ESPark Research Group...How can we fabricate HEA foam ? What are the advantages of HEA foam ? 1) Alloy design for fabricating HEA foam : Phase separating HEA 2) Fabricating method

Application of metallic foamsA promising candidate of thermal insulation and bio-compatible materials

Kook Noh Yoon

ESPark Research Group

Current status of structural materials on02APR2018

RIAM, Department of Material Science and Engineering, Seoul National University, Republic of Korea

1. Fabrication of High entropy alloy foams

OUTLINE

How can we fabricate HEA foam ?

What are the advantages of HEA foam ?

1) Alloy design for fabricating HEA foam : Phase separating HEA2) Fabricating method : Dealloying process

1) Porous structure of HEA foam2) Thermal and Mechanical properties of HEA foam

Introduction of thermal shield material

1) What is the thermal shield material ? 2) Needs and problems of metal matrix thermal shield

Current status of structural materials : Applications of metallic foams /303

Thermal shielding property : Resistance to thermal flux

High strength, large ductility and high formability are needed !


Advantages of porous structure

中国绝热节能材料网 (2015)

“Ultralight metal foams”, Scientific report (2015)

Metallic materials

Light

Insulative

Porous structure

• Strong & Ductile

• HeavyConstituted with heavy elements

• High conductivityDue to the free electrons

• High formability


High entropy alloy

Major element 2

Major element 3

Major element …

Major element 1

Minor element 2

Minor element 3

Major element

Traditional alloy

Minor element 1

High entropy alloy

(1) Thermodynamics : high entropy effect

(3) Structure : severe lattice distortion effect

(2) Kinetics : sluggish diffusion effect

(4) Property : cocktail effect

∆𝑆𝑆𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐. = 𝑅𝑅𝑅𝑅𝑅𝑅(𝑅𝑅)𝒏𝒏𝒏𝒏𝒏𝒏𝒏𝒏𝒏𝒏𝒏𝒏 𝒐𝒐𝒐𝒐 𝒏𝒏𝒆𝒆𝒏𝒏𝒏𝒏𝒏𝒏𝒏𝒏𝒆𝒆𝒆𝒆 ↑ ↔ 𝒄𝒄𝒐𝒐𝒏𝒏𝒐𝒐𝒄𝒄𝒄𝒄𝒏𝒏𝒏𝒏𝒄𝒄𝒆𝒆𝒄𝒄𝒐𝒐𝒏𝒏𝒄𝒄𝒆𝒆 𝒏𝒏𝒏𝒏𝒆𝒆𝒏𝒏𝒐𝒐𝒆𝒆𝒆𝒆 ↑

HEA is stable especially at high temperature

∆𝑮𝑮𝒄𝒄𝒐𝒐𝒏𝒏𝒐𝒐𝒄𝒄𝒄𝒄. = ∆𝑯𝑯𝒄𝒄𝒐𝒐𝒏𝒏𝒐𝒐𝒄𝒄𝒄𝒄. − 𝑻𝑻∆𝑺𝑺𝒄𝒄𝒐𝒐𝒏𝒏𝒐𝒐𝒄𝒄𝒄𝒄.

(2) Kinetics : sluggish diffusion effect


(3) Structure : severe lattice distortion effect

Dealloying process

Metal Index (V)

Most cathodic

Gold - 0.00

Copper -0.35

Iron -0.85

Zinc -1.25

Magnesium -1.85

Most anodicCorrosion Science 67 (2013) 100–108, Xuekun Luo et al.

Due to the high phase stability and high melting point of HEA, it is hard to fabricate foam with conventional methods.

Dealloying process

is a corrosion type in some solid alloys, when in suitable conditions a component of the alloys is preferentially leached from the material.


Alloy design : Phase separating HEA

+6

152 pm

Co

Cu

+12+4

NiCr

-4

-2

-7-1-0

-1Fe

166 pm

156 pm

149 pm

146 pm

+13

Phase separation occurrs due to Positive ∆Hmix correlation


FeCoCrNi-Cu alloy

40 ㎛

▼

1) SEM / EPMA analysis 2) XRD pattern

Cu-rich region is separated even at high entropy condition.

Inter-dendrite Dendrite

555045

inte

nsity

(a.u

.)2 theta (deg.)

