Ion implantation induced defects in Fe-Cr alloys studied by ......High Voltage divider START BaF 2...

Vladimír Kršjak, Stanislav Sojak, Vladimír Slugeň

Ion implantation induced defects in Fe-Cr alloysstudied by conventional positron annihilation

lifetime spectroscopyslow positron beam

Department of Nuclear Physics and TechnologySlovak University of Technology

e-mail: [email protected]

IAEA’s technical meeting, Kharkov, Ukraine, 9-13 June 2008

Krsjak, IAEA TM 34567 SMoRE, Kharkov, Ukraine 9. - 13.6.2008 2

- Experimental approach to study radiation damage- Positron Annihilation Spectroscopy - Measuring of the positron lifetime (elementary principles)- Application of conventional PALS and positron beams in the study of helium implanted Fe-Cr alloys - Contribution of complementary non-destructive experimental techniques- Summary

Content


material treatment evaluation

simple materials, model alloys,

reference materials, neutron irradiation, ion implantation (heavy, light)

experimental technique with proper sensitivity

Experimental approach to study radiation damage

conclusions for further research

Material research


~10µm ~100 nm ~10 nm < 1 nm~10-5 m ~10-7 m ~10-8 m < 10-9 m~100 Å < 10 Å~1000 Å~10 5 Å

Grain boundary Dislocations Dislocation loops, precipitates Frenkel defects

TEM Transmission electron microscopy

PAS Positron annihilation spectroscopySANS Small Angle Neutron Scattering

SEM Scanning electron microscopy APFIM Atom Probe Field Ion MicroscopyOM Optical Microscopy

XRD X-Ray Diffraction

Dime

nsion

sDe

fects

Tech

nique

sSome experimental techniques for radiation damage study


1. Standard mechanical test: simple physical test for obtaining some of tabular parameters, which itself provide some information about specimen condition. Generally destructive test. (fracture toughness, tensile, hardness etc.)

2. Image based techniques: using particles, or EM waves (reflection or transition) we can obtain microstructure image. Generally non-destructive test. (TEM, SEM, OM etc.)

3. Analytical techniques: Obtained information in the form of numerical value has to be processed to get some scientific outcome (PAS, SANS, XRD, MS…).

Well known dividing of experimental techniques is destructive and non destructive techniques, based on the specimen damaging during measurement.Better visualization about specific technique can be obtained from following experimental techniques dividing:

Different approaches to radiation damage study


1. Standard mechanical test:- Measuring of physical parameter, and comparing with large database of similar measurement on reference standards- Result cannot be interpreted incorrectly (in general)

2. Image based techniques:- Acquiring of 2D or 3D picture of material microstructure

3. Analytical techniques:- Possibility of qualitative and quantitative analysis of measuredspecimen

Advantages


1. Standard mechanical test:High reliability of acquired dataMeasurements can follow the changes in studied parameter, but doesn’t tell anything about origin of this changeQuantitative very accurate result, but from the microstructure point of view almost no qualitative information.

2. Image based techniquesQualitative and particularly also quantitative good information.Medium reliability of acquired data

3. Analytical techniques: Very accurate qualitative and quantitative analysis, but in general low reliability of acquired data.

Reliability of obtained data


BUT this low reliability can be improved by good theoretical knowledge of used technique and its data

processing!!!

Reliability of obtained data

3. Analytical techniques: Very accurate qualitative and quantitative analysis, but in general low reliability of acquired data.


Positron Annihilation Spectroscopy


- First antiparticle in physics- Positron as the antiparticle of the electron was predicted by Dirac - First observation of positron were performed California Institute of

Technology by Carl Anderson in 1932

- It was discovered early that the energy and momentum conservation during the annihilation process could be utilised to study properties of solids.

- 1945 – 1975 various experimental techniques of positron annihilation based upon the equipment of nuclear spectroscopy were developed.

PA history


• nowadays - well recognized as a powerful tool of microstructure investigations of condensed matter

• advanced applications, as positron beams, microscopy and wide range of other applications in medicine, particle physics, cosmology & astronomy were developed

• PAS technique is at the first place unique for the study of vacancy type defects

• possibility of registration also very low concentration of defects• a suitable technique for defects study in the near surface region

PA present


a) Positron-lifetime measurements (PAS LT).b) Angular correlation of annihilation gamma (PAS ACAR). c) Doppler broadening of annihilation line (PAS DB).

