Pascal Berger

Oxygen diffusion studies in oxide scales thermally grown or deposited on mechanically loaded metallic surfaces (MS-P2)

Abstract The high temperature growth of oxides on metallic surfaces mechanically in loading is not well understood yet. The knowledge of the growth of oxides on static surfaces and of the mechanical behavior of the metal/oxide system does not give account of the synergetic effects between the load and the growth of the oxide. The formation of cracks on the oxide scales, their healing and the role of the load on the oxygen diffusion processes have been studied on pure nickel and zirconium samples in creep. The use of oxygen-18 to study the oxygen diffusion and the determination of local oxygen-18 diffusion profiles with use of the nuclear reaction 18O(p, α)15N show a sharp influence of the load. The application of the load induces an increase of the oxygen diffusion coefficients until two orders of magnitude (typically from around 10 −15 cm 2 s −1 to around 10 −13 cm 2 s −1 in NiO thermally grown on nickel monocrystals). However, this enhancement decreases with the increase of the load. As the oxide scales are multilayered and as spalling and regrowth may occur, RBS mapping of the local thickness of the oxide stripes is also performed. This technique helps in understanding the formation of the microstructure and of the damaging process during the mechanical loading.

High temperature oxidation of zirconium and zircaloy-4 under applied load: Nuclear microprobe study of the growth of the oxide

Abstract The understanding of the mechanisms of high temperature growth of zirconium oxide on zircaloy alloys is of considerable importance owing to the world-wide use of zirconium in nuclear plants. Spallations and growth recovery are known to originate in structural changes of the oxide, but the induced modifications in the oxygen transport have still to be investigated. The zirconia is known to grow mainly by anionic transport, but, due to the very high solubility of oxygen in zirconium, the oxygen penetration profile may be rather complex and extend beyond the oxide/metal interface. The nuclear microprobe offers an attractive way to investigate the oxygen penetration, both from RBS and NRA. In the case of parallel mechanical loading and high temperature oxidation, the oxygen diffusivity is very sensitive to the generated defects in the oxide scales. The use of alternate 16O/18O atmosphere and subsequent local microanalysis by means of 18O(p,α)15N nuclear reaction allows to investigate the effect of the mechanical loading on the oxygen penetration.

Influence of laser–target interaction regime on composition and properties of surface layers grown by laser treatment of Ti plates

Surface laser treatment of commercially pure titanium plates was performed in air using two different Nd : YAG sources delivering pulses of 5 and 35 ns. The laser fluence conditions were set to obtain with each source either yellow or blue surface layers. Nuclear reaction analysis (NRA) was used to quantify the amount of light elements in the formed layers. Titanium oxinitrides, containing different amounts of oxygen and nitrogen, were mainly found, except in the case of long pulses and high laser fluence, which led to the growth of titanium dioxide. The structure of the layers was studied by x-ray diffraction and Raman spectroscopy. In addition, reflectance spectra showed the transition from a metal-like behaviour to an insulating TiO2-like behaviour as a function of the treatment conditions.Modelling of the laser–target interaction on the basis of the Semak model was performed to understand the different compositions and properties of the layers. Numerical calculations showed that vaporization dominates in the case of short pulses, whereas a liquid-ablation regime is achieved in the case of 35 ns long pulses.

Nuclear microprobe study of stress-oxidation of nickel

Abstract In order ot study the stress oxidation, a specific device has been designed to perform mechanical loadings under controlled temperature and oxygen pressure. Time-dependent strain-stress curves may be recorded according to the usual modes: creep, fatigue and fatigue-creep. A set of two sorption pumps allows to alternate oxygen-16 and oxygen-18. Oxygen-18 diffusion profiles have been measured by SIMS (Secondary Ion Mass Spectroscopy) and by nuclear microprobe, by means of the 18 O(p, α) 15 N nuclear reaction to characterize selected areas. For the purpose of further investigations on nickel-based alloys, initial experiments have been performed on pure nickel to obtain knowledge of the fundamental properties of nickel. Because of complex relationships between mechanical behaviour, microstructure modifications and oxygen diffusion, both polycrystalline and monocrystalline samples have been investigated.

