B. Radiguet

Comparison of radiation-induced segregation in ultrafine-grained and conventional 316 austenitic stainless steels

Due to a high number density of grain boundaries acting as point defect sinks, ultrafine-grained materials are expected to be more resistant to irradiation damage. In this context, ultrafine-grained 316 austenitic stainless steel samples have been fabricated by high pressure torsion. Their behavior under ion irradiation has been studied using atom probe tomography. Results are compared with those obtained in an ion irradiated conventional coarse-grained steel. The comparison shows that the effects of irradiation are limited and that intragranular and intergranular features are smaller in the ultrafine-grained alloy. Using cluster dynamic modeling, results are interpreted by a higher annihilation of point defects at grain boundaries in the ultrafine-grained steel.

Segregation-controlled nanocrystallization in an Al–Ni–La metallic glass

The early stages of devitrification of Al89Ni6La5 metallic glass were investigated using three-dimensional atom probe. Even in the as-quenched amorphous state, nanometer-sized fluctuations of Ni and Al were observed. The Al-rich fluctuations act as nucleation sites for the crystallization at 443K in which α-Al clusters with a number density up to 5×1023m−3 nucleate and grow to nonspherical, 12nm large crystals. α-Al particles are almost pure in Al and are surrounded by a La-rich shell. It is concluded that La segregation controls the growth and limits the size of the α-Al nanocrystals.

Atomic-level observation with three-dimensional atom probe of the solute behaviour in neutron-, ion- or electron-irradiated ferritic alloys

The three-dimensional atom probe provides one of the most effective tools to characterize, with near atomic resolution, the solute distribution and the early stage of precipitation in metallic alloys. This paper presents results on the application of this technique to pressure vessel steels and model alloys. It is shown that, in neutron irradiated samples from VVER reactors, after annealing and re-irradiation, no Cu–Si–Mn–Ni–P-enriched clusters form, in contrast to the irradiated but not annealed samples. In the case of model alloys, it is shown that under electron irradiation copper clustering may or may not be observed depending on the type of irradiated materials. On the other hand the microstructures of low and high supersaturated Fe–Cu alloys irradiated under similar Fe-ion irradiations are the same.

Analysis of Radiation Damage in Light Water Reactors: Comparison of Cluster Analysis Methods for the Analysis of Atom Probe Data

Irradiation of reactor pressure vessel (RPV) steels causes the formation of nanoscale microstructural features (termed radiation damage), which affect the mechanical properties of the vessel. A key tool for characterizing these nanoscale features is atom probe tomography (APT), due to its high spatial resolution and the ability to identify different chemical species in three dimensions. Microstructural observations using APT can underpin development of a mechanistic understanding of defect formation. However, with atom probe analyses there are currently multiple methods for analyzing the data. This can result in inconsistencies between results obtained from different researchers and unnecessary scatter when combining data from multiple sources. This makes interpretation of results more complex and calibration of radiation damage models challenging. In this work simulations of a range of different microstructures are used to directly compare different cluster analysis algorithms and identify their strengths and weaknesses.

Effect of Mn on the Nanoprecipitation in Binary Fe-Cu alloys

Effect of the third alloying element Mn on Cu-precipitation was studied in a binary Fe-1.3% Cu alloy. Precipitation in both the alloys was investigated after homogenization treatment and subsequent artificial aging. Advanced characterization techniques such as Positron Annihilation Spectroscopy (PAS) and Tomographic Atom Probe (TAP) were used to establish the chemical composition, morphology, size and number density of the Cu-rich phases. Combined results of PAS and TAP were particularly useful in order to follow the Cu precipitation in the binary alloy. At short aging times, addition of Mn significantly increased the kinetics of hardening while its effect on the magnitude of precipitation strengthening is only marginal. It further increases the over-aging kinetics.

A Round Robin Experiment: Analysis of Solute Clustering from Atom Probe Tomography Data.

1. Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI, U.S.A. 2. Groupe de Physique des Matériaux, UMR CNRS 6634, Université de Rouen, Saint Etienne du Rouvray Cedex, France 4. NRC “Kurchatov Institute”, Moscow, Russia 3. Institute of Nuclear Safety System, Inc., Kyoto, Japan 5. Commissariat à l’Energie Atomique (CEA), Saclay, France 6. Department of Materials, University of Oxford, U.K. 7. Institute for Materials Research, Tohoku University, Oarai Japan 8. Materials Science Research Laboratory, Central Research Institute of Electric Power Industry, Nagasaka, Japan 9. Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, WA, U.S.A. 10. Department of Physics, Chalmers University of Technology, Chalmers, Sweden 11. Departement Métallurgie, EDF, Moret sur Loing, France 12. Electric Power Research Institute, Palo Alto, CA, U.S.A.

Study of Nano-cluster Formation in Fe-18Cr ODS Ferritic Steel by Atom Probe Tomography

A high chromium ferritic Oxide Dispersion Strengthened steel was produced by mechanical alloying of Fe-18Cr-1W-0.3Ti-0.3Ni-0.15Si and 0.5% Y2O3 (wt.%) powders in industrial attritor, followed by hot extrusion at 1100°C. The material was characterized by Atom Probe Tomography on each step of manufacturing process: as-milled powder and in final hot extruded state. In addition, to get information on clustering kinetics the powder was also characterized after annealing at 850°C during 1 hour. Atom Probe Tomography revealed that the oxide dispersion strengthened steel Fe-18Cr contains nanometer scale yttrium- and oxygen-enriched nanoclusters in as-milled state. Their evolution is shown after subsequent annealing and hot extrusion. More well defined nanophases also enriched in Ti are observed. A mechanism of their formation is proposed. Mechanical alloying results in supersaturated solid solution with presence of small Y- and O-enriched clusters. Subsequent annealing stimulates incorporation of Ti to the nucleii that were previously formed during mechanical alloying.

Crystallization Behavior of Al87Ni5La7Zr1 Metallic Glass

Nano-crystalline-amorphous Al based alloys with minor additions of rare earth elements and transition metals are of technical interest, because of their extraordinary high mechanical strength. This strengthening effect depends strongly on the alloy composition and the pathway of crystallisation. The crystallisation behaviour of Al87Ni6La7 and of Al87Ni5La7Zr metallic glass was studied with complementary methods such as XRD, TEM and 3D-AP. The amorphous matrix of the Zr-containing glass shows fluctuations of all minor elements on the nanometer scale. It is suggested, that these fluctuations act as nucleation zones for the crystallization of the glass during annealing.

On the Use of Density-Based Algorithms for the Analysis of Solute Clustering in Atom Probe Tomography Data

Because atom probe tomography (APT) provides three-dimensional reconstructions of small volumes by resolving atomic chemical identities and positions, it is uniquely suited to analyze solute clustering phenomena in materials. A number of approaches have been developed to extract clustering information from the 3D reconstructed dataset, and numerous reports can be found applying these methods to a wide variety of materials questions. However, results from clustering analyses can differ significantly from one report to another, even when performed on similar microstructures, raising questions about the reliability of APT to quantitatively describe solute clustering. In addition, analysis details are often not provided, preventing independent confirmation of the results. With the number of APT research groups growing quickly, the APT community recognizes the need for educating new users about common methods and artefacts, and for developing analysis and data reporting protocols that address issues of reproducibility, errors, and variability. To this end, a round robin experiment was organized among ten different international institutions. The goal is to provide a consistent framework for the analysis of irradiated stainless steels using APT. Through the development of more reliable and reproducible data analysis and through communication, this project also aims to advance the understanding between irradiated microstructure and materials performance by providing more complete quantitative microstructural input for modeling. The results, methods, and findings of this round robin will also apply to other clustering phenomena studied using APT, beyond the theme of radiation damage.

Using Microscopy to Help with the Understanding of Degradation Mechanisms Observed in Materials of Pressurized Water Reactors

Even if temperature, pressure and chemistry of the cooling water are not very high and aggressive, materials used in PWRs (Pressurized Water Reactors) are exposed to different degradation mechanisms. One of the main goals of the research programs in this field is to develop physical model of the mechanisms down to the atomic scale. Such approach needs a clear description and understanding of the degradation mechanisms at the same small scale. This paper illustrates the benefits of different microscopies and of their last improvements up to the promising possibilities of monochromated and aberrations corrected TEM/STEM. A specific focus is placed on four different degradation mechanisms observed in austenitic stainless steel: irradiation ageing, corrosion fatigue, stress corrosion cracking and corrosion.