T. Hirai, M. Batilliot, J. Linke, G. Pintsuk Forschungszentrum Jülich, Euratom Association, Jülich

1. Cracking of a tungsten material exposed to single pulse thermal shock loads at elevated temperatures T. Hirai, M. Batilliot, J. Linke, G. Pintsuk Forschungszentrum Jülich, Euratom Association, Jülich Outline (1) Motivation (2) Thermal shock tests in electron beam facility JUDITH (3) Results: Cracking of tungsten (4) Summary

2. Thermal shock response of W 1 mm 1 mm Cracking and melting of W under thermal shock loads Einc = 2.3  MJm-2, t = 1.8 ms, (Pins = 1.3 GW/m2) 10 shots at JEBIS T0 = RT weight loss = 0.5 mg T0 = 650 - 700°C weight loss = 4.0 mg J. Linke et al., presented in ICFRM-10, Baden Baden Germany 2001.

3. Thermal shock loads on metals boiling Melting Cracking • Melting threshold: related to thermal properties (e.g. Dthermal),melting point (Tm) • Cracking threshold: related to thermal properties (e.g. Dthermal, a), mechanical properties, loading conditions (strain rate ~ DT.a/Dt) Dthermal: thermal diffusivity, a: thermal expansion, Dt: pulse duration Safe operation Crack propagation Re-crystallization Melting, re-solidification, Irregular shape, Melt-layer loss, boiling Thermal load Cracking threshold Melting threshold Power density

4. Thermal shock loads on W materials Melting Cracking Safe operation Crack propagation Thermal fatigue Melting, re-solidification Thermal load (120 keV e-beam) Cracking threshold Melting threshold W: ca. 5 um Power density Recrystallization Tm= 3400 oC Tsurfin thermal shock loads DBTT (300 ~ 600 oC depending on strain rate) T_(sthermal/syield>1) normalized stress (thermal stress/yield stress) > 1 bulk temp. Max surface temp. bulk temp. Max surface temp. bulk temp. Max surface temp. bulk temp. Max surface temp.

5. Aims of the work • Cracking of W materials is important • Cracking threshold is lower than melting threshold • Cracking may cause fatal destruction of brittle materials • Single pulse is advantageous for understanding • Single shot tests: simple to model by original & heat treated material parameters • Multiple shot tests: need to consider dynamic material modification, hardening • W has 4 characteristic temperatures • DBTT; 300 - 600 oC, depending on strain rate • At temperature, thermal stress/yield stress > 1 • Recrystallization temperature ~ 1300 oC • Melting point; 3400 oC • Aims: • Examine W cracking failure under single pulse by using (i) power density (DT) and (ii) bulk temperature (T0) as the parameters • Find safe operation range of the W grade under this condition

6. Outline • Introduction/Motivation • Thermal shock tests in electron beam facility • Results: Cracking of tungsten • Summary

7. Electron beam facility, JUDITH, FZJ Activated samples, Be samples T <100 GBq as gas, <250 GBq in bulk

8. Materials and Loading conditions B A C Materials: ITER-reference W grade, deformed tungsten from Plansee Ф12 mm, 5 mm thick Grain diameter: ~20 µm cross section Heat flux 200 um Loading conditions: Pulse duration 5 ms, single shot, loading area 16 mm2 0.90 0.80 0.70 DT = 2P.t0.5/(p.l.c.r)0.5 0.60 0.50 Power density (GW.m-2) DT=1697 oC 0.40 DT= 1131 oC 0.30 0.20 DT= 606 oC ΔT 0.10 DBTT 0.00 200 400 600 800 0 100 300 500 700 900 T0 Bulk temperature (°C)

9. W cracking; major cracks, micro-crack network A 0.43 GW/m2, 5ms at 200 oC

10. W cracking: major cracks, discontinuing cracks B 0.7 GW/m2, 5 ms at 200 oC

11. W cracking: No major cracks, micro-crack network C 0.43 GW/m2, 5 ms at 600 oC

12. Major cracks, microcracks and surface modification DT=1697 oC DT= 1131 oC DT= 606 oC Bulk temperature (oC) ΔT T0

13. Major cracks, microcracks and surface modification No cracks, surface modification Major cracks DT=1697 oC Microcracks DT= 1131 oC DT= 606 oC Bulk temperature (oC) ΔT T0

14. W cracking under single pulse 0.8 5 ms No cracks, Surface modification 0.6 Major cracks P [GW/m2] 0.4 Microcracks 0.2 Safe operation of the W grade 0 0 200 400 600 800 T0 [oC] 1. Major cracks Threshold temperature  brittleness of the material below DBTT 2. Microcracks Threshold power density  thermal stress > yield strength 3. Surface modification Only at high temperature  recrystallization of surface layer 4. Surface elevation

15. Mean crack distance Crack distance Microcracks at 200 oC Microcracks at 400 oC Microcracks at 600 oC Discontinuing networks Mean crack distance [µm] Microcrack networks No cracks No cracks Power density [MW/m2] • No clear dependence on power density • related material constant such as grain size Microcrack formation 1. Plastic deformation at heating phase 2. Generation of tensile stress in cooling phase 3. Rupture at grain boundary due to the tensile stress Tensile stresses  rupture at G.B.

16. Mean crack width Major cracks at 200 °C Microcracks at 200 °C Microcracks at 400 °C Microcracks at 600 °C Crack width 0.4 GW/m2 5 ms 200oC Crack width [µm] Major cracks Microcracks Power density [MW/m2] Mean microcrack width ~ 1 um Maximum major crack width at 0.4 GW/m2 at 200 oC

17. Surface elevation height 0.4 GW/m2 5 ms 200oC Height • Maximum elevation at 0.4 GW/m2 at 200 oC; same tendency as the crack width • Contribution plastic deformation, extending to the free surface • Maximum at 200oC and decease at higher temperatures •  Contribution from thermal vacancies is not dominant

18. Outline • Motivation • Thermal shock tests in electron beam facility JUDITH • Results: Cracking of tungsten • Summary

19. Summary • By using deformed W grade, crack appearance under single pulse thermal shock tests were studied in the electron beam facility JUDITH. • Two kinds of cracks: (i) major cracks, i.e. large macroscopic cracks running over the loaded area with a low crack density; (ii) microcracks, i.e. cracks appearing between major cracks and often creating a network • Major cracks were caused by brittleness of the material at the temperature • Microcracks were formed by: (1) Plastic deformation at heating phase; (2) Generation of tensile stress in cooling phase; (3) Rupture at grain boundary due to the tensile stress. The less developed micro-crack at high power density due to reduction of elastic modulus at the peak temperature. • Safe operation condition of the grade under 5 ms loads: >200oC, < 0.28 GW/m2 as far as crack initiation is concered • Crack growth rates of those cracks are important and need to be studied(multiple shot thermal shock loads).

20. Thank you for your attention