40EPMA data Cu Fe Ni Co Cr

Inter-dendrite 84.43 2.87 6.52 3.38 2.80

Dendrite 8.31 22.57 22.05 23.54 23.52

▼

5 ㎛

Cu conc.%

93.0

0.0

46.5


(2) Dealloyed

(1) As-cast

Inter-dendrite Dendrite

555045

inte

nsity

(a.u

.)2 theta (deg.)

40

20 ㎛

xy

z

HEA foam fabricated by dealloying process

1) SEM image (tilted 45o) 2) XRD pattern

Dendritic directional pores are fabricated after dealloying.

Need to confirm connectivity of pores

3-D tomography by serial sectioning


Microstructure confirmation : 3-D tomography

Pore structure(Inter-dendrite structure)

FeCoCrNi(Dendrite structure)

Copper content (at.%) 10 20 30 40

Calculated porosity (%) 8.7 18.3 27.6 39.2

Measured porosity (%) 11.1 19.4 29.3 38.5

▶ Porosity values are similar between calculated and measured one. ▶ The pore structure of interdendrite area is fully interconnected.

HEA foam dealloyed from FeCoCrNiCu


Pseudo-binary phase diagram of PS-HEA

Pseudo-binary system between FeCoCrNi and Cu shows monotectic reaction having liquid separation region.

Calculation courtesy of J.H.Kim et al.

Experiment Calculation

L

L 1 + L 2

FCC 1 + L 2

FCC 1 + FCC 2 FCC 2

Dendrite → Droplet → Dendrite

0 10 20 30 40 50 60 70 80 90 100400

600

800

1000

1200

1400

1600Te

mpe

ratu

re (

℃ )

Cu content (at%) CuFeCoCrNi


Compressive yield strength of HEA foam

3 mm

7 m

m

Hypo-monotectic Hyper-monotectic

10 20 30 4010

100

10 20 30 40

241

187

33

23

260 273 284292 303

Cu content (at.%)

Yiel

d st

reng

th (M

Pa)

100 - Realtive density (%)

As-cast Dealloyed

500

▶ At low porosity, HEA foam preserves its yield strength which was about 200 MPa.▶ However, after about 25 % of Cu content, the yield strength drops severely.

←Ve

rtic

al to

den

drit

es→


Pseudo-binary phase diagram of PS-HEA

Pseudo-binary system between FeCoCrNi and Cu shows monotectic reaction having liquid separation region.

Calculation courtesy of J.H.Kim et al.

Experiment Calculation

L

L 1 + L 2

FCC 1 + L 2

FCC 1 + FCC 2 FCC 2

0 10 20 30 40 50 60 70 80 90 100400

600

800

1000

1200

1400

1600Te

mpe

ratu

re (

℃ )

Cu content (at%) CuFeCoCrNi

25


Strength drop due to monotectic reaction

a) Hypo-monotectic reaction (20 at.% of Cu)

Nuclei of FCC 1

Liquid

Nucleation of FCC 1

Liquid of FCC 2

Dendrite of FCC 1

Dendritic growth of FCC 1 Thick & Long dendrites

Liquid phase separation Dendritic growth of FCC 1 Thin and short dendrites

b) Hyper-monotectic reaction (40 at.% of Cu)

Longer than 100 ㎛

Shorter than 30 ㎛

Dendrites of hypo-monotectic condition are interconnected well.

Each small dendrite is separated in short period, since each of it is nucleated after liquid separation.

High strength

Low strength


Thermal diffusivity/conductivity of HEA foam

0 100 200 3000

10

100

3.8 3.8 4.0 4.1 4.2 4.4 4.5

11.4 11.9 12.413.1 13.7

14.3 14.9

108.3

104.7 103.6102.2

100.198.43

96.64

0.81 0.83 0.88 0.92 0.93 0.96 1.0

Ther

mal

diff

usiv

ity (m

m2 /

sec)

Temperature (oC)

pure Copper FeCoCrNi (FeCoCrNi)80Cu20 HEA foam

110LFA: ASTM E1461

Material 𝜶𝜶(mm2/s)Carbon 216.5Silver 165.63Copper 111Molybdenum 54.3Air 19Steel, 1% C 11.72Quartz 1.4Sandstone 1.15Brick, common 0.52Glass, window 0.34

HEA foam 0.1 ~ 5Rubber 0.089 - 0.13 Wood 0.082

300 K, 1atm

Cu contents (%) 0 10 20 30 40

Thermal cond. 15.4 5.1 2.6 0.68 0.35

Thermal diff. 4.9 1.6 0.81 0.22 0.13

0 10 20 30 400

1

2

3

4

5

Ther

mal

diff

usiv

ity (m

m2 /

sec)

Cu contents (at. %)

𝜶𝜶 =𝜿𝜿𝝆𝝆𝑪𝑪𝒆𝒆

Thermal diffusivity decreases exponentially against the composition of Cu which is proportional to porosity.


