Vacancy type defects in oxide dispersion strengthened steels

Vacancy type defects in oxide dispersion strengthened steels V. Slugeň, J. Veterníková, V. Sabelová, J. Degmová, R. Hinca, M. Petriska and S. Sojak Institute of Nuclear and Physical Engineering Faculty of Electrical Engineering and Information Technology Slovak University of Technology Bratislava, Slovakia

Institute of Nuclear and Physical Engineering Slovak University of Technology Available techniques for material studies: Positron Annihilation Spectroscopy: Conventional PALS 2 -det. or 3 -det. Setups (for irradiated materials), digital Doppler Broadening set-up, access and experiences with PLEPS measurements at FRM-II in Garching Moessbauer spectroscopy, Atomic force microscopy, Alfa, beta, gamma spectroscopy including low/background chamber, X-ray diffraction, Barkhausen Noises measurements, TEM, SEM, Operating cascade accelerator for simulation of radiation induced defects via ion implantation

What kind of information we can obtain from Positron Annihilation Spectroscopy? Defects density Annealing effectiveness Precipitation Types of defects Near surface region study Microstructural changes due to irradiation, ageing, . . . Report: EUR 22468 EN Vladimír Slugeň JRC-Petten, 30. 8. 2006

Irradiation-induced changes of microstructure ü Neutron-irradiation – Defect production • Self-interstitial atom (SIA) & vacancy (V) rich regions xx x – Matrix damage • SIA-clusters, SIA-loops • Micro voids x x – Solute atom diffusion • Precipitates • Complex defect-solute configurations • GB segregation xx x

Experiment Radiation treatment To obtain cascade collisions in the microstructure of studied materials without neutron activation, accelerated helium ions have been used Dose [ions/cm 2] (C/cm 2) 6, 24. 1017 (0. 1) 1, 25. 1018 (0. 2) 1, 87. 1018 (0. 3) 2, 5. 1018 (0. 4) DPAPALS 0, 15 0, 30 0, 45 0, 60 DPAPLEPS 18. 55 37. 10 55. 64 74. 19 3, 12. 1018 (0. 5)Depth profile of the helium implantation, E=250 ke. V (SRIM simulation 105 ions) 0, 74 92. 74 DPA (average) calculation for different level of implantation in first 100 m layer (DPAPALS) and 800 nm (DPAPLEPS) of studied Fe-Cr alloys

Experiment Complementary techniques results SEM 1 mm Z x SEM confirms the PLEPS results of large voids in the depth >500 nm which correspondent to the helium implantation profile maxima.

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 ke. V to 1 Me. V Energy spread: 70 ke. V – 1 Me. V: 2 ke. V < 70 ke. V: < 0, 1% Beam current: 1 - 100 A

PALS equipment

Pulsed low energy positron system (PLEPS) remoderated positrons [1] P. Sperr, W. Egger, G. Kögel, G. Dollinger, Ch. Hugenschmidt, R. Repper, C. Piochacz, Applied Surface Science 255 (2008) 35– 38 [2] Hugenschmidt C. , Dollinger G. , Egger W. , Kögel G. , Löwe B. , Mayer J. , Pikart P. , Piochacz C. , Repper R. , Schreckenbach K. , Sperr P. , Stadlbauer M. , Applied Surface Science, Volume 255, Issue 1, p. 29 -32

Defects depth profiling study and studies of near-surface region. PAS and TEM results are useable for microstructural evaluation of new materials SLUGEŇ, V. et al. , Nuclear Fusion 44, 2004, 93.

HV 10 PAS parameters in comparison to results from other techniques (TEM, SEM, HV 10, MS, XRD). Ø Stress-strain experiments on Fe Ø Distinct positron trapping after 80% Hooks range (fully elastic region) Ø Early stage of fatigue 200 150 160 Stress-strain diagram of pure Fe Average positron LT in Fe after tensile strain 120 100 0 20 40 60 80 100 2 140 s[N/mm] t[ps] 180 MS 250 t [ps] 2 s [N/mm] 50 0 100 e[% fracture strain] SLUGEŇ, V. , MAGULA, V. : Nuclear engineering and design 186/3, 1998

Lattice parameter vs. positron lifetime in defects in helium implanted Fe-Cr alloys. a. ) Fe 2. 56%Cr; b. ) Fe 11. 62%Cr a. ) b. ) KRSJAK, V. : Ph. D Thesis, STU Bratislava, 2008

PAS Results - Annealing temperature for WWER-steels at 475 °C is acceptable, but PAS gives more information. The 3 D presentation of PLEPS results (Tau 1) of irradiated (1. 25 x 10 E 24 m-2) and annealed Sv -10 Kh. MFT steel (WWER-440 weld). The effectiveness of the annealing process to removing of small defects (mono/di-vacancies or Frenkel pairs) can be followed via significant decrease of parameter tau 1. This figure also shows rapid increase of mentioned small defects in WWER type of RPV steels after about 480 ºC. Slugen et al: NTD&E Int. 37 (2004) 651

