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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”. Outline of the talk. Introduction into the project in the frame of CLIC Main goals of the PhD thesis - PowerPoint PPT Presentation
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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”
Markus Aicheler 18.02.2011CERN
- 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
Markus Aicheler 18.02.2011CERN
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)
Markus Aicheler 18.02.2011CERN
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
Markus Aicheler 18.02.2011CERN
• 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”
Introduction: CLIC surface heating phenomenon
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)
Markus Aicheler 18.02.2011CERN
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
Markus Aicheler 18.02.2011CERN
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
- Ultimate tensile strength: Rm = 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: Rp0.2 ≈ 72 MPa
- Ultimate tensile strength: Rm = 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)
Markus Aicheler 18.02.2011CERN
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
Markus Aicheler 18.02.2011CERN
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
Markus Aicheler 18.02.2011CERN
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
Markus Aicheler 18.02.2011CERN
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
Markus Aicheler 18.02.2011CERN
• 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
Markus Aicheler 18.02.2011CERN
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)
1st result: Hardening threshold of Cu [100] single crystal
Markus Aicheler 18.02.2011CERN
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
1st result: Hardening threshold of Cu [100] single crystal
Markus Aicheler 18.02.2011CERN
2nd result: Orientation dependent surface roughening
- 5 x 104 shots @ 0.3 J/cm2
- ΔT = 180 K
- εtot,cycl = 5.13*10-3
Markus Aicheler 18.02.2011CERN
[1 0 0]
[1 1 1]
2nd result: Orientation dependent surface roughening
Markus Aicheler 18.02.2011CERN
- 5 x 104 shots @ 0.3 J/cm2
- ΔT = 180 K
- εtot,cycl = 5.13*10-3
2nd result: Orientation dependent surface roughening
Markus Aicheler 18.02.2011CERN
[1 0 0]
[1 1 0]
2nd result: Orientation dependent surface roughening
Markus Aicheler 18.02.2011CERN
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
2nd result: Orientation dependent surface roughening
Markus Aicheler 18.02.2011CERN
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 Þ lower local strain
2nd result: Orientation dependent surface roughening
with Smax = 0.408
with Smax = 0.272
with Smax = 0.408
High Schmid factorÞ lower local strain
Markus Aicheler 18.02.2011CERN
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
Markus Aicheler 18.02.2011CERN
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
Markus Aicheler 18.02.2011CERN
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.
Markus Aicheler 18.02.2011CERN
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
Markus Aicheler 18.02.2011CERN
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
Markus Aicheler 18.02.2011CERN