : 100um, Zero index : 0.1kgf

. Normalized curve

Cantor 1 Cantor 2 Cantor 3 2500 2500 2500 Cantor 1 Cantor 2 Cantor 3

2000 2000 2000

1500 1500 1500

1000 1000 1000 Pm (MPa) Pm Pm (MPa) Pm (MPa) Pm

500 500 500

0 0 0 0.0 0.1 0.2 0.3 0.4 0.0 0.1 0.2 0.3 0.4 0.0 0.1 0.2 0.3 0.4 h/a h/a h/a

Seoul National University Result of indentation test

This work 260 Ref

240

) 245 1/2 220

217

MPa·m 200 ( 217

jIc Mpa·m1/2

K 200

180

0 Cantor1 Cantor2 Cantor3

Ave. Grain size Kjc Test 1 Test 2 Test 3 Test 4 (Mpa·m1/2) (μm)

Ref. - - - - 217 ~ 6

Cantor1 229.913 225.478 204.192 208.74 217.0808 ~ 97

Cantor2 195.559 203.835 198.548 203.291 200.3083 ~ 107

Cantor3 230.931 241.726 254.396 252.385 244.8595 ~ 4

Ref. and cantor3 which are similar processing condition show 12% deviation

Seoul National University Strength of Ni-Fe-Cr systems

H.S.Oh / ph.D graduation paper 1100 V Ni 1200 36.8 63.2 1000 Ni40Fe30Cr30 (at%) ~8.3 μm 900 add V5 1000 add V10 CrCoNi (Acta Mater., 2017) 800

~6.5 μm 700 800 CrMnFeCoNi (Science, 2014) 600 ~6 μm 600 500

400 avg. grain size : 5.75 m (111) V5 400 300 μ

(311) (200) Tensile stress (MPa)

(220) 200 Engineering stress (MPa)

200 Intensity (a.u.) 100 1.00 1.25 1.50 1.75 2.00 2.25 FCC d-spacing (A) 0 phase 0 0 5 10 15 20 25 30 35 40 45 50 0 10 20 30 40 50 60 Engineering strain (%) strain (%)

at% Y.S (MPa) U.T.S (MPa) Elongation (%) Grain size (μm)

Cantor 331.96 693.49 41.3 ~ 6 CoCrNi 452.9 927.36 45.21 ~ 6.5 NiV 743.49 1174.05 44.53 ~ 8.3

Ni40Fe30Cr30 470.2 736.8 28.3 ~ 3.65

Ni40Fe30Cr25V5 476.4 785.7 28.6 ~ 4.18

Ni40Fe30Cr20V10 682.7 954.7 19.4 ~ 5.75

Seoul National University Heat treatment of specimen

1 2 3

Heat treatment Phase Grain size (μm)

1 Ni40Fe30Cr30 _1 As-cast FCC - homogenization + Cold 2 Ni Fe Cr _3 FCC ~ 3.7 40 30 30 rolling + Recrystallization

3 Ni40Fe30Cr20+V10 _1 As-cast FCC - homogenization + Cold 4 Ni Fe Cr +V _3 FCC ~ 4 40 30 20 10 rolling + Recrystallization

Seoul National University Fracture toughness and tensile property

. Yield Strength vs Fracture toughness . Ductility vs Fracture toughness

800

50 V36.8Ni63.2 700 Ni40Fe30Cr20V10 CoCrNi 40 V36.8Ni63.2 600 Cantor

500 30 Ni40Fe30Cr25V5

CoCrNi Ni40Fe30Cr25V5 Ni40Fe30Cr30 Ni40Fe30Cr30 400 20 Elongation (MPa)

Yield Strength (MPa) Ni40Fe30Cr20V10 Cantor 300 10 200 250 300 350 400 250 300 350 400 Fracture toughness Fracture toughness

Fracture toughness through IIT yield strength에 의한 영향이 더 큰 것으로 보임. Grain size effect on fracture toughness

260 This work 250

240 )

1/2 230

220 MPa·m (

jIc 210 K

200

190 0.0 0.1 0.2 0.3 1 / Grain size(um)

Grain size 가 작아질 수록 Fracture toughness 가 증가하는 것으로 보임. Grain boundary and fracture toughness Zhongyun Fan, Mat. Sci. Eng. A, 191 (1995) . . Grain size vs Yield Strength Grain size vs Fracture toughness

Hall-petch relation : Grain boundary and fracture toughness

Zhongyun Fan, Mat. Sci. Eng. A, 191 (1995) Seoul National University