A. Barbu

Experimental study and modelling of copper precipitation under electron irradiation in dilute FeCu binary alloys

In order to study the effect of the supersaturation of freely migrating defects on the precipitation of copper in high residual copper pressure vessel steels under neutron irradiation, FeCu (1.34, 0.30 and 0.11 at.% Cu) model alloys were irradiated between 175 and 360°C with high energy electrons. On-line electrical resistivity measurements and small angle neutron scattering (SANS) under magnetic field experiments were carried out. The use of the latter method to get the chemical composition of the precipitates is discussed. We show that the mechanisms of precipitation are identical under electron irradiation and under thermal aging. The sole effect of electron irradiation on the precipitation is to enhance the kinetics. A modelling of copper precipitation based on cluster dynamics is proposed. A comparison between our SANS results obtained under electron irradiation and those under neutron irradiation published in the literature shows that the mechanisms of precipitation are very likely different with both kinds of particles.

Latent Tracks Do Exist in Metallic Materials

Until now experiments have failed to show that latent tracks can be formed in metallic crystals. For the first time, discontinuous tracks have been observed in crystalline Ni-Zr alloys irradiated by GeV heavy ions. It is shown that their formation results from the very high value of the energy deposited in electronic excitations.

Phonon Soft Modes and Damage Production by High Electronic Excitations in Pure Metals

It is now well known that during high-energy heavy-ion irradiation, the very high-energy deposition in electronic excitation induces a spectacular damage creation in some types of metallic targets. A selected number of pure metals has been irradiated by GeV ions in order to test some possible criteria which might be pertinent to explain such effects: electron-phonon interaction, electrical conductivity, existence of various allotropic phases,.... The present results show that the latter criterion or more precisely that the existence of a displacive transformation associated with a soft mode in the phonon spectrum seems to favour efficient energy transfers between highly excited electrons and target atoms. For titanium targets, electron microscopy observations show striations which are parallel to the incident ion beam direction.

Simulation of Irradiation Effects in Reactor Pressure Vessel Steels: the Reactor for Virtual Experiments (REVE) Project

Components of commercial nuclear reactors are subjected to neutron bombardments that can modify their mechanical properties. Prediction of in-service and post-service behaviors generally requires irradiation in so-called “test reactors” as well as subsequent mechanical testing in specialized hot cell facilities. However, the use of these research facilities is becoming more problematic, in particular due to increasing costs and decreasing availability. One way of partially mitigating these problems is to complement the empirical approach by developing tools for numerical simulation of irradiation effects in materials. The development of such tools is clearly an ambitious task that will require a long-term international collaborative effort. In this paper, we present an outline of the Reactor for Virtual Experiments (REVE) project, a collaborative European and American effort aimed at developing quantitative simulations of irradiation effects in materials. The first demonstration phase of REVE will target embrittlement of reactor pressure vessel (RPV) steels, since the effects and mechanisms of irradiation damage in this material are relatively well understood and many modeling tools have been developed or are under development in this field. As for any experiment, the input variables of the REVE simulation will be the neutron spectrum, time and temperature of irradiation, the alloy composition (e.g., Cu, Ni, Mn, and C contents) and microstructure and the unirradiated mechanical properties. The simulations will predict the irradiation-induced increases of yield stress and Charpy transition temperature as well as the decrease of toughness due to the concomitant evolution of the microstructure.

Electron microscopy observations of titanium irradiated with GeV heavy ions

Abstract GeV heavy ions induce the creation of damage in some metallic targets via electronic excitation. We report here on room temperature electron microscopy observations of titanium irradiated at 15 or 90 K by xenon, tantalum and lead ions. For sufficiently high electronic energy losses ( ⪆ 2.5 keV A −1 ), black dots aligned along the incident ion beam direction are observed. The ratio of the number of such alignments to that of impinging ions depends strongly on the irradiation conditions, namely the irradiation temperature and fluence. A tentative explanation of these observations is proposed. It involves the mechanism of point defect clustering resulting either from thermally activated migration or from athermal processes occuring in the wake of the incident ions.

Evolution of microstructure resulting from high electronic excitation during swift heavy ion irradiations

Abstract This paper is devoted to the specific damage processes resulting from high levels of energy deposition in electronic excitation (a few 10 keV/nm) in metallic targets. Such conditions are fulfilled during irradiations with GeV heavy ions. After a short discussion about the mechanisms of energy deposition in elastic and inelastic collisions, the various microstructural changes observed during swift heavy ion irradiations of metals are described. The damage resulting from electronic excitation is localized along the ion wake and consists mainly in local or long-range order modifications or even phase transformation in crystalline materials and in anisotropic growth in amorphous materials. A tentative model based on the Coulomb explosion mechanism explains how part of the energy deposited in electronic excitation can be transferred to lattice atoms and accounts for the available experimental results.

A comparison of the effect of electron irradiation and of thermal aging on the hardness of FeCu binary alloys

The hardening of binary FeCu alloys under 2.5 MeV electron irradiation between 175°C and 360°C was investigated. We show that an extra-hardening component which cannot be ascribed to copper precipitation is induced during irradiation with electrons. We discuss the possible nature of the objects responsible for this extra-hardening component. They have to be very small since they are not visible by transmission electron microscopy. We come to the conclusion that they most likely are very small interstitial clusters nucleated by random encounter of interstitials. An important issue from the technological point of view raised by this work is to know whether it is reasonable to totally ignore any hardening component of the pressure vessel steels attributed to the clustering of freely migrating point defect escaping from the core of the displacement cascades or created between them.

Precipitation of copper in iron under swift ion irradiations

Abstract After irradiation was carried out on the Fe 1.34 at% Cu supersaturated solution with 2.5 MeV electrons and 188 MeV oxygen ions at 290°C, we report new results about copper precipitation under 5.07 GeV krypton irradiation. The precipitation observed on line by electrical resistivity and after irradiation by tomographic atom probe experiments and microhardness measurements, shows that the kinetics of copper precipitation (in term of dpa calculated using the modified Kinchin–Pease model) is hundred times larger with GeV krypton ions than with MeV electrons and is the same under oxygen and electron irradiations. We show that such a behaviour can be explained only by assuming that the energy deposited in electronic excitations along the path of swift ions is actually able to induce precipitation in metallic supersaturated solid solutions.

Cluster dynamics modeling of materials: Advantages and limitations

The aim of this paper is to give a short review on cluster dynamics modeling in the field of atoms and point defects clustering in materials. It is shown that this method, due to its low computer cost, can handle long term evolution that cannot, in many cases, be obtained by Lattice Kinetic Monte Carlo methods. Indeed, such a possibility is achieved thanks to an important drawback that is the loss of space correlations of the elements of the microstructures. Some examples, in the field of precipitation and irradiation of metallic materials are given. The limitations and difficulties of this method are also discussed. Unsurprisingly, it is shown that it goes in a very satisfactory way when the objects are distributed homogeneously. Conversely, the source term describing the primary damage under irradiation, by nature heterogeneous in space and time, is tricky to introduce especially when displacement cascades are produced.

First transmission electron microscopy observation of latent tracks in a metallic compound

Abstract Until now latent tracks have never been observed by transmission electron microscopy in metallic targets. For the first time we observe discontinuous tracks in a crystalline nickel-zirconium ordered alloy irradiated at 80 K by 3 GeV uranium ions. The diameter of the tracks is of about 3 nm and their number is close to the number of incoming ions. This result is due to the very high value of the electronic stopping power of the target for uranium ions and probably to specific properties of the compound.