Positron Microbeams and their applications

Matz Haaks

H e l m h o l t z - I n s t i t u tf ü r S t r a h l e n - u n d K e r n p h y s i k

Nussallee 14 - 16 53173 Bonn Germany

Down to the micron range: ideas and techniques

Application to material science

Scanning Positron Microscopy (SPM):

The Bonn Positron Microprobe (BPM)

A commercial SPM?

Building a positron microscope...

Intense positron source (radioactive isotope, accelerator, reactor)with small phase space

Mono-energetic positrons neededefficient moderation.solid noble gases (Ne), some pure metals (W, Mo)

→

Remoderation: brightness enhancement

Physical resolution limits:

Electrostatic acceleration

Electromagnetic beam guiding

Focusing into the micron range

Scanning the beam (electromagnetic) orscanning the sample (motorized stage)

Lateral: 0.2 - 2 µm (depending on positron energy and defect density)

Depth: 0.1 - 5 µm (depending on positron energy)

R.H. Howell, W. Stoeffl, A. Kumar, P.A. Sterne, T.E. Cowan, J. Hartley, Mater. Sci. Forum 255-257 (1997) 644

Positron microscope at the LLNL (e -lifetime)+

W. Triftshäuser, G. Kögel, P. Sperr, D.T. Britton, K. Uhlmann, P. Willutzki, Nucl. Instr. Meth. B 130 (1997) 264

Positron microscope at the FRM (e -lifetime)II +

Positrons

Krause-Rehberg et al., 2002

SEM

Positrons

Krause-Rehberg et al., 2002

SEM

Microhardness indentation in GaAs

H. Greif, M. Haaks, U. Holzwarth, U. Männig, M. Tongbhoyai, T. Wider, K. Maier, APL 15 (1997) 2115

Specifications

1500 cps

Background: 0.7 cps

e+: 4.5 � 30 keV

e-: 0.3 � 30 keV

e+: 5 � 200 µm

e-:

Gammas in photopeak:

Energy range

Beam diameter�12 nm

condensor lenscondensor lens prism

intermediatevoltage

high voltage elektron gun

condensor zoom

positronsource

objektive lens

moderator

sample

motorised tablex = 1 m

Ge - detector

��������������

The Bonn Positron Microprobe (Doppler spectroscopy)

electrode onground potential

beam axis

intermediatepotential

22Na source 10 mCiØ 500 µm

Halter underste Abschirmung (Ta)

accelerationpotential

slit 10 µm

2 mm

Au

fixation andshielding (Ta)

compressionpin (Ta)

W-moderator:single crystalwith 500 nmW-foil

Source and moderator

300

µm

mo

de

rato

r(W

)

inte

rme

dia

tep

ote

ntia

l

ground potential

2. Cross-Over

Positron beam geometry (Simion 7)

electrons

positrons

samples

PhD student

Tensile test: stress-strain-curve

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

0

100

200

300

400

500

600

700

strain [%]

1.00

1.01

1.02

1.03

1.04

1.05

1.06

0.0 0.5 1.0 1.5

400

stre

ss[M

Pa

]�

S-p

ara

me

ter

300

200

100

well annealed carbon steel AISI 1045

1.00

1.02

1.04

1.06

1.08

S-p

ara

mete

r

-1500-1000

-5000

5001000

1500

y [µm]

-500

0

500

1000

1500

x[µm

]

(a) (b)

(c)

1.10

cracktip

slip lines

crack tip

20 µm

20 µm

Cyclic plastic zone at a fatigue crack

Compact tension fatigue: stainless steel AISI 321

Longitudinal axis [µm]

1.07

1.06

1.05

1.04

1.03

1.02

1.01

1.00Tra

nsv

ers

ala

xis

[µm

]

-800 -400 0 400 800

S-p

ara

mete

r

1000

800

600

400

200

0

-200

Cyclic plastic zone at a fatigue crack Lo

ad

Rotating bending fatigue: TiAl6V4

-5 -4 -3 -2 -1 0 1 2 3 4 5

1.00

1.01

1.02

1.03

1.04

1.05

-5 -4 -3 -2 -1 0 1 2 3 4 50.004

0.005

0.006

0.007

0.008

neut

ralf

ibre

S-p

aram

eter

FW

HM

{200

}[Å

]

compression elongation

distance from sample center x [mm] distance from sample center x [mm]

Three-point bending test on AISI 1045: Positrons / X-rays

F

�

�max

�

�max

Linear stressgradient

Neutral plane inthe center of thesample

Positrons from BPM

Lateral resolved Debye-Scherrerdiffraction at 67 keVBeam diameter: 1.5 × 0.1 mmPowder condition: ~40000 grains

2

(hard X-ray beam-line at PETRA II,Desy/Hasylab, Hamburg)

FW

HM

{200

}[Å

]

{200} reflections of -iron (bcc)�

X-rays:

Cracktip in CT geometry (AISI 1045): Positrons / X-rays

X-ray beam diameter: 0.1 × 0.1 mmpowder condition: ~1500 grains

2

Bad

-500 0 500 1000

-600

-400

-200

0

200

400

600

-500 0 500 1000

direction of crack growth [µm]