Advanced positron techniques• Low energy positron diffraction (LEPD)• Positron annihilation induced Auger

electron spectroscopy (PAES)• Reemited – positron Energy-Loss

spectroscopy (REPELS)• Slow Positron Beam (SPB).

PAS TECHNIQUES


- measures deviation of annihilation gamma quanta from the 180°- annihilation photons are detected in coincidence by scintillation counters- lead collimators in front of the detectors define the instrumental angular resolution (1 mrad)- slits are made in the x direction as long as possible- single channel analyser (SCA) is tuned for 511 keV photons and the device simply counts the coincidence pulses as a function of the angle Θz.

(Mogensen 1995)

PAS TECHNIQUESAngular correlation of annihilation radiation (PAS ACAR).


- the motion of the annihilating pair causes a Doppler shift (Doppler shift of 1keV correspondence to an angle deviation of 3,91 mrad)

- experimental measurements - Liquid-nitrogen-cooled pure Ge crystals

PAS TECHNIQUESDoppler broadening of annihilation line (PAS DB)

S – “Shape” parameter (recently also called the valence annihilation parameter).W - “Wing” or core annihilation parameter.


J. Kansy

Measuring of time between the generation of positron and the electron-positron annihilation

The positron lifetime (usually less than 1 ns) is determined with the environment (electron density)

Positron Annihilation Lifetime Spectroscopy (PALS)



- positron wave function is localized at the vacancy site until annihilation

- positron annihilation parameters change when annihilation in defects

- defects can be detected (qualitative and quantitative)

R.K. Rehberg

Delocalized positron in Fe

Positron wave function localized at the vacancy site (Fe)

Troev et al.


� The most common source of positrons is from the β+ decay of 22NaCl, which results in the simultaneous emission of a positron and a birth quantum of 1.27 MeV.

22Na 22Ne + β+ + νe + γ(1.27MeV)� High-energy photons impinging upon a material generate positrons,

neutrons and radionuclides.� Pair production using a beam of MeV electrons impinging upon a

target.

Positron Annihilation Lifetime Spectroscopy (PALS)Positrons sources


A problem when using a radioactive positron source is that the implantation depth is of the order of several tens of a micrometer and is not controllable.

A beam of moderated positrons can be accelerated to the desired energy to form a beam that can implant positrons to the desired depth.

Positron Annihilation Lifetime Spectroscopy (PALS)Positrons sources

Radioisotope positron source

Moderated positrons

… from the experimental point of view (positron lifetime measurements)


Positron Annihilation Lifetime Spectroscopy (PALS)Conventional PALS

- 22Na positron source - continuous spectra 0 – 545 keV- correspondent depth in bcc iron 0 – 130 µm- mean implantation depth < 10 µm

McCann and SMITH, Direct measurement of the K electron capture to positron emission ratio in the decay of 22Na, J. PHYS. A (GEN.PHYS.), 1969, SER. 2, VOL.

2.

However, considerable amount of positrons (>10%) annihilate in near surface area up to 1 µm*. With the statistic at least 106 counts, observation of microstructure changes in this region can be performed.



Typical spectra ~ 106 counts~ 6 - 8 hours

Conventional PALS

HV sourceOrtec 556

HV sourceOrtec 556

SCAOrtec 583

SCAOrtec 583

TACOrtec 566Delay Canberra 1458

MCAAccuspec

SSTTOOPP

22Na positron source█

Photomultiplier tube █

High Voltage divider █

STARTSTART

BaF2 Scintilator █


Positron Annihilation Lifetime Spectroscopy (PALS)Pulsed low energy positron beams

The sketch of Makhov profiles P(z, E) in silicon calculated for four incident positron energies [Bauer-Kugelmann et. al, 2001 ]

- no accompanying gamma to indicate the creation of positron → positrons are in time pulses implanted into materials

- variable energy - positron beam energies range typically from 10 eV to 100 keV (mean stopping depths from 10 nm to a few µm )

- the variation of the positron energy allows the detection of defects as a function of the penetration depth (defect depth profiling )

- monoenergetic positrons

Krsjak, IAEA TM 34567 SMoRE, Kharkov, Ukraine 9. - 13.6.2008 22Werner Egger

remoderated Positrons

W-Remoderator

Energy-filterPositron Annihilation Lifetime Spectroscopy (PALS)PLEPS at FRM-IIPLEPS - Pulsed Low Energy Positron System

Neutron induced positron source at Munich NEPOMUC is based on absorption of high-energy prompt -rays from thermal neutron capture in 113Cd.

