Member of the Helmholtz Association Takeshi Hirai | Institute of Energy Research | Association EURATOM – FZJ 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

Takeshi Hirai | Institute of Energy Research | Association EURATOM – FZJ26 May 2008No 2 1 mm T 0 = RTweight loss = 0.5 mgT 0 = °Cweight loss = 4.0 mg E inc = 2.3 MJm -2,  t = 1.8 ms, (P ins = 1.3 GW/m 2 ) 10 shots at JEBIS J. Linke et al., presented in ICFRM-10, Baden Baden Germany Cracking and melting of W under thermal shock loads Thermal shock response of W

Takeshi Hirai | Institute of Energy Research | Association EURATOM – FZJ26 May 2008No 3 Cracking Power density Melting boiling Thermal load Cracking threshold Melting threshold Safe operationCrack propagation Re-crystallization Melting, re-solidification, Irregular shape, Melt-layer loss, boiling Melting threshold: related to thermal properties (e.g. D thermal ), melting point (T m ) Cracking threshold: related to thermal properties (e.g. D thermal,  ), mechanical properties, loading conditions (strain rate ~  T.  /  t) D thermal : thermal diffusivity,  : thermal expansion,  t: pulse duration Thermal shock loads on metals

Takeshi Hirai | Institute of Energy Research | Association EURATOM – FZJ26 May 2008No 4 Cracking Power density Melting Thermal load (120 keV e-beam) W: ca. 5 um Cracking threshold Melting threshold Safe operationCrack propagation Thermal fatigue Melting, re-solidification T_(  thermal /  yield >1) normalized stress (thermal stress/yield stress) > 1 bulk temp.Max surface temp. T surf in thermal shock loads T m = 3400 o C DBTT (300 ~ 600 o C depending on strain rate) bulk temp.Max surface temp. bulk temp.Max surface temp. bulk temp.Max surface temp. Thermal shock loads on W materials Recrystallization

Takeshi Hirai | Institute of Energy Research | Association EURATOM – FZJ26 May 2008No 5 Aims: Examine W cracking failure under single pulse by using (i) power density (  T) and (ii) bulk temperature (T 0 ) as the parameters Find safe operation range of the W grade under this condition 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; o C, depending on strain rate At temperature, thermal stress/yield stress > 1 Recrystallization temperature ~ 1300 o C Melting point; 3400 o C Aims of the work

Takeshi Hirai | Institute of Energy Research | Association EURATOM – FZJ26 May 2008No 6 Outline (1)Introduction/Motivation (2)Thermal shock tests in electron beam facility (3)Results: Cracking of tungsten (4)Summary

Takeshi Hirai | Institute of Energy Research | Association EURATOM – FZJ26 May 2008No 7 Activated samples, Be samples T <100 GBq as gas, <250 GBq in bulk Electron beam facility, JUDITH, FZJ

Takeshi Hirai | Institute of Energy Research | Association EURATOM – FZJ26 May 2008No Bulk temperature (°C) Power density (GW.m -2 ) DBTT  T=1697 o C  T= 1131 o C  T= 606 o C  T = 2P.t 0.5 /( ..c.  ) 0.5 A B C Materials and Loading conditions Loading conditions: Pulse duration 5 ms, single shot, loading area 16 mm 2 Materials: ITER-reference W grade, deformed tungsten from Plansee Ф12 mm, 5 mm thick Grain diameter: ~20 µm ΔTΔT T0T0 200 um Heat flux cross section

Takeshi Hirai | Institute of Energy Research | Association EURATOM – FZJ26 May 2008No 9 A 0.43 GW/m 2, 5ms at 200 o C W cracking; major cracks, micro-crack network

Takeshi Hirai | Institute of Energy Research | Association EURATOM – FZJ26 May 2008No 10 B 0.7 GW/m 2, 5 ms at 200 o C W cracking: major cracks, discontinuing cracks

Takeshi Hirai | Institute of Energy Research | Association EURATOM – FZJ26 May 2008No 11 C 0.43 GW/m 2, 5 ms at 600 o C W cracking: No major cracks, micro-crack network

Takeshi Hirai | Institute of Energy Research | Association EURATOM – FZJ26 May 2008No 12  T= 606 o C  T= 1131 o C  T=1697 o C Bulk temperature ( o C) Major cracks, microcracks and surface modification ΔTΔT T0T0

Takeshi Hirai | Institute of Energy Research | Association EURATOM – FZJ26 May 2008No 13  T= 606 o C  T= 1131 o C  T=1697 o C Bulk temperature ( o C) Major cracks, microcracks and surface modification No cracks, surface modification Major cracks Microcracks ΔTΔT T0T0

Takeshi Hirai | Institute of Energy Research | Association EURATOM – FZJ26 May 2008No Microcracks 1. Major cracks 3. Surface modification W cracking under single pulse P [GW/m 2 ] T 0 [ o C] No cracks, Surface modification Major cracks Microcracks Threshold temperature  brittleness of the material below DBTT Threshold power density  thermal stress > yield strength Only at high temperature  recrystallization of surface layer 4. Surface elevation Safe operation of the W grade 5 ms

Takeshi Hirai | Institute of Energy Research | Association EURATOM – FZJ26 May 2008No 15 No clear dependence on power density  related material constant such as grain size Crack distance Mean crack distance Mean crack distance [µm] Power density [MW/m 2 ] Microcracks at 200 o C Microcracks at 400 o C Microcracks at 600 o C Microcrack networks No cracks Discontinuing networks No cracks Tensile stresses  rupture at G.B. 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

Takeshi Hirai | Institute of Energy Research | Association EURATOM – FZJ26 May 2008No 16 Mean microcrack width ~ 1 um Maximum major crack width at 0.4 GW/m 2 at 200 o C Crack width Mean crack width Crack width [µm] Power density [MW/m 2 ] Major cracks at 200 °C Microcracks at 200 °C Microcracks at 400 °C Microcracks at 600 °C 0.4 GW/m 2 5 ms 200 o C Major cracks Microcracks

Takeshi Hirai | Institute of Energy Research | Association EURATOM – FZJ26 May 2008No 17 Maximum elevation at 0.4 GW/m 2 at 200 o C; same tendency as the crack width  Contribution plastic deformation, extending to the free surface Maximum at 200 o C and decease at higher temperatures  Contribution from thermal vacancies is not dominant Height Surface elevation height 0.4 GW/m 2 5 ms 200 o C

Takeshi Hirai | Institute of Energy Research | Association EURATOM – FZJ26 May 2008No 18 Outline (1)Motivation (2)Thermal shock tests in electron beam facility JUDITH (3)Results: Cracking of tungsten (4)Summary

Takeshi Hirai | Institute of Energy Research | Association EURATOM – FZJ26 May 2008No 19 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: >200 o C, < 0.28 GW/m 2 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). Summary

Takeshi Hirai | Institute of Energy Research | Association EURATOM – FZJ26 May 2008No 20 Thank you for your attention