Jean-Marc Cloué

REFLET Experiment in OSIRIS: Relaxation under Flux as a Method for Determining Creep Behavior of Zircaloy Assembly Components

A stress relaxation on bent-beam specimen irradiation campaign was performed in the French material testing reactor OSIRIS in order to screen different materials according to their in-flux behavior and to derive constitutive laws that are used to describe the in-reactor behavior of assembly components under axial stress. This paper presents the methodology used, with particular emphasis on the validation of the different hypotheses. The methodology is illustrated with the results obtained on an industrial Zircaloy-4 (Zy-4) alloy used in guide thimbles. As an illustration of the capability of the method, the parameters of simple creep behavior laws are adjusted to the relaxation results. The creep-law predictions are compared to results obtained previously in a creep experiment.

Multiscale Analysis of Viscoplastic Behavior of Recrystallized Zircaloy-4 at 400 ° C

Zirconium alloys are used in the nuclear industry as cladding tubes to prevent the fissile material from leaking into the coolant as the first safety wall of nuclear fuel. More and more requirements on fuel performance lead to stronger mechanical solicitations and integrity of cladding tubes has to be guaranteed. In this framework, the polycrystalline models, which are based on plasticity mechanisms, have interesting advantages compared to phenomenological ones. Some previous studies have shown that a polycrystalline approach could be very useful to describe the mechanical behavior of zirconium alloys. This modelling strategy has been successfully applied to fresh material and also, more recently, to irradiated material. The micromechanical approach has been developed in the light of transmission electron microscopy (TEM) observations. These experiments have been achieved to identify the main deformation mechanisms, which occur in several grains of relaxed and crept Zircaloy-4 samples. The main purpose of this paper is to describe an improved micromechanical model able to reproduce both the anisotropic creep behavior and the elasto-plastic behavior of unirradiated recrystallized Zircaloy-4 at 400 ° C . Finally, the quantitative analyses, which have been carried out with TEM correspond well with the results provided by the micromechanical approach.

Evolution of microstructure and impact-strength energy in thermally and thermomechanically aged 15-5 PH

Due to its outstanding mechanical resistance and resistance to corrosion, alloy 15-5 PH can be beneficially used for manufacturing aerospace structural parts. Following exposure to intermediate temperature, from 300°–400 °C, the alloy embrittles through the decomposition of the martensite into iron-rich and chromium-rich domains. Depending on the ageing time, these domains are either interconnected or unconnected with each other. The embrittlement results in a drastic drop of the impact strength-energy and an increase of the ductile-to-brittle transition temperature. The initial microstructure and mechanical properties can be recovered through a re-homogenization of the distribution of chromium and iron atoms in the material in the case where the decomposition of the matrix is not too pronounced. The application of a stress higher than 60 per cent of the yield strength further enhances the ageing kinetics in the case where the combined effect of temperature and time results in the spinodal decomposition of the martensite.

Atomic Species Associated with the Portevin–Le Chatelier Effect in Superalloy 718 Studied by Mechanical Spectroscopy

In many Ni-based superalloys, dynamic strain aging (DSA) generates an inhomogeneous plastic deformation resulting in jerky flow known as the Portevin–Le Chatelier (PLC) effect. This phenomenon has a deleterious effect on the mechanical properties and, at high temperature, is related to the diffusion of substitutional solute atoms toward the core of dislocations. However, the question about the nature of the atomic species responsible for the PLC effect at high temperature still remains open. The goal of the present work is to answer this important question; to this purpose, three different 718-type and a 625 superalloy were studied through a nonconventional approach by mechanical spectroscopy. The internal friction (IF) spectra of all the studied alloys show a relaxation peak P718 (at 885 K for 0.1 Hz) in the same temperature range, 700 K to 950 K, as the observed PLC effect. The activation parameters of this relaxation peak have been measured, Ea(P718) = 2.68 ± 0.05 eV, τ0 = 2·10−15 ± 1 s as well as its broadening factor β = 1.1. Experiments on different alloys and the dependence of the relaxation strength on the amount of Mo attribute this relaxation to the stress-induced reorientation of Mo-Mo dipoles due to the short distance diffusion of one Mo atom by exchange with a vacancy. Then, it is concluded that Mo is the atomic species responsible for the high-temperature PLC effect in 718 superalloy.

Influence of Chloride Ions as Contaminants on the Corrosion Behavior of Alloy 718 in Pool Water of Nuclear Power Plants

The electrochemical behavior of alloy 718 in a chloride-containing boric acid solution was studied to determine the influence of chloride ions as contaminants of pool water of nuclear power plants on the corrosion behavior of the alloy. Experiments were performed at 20°C and 60°C with chloride concentrations from 1.5 to 15 000 ppm, using stationary measurements i.e. OCP versus time measurements and plotting of current-potential curves. After the electrochemical tests, the samples were observed using optical microscopy. Immersion tests in chloride-containing boric acid solutions were also carried out: samples were immersed for a time as long as 17 weeks at open circuit potential and their residual mechanical properties were measured. Results showed that, whatever the chloride concentration, there was no corrosion for samples immersed at open circuit potential. However, when the samples were polarized at high potentials, intergranular corrosion might be observed in occluded zones.