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Improved Fracture Behaviour of Thin Tungsten Foils

Vladica Nikolić 1*, Stefan Wurster 2, Reinhard Pippan1

1 Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Leoben, Austria 2 Department of Materials Physics, Montanuniversität Leoben, Austria

I – Motivation II – Why tungsten?

III – Results & Discussion

IV – Outlook References

• Future fusion reactors are associated with challenging

operating conditions which will require the utilization of

advanced materials with extreme mechanical, heat and

neutron load capacities.

• Tungsten and tungsten based materials have been

investigated in the last few years as potential candidates for

divertor and plasma facing components.

• Tungsten is a metal with an interesting combination of many advantageous properties, which

makes it a premium candidate for high temperature applications. W can be used as :

structural material

armour material

LOW DBTT HIGH FRACTURE TOUGHNESS

• Problem?! Its inherent brittleness. A promising ductilization strategy is the synthesis of:

Composite materials W laminates assembled from layers of thin tungsten foils (superior fracture behaviour!) [2]

• 99.97% pure tungsten foil, with a thickness of 100μm.

• Elongated grains along the rolling direction (RD).

• Pronounced texture with an orientation in (100)<011>.

ITER Tokamak, Cadarache (France) [1]

Investigated Material Sample Preparation Testing Procedure

• Crack initiation was done in the following steps:

• Samples were vacuum brazed (at 780°C and 1100°C)

to steel holders using AgCu and Cu foils.

• Experiments were conducted taking into

account several parameters:

testing direction ∥ , ⊥, 45° to RD

testing temperature -196°C to 800°C

testing speed 0.4mm/min, 20mm/min

𝐾𝑞 =𝐹

𝐴× 𝜋 𝑎 × 𝐹𝐼(𝛼)

This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect

those of the European Commission or those of the European Commission.

• Future experiments:

Fracture experiments of potassium doped WVM.

Annealed materials for investigating the fracture behaviour of recrystallized W foils.

Two additional crack systems, with crack propagating perpendicular to the foil plane and within

the foil plane.

taken from [3]

* corresponding author: vladica.nikolic@oeaw.ac.at

II – Experimental work

-196ºC

RT

200ºC

400ºC

600ºC

800ºC

K=16,4

Temperature [°C]

diamond – wire saw

razor blade

FIB

• Fracture mechanical tests were performed on an universal tensile

testing device (Zwick). Testing at 400°C and above was performed

in vacuum. Steel holder

Tungsten foil

Pre – crack

• The stress intensity factor K is used in fracture mechanics to predict the stress state near the tip of a crack caused by loading.

Conditional fracture toughness values Kq were calculated from the maximum force.

Fracture Toughness

-196°C

200°C

400°C

600°C

RT

800°C

Fracture Surfaces

Kq [

MP

am1

/2]

CTOD Measurements

𝐾𝑞~ 𝐶𝑇𝑂𝐷 × 𝜎𝑌 × 𝐸

20µm

• An improved fracture behaviour Higher degree of deformation leads to higher

toughness.

• Ductile – brittle transition temperature (DBTT) of investigated crack system is

expected to be at RT.

• Experimental results are in correlation with tensile experiments [4] of the same

material.

• Decrease in toughness at high temperatures attributable to increase in in-plane

toughness?

[1] www.efda.org

[2] J. Reiser et al, J. Nucl. Mat 423 (2012) 1-8

[3] Y. Murakami, Stress Intensity Factors

Handbook, Volume 1, Pergamon Press

[4] J. Reiser et al, J. Nucl. Mat 434 (2012) 357-

366

[5] T.L.Anderson, Fracture mechanics, 2nd

edition, CRC Press

brittle fracture (cleavage)

F - maximum load A - area

a - crack length FI - geom. factor

α = a/W W - width

delamination

K=64,3

K=77,5

K=66,7

K=54,7

K=41,5

delamination

delamination ductile

ductile fracture

ductile fracture

10µm

10µm

10µm

10µm

10µm

10µm

• The crack tip opening displacement (CTOD) is a parameter used to determine fracture toughness of materials that show some plastic deformation

before failure occurs, causing the tip to stretch open.

E - Young‘s modulus

σY - tensile strength

• A stereophotogrammetric reconstruction of fracture surfaces

done by analyzing scanning electron microscope (SEM)

images.

• Digital Elevation Models (DEMs) of the investigated surface

degree of deformation of the metallic fracture surface.

taken from [5]

5 µm 5 µm