1.000

1.004

1.014

1.025

1.035

1.045

1.055

S-parameter

positronsDebye-Scherrer

-iron {200}�

0.00

0.02

0.04

0.06

0.08

relativeLatticedistortion

crack tipopening

plastic zone

cycl

iclo

ad

Compact Tension (CT)

High cycle fatigue: 7×10 cycles5

direction of crack growth [µm]

loa

dd

ire

ctio

n[µ

m]

X-rays

-600

-400

-200

0

200

400

600

loa

dd

ire

ctio

n[µ

m]

[µm]

AA 2024 (At%: Mg 1.6 Cu: 4.4)

1.000

1.015

1.012

1.009

1.006

1.003

1.018

S/SAl bulk

400

[µm

]

200

100

0

-100

-200

3002001000

directly after crack production

400300200100

2 weeks later

400300200100

6 weeks later

S/SAl bulk

1.028

1.032

1.036

1.040

1.044

1.048

[µm]

AA 6013 (At%: Mg: 1.0 Si: 0.8 Cu: 0.9)

[µm

]

200

100

0

-100

-200

4003002001000

directly after crack production

400300200100

3 weeks later

400300200100

9 weeks later

Hydrogen in aluminum alloys: AA 2024 and AA 6013cyclic plastic zones produced in corrosive environment:diffusion of vacancies hindered by hydrogen

Micro-scratch on GaAs surface

0.995

1.000

1.005

1.010

1.015

1.020

1.025

1.030

1.035

0 200 400 600 800 1000

1.000

1.005

1.010

1.015

1.020

1.025

1.030

1.035

100

50

0

0 200 400 600 800 1000

S/Sbulk

scratch direction [µm]

zone 1

30 mm

zone 3zone 2zone 1

156 mm 166 mm

du

ctile

brittle

[011]--

[µm

]

zone 2 zone 3

Ductile behavior:Plasticity due tohydrostatic pressure

Prediction of fatigue failure:Defect density as precursor for fatigue failure

Critical S-parameterCritical defect densityMaterial failure

(a)

310 MPa300 MPa290 MPa210 MPa

1.08

1.06

1.04

1.02

1.00

S-p

ara

me

ter

S-p

ara

me

ter

cycle number

1.05

1.04

1.03

1.02

1.01

1.00

390 MPa350 MPa320 MPa260 MPa

10 10 10 10 10 10 10

2 3 4 5 6

Ferr

itic

steel

AIS

I1045

Au

ste

niti

cst

ee

lA

ISI

32

1

OK!Failure at criticaldefect density

Uncertain!Fatigued zonestrongly localised

Localization: Lateral defect structures during fatigue

0 1 2 3 4 5 64

3

2

1

0

noda

ta

1.000

1.004

1.008

1.012

1.015

1.019

1.023

1.027

cyclic load direction

30 cycles

3500 cycles

4

3

2

1

0

4

3

2

1

0

0 1 2 3 4 5 60 1 2 3 4 5 6

load direction [mm] load direction [mm]

load direction [mm]

wid

th[m

m]

wid

th[m

m]

wid

th[m

m]

1

32

AISI 1045

S-p

ara

mete

r350 cycles

Geometry with defined stress concentration

VonMieses stress: FEM simulation

Stress concentratedin small volume

cycl

iclo

ad

Strongly localised defectdistribution expected

FEM: VonMieses-stress �VM

300 µm

200

µm

1.000

1.012

1.024

1.033

1.047

S-p

ara

me

ter

FEM: �vM

S-parameter

N = 2.2×104

Cyclic load = 160 MPa�

N = 7.0×103

0 2 4 6 8

1.00

1.01

1.02

4

6

8

10

�V

M[M

Pa]

S-p

ara

mete

r

distance from central hole [mm]

line scan

FEM

Positron results

from crack tip: = 1.052(2)Scrit

1.00

1.01

1.02

1.03

1.04

1.05

1.06S

-para

mete

rextrapolated failure: =1.0×10Nf

5

annealed

100 101 102 103 104 105

load cycles

� = 160 MPa

Failure prediction using the critical defect density

1.00

1.01

1.02

1.03

1.04

1.05

1.06S

-para

mete

rextrapolated failure: =1.0×10Nf

5

annealed

100 101 102 103 104 105

load cycles

measu

red

failu

re:

=1.2

×10

Nf

5

� = 160 MPa

Failure prediction using the critical defect density

from crack tip: = 1.052(2)Scrit

A commercial SPM...

moderatorpreparation

linear

guiding

preparationchamber

source & moderator

prism

electrons

EM column

sample chamber

Ge-detector/LN2

A commercial SPM...

source & moderator:prepared and maintainedexternally

prism

electrons

EM column

sample chamber

Ge-detector/LN2

Positron Microbeams and their applications

Matz Haaks

H e l m h o l t z - I n s t i t u tf ü r S t r a h l e n - u n d K e r n p h y s i k

Nussallee 14 - 16 53173 Bonn Germany

Down to the micron range: ideas and techniques

Application to material science

Scanning Positron Microscopy (SPM):

The Bonn Positron Microprobe (BPM)

A commercial SPM?