P. Pareige

Synthesis of atom probe experiments on irradiation-induced solute segregation in French ferritic pressure vessel steels

Abstract Microstructural changes due to neutron irradiation cause an evolution of the mechanical properties of reactor pressure vessels (RPV) steels. This paper aims at identifying and characterising the microstructural changes which have been found to be responsible in part for the observed embrittlement. This intensive work relies principally on an atom probe (AP) study of a low Cu-level French RPV steel (Chooz A). This material has been irradiated in in-service conditions for 0–16 years in the frame of the surveillance program. Under this aging condition, solute clustering occurs (Cu, Ni, Mn, Si, P, …). In order to identify the role of copper, experiments were also carried out on Fe–Cu model alloys submitted to different types of irradiations (neutron, electron, ion). Cu-cluster nucleation appears to be directly related to the presence of displacement cascades during neutron (ion) irradiation. The operating basic physical process is not clearly identified yet. A recovery of the mechanical properties of the irradiated material can be achieved by annealing treatments (20 h at 450°C in the case of the RPV steel under study, following microhardness measurements). It has been shown that the corresponding microstructural evolution was a rapid dissolution of the high number density of irradiation-induced solute clusters and the precipitation of a very low number density of Cu-rich particles.

A model accounting for spatial overlaps in 3D atom-probe microscopy

The spatial resolution of three-dimensional atom probe is known to be mainly controlled by the aberrations of ion trajectories near the specimen surface. An analytical model accounting for the spatial overlaps that occur near phase interfaces is described. This model makes it possible to correct the apparent composition of small spherical precipitates in order to determine the true composition. The prediction of the overlap rate as a function of the particle size was found in remarkably good agreement with the simulations of ion trajectories that were made. The thickness of the mixed zone around beta precipitates was found to be of 0.3 nm for a normalised evaporation field of beta phase of 0.8. Using simulations, the overlap rate could be parameterised as a function of the apparent atomic density observed in particles. This model has been applied to copper precipitation in FeCu.

APFIM investigation of clustering in neutron-irradiated FeCu alloys and pressure vessel steels

Abstract Pressure vessel steels used in PWRs are known to be prone to hardening and embrittlement under neutron irradiation. The changes in mechanical properties are commonly supposed to result from the formation of point defects, dislocation loops, voids and copper-rich precipitates. However, the real nature of the irradiation induced damage, in these particularly low copper steels ( A new experimental work has been carried out thanks to atom probe and field ion microscopy (APFIM) facilities and, more particularly with a new generation of atom probe recently developed, namely the tomographic atom probe (TAP), in order to improve: - the understanding of the complex behavior of copper precipitation which occurs when low-alloyed Fe Cu model alloys are irradiated with neutrons; - the microstructural characterization of the pressure vessel steel of the CHOOZ A reactor under various fluences (French Surveillance Programme). The investigations clearly reveal the precipitation of copper-rich clusters in irradiated Fe Cu alloys while more complicated Si, Ni, Mn and Cu-solute ‘clouds’ were observed to develop in the low-copper ferritic solid solution of the pressure vessel steel.

Atom probe characterization of the microstructure of nuclear pressure vessel surveillance materials after neutron irradiation and after annealing treatments

Abstract Microstructural changes due to neutron irradiation of weld and forging materials were characterized using the atom probe field ion microscope (APFIM). Neutron-induced clusters containing Cu, P, Ni, Mn and Si were detected in the high copper weld (0.24 at.% Cu) after irradiation to fluences of 6.6 × 1022 and 3.47 × 1023 n m−2; only phosphorus atmospheres were observed in the low copper forging material (0.02 at.% Cu) irradiated to an intermediate fluence of 1.5 × 1023 n m−2. These results are in agreement with previous studies and with their respective measured transition temperature shifts. In addition, APFIM experiments were carried out on the high fluence weld material after two post-irradiation annealing treatments. The first annealing treatment of 168 h at 454°C is similar to the proposed condition for in situ pressure vessel annealing and the second, 29 h at 610°C, is similar to the final stress relief heat treatment employed in vessel fabrication. Annealing at 454°C led to coarsening of the copper-enriched precipitates and a 92% recovery of the radiation-induced transition temperature shift. Essentially complete rehomogenization of the solutes was obtained in the simulated stress relief treatment at 610°C.