The EPR pressure vessel in Olkiluoto 3 (Finland) – 2009 VVER-440 annealing facility VVER-440 V-213 Pressure vessel 15

How should the ideal WWER-reactor steel look like and why? • Corrosion reistance, • Radiation resistance – negligible radiation swelling, small ΔDBTT (RTNDT, NDTT) and upper shelf energy (USE) decrease • Thermal resistance, • Reduced activated steel – with eliminated content of long-term radiactive Ni, C, Cu and Co replaced by V, Mn, Cr, Ti, W. Optimization of all factors affected on properties

Difficulties with irradiated RPV steels and advantages for implantation • Radioactivity ―> special rules for handling, transport, polishing, storage. . . (PROBLEMS), – Reducing of volume, – Reducing of number of samples, – Application of other techniques if possible. • PAS disturbing 60 Co contribution (photopiks 1. 17 and 1. 33 Me. V) – 1. Measurement using PLEPS (very thin samples of about 20 μm are necessary), – 2. Measurement using 3 detector set-up in coincidence mode (takes about 2 weeks), – 3. To wait. . . (T 1/2(Co-60)=5. 27 a). • Ion implantation – none transmutations = none 60 Co, very short half-time of decay for radionuclides, only 2 detectors measurement equipment for PAS In ODS steels – 0 Co content (theory)

Chemical composition of studied ODS steels (in % wt. ). C Mn Ni Cr Mo Ti Al Si W Y 2 O 3 MA 956 0. 07 0. 12 0. 07 20 0. 1 0. 3 3. 4 0. 04 - 0. 5 ODM 751 0. 07 0. 02 16 1. 74 0. 7 3. 8 0. 06 - 0. 5 ODS Eurofer 0. 1 0. 44 - 9 0. 01 - - 0. 01 1. 1 0. 3 • Addition of stable oxides (Al 2 O 3, Cr 2 O 3, Y 2 O 3) • Better mechanical properties – strength, toughness • Better corrosion resistance and resistance to thermal loading • Candidate materials for fuel cladding in new reactors (fast reactors) Oxide Dispersion Strengthening might improve the swelling resistance of F/M steels

ODS 14%Cr ferritic steels: MA 957 Less hardening than conventional and low activation F/M Steels J. L. Boutard, IAEA Technical Meeting, Vienna, 27 -29 June 2011 19

Experimental techniques • Positron annihilation lifetime spectroscopy (PALS) − Slovak University of Technology, Slovakia • Doppler Broadening Spectroscopy (DBS) − Aalto University, Finland • Magnetic Barkhausen Noise (MBN) − JRC, Petten, Netherland

PALS meaurement – MLT

Results of Positron Annihilation Lifetime Spectroscopy: Lifetimes (a); Intensities (b). a) b) ODM 751 had visible higher values – 250 ps. This signifies that MA 956 and ODS Eurofer contain defects probably with the similar size of di-vacancies, although the lifetime of MA 956 has much higher deviation. According to ΔLT 2, MA 956 can also contains three-vacancies. ODM 751 has three- and four-vacancy clusters. The intensities (percentages) of positron annihilation in the defects (I 2) differ significantly for investigated steels, i. e. for MA 956 ~ 60%, ODM 751 - 51% and ODS Eurofer – over 70%. Observed defects are categorical and they are formed during manufacture.

DBS results

Behavior of S- W parameters. The highest gradient ODM 751 is probably by the lowest defect presence.

The signal envelope of Magnetic Barkhausen noise for frequency up to 50 Hz (depth ~ 1 mm). The highest signal amplitude – BNA: ODM 751, which demonstrates the lowest concentration of all structural defects (vacancies, precipitations, grain boundaries) than in ODS Eurofer and MA 957. It can also denote lower hardness or lower level of residual stress in ODM 751. The highest residual stress belongs to ODS Eurofer. The smallest grains were found in MA 956. The highest Hpeak as well as the coarsest grains were detected for ODM 751.

Conclusion • Defect concentration, defet size (PALS) • Defect concentration, defet size (DBS) SSame or similar results • Hardness = residual stress (MBN) HHardness (residual stress) increases with defect concentration growth, no with defect size growth (precipitation vs dislocation) • ODM 751 – the lowest defect concentration, but the largest defects, • ODS Eurofer – the higher defect concentration as well as hardness. • No relation to chromium content as was assumed • Influence of Cr + Mo + W and aslo Al on hardness (creation of precipitates

Thank you! Vladimir. Slugen@stuba. sk