Oxidation kinetics and mechanisms of Ni-base alloys in pressurised water reactor primary conditions: Influence of subsurface defects

International audienceOxidation of Alloy 690 in PWR primary water conditions has been investigated, considering particularly the role played by subsurface structural defects. To simulate a defective surface state, Xe implantation has been set up on samples. Corrosion experiments were thereafter performed in a corrosion loop simulating the PWR medium with durations between 24 h and 1000 h. Microstructural observations and NRA measurements underlined the role played by defects on the crystallinity of the continuous oxide spinel layer, on the nucleation of Cr2O3 nodules and on the oxidation rate. The higher defects concentration seemed to modify the oxygen diffusion in the oxide scale

SIMS as a new methodology to depth profile helium in as-implanted and annealed pure bcc metals?

International audienceReliable He profiles are highly desirable for better understanding helium behavior in materials for future nuclear applications. Recently, Secondary Ion Mass Spectrometry (SIMS) allowed the characterization of helium distribution in as-implanted metallic systems. The Cs+ primary ion beam coupled with CsHe+ molecular detector appeared to be a promising technique which overcomes the very high He ionization potential.In this study, 4He depth profiles in pure body centered cubic (bcc) metals (V, Fe, Ta, Nb and Mo) as-implanted and annealed, were obtained by SIMS. All as-implanted samples exhibited a projected range of around 200 nm, in agreementwith SRIM theoretical calculations. After annealing treatment, SIMS measurements evidenced the evolution of helium depth profile with temperature. The latter SIMS results were compared to the helium bubble distribution obtained by Transmission Electron Microscopy (TEM).This study confirmed the great potential of this experimental procedure as a He-depth profiling technique in bcc metals. Indeed, the methodology described in this work could be extended to other materials including metallic and non-metallic compounds. Nevertheless, the quantification of helium concentration after annealing treatment by SIMS remains uncertain probably due to the non-uniform ionization efficiency in samples containing large bubbles

Implementation of heavy-ion elastic recoil detection analysis at JANNUS-Saclay for quantitative helium depth profiling

International audienceQuantitative depth profiling measurements of implanted light elements is an important issue for electronics and nuclear applications. Conventional elastic recoil detection analysis (ERDA) has been improved by using heavy ions as incident particles for quantitatively profiling helium in materials.A new system has been implemented on the triple beam irradiation platform JANNUS at Saclay devoted to carry out HI-ERDA measurements. This device is dedicated to helium depth profiling using a 15 MeV 16O5+ incident ion beam. Capabilities of the technique (quantitative analysis, resolution and limit of detection) were tested on samples of known composition.For the first time, 4He depth profiles in pure α-iron, as-implanted and annealed, are obtained. HI-ERDA measurements have shown that helium release in pure α-iron can be described by a succession of two steps, the first having a slow kinetics below 700C and the second with a fast kinetics above 700C

Diffusion processes as possible mechanisms for Cr depletion at SCC crack tip

International audienceTwo mechanisms are studied to explain the asymmetrical chromium depletions observed ahead of SCC crack tips in nickel-base alloys: diffusion-induced grain boundary migration (DIGM) and plasticity-enhanced diffusion. On the one hand, DIGM is evidenced in a model Alloy 600 by focused ion beam (FIB) coupled with scanning electron microscopy (SEM) cross-section imaging and analytical transmission electron microscopy (TEM) after annealing at 500 °C under vacuum and at 340 °C after exposure to primary water. The occurrence of grain boundary migration depends on the grain boundary character and misorientation. On the other hand, the effect of plasticity on chromium diffusion in nickel single-crystals is investigated by performing diffusion tests during creep tests at 500 and 350 °C. An enhancement of Cr diffusion is observed and a linear relationship between the diffusion coefficient and strain rate is evidenced. At last, in an attempt to discriminate the two mechanisms, an analytical modeling of the Cr-depleted areas observed at propagating SCC crack tips is proposed