Jarmila Degmová

Defect Detection in Fe-Cr Alloys with Positron Annihilation Doppler Broadening Spectroscopy

Positron annihilation Doppler broadening spectroscopy (DBS) has been used for the detection of structural defects in Fe-9wt%Cr (Fe-9Cr) alloy in the as-prepared and implanted states. Defects were created by He and H ion implantation with a kinetic energy of 250 keV. DBS is a non-destructive method and is a unique tool for the observation of open volume defects like vacancies and vacancy clusters in solids. A positron beam with variable positron energy was used for the measurement of defect depth profiles up to 1.5 µm. The obtained results provide qualitative and semi-quantitative information about radiation induced defects and their chemical environment. Although the collision damage from helium implantation was one order of magnitude higher than for the case of hydrogen, the changes in S and W parameters are much less significant, probably due to considerably lower mobility of helium in the implanted materials, which results in helium capture by the created open volume defects.

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.

Investigation of laboratory produced ODS alloys

Irradiation, heat and mechanical stresses are factors which influence structural materials of nuclear power plants (NPP), e.g. reactor pressure vessel steels and may reduce lifetime of NPP operation [1-3]. High radiation and thermal loads are expected in the newest generation of nuclear power plants, such as Generation IV (GEN IV) and fusion reactors, which will be operated at temperatures between 550 - 1 000 °C and will be exposed to irradiation over 100 DPA during planned lifetime (more than 60 years) [4]. Consequently, the demands on the structural materials are very high and so the research and development needs to be focus on their improved characteristics. The advanced structural materials, as oxide-dispersion-strengthened (ODS) steels, are developed for application in cooling systems, reactor pressure vessel or fuel cladding of the GEN IV nuclear power plants. The ODS steels fulfill demands on radiation, thermal and mechanical resistance during operation of nuclear reactor. ODS steels have high the...

Positron and nanoindentation study of helium implanted high chromium ODS steels

Abstract Three oxide dispersion strengthened (ODS) steels with different chromium content (MA 956, MA 957 and ODM 751) were studied as candidate materials for new nuclear reactors in term of their radiation stability. The radiation damage was experimentally simulated by helium ion implantation with energy of ions up to 500 keV. The study was focused on surface and sub-surface structural change due to the ion implantation observed by mostly non-destructive techniques: positron annihilation lifetime spectroscopy and nanoindentation. The applied techniques demonstrated the best radiation stability of the steel ODM 751. Blistering effect occurred due to high implantation dose (mostly in MA 956) was studied in details.

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.

Positron Annihilation Measurements Performed on Oxide-Dispersion Strengthened Steels

This paper is focused on investigation of oxide-dispersion strengthened steels in a basic state (before an experimental treatment) by two positron annihilation techniques – positron annihilation lifetime spectroscopy and Doppler broadening spectroscopy. Three high chromium commercial steels – MA 956, MA 957 and PM 2000 were measured in order to identify and quantify defects in samples as well as to demonstrate defect depth profile in subsurface layers up to 1.6µm. Results indicate presence of di-vacancies in PM 2000, three-vacancies in MA 956 and six-vacancies together with dislocations in MA 957. Positron techniques were supplemented by scanning electron microscopy, which demonstrated also presence of larger defects as pores and different density of precipitates.

Analysis of Steam Generators Corrosion Products from Slovak NPP Bohunice

One of the main goals of the nuclear industry is to increase the nuclear safety and reliability of nuclear power plants (NPPs). As the steam generator (SG) is the most corrosion sensitive component of NPPs, it is important to analyze the corrosion process and optimize its construction materials to avoid damages like corrosion cracking. For this purpose two different kinds of SGs and its feed water distributing systems from the NPP Jaslovske Bohunice were studied by nondestructive Mossbauer spectroscopy. The samples were scraped from the surface and analyzed in transmission geometry. Magnetite and hematite were found to be the main components in the corrosion layers of both SGs. Dependant of the material the SG consisted of, and the location in the system where the samples were taken, the ratios between magnetite and hematite and the paramagnetic components were different. The obtained results can be used to improve corrosion safety of the VVER-440 secondary circuit as well as to optimize its water chemistry regime.

Characterisation of as-cast model steels with parametric variation of Ni, Mn, Si and Cr content

In order to understand the role and influence of Cr, Ni, Si and Mn alloying elements on the mechanical properties of steels a large spectrum of ferritic steels with parametric variation of alloying elements as well as impurities content were prepared in the frame of the SAFELIFE Action of JRC-IE and the AMES Network. The composition of the 12 prepared model steels were inspired by typical base composition of WWER-1000 and PWR materials. The testing results showed the important role of Cr and Ni concentration in model steels composition. The ranges of ductile-to-brittle transition temperature differ between high and low Cr containing steels. An increase of Cr content in model steels leads to an increase of root mean square values of the Barkhausen noise measurements, independently on Mn and Si content. On the contrary, amount of Ni influences mostly the results of Barkhausen noise and relative Seebeck coefficient measurements.

Influence of External Factors on Amorphous and Nanocrystalline Soft Magnetic Alloys Studied by Mössbauer Spectroscopy

In this paper, a review of recent 57Fe Mössbauer spectroscopy (MS) studies of external influence on the properties of amorphous and nanocrystalline Fe- and Co-based alloys is submitted. Different types of alloys (FeCuNbZr, FeCuNbSiB, FeCoCuNbB, CoFeZrB and CoFeSiB) in the form of original amorphous and nanocrystalline ribbons were subjected to different external factors: different annealing atmospheres, mechanical stress (for example influence of ball-milling) and tensile stress. It will be shown that the Mössbauer spectrometry is a suitable tool for such studies because the measured spectral parameters are very sensitive to the changes in the vicinity of the probe 57Fe-nuclei and thus, this technique provides a wide variety of information about structural and magnetic behavior of Fe-containing materials.