Ion implantation induced defects in Fe-Cr alloys studied by conventional positron annihilation lifetime spectroscopy slow positron beam

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1 Ion implantation induced defects in Fe-Cr alloys studied by conventional positron annihilation lifetime spectroscopy slow positron beam Vladimír Kršjak, Stanislav Sojak, Vladimír Slugeň Department of Nuclear Physics and Technology Slovak University of Technology IAEA s technical meeting, Kharkov, Ukraine, 9-13 June 2008

2 Content - 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 Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

3 Experimental approach to study radiation damage Material research material treatment evaluation simple materials, model alloys, reference materials, neutron irradiation, ion implantation (heavy, light) experimental technique with proper sensitivity conclusions for further research Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

4 Some experimental techniques for radiation damage study TEM Transmission electron microscopy Dimensions Defects Techniques SEM Scanning electron microscopy OM Optical Microscopy Grain boundary Dislocations XRD X-Ray Diffraction SANS Small Angle Neutron Scattering PAS Positron annihilation spectroscopy Dislocation loops, precipitates APFIM Atom Probe Field Ion Microscopy Frenkel defects ~10µm ~100 nm ~10 nm < 1 nm ~10-5 m ~10-7 m ~10-8 m < 10-9 m ~10 5 Å ~1000 Å ~100 Å < 10 Å Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

5 Different approaches to radiation damage study 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: 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 ). Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

6 Advantages 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 measured specimen Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

7 Reliability of obtained data 1. Standard mechanical test: High reliability of acquired data Measurements can follow the changes in studied parameter, but doesn t tell anything about origin of this change Quantitative very accurate result, but from the microstructure point of view almost no qualitative information. 2. Image based techniques Qualitative 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. Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

8 Reliability of obtained data 3. Analytical techniques: Very accurate qualitative and quantitative analysis, but in general low reliability of acquired data. BUT this low reliability can be improved by good theoretical knowledge of used technique and its data processing!!! Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

9 Positron Annihilation Spectroscopy Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

10 PA history - 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 It was discovered early that the energy and momentum conservation during the annihilation process could be utilised to study properties of solids various experimental techniques of positron annihilation based upon the equipment of nuclear spectroscopy were developed. Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

11 PA present 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 Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

12 PAS TECHNIQUES 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). Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

13 PAS TECHNIQUES Angular correlation of annihilation radiation (PAS ACAR). - measures deviation of annihilation gamma quanta from the 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 Θ z. (Mogensen 1995) Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

14 PAS TECHNIQUES Doppler broadening of annihilation line (PAS DB) - 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 - Liquidnitrogen-cooled pure Ge crystals S Shape parameter (recently also called the valence annihilation parameter). W - Wing or core annihilation parameter. Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

15 Positron Annihilation Lifetime Spectroscopy (PALS) 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) J. Kansy Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

16 Positron Annihilation Lifetime Spectroscopy (PALS) Delocalized positron in Fe Positron wave function localized at the vacancy site (Fe) R.K. Rehberg - 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) Troev et al. Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

17 Positron Annihilation Lifetime Spectroscopy (PALS) Positrons sources The most common source of positrons is from the β + decay of 22 NaCl, which results in the simultaneous emission of a positron and a birth quantum of 1.27 MeV. 22 Na 22 Ne + β + + ν e + 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. Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

18 Positron Annihilation Lifetime Spectroscopy (PALS) Positrons sources from the experimental point of view (positron lifetime measurements) Radioisotope positron source 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. Moderated positrons A beam of moderated positrons can be accelerated to the desired energy to form a beam that can implant positrons to the desired depth. Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

19 Positron Annihilation Lifetime Spectroscopy (PALS) Conventional PALS - 22 Na positron source - continuous spectra kev - correspondent depth in bcc iron µm - mean implantation depth < 10 µm However, considerable amount of positrons (>10%) annihilate in near surface area up to 1 µm *. With the statistic at least 10 6 counts, observation of microstructure changes in this region can be performed. McCann and SMITH, Direct measurement of the K electron capture to positron emission ratio in the decay of 22 Na, J. PHYS. A (GEN.PHYS.), 1969, SER. 2, VOL. 2. Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

20 Positron Annihilation Lifetime Spectroscopy (PALS) Conventional PALS START 22 Na positron source HV source Ortec 556 SCA Ortec 583 BaF 2 Scintilator Photomultiplier tube High Voltage divider HV source Ortec 556 TAC Ortec 566 SCA Ortec 583 Delay Canberra 1458 S T O P MCA Accuspec Typical spectra ~ 10 6 counts ~ 6-8 hours Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

21 Positron Annihilation Lifetime Spectroscopy (PALS) Pulsed low energy positron beams - 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 The sketch of Makhov profiles P(z, E) in silicon calculated for four incident positron energies [Bauer-Kugelmann et. al, 2001 ] Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