Low thermal diffusivity of HEA foam

HEA foam shows similar thermal diffusivity value with ceramic foams


2. Fabrication of Biocompatible Co-Cr foams

Outline

Fabrication of porous Co-Cr alloy by LMD process

Introduction of bio-compatible Co-Cr alloy Fabrication method : Liquid metal dealloying process, LMD Microstructure optimization - Process condition control

Properties of Co-Cr alloy foams

Biocompatibility Hydrophilic test : Water drop test Osteoblast cultivation on Co-Cr alloy foam

Mechanical properties Compressive properties of Co-Cr alloy foam


Porous Co-Cr alloy for biomedical applications

Porous Co-Cr alloy can be a good candidate for implant materials

Co-Cr alloy

Excellent bio-compatibility

High corrosion and fatigue resistance

Good mechanical properties - fracture toughness and strength

+ Porous structure?

Co-Cr alloy

Surface Engineering 443.45 (2015) Int J Med Robotics Comput Assist Surg 2014; 10:93–97.

E ↓

Cell attachment ↑


Limitation of sintering for porous Co-Cr alloy

M.T. Dehaghani et al. / Materials and Design 88 (2015) 406–413

Porous Co-Cr alloy

Compacted Co-Cr powder

Porous Co-Cr alloy is normally fabricated by sintering powder It is hard to control property of porous structure with sintering technique - Connectivity A material developed by sintering is normally exhibiting brittleness due to defects

New method to fabricate porous structure of Co-Cr alloy is needed !


Liquid metal dealloying process – Thermodynamics for pore fabrication

5 ㎛

Metallic materials can be dealloyed by a liquid metal as well as an etchant


Alloy design for LMD precursor - (Co4Cr1)-Ni “single solid solution alloy”

0 20 40 60 80 100

800

1000

1200

1400

1600

1800

2000

2200

Liquid

Sigmaphase

HCP solid solution

BCC solid solution

Cr

Tem

pera

ture

(K)

Cr content (at.%)Co

FCC solid solution

Ca Ni Co Cr

Ca 0 -7 +2 +38

Ni - 0 -1 -7

Co - - 0 -4

Cr - - - 0

Co80

Cr20

Co75

Cr25

Leachableelement

Solid solutionformer

To obtain target composition which is Co3Cr, precursor was prepared with composition of Co4Cr-Ni. The precursor alloy was ternary single solid solution of cobalt, chromium and nickel.

0 20 40 60 80 100

800

1200

1600

2000

Ni

Liquid

SigmaphaseHCP

FCC solid solution

Tem

pera

ture

(K)

Ni content (at.%)

CrNi2

Co4Cr

Ternary alloy was designed with consideration of thermodynamic correlation


Microstructure analysis of porous Co-Cr alloy : Co40Cr10Ni50 (50% porosity)

The precursor alloy, Co40Cr10Ni50, was immersed in 1000oC Ca melt for 10 minutes After LMD process, well-connected Ca rich phase was formed in several mm depth Etched in 0.3M nitric acid, the Ca-rich phase disappeared and only Co-Cr alloy phase remains

20 30 40 50 60 70 80 90 100

Co-Cr alloy Ca-rich phase Oxide

(3) after etching

(2) after LMD

Inte

nsit

y (a

.u)

2 thtea (deg.)

(1) (Co3Cr)Ca

5 𝝁𝝁𝒏𝒏

1) SEM image 2) XRD pattern

Porous Co-Cr alloy was successfully fabricated by LMD process

5 ㎛


Porosity control by changing precursor composition

Co Cr Ni Ca

Content(at.%) 68.79 25.79 5.42 0.00

The porosity was successfully controlled by changing Ni content which has negative enthalpy of mixing with Ca.

