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Development of chromium for mid-flux region PFCs for DEMO divertor

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Development of chromium for mid-flux region PFCs

for DEMO divertor

T. Khvan 1,2,*, D. Terentyev 1,**, J.-H. You 3 & N. Van Steenberge 4

1 Belgian Nuclear Research Centre, SCK•CEN, Mol, 2400, Belgium

2 University of Liège, Place du 20 Août 7, 4000 Liège, Belgium

3 Max-Planck-Institute für Plasmaphysik, 85748, Garching, Germany

4 OCAS N.V., Pres J.F. Kennedylaan 2, 9060 Zelzate, Belgium

First e-mail: tkhvan@sckcen.be Second e-mail: dterenty@sckcen.be

Introduction

Designing of plasma-facing components (PFC) for DEMO divertor unravel new challenges to be met by the in-vessel materials. For designing mid heat flux PFCs, chromium (Cr) is currently considered as material for the main body connected to tungsten armour tile and cooling pipe.

Objectives

The main objective was to test and investigate mechanical properties as well as microstructure of pure Cr and Cr-10W alloy produced in Vacuum Arc Melter and compare them with commercial high purity Cr produced by PlanseeTM with sintering and

mechanical treatment.

• 3-point bending flexural test for determination of ductile to brittle transition region. • Vickers hardness and nanoindentation. Estimation of σ0.2 as H ≈ 3σ0.2.

• Fracture surface analysis with SEM and grain pattern analysis with EBSD.

Materials and methods

Results

Conclusions

• As-casted pure Cr (without any further treatment) produced by VAM has shown very similar mechanical properties to pure Cr of Plansee, which was produced by sintering and rolled into plate. The DBTT of VAM Cr is around room temperature.

− Further thermo-mechanical treatment applied to VAM Cr should considerably improve its properties in terms of strength increase and DBTT reduction. − The Vacuum Arc Melter produces enough volume of chromium to fabricate a monoblock.

− Due to the large grain size, the main fracture mechanism of VAM Cr is transgranular cleavage

• As-casted Cr-10W produced by VAM has shown much higher strength than pure Cr, capacity for significant work-hardening and DBTT around 250°C. − EBSD examination reveals the presence of W-rich particles, which implies that perfect solid solution was not reached.

− Fracture mode is a mixture of trans- and intergranular cleavage.

Mechanical properties

SCK•CEN || Boeretang 200 || BE-2400 Mol || www.sckcen.be || info@sckcen.be || Posternr:

Arc Melter system

The Arc Melter

An innovative solution acquired by OCAS

TM

which

uses principles of an electric arc furnace in

combination with induction furnace:

• Allows on to melt W and Cr in order to create

solid solution, instead of sintering;

• Vacuum chamber significantly reduces oxidation;

• Induction furnace involved in a melting process

helps to cool down material slower, ensuring

homogeneity of microstructure and mechanical

properties.

Chromium flakes and tungsten lumps were

used to produce tested materials.

Chromium flakes

(99,97% purity)

Tungsten lumps

(99,99% purity)

Results

Microstructure

0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 0 2 4 6 8 10 Hard ne ss (G Pa)

Displacement Into Surface (um)

IGP tungsten VAM Cr VAM Cr-10% W 0 100 200 300 400 500 0 2 4 6 8 10 12 14 16 18 20 22 24 26 Plansee Cr VAM Cr VAM Cr-10% W IGP-L IGP-T Fle xur al str ain (%) Test temperature (oC) 5% 0,00 0,05 0,10 0,15 0 200 400 600 800 1000 Fle xur al str ess ( MPa) Flexural strain 150oC 200oC 250oC 300oC 350oC 500oC 0,00 0,05 0,10 0,15 0 200 400 600 800 1000 Fle xur al str ess ( MPa) Flexural strain RT 50 oC 75 oC

Plansee Cr VAM Cr VAM Cr-10% W

Nanoindentation DBTT derivation 0,00 0,05 0,10 0,15 0,20 0,25 0,30 0 100 200 300 400 500 600 700 800 900 1000 1100 Fle xur al str ess ( MPa) Flexural strain VAM Cr VAM Cr-10% W Plansee Cr IGP-T Tungsten 0,00 0,05 0,10 0,15 0,20 0,25 0,30 0 100 200 300 400 500 600 700 800 900 1000 1100 Fle xur al str ess ( MPa) Flexural strain VAM Cr VAM Cr-10% W Plansee Cr IGP-T Tungsten 0,00 0,05 0,10 0,15 0,20 0,25 0,30 0,35 0 100 200 300 400 500 600 700 800 900 1000 1100 Fle xur al str ess ( MPa) Flexural strain VAM Cr VAM Cr-10% W Plansee Cr

Flexural σ0.2 vs Test temperature Flex. stress vs flex. strain at RT Flex. stress vs flex. strain at 300℃ Flex. stress vs flex. strain at 500℃

Materials investigated in this work are on the first stage of

future improvements by thermo-mechanical treatments.

Cr has better toughness property in the low temperature range where the commercial tungsten (W) products are brittle. Cr is an excellent material to reduce the oxidation in case of loss-of-vacuum accident. Cr does not experience such harsh transmutation due to thermal neutrons unlike transmutation of Re and Os in tungsten. However, the mechanical properties of Cr are extremely sensitive to purity. Here, we employ vacuum arc melting (VAM) equipment for fabrication of Cr and Cr-W alloy suitable for PFCs. VAM represents new promising alternative route with a high upscale potential. VAM fabrication might improve Cr quality by avoiding the introduction of interstitial impurities, while the produced grades can be further mechanically treated to design dedicated microstructure and enhance the yield strength and toughness.

Material [mass %]Cr [mass %]W [mass %]Fe [mass %]O O in raw material

VAM Cr 99,5 0,0025 0,005 0,003

VAM

Cr-10% W 90,50 9,1 0,1 0,07 0,003

Plansee Cr 99,8 0,078 0,05

Results

Chemical analysis by XRF and O contamination

Cr sample produced

after melting

(271 g)

Material Flexural σ0.2 at 300oC , MPa Flexural σ0.2 at 500oC , MPa VH σ0.2 ≈ 3VH, MPa NI hardness, GPa T5%in 3PB test, oC VAM Cr 203 164 132 396 2,6 27 Plansee Cr 224 152 151 453 TBD* 27 VAM Cr-10%W 339 328 263 789 4 256 IGP-T Tungste n Brittle 666 460 1380 6,8 405 IGP-L Tungste n TBD* TBD* 460 1380 6,8 162 0 100 200 300 400 500 200 400 600 800 1000 1200 1400 Fle xur al s0.2 (MPa ) Test Temperature (oC) VAM Cr Cr VAM Cr-10% W Plansee IGP-L Tungsten IGP-T Tungsten Expected s0.2 VAM Cr at RT Expected s0.2 VAM Cr-10% W at RT Expected s0.2 Plansee Cr at RT 0,00 0,02 0,04 0,06 0,08 0,10 0,12 0,14 0 200 400 600 800 1000 Fle xur al str ess ( MPa) Flexural strain RT 50 oC 75 oC VAM Cr-10% W

Avg. grain size ≈ 272,46 µm

VAM Cr

Avg. grain size ≈ 522,48 µm

VAM Cr-10% W 200oC 300oC Plansee Cr RT 75oC VAM Cr RT 75oC VAM Cr-10% W (x300) Possible precipitates of W TBD* - To be determined

Références

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