Vladimir Krsjak

NDT study of oxide dispersion strengthened steels

Various oxide dispersion strengthened (ODS) steels (PM2000, MA956, ODM751 and ODS Eurofer) have been investigated using positron annihilation lifetime spectroscopy. Preliminary characterization of the vacancy type defects and the yttria nanoparticles are reported in this paper. The interpretation of the experimental data considers also the results of magnetic Barkhausen noise (BN) measurements as well as SEM and TEM investigations. Significant differences due to the presence of the yttria oxides (namely Y2O3) were observed in the studied materials in comparison to conventional ferritic/martensitic steels (e.g. Eurofer). Higher positron mean lifetime in ODS steels is, however, not only due to the presence of dispersoids, but also a result of vacancy agglomeration (clusters of 6–8 vacancies) which have been confirmed in recrystallised ODM751 and MA956 materials. On the other hand, positron trapping at dislocations was observed in the as-extruded ODS Eurofer in contrast to the other, recrystallised, materials where the high temperature treatment had led to the static recovery of these defects. It is suggested that some defects which are present (dislocations, vacancy clusters and dispersoids) and affect positron trapping in the materials are also important pinning sites for the magnetic domains. This is reflected by a shift of the signal peak in the BN spectra. The present complementary study provides more comprehensive information about materials microstructure and can support the interpretation of the physical/mechanical testing results (hardness, fracture mechanics etc.) obtained on these materials.

Vacancy Type Defects in Oxide Dispersion Strengthened Steels

This study was focused on commercial oxide-dispersion strengthened (ODS) steels - MA 956 (20%Cr), PM 2000 (19%Cr), ODM 751 (16%Cr) and MA 957 (14%Cr) developed for fuel cladding of GEN IV reactors. The ODS steels are described in order to comparison of their microstructure features. Vacancy defects were observed by Doppler Broadening Spectroscopy (DBS) and Positron Annihilation Lifetime Spectroscopy (PALS). Residual stress proportional to all kinds of defects was investigated by Magnetic Barkhausen Noise (MBN) measurement. The highest presence of open volume defects was found in MA 956 and the lowest defect concentration in MA 957, although this steel contains the largest defects (six-vacancies together with dislocations). Other investigated steels demonstrated probably three- or four-vacancy clusters. Further, results from positron technique indicated proportionality of chromium content to defect concentration. Magnetic Barkhausen noise results also showed that Hpeak value (describing grain size) increased with growth of chromium content. However residual stress was independent on chromium level.

Thermal Annealing Influence on Ions Implanted Fe-Cr Model Alloys

Abstract. Binary Fe-11.62wt%Cr alloys were investigated in as-received state as well as after a two step helium ion implantation at different energies (100 keV and 250 keV) with doses up to 3.12×1018 cm-2. In order to study changes in alloys in dependence on the temperature, thermal annealing was performed at temperatures of 400, 475, 525 and 600 °C and specimens were afterwards measured by a pulsed low energy positron system (PLEPS). Annealing out of defects at lower temperatures was not as significant as expected, and we also encountered difficulties with defect identification. However, an apparent decrease of defect size was observed in the specimen annealed at a temperature of 600 °C.

Advanced Fe-Cr Alloys Studied by Pulsed Low Energy Positron System Before and After Helium Ions Implantation

Positron annihilation spectroscopy (PAS) is a non-destructive technique which provides information about microstructural damage of structural materials. In this paper, the Pulsed Low Energy Positron System (PLEPS) at the research reactor FRM-II at TU Munich was used to study depth profiling of binary Fe-Cr alloys. Fe-Cr model alloys with different chromium content were investigated in the as-received state as well as after helium ion implantation (dose up to 6.24×1017 ions/cm−2 ). Measured results show changes in the size of defects after implantation and also in non-implanted specimens depending on the Cr content.Copyright

Application of Mossbauer and positron annihilation spectroscopies for monitoring of WWER RPV steel degradation

This paper describes the application of Mossbauer Spectroscopy (MS) and Positron Annihilation Spectroscopy (PAS) to evaluate the microstructure parameters of materials. These methods are used to collect additional characteristics for understanding of degradation processes in Reactor Pressure Vessel (RPV) steels. Samples from the Russian 15Kh2MFA and Sv10KhMFT steels, commercially used at WWER-440 reactors, were irradiated near the core at NPP Bohunice (Slovakia) to neutron fluences in the range 7.8 ? 1023 m−2 to 2.5 ? 1024 m−2. Systematic changes in the MS and PAS spectra were observed mainly during the early period of irradiation. These could be due to changes caused by the precipitation of elements such as Cu, P or Cr, mainly in carbides to the surface. The MS results confirm that the close environment of Fe atoms in the body-centred cubic lattice of RPV steels remain almost stable after initial changes and can be correlated with the ductile?brittle transition temperature curve from mechanical tests. Po...