Download - Markus Aicheler, Ruhr-University Bochum and CERN

Markus Aicheler 18.02.2011CERN

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”


- 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


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)


CLIC accelerating structure (AS):• Shape accuracy ± 2.5 µm• Roughness Ra 0.02 µm• Very high conductivity material


Assembly by:

brazing

bolting


• Pulsed magnetic field induces currents (200 ns, repetition rate 50 Hz)• Superficial Joule heating for electrical conductivity of copper: ΔT ≈ 60 K Þ cyclic heating- and cooling phases (biaxial strain)Þ thermal fatigue with σ ≈ 0 MPa to 150 MPa (comp.)Þ skin depth several µmÞ surface roughness degrades operation conditions “functional fatigue”


Estimated CLIC life time 2 x 1010 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)


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


Experimental: Observation material

40% cold worked- Round bar cold rolled Ø40 mm and

Ø100 mm

- Yield Strength: Rp0.2 = 316 MPa

- Ultimate tensile strength: Rm = 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: Rp0.2 ≈ 72 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: Rp0.2 ≈ 72 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)


Experimental: Conventional fatigue test (CVF)

2 mm

- Mechanical fatigue; R = -1 (R = σmin /σmax)

- 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


Experimental: Ultrasound swinger device (USS)

- Mechanical fatigue; R = -1 (R = σmax/ σmin)

- Piezo electric resonant attenuator- Repetition rate 24 kHz - Cycles: 2 x 1010

- σmax = +/-60 MPa ε = 6 x 10-4

- Samples: special designed sonotrodes


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 104 shots @ 0.3 J/cm2

- ΔT = 280 K εtot = 7 x 10-3

- Round disc diameter 40 mm- 25 discrete spots per disc


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 107 Pulses @ 50 MW

- ΔTmax = 110 K εtot = 1.8 x 10-3

- Round disc diameter 100 mm- Continuous radial distribution of ΔT

ΔT

r


• 107 Pulses • ΔTmax = 110 K εtot = 3.13 x 10-3

• Radial micro hardness distribution

1st result: Hardening threshold of Cu [100] single crystal

ΔT

r


0 5 10 15 20 2540

45

50

55

60

65

70

75

80

0

15

30

45

60

75

90

105

120[1 0 0] single crystalT110

Radial position / mm

Har

dnes

s /

HV

ΔT

/ K

Courtesy of KEK

Threshold of cyclic temperature rise for

hardening (58 K)



0.0E+00 1.0E-03 2.0E-03 3.0E-0340

45

50

55

60

65

70

75

80

Equivalent strain εcycl.max / -

Har

dnes

s H

/ H

V

Threshold of cyclic strain for hardening

1.7 x 10-3

ΔH / Δεcycl.max = 1.83 x 104 HV/1



2nd result: Orientation dependent surface roughening

- 5 x 104 shots @ 0.3 J/cm2

- ΔT = 180 K

- εtot,cycl = 5.13*10-3


[1 0 0]

[1 1 1]



- 5 x 104 shots @ 0.3 J/cm2

- ΔT = 180 K

- εtot,cycl = 5.13*10-3



[1 0 0]

[1 1 0]



Rz Surface index = true surface

projected surface

unfatigued (ref.) [1 0 0] fatigued [1 1 1] fatigued [1 1 0] fatigued0

200

400

600

800

1000

1200

1400

1600

1800

2000

2200

1.0E+00

1.0E+01

Rz in µm Si-1



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



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 Þ lower local strain


with Smax = 0.408

with Smax = 0.272

with Smax = 0.408

High Schmid factorÞ lower local strain


3rd 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%)

[100] [111] [110]40

45

50

55

60

65

70

75before cycling

after cycling

Har

dnes

s / H

V0.0

1- 5 x 104 shots @ 0.3 J/cm2

- ΔT = 180 K

- εtot,cycl = 5.13*10-3


45 50 55 60 65 70 750

500

1000

1500

2000

2500[100] initial state[100] fatigued[111] initial state[111] fatigued[110] initial state[110] fatigued

Hardness / HV0.01

Rou

ghne

ss R

z / n

m

5 7 9 11 13 15 17 19 21 23 250

500

1000

1500

2000

2500[100][111][110]

Hardness increase / HV0.01

Rou

ghne

ss in

crea

se Δ

Rz

/ nm

3rd result: Orientation dependent hardening/roughening

• Initially similar roughness and sligthly different hardnessÞ Same notch free surface• Very different roughening / hardening behaviourÞ The rougher, the harder!

• Linear relation of hardening and rougheningÞ Indication of fundamental link

between both mechanisms• Offset of hardnessÞ Indication of microstructural

activity before roughness detectable on surface

Þ Hardness more sensitive criteria


Summary and Conclusion

Laser fatigue RF fatigue USS 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.


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


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