Papers by Author: Pascal Berger

Abstract: In order to study the nucleation and growth of cracks in the outer oxide scale which expand into the underlying alloy, deformation in creep in oxygen or in vacuum of zirconium and Zircaloy-4 has been studied mainly at 500°C. Influence of applied stresses, atmosphere and alloy’s grade on the deformation and oxidation processes are especially analyzed. The results underline the presence of two distinct deformation domains for both alloys grades, depending on the applied stress value. The presence of the oxide scale leads only to slight modifications on the deformation mechanism but it induces an increase of the deformation rate. This enhancement is especially observed in the case of the pre-oxidized Zircaloy-4 whose cracks remain mainly located in the outer part of the oxide. In opposite, the pre-oxidized zirconium shows cracks located down to the underlying metal. Acoustic emission is used to follow, in situ, in temperature the damage process of the outer zirconia layer during creep, and precisions about the oxidation mechanism and the effect of applied stress on oxygen diffusion and oxide growth rate are obtained thanks to the use of 18O as a marker.

Abstract: 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.

Abstract: 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.