Krsjak, IAEA TM 34567 SMoRE, Kharkov, Ukraine 9. - 13.6.2008 23Werner Egger

Positron Annihilation Lifetime Spectroscopy (PALS)PLEPS at FRM-II


Data treatment, potentialities and limitations,


Positron Annihilation Lifetime Spectroscopy (PALS)Data treatmentIn the case of one type defect in pure material the final spectra includes two components (exponential functions) τ1 and τ2. In the logarithmic scale we get two lines with different slope. Intersection of this line with time axis provide the searched lifetimes.



Trapping model (assumption that microstructure bulk contain only one type of defect)Annihilation in bulk with the rate λb= τb-1.

Trapping in defects with the rate κAnnihilation in defects with the rate λd= τd-1.

bbbb PPdttdP ..)( κλ −−=

bddd PPdttdP ..)( κλ +−=

Pb and Pd are the probabilities that the positron is in the bulk and in the trap, respectively

11 )( −+= κλτ b

12

−

= dλτ

21 1 II −=

)(2 κλλκ+−

=db

I

lifetimes intensities

2211 .. τττ IIm +=

Positron mean lifetime

Data treatment



• Point defects: atoms missing or in irregular places in the lattice (vacancies, interstitials, impurities)

• 1D, Linear defects: groups of atoms in irregular positions (e.g. screw and edge dislocations)

• 2D, Planar defects: the interfaces between homogeneous regions of the material (grain boundaries, internal and external surfaces)

• 3D, Volume defects: extended defects (voids, Stacking Fault Tetrahedra, pores, cracks)

PALS defectoscopy

- Practically only two different types of defects can be described in lifetime spectrum- even then the separation of these two lifetimes can be successful only if

- the voids of about 50 vacancies and more or any other defects with positron lifetime < 500 cannot be distinguish with this technique anymore

5.12

1f

λλ

Data treatment

Limitations !!


Experiments


ExperimentMaterials

0.000.0390.030.010.0911.620.0030.0030.0060.050.03L2530.0020.0350.020.010.078.390.0030.0070.00070.0120.03L2520.0010.0340.020.010.064.620.0030.0030.0060.0110.02L2590.0010.0170.0080.0050.0442.360.0040.0030.0020.0130.009L251

V [wt%]

N [wt%]

C [wt%]

Cu [wt%]

Ni [wt%]

Cr [wt%]

Ti [wt%]

Al [wt%]

Si[wt%]

P [wt%]

Mn[wt%]

Material

Specimens preparation:Dimensions 10x10x0,4 mm,One side mirror-like polished

*Manufactured in Dept. of Metallurgy of Ghent University, Belgium

Object of study

To study the influence of chromium concentration on the radiation resisistance, four Fe-Cr binary alloys with different Cr content have been prepared*.


Collision events

00.00050.001

0.00150.002

0.00250.003

0.00350.004

0.00450.005

01 0

02 0

03 0

04 0

05 0

06 0

07 0

08 0

09 0

01 0

0 0

target depth [nm]

counts

Depth profile of the helium implantation, E=250keV (SRIM simulation 105 ions)

ExperimentRadiation treatment

92.7474.1955.6437.1018.55DPAPLEPS0,740,600,450,300,15DPAPALS

3,12.1018(0.5)

2,5.1018(0.4)

1,87.1018(0.3)

1,25.1018(0.2)

6,24.1017(0.1)

Dose [ions/cm2](C/cm2)

DPA (average) calculation for different level of implantation in first 100µm layer (DPAPALS) and 800nm (DPAPLEPS) of studied Fe-Cr alloys

To obtain cascade collisions in the microstructure of studied materials without neutron activation, accelerated helium ions have been used


