Ductilisation of tungsten through cold rolling: Change of brittle to ductile transition temperature in highly deformed tungsten

Carsten Bonnekoh, Jens Reiser, Simon Bonk, Jan Hoffmann

INSTITUTE OF APPLIED MATERIALS, APPLIED MATERIALS PHYSICS, HIGH TEMPERATURE MATERIALS, CAMPUS NORTH, KIT SFB-TR 103 YOUNG RESEARCHERS INTERACTION WEEK, IRSEE, 06.12.2016

KIT – The Research University in the Helmholtz Association www.kit.edu Outline

Improvements through severe cold rolling

Theory of BDT

Motivation

Microstructure Inverse pole figure maps ODF maps Grain boundary character

Mechanical testing Influence of specimen thickness Influence of microstructure

Summary

2 07.12.2016 Carsten Bonnekoh Institute for Applied Materials, Campus North, KIT Improvements through severe cold rolling

BCC metal with melting temperature of 3422 °C High heat conductivity, high temperature strength and low thermal expansion coefficient Tungsten (W) - perfect material for high temperature vacuum applications

[1] [2] Poor oxidation resistance Brittle fracture at ambient temperature Not in use as structural material, only applied as functional material nowadays

[1] Wendelstein X7 Newsletter [2] www.euro-fusion.org

3 07.12.2016 Carsten Bonnekoh Institute for Applied Materials, Campus North, KIT Improvements through severe cold rolling

20 °C 20 °C

Hot rolled Severely cold rolled Coarse grained W Ultra-fine grained W

4 07.12.2016 Carsten Bonnekoh Institute for Applied Materials, Campus North, KIT Improvements through severe cold rolling

2000 [3] 20 °C

1500

1000 tests at room temperature

500 rolled at 600°C

Engineering stress [MPa] rolled at 1000°C hot rolled 0 Severely cold rolled 0 1 2 3 4 5 Ultra-fine grained W Engineering strain [%]

Tensile test: Ductility and yield strength improved

[3] Reiser, J. et al.: Ductilisation of tungsten (W): On the increase of strength and room-temperature tensile ductility through cold-rolling

5 07.12.2016 Carsten Bonnekoh Institute for Applied Materials, Campus North, KIT Improvements through severe cold rolling

200 [4] 180 W plate, 1 mm 160

] 140

1/2 120

100 tests at

[MPa m 80 room temperature

IQ

K 60 40 20 a 0 W 0 1 2 3 4 a [mm]

Tensile test: Ductility and yield strength improved Fracture behavior: Stable crack growth at room temperature achieved

[4] Reiser, J. et al.: Ductilisation of tungsten (W) through cold-rolling: R-curve behaviour

6 07.12.2016 Carsten Bonnekoh Institute for Applied Materials, Campus North, KIT Improvements through severe cold rolling

1,5 [5]

1,0

0,5

Charpy Energy [J]

0,0 0 200 400 600 800 1000 Temperature [°C]

Tensile test: Ductility and yield strength improved Fracture behavior: Stable crack growth achieved Charpy tests: BDTT shifted to lower temperatures

[5] Reiser, J. et al.: Ductilisation of tungsten (W): On the shift of the brittle-to-ductile transition (BDT) to lower temperatures through cold rolling

7 07.12.2016 Carsten Bonnekoh Institute for Applied Materials, Campus North, KIT Improvements through severe cold rolling

1,5

1,0 Outlet

Inlet 0,5

Charpy Energy [J]

0,0 0 200 400 600 800 1000 100 mm H. Greuner, IPP Temperature [°C]

3 x 4 x 27 mm3

8 07.12.2016 Carsten Bonnekoh Institute for Applied Materials, Campus North, KIT Theory of BDT

BCC metals exhibit two kinds of fracture Low energy fracture – brittle High energy fracture – tough Competition between critical resolved shear stress and cleavage stress Mobility of ⟨111⟩ screw dislocation depends on temperature, loading rate Cleavage stress independent on temperature If CRSS reaches cleavage stress, transition in fracture behaviour

9 07.12.2016 Carsten Bonnekoh Institute for Applied Materials, Campus North, KIT Theory of BDT

BCC metals exhibit two kinds of fracture Low energy fracture – brittle High energy fracture – tough Competition between critical resolved shear stress and cleavage stress Mobility of ⟨111⟩ screw dislocation depends on temperature, load rate Cleavage stress independent on temperature If CRSS reaches cleavage stress, transition in fracture behavior Activation energy for BDT can be calculated

푄 퐾 = 퐴 exp (− 퐵퐷푇 ) kB 푇퐵퐷푇

[6]

[6] Hartmaier, A. et al.: On the Activation Energy for the Brittle/Ductile Transition

10 07.12.2016 Carsten Bonnekoh Institute for Applied Materials, Campus North, KIT Motivation

Observation: UFG microstructure affects strain rate sensitivity of BDT Hypothesis: Change of controlling mechanism

coarse grained [7] ultra-fine grained [7]

{

Question: Identification of controlling mechanism of BDT in ultra-fine grained W Methods: Indirect by K-tests; direct via electron microscopy

[7] Németh, A. et al.: The nature of the brittle-to-ductile transition of ultra fine grained tungsten (W) foil

11 07.12.2016 Carsten Bonnekoh Institute for Applied Materials, Campus North, KIT Material

Five W sheets produced exclusively at PLANSEE SE, Reutte, Austria Processing through hot and cold rolling out of one single sintered compact Sheet thickness s /mm 1.0 0.5 0.3 0.2 0.1

Degree of cold work φ /- 1.8 2.5 3.0 3.4 4.1 CR

Extremely high degree of deformation through cold-rolling Five degrees of deformation causing Five sheet thicknesses Five microstructures

No further heat treatment after rolling applied

12 07.12.2016 Carsten Bonnekoh Institute for Applied Materials, Campus North, KIT Outline

Improvements through severe cold rolling

Theory of BDT

Motivation

Microstructure Inverse pole figure maps ODF maps Grain boundary character

Mechanical testing Influence of specimen thickness Influence of microstructure

Summary

13 07.12.2016 Carsten Bonnekoh Institute for Applied Materials, Campus North, KIT Summary

Tungsten: Designated material for plasma facing components in fusion devices but inherently brittle up to 400 °C

Motivation: Dramatic improvement of its mechanical properties through cold rolling

Goal: Identification of mechanism causing BDTT in ultra-fine grained tungsten

Experimental: Five cold rolled sheets, five microstructures made of same sintered compact

Result 1: Grain refinement down to 300 nm Result 2: Alpha and gamma fiber, pronounced rotated cube orientation Result 3: BDTT less dependent of specimen thickness Result 4: BDTT shift of 500 K downwards to -100 °C for the 0.1 mm UFG W

14 07.12.2016 Carsten Bonnekoh Institute for Applied Materials, Campus North, KIT Thank you for your attention

The author is grateful to: PLANSEE SE, University of Oxford, Erich Schmid Institute of Materials Science, DFG RE3551/4-1

15 07.12.2016 Carsten Bonnekoh Institute for Applied Materials, Campus North, KIT