Very small amount of Ni is still remaining in Co-Cr alloy foam. Because Ni has solubility limit, 7at.%, to Co-Cr alloy, small amount of Ni can be remained. And the ratio between Co and Cr was changed to 3:1 after LMD process

Co Cr Ni Ca

Content(at.%) 66.57 25.77 7.65 0.00

Co Cr Ni Ca

Content(at.%) 68.47 24.30 6.89 0.34

(Co4Cr)75Ni25 (Co4Cr)50Ni50 (Co4Cr)25Ni75

It is possible to control porosity by changing composition of precursor

Relative density : 0.784 Relative density : 0.561 Relative density : 0.320


Surface morphology control by changing etchant – Water etching

• 4 Ca + 10 HNO3= 4 Ca(NO3)2 + N2O + 5 H2O

• Ca + 2 H2O = Ca(OH)2 + H2

𝑲𝑲𝜶𝜶 =𝑨𝑨− 𝑯𝑯𝟑𝟑𝑶𝑶+

𝑯𝑯𝑨𝑨 [𝑯𝑯𝟐𝟐𝑶𝑶]

▶𝑲𝑲𝜶𝜶,𝑯𝑯𝟐𝟐𝑶𝑶 = 𝟏𝟏𝟏𝟏−𝟏𝟏𝟏𝟏

▶𝑲𝑲𝜶𝜶,𝑯𝑯𝟐𝟐𝑶𝑶 = 𝟐𝟐𝟏𝟏

Acid etching

Water etching

Faceted surface

Etchant can affect surface morphologyof Co-Cr alloy foam

Free energy of dealloying

F = 𝜶𝜶 c(1–c) + 𝑲𝑲 ·T[clnc + (1–c)ln(1–c)]

𝛼𝛼 = 6(Ea-b – (1/2)(Ea-a+ Eb-b))

: Meaningless value

: Considerable influence


Hydrophilic test for Co-Cr alloy foam with various pore structure

0

20

40

60

80

75W50W25W75N50N25N

Cont

act a

ngle

(deg

.)

CoCr

DI water droplet (1g)90

𝑯𝑯𝒆𝒆𝑯𝑯𝒏𝒏𝒐𝒐𝒆𝒆𝑯𝑯𝒄𝒄𝒆𝒆𝒄𝒄𝒄𝒄 𝒆𝒆𝒏𝒏𝒐𝒐𝒆𝒆𝒏𝒏𝒏𝒏𝒆𝒆𝒆𝒆

~ 𝟏𝟏/𝒘𝒘𝒏𝒏𝒆𝒆𝒆𝒆𝒄𝒄𝒏𝒏𝒄𝒄 𝒄𝒄𝒏𝒏𝒄𝒄𝒆𝒆𝒏𝒏

Every foam is more hydrophilic than Co-Cr alloy because of its porous structure This is because of not only intrinsic property of Co-Cr alloy but also the porous structure

Biocompatibility can be enhanced by surface morphology of porous structure

Biocompatibility ↑


Osteoblast cultivation on Co-Cr alloy foams

𝟐𝟐𝟏𝟏 𝝁𝝁𝒏𝒏

𝟐𝟐𝟏𝟏 𝝁𝝁𝒏𝒏 𝟏𝟏𝟏𝟏 𝝁𝝁𝒏𝒏

𝟐𝟐𝟏𝟏 𝝁𝝁𝒏𝒏

Co66.6Cr33.3 25W

50W 75W

Co-Cr alloy shows very high bio-compatibility itself – Several cells are well-attached on the surface Every sample with various porosity shows good bio-compatibility which is similar to Co-Cr alloy


Mechanical test : Compressive stress-strain curve

𝑬𝑬 = 𝑬𝑬𝟏𝟏 𝟏𝟏 +𝑨𝑨𝑨𝑨

𝟏𝟏 − 𝑨𝑨 + 𝟏𝟏 𝑨𝑨

Elastic modulus of Co-Cr alloy foam decreased according to increase of porosity Compressive strength of Co-Cr alloy foam was enough high by comparison with Ti alloy foam

Cortical bone

85% Ti foam

50% Ti foam

0 1 2 3 4 5

200

400

600

Com

pres

sive s

tres

s (M

Pa)

Compressive strain (%)

(Co-Cr)75Ni25 (Co-Cr)50Ni50 (Co-Cr)25Ni75


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[email protected] | http://espark.snu.ac.kr

YOUR KIND ATTENTION

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Current status of structural materials : Applications of metallic foams 30

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ESPark Research Group...How can we fabricate HEA foam ? What are the advantages of HEA foam ? 1) Alloy design for fabricating HEA foam : Phase separating HEA 2) Fabricating method