Evidence of superparamagnetic co clusters in pulsed laser deposition-grown Zn0.9Co0.1O thin films using atom probe tomography.

Nanosized Co clusters (of about 3 nm size) were unambiguously identified in Co-doped ZnO thin films by atom probe tomography. These clusters are directly correlated to the superparamagnetic relaxation observed by ZFC/FC magnetization measurements. These analyses provide strong evidence that the room-temperature ferromagnetism observed in the magnetization curves cannot be attributed to the observed Co clusters. Because there is no experimental evidence of the presence of other secondary phases, our results reinforce the assumption of a defect-induced ferromagnetism in Co-doped ZnO diluted magnetic semiconductors.

Microstructural characterization of atom clusters in irradiated pressure vessel steels and model alloys

Abstract In order to characterize the microstructural evolution of the iron solid solution under irradiation, two pressure vessel steels irradiated in service conditions and, for comparison, low copper model alloys irradiated with neutrons and electrons have been studied. The characterization has been carried out mainly thanks to small angle neutron scattering and atom probe experiments. Both techniques lead to the conclusion that clusters develop with irradiations. In Fe-Cu model alloys, copper clusters are formed. These clusters do not appear to be pure copper. Nevertheless, their actual shape and composition are still a matter of investigation. In the low copper industrial steels, the feature is more complex. Solute atoms like Ni, Mn and Si, sometimes associated with Cu, segregate as clusters. These Si, Ni, Mn associations, may facilitate the copper segregation although the initial iron matrix contains a low copper concentration.

Si nanoparticles in SiO2 An atomic scale observation for optimization of optical devices

Three-dimensional imaging of silicon nanoclusters array in silicon-rich silicon oxide layers was evidenced and studied. The atom probe tomography technique allows to give the composition of the nanoclusters and the composition of the interface with the silica matrix. These results give new insights for the understanding of the properties of Si-based photonic devices.

Incorporation and redistribution of impurities into silicon nanowires during metal-particle-assisted growth

The incorporation of metal atoms into silicon nanowires during metal-particle-assisted growth is a critical issue for various nanowire-based applications. Here we have been able to access directly the incorporation and redistribution of metal atoms into silicon nanowires produced by two different processes at growth rates ranging from 3 to 40 nm s(-1), by using laser-assisted atom probe tomography and scanning transmission electron microscopy. We find that the concentration of metal impurities in crystalline silicon nanowires increases with the growth rate and can reach a level of two orders of magnitude higher than that in their equilibrium solubility. Moreover, we demonstrate that the impurities are first incorporated into nanowire volume and then segregate at defects such as the twin planes. A dimer-atom-insertion kinetic model is proposed to account for the impurity incorporation into nanowires.

Boron distribution in the core of Si nanowire grown by chemical vapor deposition

The boron dopant distribution in Si nanowires grown by the Au-catalyzed chemical vapor deposition is characterized by laser-assisted atom probe tomography. A convenient and an effective method for performing the atom probe tomography of an individual nanowire is developed. Using this technique, we demonstrate that when Si nanowires are doped with boron at high silane partial pressure, the radial distribution of boron atoms is rather inhomogeneous. Much more boron atoms incorporate at the periphery than in the center, with the concentration increasing by an order of magnitude as the distance from the nanowire axis increases from zero to only 15 nm. A theoretical model is presented that is capable of describing the observed spatial inhomogeneity of boron dopant. We also consider different kinetic pathways of boron incorporation and discuss the values of diffusion length and diffusion coefficients obtained by fitting the experimental data.

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.