markus aicheler 18.02.2011 cern markus aicheler, ruhr-university bochum and cern “surface...
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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
Ha
rdn
es
s
/ H
V
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 / -
Ha
rdn
es
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
75
before cycling
after cycling
Ha
rdn
es
s /
HV
0.0
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
Ro
ug
hn
es
s 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
Ro
ug
hn
es
s in
cre
as
e 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