1. Markus Aicheler 18.02.2011 CERN Markus Aicheler, Ruhr-University Bochum and CERN “Surface phenomena associated with thermal cycling of copper and their impact on the service life of particle accelerator structures”

2. Markus Aicheler 18.02.2011 CERN - Introduction into the project in the frame of CLIC - Main goals of the PhD thesis - Experimental: Material and Fatigue devices - Discussion of 3 results - Hardening threshold of Cu [100] single crystal - Orientation dependent cyclic roughening - Orientation dependent cyclic hardening/roughening - Summary and Conclusion Outline of the talk

3. Markus Aicheler 18.02.2011 CERN Introduction: CLIC surface heating phenomenon CLIC (Compact Linear Collider) two beam scheme: Electron – positron collider at center-of-mass energy of 3 TeV (LHC: 7 TeV but nonelementar head on collisions)

4. Markus Aicheler 18.02.2011 CERN CLIC accelerating structure (AS): Shape accuracy ± 2.5 µm Roughness Ra 0.02 µm Very high conductivity material Introduction: CLIC surface heating phenomenon Assembly by: brazing bolting

5. Markus Aicheler 18.02.2011 CERN Pulsed magnetic field induces currents (200 ns, repetition rate 50 Hz) Superficial Joule heating for electrical conductivity of copper: ΔT ≈ 60 K  cyclic heating- and cooling phases (biaxial strain)  thermal fatigue with σ ≈ 0 MPa to 150 MPa (comp.)  skin depth several µm  surface roughness degrades operation conditions  “functional fatigue” Introduction: CLIC surface heating phenomenon Estimated CLIC life time 2 x 10 10 cycles @ 50Hz (= 20 years of operation) => No mean to test a “real” structure under “real” conditions for whole life time! Surface a) magnetic and b) electric field distribution in CLIC AS cell a)b)

6. Markus Aicheler 18.02.2011 CERN Main goals of the thesis - understand the basic mechanism of fatigue observed when low loads induced by very superficial cyclic heating are applied to copper alloys - put them in relation with the conventional fatigue induced by bulk cyclic loads - determine if superficial pulsed laser and bulk ultrasonic fatigue tests may be extrapolated for selection of a best candidate material for the application to CLIC structures “Study of surface thermo-mechanical fatigue phenomena applied to materials for CLIC accelerating structures” PhD program, Markus Aicheler

7. Markus Aicheler 18.02.2011 CERN Experimental: Observation material 40% cold worked - Round bar cold rolled Ø40 mm and Ø100 mm - Yield Strength: R p0.2 = 316 MPa - Ultimate tensile strength: R m = 323 MPa - Average grain size: Ø110 µm - Relevance: state with best properties Brazed - Heat treatment in vacuum furnace: 300 K/h -> 795 °C; 60 min hold 100 K/h -> 825 °C; 6 min hold Natural cooling in vacuum - Yield Strength: R p0.2 ≈ 72 MPa - Ultimate tensile strength: R m = 270 MPa - Average grain size: Ø400 µm - Relevance: state after brazing assembly 2h@1000 °C - Heat treatment in vacuum furnace: 300 K/h -> 1000 °C; 120 min hold Natural cooling in vacuum - Yield Strength: R p0.2 ≈ 72 MPa - Ultimate tensile strength: R m = 257 MPa - Average grain size: Ø1400 µm - Relevance: state after bonding assembly C10100 (OFE Copper) - Reference material - Well known - Results comparable to other researchers - Supplementary fatigue data needed (CuZr well tested by predecessor)

8. Markus Aicheler 18.02.2011 CERN Experimental: Conventional fatigue test (CVF) 2 mm - Mechanical fatigue; R = -1 (R = σ min /σ max ) - UTS electro-mechanical universal-test machine - Repetition rate 0.5 Hz - Tested in loads up to +/-250 MPa; stress controlled - Sample shape conform ISO 12106 - 3-5 samples for one data point - Damage criterion: rupture

9. Markus Aicheler 18.02.2011 CERN Experimental: Ultrasound swinger device (USS) - Mechanical fatigue; R = -1 (R = σ max / σ min ) - Piezo electric resonant attenuator - Repetition rate 24 kHz - Cycles: 2 x 10 10 - σ max = +/-60 MPa  ε = 6 x 10 -4 - Samples: special designed sonotrodes

10. Markus Aicheler 18.02.2011 CERN Experimental: Laser fatigue device (LAF) - Thermal fatigue through irradiation - OPTEX Excimer Laser; λ = 248 nm - Repetition rate 200 Hz - Pulse length: 40 ns - 5 x 10 4 shots @ 0.3 J/cm 2 - ΔT = 280 K  ε tot = 7 x 10 -3 - Round disc diameter 40 mm - 25 discrete spots per disc

11. Markus Aicheler 18.02.2011 CERN Experimental: SLAC RF heating device (Stanford) - Thermal fatigue due to RF heating - Mushroom cavity @ 11,4 GHz - Repetition rate 60 Hz - Pulse length 1.5 µs - 1 x 10 7 Pulses @ 50 MW - ΔT max = 110 K  ε tot = 1.8 x 10 -3 - Round disc diameter 100 mm - Continuous radial distribution of ΔT ΔT r