Technical specificationTotal accelerating voltage: 0 - 1 MVRipple factor: < 1%Energy rangefor singly charged particles: 5 keV to 1 MeV Energy spread: 70 keV – 1 MeV: ≈ 2 keV

< 70 keV: < 0,1%Beam current: 1 - 100 µA

Cascade accelerator, laboratory of ion beams, Slovak University of

Technology

Radiation treatmentExperiment


Results of the positron lifetime measurements


ExperimentConventional PALS Results

Positron lifetime in defects (2. component)

• This lifetime can be associated with the trapping of positrons in dislocations and small vacancy type defects. In the low chromium alloys (L251, L259) the defect lifetime increased with the implantation dose up to 235 ps. This value may be associated to small clusters of 4 – 5 vacancies or slightly larger clusters containing helium.



Intensity of the annihilation in defects

• The intensity I2 of the second lifetime τ2 is almost independent from the implantation dose. However, it is increased for the high chromium alloys. This points to a higher density of uniformly distributed defects, which are smaller than in the low chromium alloys.



• MLT parameter is increasing with the implantation dose in all materials (successful application of conventional PALS in the near surface study).

• Unequal behavior of the different materials under radiation treatment.

Positron mean lifetime in the He implanted FeCr alloys


PLEPS ResultsExperiment

Defects depth profile investigation has been performed on the L253 alloy, Fe11.62%Cr

• The positron mean lifetime (MLT) is increasing with the implantation dose, thus indicating the creation of defects due to implantation. • The increase of the MLT close to the surface (< 200nm below the surface) is probably due to positrons annihilating in surface oxide layer. • At higher depths the course of the MLT depth profile corresponds to the expected zone of maximum damage.


ExperimentPLEPS Results

Intensity of annihilation in large defects (voids) measured in Fe11.62%Cr alloy.

• The component with the lifetime 400 to 500 ps (large voids > 1 nm) has been observed in all implantation level.

• The intensity of this component (I3) increases dramatically with the helium implantation dose.• The course of the I3 depth profile again corresponds to the expected zone of maximum

damage.

Collision events

00.00050.001

0.00150.002

0.00250.003

0.00350.004

0.00450.005

01 0

02 0

03 0

04 0

05 0

06 0

07 0

08 0

09 0

01 0

0 0

target depth [nm]

counts


Complementary techniques contribution


Complementary techniques resultsExperiment L259 (Fe4.62%Cr) He implanted 0.3C

Implanted layer

bulk(Z scale is 4.8x magnified due to angle of specimen cutting ~12°)

x

Y

x

Z

Metallography

-Material desintegration and “flakes”creation after dose approx. 1.1017 cm-2

- “flakes” thickness 0.6 - 0.8 µm


Complementary techniques resultsExperiment

SEM

x

Z SEM confirms the PLEPS results of large voids in the depth >500nm which correspondent to the helium implantation profile maxima .

1µm


Mössbauer spectroscopy- was confirmed as a good for distinguishing between materials, however the surface study in backscattered geometry showed only slight changes in Mössbauer parameters due to helium implantation

X-Ray diffraction- possible distinguishing between materials and observable changes(increasing) in lattice parameter due to helium implantation (near surface study in grazing incidence geometry)

Complementary non-destructive

Any relevant information from different experimental techniques can contribute to the creation of complex image about material microstructure processes under radiation treatment.


Summary- There is no experimental technique, sufficient to describe the complicated behavior of real materials microstructure under radiation treatment, therefore different techniques needs to be combined for this purpose

- Light ion implantation can be successfully applied as a material degradation tool (the simulation of neutron flux) and created defects can be experimentally studied by various techniques.

- Positron lifetime experiments show that chromium plays an important role in the formation of the microstructure under radiation treatment. In particular, higher chromium content in FeCr alloys leads to a uniformly distribution of small defects rather then defects clustering.

- Depth profiles of defects, obtained with PLEPS, in the helium implanted region reflect the helium implantation profiles and show the creation of small vacancy clusters and large voids. These defects cannot be observed by any other technique in a non-destructive way.


Thank you for attention

Ion implantation induced defects in Fe-Cr alloys studied by ......High Voltage divider START BaF 2...

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