markus aicheler, ruhr-university bochum and cern metallurgy of pulsed surface heating

Markus Aicheler 07.05.2010

BDWS 2010

Markus Aicheler, Ruhr-University Bochum and CERN

Metallurgy of Pulsed Surface Heating


BDWS 2010

Consequences of PSH

PSH

Intra pulse effects:• Heating surface in E+B

area enhancing arcing?• Heating in surface

imperfections (crack, scratch)

• Increased ohmic losses• Change of Workfunction

Long term/accumulative effects:• Surface extrusions and tips

enhanced probability for el. Breakdown?

influence on RF-performance?• Surface intrusions

preferred sites for fatigue crack initiation

• Surface cracks obstacle for currents; enhanced

probability for el. breakdown• Increase of dislocation density in

surface • Nano sized field emitters (?)


BDWS 2010

Outline of the talk

- Introduction Pulsed Surface Heating (PSH)- Experimental and Results- Discussion of Results- Summary and Outlook


BDWS 2010

Surface magnetic field distribution in HDS cell (Courtesy A. Grudiev)

• Pulsed magnetic field induces currents• Superficial Joule heatingÞ cyclic heating and cooling phasesFor conductivity of copper: ΔT ≈ 60 K Þ σ ≈ 0 MPa to 150 MPa (comp.)Þ Heated layer depth several µm

Origin and nature of PSH

Estimated CLIC life time 2x1010 cycles @ 50Hz (= 20 years of operation)=> No mean to test a “real” structure under “real” conditions for whole life time!

Calculation of stress components during pulse=> Biaxial load case!


BDWS 2010

Observation material C10100 (OFE Copper)

• Heat treatment in vacuum furnace:

300 K/h -> 1000 °C; 120 min hold; Natural cooling in vacuum

• Rp0.2 ≈ 72 MPa

• Rm = 257 MPa

• GS: Ø 1400 um

2h@1000 °C

• Round bar cold rolled:

Ø 40 mm and Ø 100 mm

• Rp0.2 = 316 MPa

• Rm = 323 MPa

• GS: Ø 110 um

40% cold worked (H02)


BDWS 2010

RF heating device (SLAC Stanford)

- Thermal fatigue due to RF heating- Mushroom cavity @ 11,4 GHz- Repetition rate 60 Hz - Pulse length 1.5 µs- 2 x 106 Pulses @ 50 MW

- ΔTmax = 110 K ε = 1.8*10-3

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

Photos: Sami TantawiPresentation 23 Jan. 2008

ΔT

r


BDWS 2010

0 5 10 15 20 2540

50

60

70

80

90

100

110

120

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 temperature for hardening

Hardening in RF fatigue Cu [100] single crystal


BDWS 2010

0 5 10 15 20 2540

50

60

70

80

90

100

110

120

0

15

30

45

60

75

90

105

120Cu H02

Cu an1

Cu an2

T110

Radial position / mm

Har

dnes

s /

HV

ΔT

/ K

Hardness of H02 unaffected by cycling

Threshold Temp

Hardening in RF fatigue OFE Cu (hard and soft!)

Damage


BDWS 2010

Laser fatigue device

- 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 ε = 7*10-3

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


BDWS 2010

Hardening and roughening on Cu OFE 2h@300°C

1.0E+03 1.0E+04 1.0E+0545

50

55

60

65

70

75

80

0

50

100

150

200

250

300

350HV

Ra

Shot numbers

Hard

ness

/ HV

0.01

Roug

hnes

s Ra

/ nm

45 50 55 60 65 70 750

50

100

150

200

250virgin20005000100002000050000100000200000

Hardness / HV0.01

Rou

ghne

ss R

a / n

m

Micro hardness inprints directly in fatigued surface

No delayed increase of roughness with respect to hardness!

Large scatterÞ correlation hard…

… but trend is:„The the rougher it appears, the harder it gets!“


BDWS 2010

Crystallography

A cube with atoms on its corners and its faces (so called face centered cubic FCC)

Plenty of these elementary cells

x

y

z

[1 0 0]

[1 1 0]

[1 1 1]

Crystallographic description of directions: Crystallographic description of planes: Normal vector of planes!

(elementary cell)

Anisotropy: direction dependant properties

Primary slip system (111) with [-110]

Grain


BDWS 2010

Roughness developing on main orientations

[1 0 0]

[1 1 1]

[1 1 0]

- 5 x 104 shots @ 0.3 J/cm2 - ΔT = 280 K ε = 7*10-3


BDWS 2010

Roughness developing on main orientations

Rz Surface index = true surface

projected surface


BDWS 2010

Hardness and roughness on main orientations

[100] [111] [110] [100]2h@1000-5-C5 2h@1000-45deg-3-C1

40

45

50

55

60

65

70

75before cycling

after cyclingHa

rdne

ss /

HV0.

01

55 57 59 61 63 65 67 69 71 73 750

500

1000

1500

2000

2500[100][111][110]

Hardness / HV0.01

Rou

ghne

ss R

a / n

m

Hardening rates:[100] 18% < [111] 32% < [110] 42%

Good reproducibility!

Higher roughness developement goes along with higher hardening!


BDWS 2010

Discussion thermal fatigue results

=>[110] severe roughening / hardening

=>[111] moderate roughening / hardening

=>[100] low roughening / hardening

Possible explanations:

1. Isotropic thermal expansion causes due to anisotropic module different shear stresses (τ[100] < τ[111] < τ[110])

2. Different Schmid factor configurations on slip systems (active slip systems: [100] = 8; [111] = 6; [110] = 4)

3. Different dislocation substructures form as a function of out-of-plane orientation (multiple slip => more stable structures)

Schmid factorS=τ/σ

σ

τ

τ

ε

[111]

[100]τ[111]

τ [100]

εth

Ideal example:(in reality much more complex…)

[1 0 0]

[1 0 1]

[1 0 0]

[1 1 1]


BDWS 2010

Summary and Outlook

Þ Quantification of long (medium) term surface degradation

Þ Crystallographic orientation of Cu has strong influence on surface behaviour (roughening and hardening) during thermal cycling

Þ Grain boundary configuration very important for surface behaviour in GB vicinity (needs additional work)

Þ Combination of different available fatigue techniques allow description of non standard fatigue load case in CLIC accelerating structures

PSH

Intra pulse effectsLong term/accumulative effects


BDWS 2010

Thank you for the attention and have a nice weekend!!!


BDWS 2010

C10100_2h@1000_EP_Probe5_C5 Virgin Surface


BDWS 2010

C10100_2h@1000_EP_Probe5_C5


BDWS 2010

C10100_2h@1000_EP_45°Probe3_C1

markus aicheler, ruhr-university bochum and cern metallurgy of pulsed surface heating

Documents

heating surface

disc markus aicheler

outlookmarkus aicheler

damagemarkus aicheler

trmarkus aicheler

grainmarkus aicheler

surface extrusions

fatigued surface