12. Markus Aicheler 18.02.2011 CERN 10 7 Pulses ΔT max = 110 K  ε tot = 3.13 x 10 -3 Radial micro hardness distribution 1 st result: Hardening threshold of Cu [100] single crystal ΔT r

13. Markus Aicheler 18.02.2011 CERN Courtesy of KEK Threshold of cyclic temperature rise for hardening (58 K) 1 st result: Hardening threshold of Cu [100] single crystal

14. Markus Aicheler 18.02.2011 CERN Threshold of cyclic strain for hardening 1.7 x 10 -3 Δ H / Δε cycl.max = 1.83 x 10 4 HV/1 1 st result: Hardening threshold of Cu [100] single crystal

15. Markus Aicheler 18.02.2011 CERN 2 nd result: Orientation dependent surface roughening - 5 x 10 4 shots @ 0.3 J/cm 2 - ΔT = 180 K - ε tot,cycl = 5.13*10 -3

16. Markus Aicheler 18.02.2011 CERN [1 0 0] [1 1 1] 2 nd result: Orientation dependent surface roughening

17. Markus Aicheler 18.02.2011 CERN - 5 x 10 4 shots @ 0.3 J/cm 2 - ΔT = 180 K - ε tot,cycl = 5.13*10 -3 2 nd result: Orientation dependent surface roughening

18. Markus Aicheler 18.02.2011 CERN [1 0 0] [1 1 0] 2 nd result: Orientation dependent surface roughening

19. Markus Aicheler 18.02.2011 CERN Rz Surface index = true surface projected surface 2 nd result: Orientation dependent surface roughening

20. Markus Aicheler 18.02.2011 CERN 1. Isotropic thermal expansion causes different shear stresses (anisotrope moduli) (Thesis Reiner Mönig)  110 /  100 = 1.60  111 /  100 = 1.51 maximum resolved shear stress as a function of out- of-plane grain orientation in Cu due to an equibiaxial in-plane strain of 0.1% and zero out-of-plane stress 2. Different Schmid factor configurations on slip systems (local strain) Schmid factor S=τ/σ σ τ [1 0 0]: 8 Systems active [1 1 1]: 6 Systems active [1 1 0]: 4 Systems active a) Straining of a body with ΔL. Illustration of local strain in slip system with b) low and c) high Schmid factor High number of slip systems  lower local strain 2 nd result: Orientation dependent surface roughening with S max = 0.408 with S max = 0.272 with S max = 0.408 High Schmid factor  lower local strain

21. Markus Aicheler 18.02.2011 CERN 3 rd result: Orientation dependent hardening/roughening [1 1 0] [1 0 0] non irradiated area irradiated area Micro hardness indents Micro hardness indents in fatigued surface Hardness increase: [1 0 0]: 49 HV -> 58 HV (+17%) [1 1 1]: 49 HV -> 65 HV (+32%) [1 1 0]: 47 HV -> 68 HV (+44%) - 5 x 10 4 shots @ 0.3 J/cm 2 - ΔT = 180 K - ε tot,cycl = 5.13*10 -3

22. Markus Aicheler 18.02.2011 CERN 3 rd result: Orientation dependent hardening/roughening Initially similar roughness and sligthly different hardness  Same notch free surface Very different roughening / hardening behaviour  The rougher, the harder! Linear relation of hardening and roughening  Indication of fundamental link between both mechanisms Offset of hardness  Indication of microstructural activity before roughness detectable on surface  Hardness more sensitive criteria

23. Markus Aicheler 18.02.2011 CERN Summary and Conclusion Laser fatigueRF fatigueUSS fatigue Summary of Thesis Test campaign on different states of OFE copper with 4 different fatigue devices Phenomenon of orientation dependent roughening/hardening identified Influence of grain boundaries identified (not shown here) Influence of initial hardness identified (not shown here) Results obtained and phenomena observed allowed to compare different fatigue techniques and to make a suggestion for the best material candidate for CLIC accelerating structures.

24. Markus Aicheler 18.02.2011 CERN Conclusions Grain boundaries start to play important role in fine structures (grain sizes 1 µm - 5 µm). High local stresses arising from the effect of anisotropy of moduli are averaged out. The [1 0 0] crystallographic orientation of surface grains shows the smallest amount of surface roughening and sub-surface hardening. Copper materials with high initial hardness show no further cyclic strengthening, while significant cyclic hardening accompanied cycling of soft material states. Results obtained by mechanical techniques cannot be directly related to thermal fatigue data. Possible material candidates for the CLIC accelerating structure: 1) A strongly textured and fine grained OFE copper, e.g. equal-angular-channel- pressed (ECAP) OFE copper (currently fabricated up to Ø 50 mm) 2) A strongly [1 0 0] orientation textured pure copper thin film (observed and looks promising!) Summary and Conclusion

25. Markus Aicheler 18.02.2011 CERN Acknowledgements Prof. Eggeler and Dr. Sgobba Prof. Theisen CERN and especially the CLIC study All my collegues and friends at RUB and CERN My parents My better half: Anne-Laure

26. Markus Aicheler 18.02.2011 CERN