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Direct view of self healing in creep alloys

TU Delft: Haixing Fang, Ekkes Brück, Sybrand van der Zwaag, Niels van Dijk ESRF: Peter Cloetens MPIE: Michael Herbig

Dutch Materials 2018, Utrecht, The Netherlands

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Creep damage in metals:

Isolated

σ σ

Oriented

σ

Micro crack

σ

Macro crack

• time-dependent deformation • occurs for T > 0.4 Tmelting (in K) • under constant load • Damage preferentially nucleate

and grow at grain boundary (GB)

σ

3

mobile solute atoms precipitation at free crack surface

Self healing approach: manage the damage

Cavity nucleates and grows at GB

Supersaturated mobile atoms diffuse towards cavity Precipitation at cavity surface

Precipitation completely close the open cavity

fully filled

partially filled

unfilled

1 μm

Fe-Au Fe-Mo

Self healing model alloys

S. Zhang, Adv. Eng. Mater., 2015. S. Zhang, Metall. Mater. Trans. A, 2016.

Au-depleted zone

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• Could be solutionized at high T

• Supersaturation: wX > wX (sol.)

• Diffusivity: D(X) > D(Fe)

• A tendency to precipitate at free surface

- Atomic radius: R(X) > R(Fe)

- Atomic volume: V(X of prec.) > V(Fe)

Requirements of element X to be healing agent

Schematic phase diagram of potential self healing Fe-X alloy

Tem

pera

ture

(oC

)

Weight % X

BCC

BCC + Precipitate

Super-saturation

Solutionized temperature

Service temperature

5

Preferred X elements for self healing Fe-X alloys

Fe-Cu

Fe-Mo Fe-W

Fe-Au

BCC+Au BCC+Cu

BCC+Fe2Mo

BCC+Fe2W

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0 10 20 30 40-1.5x10-13

-1.0x10-13

-5.0x10-14

0.0

5.0x10-14

0 10 20 30 40-2.0x10-14

-1.5x10-14

-1.0x10-14

-5.0x10-15

0.0

5.0x10-15

0 10 20 30 40-2.0x10-14

-1.5x10-14

-1.0x10-14

-5.0x10-15

0.0

5.0x10-15

0 10 20 30 40-2.0x10-14

-1.5x10-14

-1.0x10-14

-5.0x10-15

0.0

5.0x10-15

Gib

bs e

nerg

y (J

)

Precipitation in matrix Precipitation at free surface

(a) Fe-Cu

Precipitation in matrix Precipitation at free surface

(b) Fe-AuG

ibbs

ene

rgy

(J)

Cluster size (nm)

Precipitation in matrix Precipitation at free surface

(c) Fe-Mo

Cluster size (nm)

Precipitation in matrix Precipitation at free surface

(d) Fe-W

Nucleation at free surface than in the matrix is easier for larger atoms, e.g. T = 550 oC, Fe-(sat.+1) at.% X alloys

Surface precipitation of gold in Fe-Au

W.W. Sun et al, Metall. Mater. Trans. A, 2017.

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Advantages of W as healing agents in steels

• Be able to be tuned into supersaturation state

• Dw > DFe (diffusivity)*

• RW ≈ 1.10 RFe, RFe2W ≈ 1.06 RFe

(atomic radius)

• Less expansive than Au

• Lower neutron activation than Mo

• Widely used in most advanced creep-resistant steels as solid solution strengthening solute

Fe-W

BCC+Fe2W

*C.D. Versteylen et al, Phys. Rev. B, 2017 0 10 20 30 40-2.0x10-14

-1.5x10-14

-1.0x10-14

-5.0x10-15

0.0

5.0x10-15

Fe-W

Gib

bs e

nerg

y (J

)

Cluster size (nm)

Precipitation in matrix Precipitation at free surface

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Surface precipitation without damage (ageing at 700 oC for 30 min)

Precipitation of Laves Fe2W at external surface in Fe-W alloys

Surface precipitation at surface indentation damage (ageing at 600 oC for 140 h)

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0 50 100 150 200 2500.00

0.05

0.10

0.15

0.20

0.25

0 20 40 60 80 100 1200.00

0.05

0.10

0.15

0.20

0.25

(a)

Stra

in

Time (h)

tR = 236 h 0.75tR = 177 h 0.50tR = 118 h 0.25tR = 59 h

σ = 140 MPa

σ = 160 MPa

Stra

in

Time (h)

tR = 104 h 0.50tR = 52 h

(b)

Creep tests of Fe-3.8 wt. % W

Sample T (oC) σ (MPa) tcreep (h) Scanning resolutions

S1 550 140 tR = 236 h 100 nm & 30 nm

S2 550 140 0.75tR = 177 h 100 nm & 30 nm

S3 550 140 0.5tR = 118 h 100 nm & 30 nm

S4 550 140 0.25tR = 59 h 100 nm & 30 nm

S5 550 160 tR = 104 h 100 nm & 30 nm

S6 550 160 0.5tR = 52 h 100 nm & 30 nm

Samples for synchrotron X-ray nano-tomography

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Synchrotron X-Ray Tomography on Fe-4W alloy after creep tests