22 Positron Annihilation Lifetime Spectroscopy (PALS) PLEPS at FRM-II Energy-filter PLEPS - Pulsed Low Energy Positron System W-Remoderator remoderated Positrons Neutron induced positron source at Munich NEPOMUC is based on absorption of high-energy prompt -rays from thermal neutron capture in 113 Cd. Werner Egger Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

23 Positron Annihilation Lifetime Spectroscopy (PALS) PLEPS at FRM-II Werner Egger Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

24 Data treatment, potentialities and limitations, Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

25 Positron Annihilation Lifetime Spectroscopy (PALS) Data treatment In the case of one type defect in pure material the final spectra includes two components (exponential functions) τ 1 and τ 2. In the logarithmic scale we get two lines with different slope. Intersection of this line with time axis provide the searched lifetimes. Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

26 Positron Annihilation Lifetime Spectroscopy (PALS) Data treatment Trapping model (assumption that microstructure bulk contain only one type of defect) Annihilation in bulk with the rate λ b = τ -1 b. Annihilation in defects with the rate λ d = τ -1 d. Trapping in defects with the rate κ dpb ( t) dt = λ. P κ. P b b b P b and P d are the probabilities that the positron is in the bulk and in the trap, respectively dpd ( t) dt = λ. P+κ. P d d Positron mean lifetime b τ 1 1 = ( λb+ κ) τ lifetimes 2 = λ d 1 I1= 1 I 2 I intensities 2 = ( λ b κ λ d + κ) τ m = I1 τ1+ I2.. τ 2 Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

27 Positron Annihilation Lifetime Spectroscopy (PALS) Data treatment PALS defectoscopy 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) Limitations!! - 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 λ 1 f 1.5 λ - the voids of about 50 vacancies and more or any other defects with positron lifetime < 500 cannot be distinguish with this technique anymore Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

28 Experiments Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

29 Object of study Experiment Materials To study the influence of chromium concentration on the radiation resisistance, four Fe-Cr binary alloys with different Cr content have been prepared *. Material Mn [wt%] P [wt%] Si [wt%] Al [wt%] Ti [wt%] Cr [wt%] Ni [wt%] Cu [wt%] C [wt%] N [wt%] V [wt%] L L L L * Manufactured in Dept. of Metallurgy of Ghent University, Belgium Specimens preparation: Dimensions 10x10x0,4 mm, One side mirror-like polished Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

30 0 Experiment Radiation treatment To obtain cascade collisions in the microstructure of studied materials without neutron activation, accelerated helium ions have been used counts Collision events Dose [ions/cm 2 ] (C/cm 2 ) 6, (0.1) 1, (0.2) 1, (0.3) 2, (0.4) 3, (0.5) DPA PALS 0,15 0,30 0,45 0,60 0,74 DPA PLEPS target depth [nm] Depth profile of the helium implantation, E=250keV (SRIM simulation 10 5 ions) DPA (average) calculation for different level of implantation in first 100µm layer (DPA PALS ) and 800nm (DPA PLEPS ) of studied Fe-Cr alloys Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

31 Experiment Radiation treatment Cascade accelerator, laboratory of ion beams, Slovak University of Technology Technical specification Total accelerating voltage: 0-1 MV Ripple factor: < 1% Energy range for singly charged particles: 5 kev to 1 MeV Energy spread: 70 kev 1 MeV: 2 kev < 70 kev: < 0,1% Beam current: µa Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

32 Results of the positron lifetime measurements Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

33 Experiment Conventional 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. Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

34 Experiment Conventional PALS Results Intensity of the annihilation in defects The intensity I 2 of the second lifetime τ 2 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. Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

35 Experiment Conventional PALS Results Positron mean lifetime in the He implanted FeCr 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. Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

36 Experiment PLEPS Results 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. Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

37 0 Experiment PLEPS Results counts Intensity of annihilation in large defects (voids) measured in Fe11.62%Cr alloy Collision events target depth [nm] 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 (I 3 ) increases dramatically with the helium implantation dose. The course of the I 3 depth profile again corresponds to the expected zone of maximum damage. Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

38 Complementary techniques contribution Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

39 Experiment Complementary techniques results Metallography Y L259 (Fe4.62%Cr) He implanted 0.3C -Material desintegration and flakes creation after dose approx cm -2 x - flakes thickness µm Implanted layer Z x (Z scale is 4.8x magnified due to angle of specimen cutting ~12 ) bulk Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

40 Experiment Complementary techniques results SEM 1µm Z x SEM confirms the PLEPS results of large voids in the depth >500nm which correspondent to the helium implantation profile maxima. Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

41 Complementary non-destructive 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) Any relevant information from different experimental techniques can contribute to the creation of complex image about material microstructure processes under radiation treatment. Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

42 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. Krsjak, IAEA TM SMoRE, Kharkov, Ukraine

43 Thank you for attention Krsjak, IAEA TM SMoRE, Kharkov, Ukraine