Atomic-resolved depth profile of strain and cation intermixing around LaAlO 3 /SrTiO 3 interfaces

Novel behavior has been observed at the interface of LaAlO3/SrTiO3 heterostructures such as two dimensional metallic conductivity, magnetic scattering and superconductivity. However, both the origins and quantification of such behavior have been complicated due to an interplay of mechanical, chemical and electronic factors. Here chemical and strain profiles near the interface of LaAlO3/SrTiO3 heterostructures are correlated. Conductive and insulating samples have been processed, with thicknesses respectively above and below the commonly admitted conductivity threshold. The intermixing and structural distortions within the crystal lattice have been quantitatively measured near the interface with a depth resolution of unit cell size. A strong link between intermixing and structural distortions at such interfaces is highlighted: intermixing was more pronounced in the hetero-couple with conductive interface, whereas in-plane compressive strains extended deeper within the substrate of the hetero-couple with the insulating interface. This allows a better understanding of the interface local mechanisms leading to the conductivity.

On the 22–23 K superconducting phase in the Y–Pd–B–C system

Abstract Superconducting and non-superconducting (annealed) samples of YPd 5 B 3 C 0.35 have been investigated using electrical resistance and magnetization measurements, X-ray diffraction, electron diffraction with energy dispersive X-ray spectrometry and electron probe micro analysis (EPMA). In the superconducting sample, six phases were observed out of which two were clearly decomposed by annealing. Composition and unit cell of these phases were determined. The concentrations of boron and carbon are uncertain, due to the small atomic weight of these elements. Therefore, we report for the first time, nuclear probe micro analysis (NPMA) measurements. Preliminary results of NPMA are consistent with EPMA. At last, we suggest that the superconducting phase has a composition close to YPd 2 B 2 C and an I -centred tetragonal unit cell with a =3.751(1) and c =10.725(3) A.

Diffusion-controlled liquid bismuth induced intergranular embrittlement of copper

The consequences of the contact between liquid bismuth and a copper bicrystal are investigated at 500°C. Atoms of bismuth are shown to penetrate and embritlle the copper grain boundary. Grain boundary concentration profiles of bismuth are obtained on fracture surfaces by both Auger electron spectroscopy and He4+ Rutherford backscattering spectroscopy. The maximum bismuth intergranular concentration is calculated from experimental data to be about 1.7 monolayers (near the liquid bismuth / solid copper interface). The overall profiles are significantly different from typical erfc profiles and an interpretation is proposed, based on the coupling effect between grain boundary diffusion and non-linear segregation. These results allow us to conclude on the absence of grain boundary wetting for the Cu / Bi system at 500°C.

Evidence for a Diffusion-Based Mechanism of Liquid Metal Intergranular Penetration: Case Study of a Ni-Bi Model System

A model Ni-Bi system has been used to investigate intergranular penetration (IGP) phenomenon. All experiments have been done on Ni 26°<110> bicrystal at 700°C using bismuth vapour condensation as a source of liquid bismuth. Such a procedure results at room temperature in either partial or total Liquid Metal Induced Embrittlement (LMIE) of a unique grain boundary, depending on the duration of liquid Bi / solid Ni contact at 700°C. Auger Electron Spectrometry (AES) and Rutherford Backscattering Spectrometry (RBS) have been used to measure the Bi concentration profile between the source of liquid bismuth and the penetration front. Two zones have been clearly identified : the first one of almost constant Bi concentration called nanometrethick film which is interpreted in terms of Fowler-Guggenheim multi-layer segregation under local equilibrium conditions and the second one with a progressive decrease of Bi concentration over a distance of the order of 20-200µm. Such a long transition zone, together with parabolic diffusion kinetics indicates diffusion-based mechanism of intergranular penetration as opposed to the direct grain boundary wetting.

Noise-induced local heatings in microbeam analysis

Heatings produced by the irradiation with light ion microbeams were measured in situ by using microthermocouples deposited and etched onto a SiO2 glass substrate. For a given distance r between the microbeam centre and the thermocouple, we show that the heating averaged over the irradiation time decreases as 1/r, in agreement with theory. Moreover, we report strong heating random fluctuations close to the beam resulting from the fluctuations of the beam intensity. We show that, close to the beam, these heating fluctuations dominate largely over the averaged heating, as predicted by the theory of heat propagation with a random source.

Noise-induced local heatings in beam irradiation

In every kind of beam irradiation, a part of the incident power is eventually dissipated within the irradiated sample, leading to a heating which has to be evaluated, in order to be sure that thermal effects (such as diffusion, phase transitions, etc.) are under control. Expanding on usual thermal estimations based upon the coherent part of the incident power, we focus on the thermal effects due to the noisy (i.e., incoherent) part of the power. Starting from the autocorrelation of the beam, we show analytically that white noise induces an extra heating whose relative influence increases as the distance to the center of the beam decreases. Relying on the physical results derived for white noise, a simple method is proposed to extract the main aspects of these extra heatings from experiments where the autocorrelation function of the beam is not known a priori. Experimental data from MeV light ion microbeam are briefly presented as an example.