3D image with a nanometer (30 and 100 nm) resolution

Synchrotron ring Photons Detector Projection images Sample

33.6 keV ω

3D rendering

• ESRF ID16A-NI nano-imaging beamline

• Exposure time: 1 s

• 1800 projections with 3216×3216 pixels

• One measurement renders 3216 slices with 3216×3216 pixels

• 3D rendering and visualization by FEI Avizo 8.1

• Voxel size: 100 nm and 30 nm

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Top

view

Fr

ont v

iew

Si

de v

iew

(a) 0.25 tR (b) 0.50 tR (c) 0.75 tR (d) tR

100 μm 100 μm 100 μm 100 μm

precipitate cavity Creep at 550 oC and 140 MPa

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Sphere Equiaxed Rod Sheet Complex

(b) tR

(a) 0.25tR

0.5 μm 1 μm 1 μm 5 μm 10 μm

0.2 μm 0.5 μm 1 μm 2 μm 10 μm

Identification of cavities in different shapes using complexity (Ω3), elongation (E) and flatness (F)

Sphere + Equiaxed: form isolated; Rod + Sheet + Complex: more probably form by linking with neighbors

H. Fang et al., Acta Mater. 121 (2016) 352.

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Movie showing shape classification of cavities

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By shape classification and comparing cavity size to cavity spacing, we can identify isolated and linked cavities.

High number density of non-linked cavities

Low volume fraction of non-linked cavities

Linked cavities fulfill two criteria: 1) Shape is rod, sheet or complex 2) Major axes of cavity > λcavity

Coalescence of cavity increases by one magnitude at failure

40 80 120 160 200 2401E-7

1E-6

1E-5

1E-4

1E-3

40 80 120 160 200 240

40 80 120 160 200 2401E-7

1E-6

1E-5

1E-4

1E-3

0.01

40 80 120 160 200 240

Num

ber d

ensi

ty (µ

m-3)

Sphere Equiaxed Rod Sheet Complex Total

(a) Number density (b) Number density

Total Isolated Linked

(c) Volume fraction

Vol

ume

fract

ion

Creep time (h)

Sphere Equiaxed Rod Sheet Complex Total

(d) Volume fraction

Creep time (h)

Total Isolated Linked

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(a) (b) (c) (d)

(e) (f) (g) (h)

(i) (j) (k) (l)

precipitate cavity

Top

view

Fr

ont v

iew

Si

de v

iew

V = 0.20909 μm3

FR = 0.372 V = 0.64973 μm3

FR = 0.027 V = 1.07654 μm3

FR = 0.055 V = 1.32538 μm3

FR = 0.392

Partial filling cavities in S3_140MPa_0.5tR

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(a) (b) (c) (d)

(e) (f) (g) (h)

(i) (j) (k) (l)

precipitate cavity

Top

view

Fr

ont v

iew

Si

de v

iew

V = 1.60272 μm3

FR = 0.209 V = 8.72035 μm3

FR = 0.180 V = 938.68800 μm3

FR = 0.017

Partial filling cavities in S1_140MPa_tR

V = 44.63120 μm3

FR = 0.087

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Filling ratio as a function of particle ( = cavity+precipitate) volume

T = 550 oC σ = 140 MPa

0.0

0.2

0.4

0.6

0.8

1.0

10-3 10-2 10-1 100 101 102 103 104

0.0

0.2

0.4

0.6

0.8

1.0

10-3 10-2 10-1 100 101 102 103 104

(a) 0.25tR

Isolated Linked

Filli

ng ra

tio(b) 0.50tR

Isolated Linked

(c) 0.75tR

Isolated Linked

Filli

ng ra

tio

Particle volume (µm3)

(d) tR

Isolated Linked

Particle volume (µm3)

isolated cavities

linked cavities

Yellow region: filling ratio continuously increases until fully filled. Blue region: filling ratio increases to some extent and then drops down.

Resolution limit

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Filling ratio as a function of particle ( = cavity+precipitate) volume

T = 550 oC

σ = 160 MPa

0.0

0.2

0.4

0.6

0.8

1.0

10-3 10-2 10-1 100 101 102 103 104

0.0

0.2

0.4

0.6

0.8

1.0

Isolated Linked

Filli

ng ra

tio

(a) 0.50tR

(b) tR

Isolated Linked

Filli

ng ra

tio

Particle volume (µm3)

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Conclusions

Creep cavities can be filled autonomously by precipitation of supersaturated solute in bcc iron matrix

Self healing is most efficient in Fe-Au, but Fe-W provides a promising (and technologically more relevant) alternative for self healing alloys

The filling behavior for isolated and linked cavities is distincly different

X-ray nanotomography provides a unique insight in the damage formation and healing operation in these alloys

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Aknowledgements

TU Delft: Nikodem Szymanski, Shasha Zhang, Casper Versteylen, Youp van Goozen, Gijs Langelaan, Frans Tichelaar,

Kees Kwakernaak, Hans Brouwer, Wim Sloof PSI: Joachim Kohlbrecher ESRF: Peter Cloetens, Yang Yang MPIE: Shanoob Balachandran Nair, Margarita Kuzmina,

Michael Herbig, Dierk Raabe Funding: Collaboration: