Papers by Author: Jérôme Favergeon

Abstract: There are few articles that mention fractal dimension in grain growth mechanism. Some authors build a simplified analytic model showing that initial fractal dimension of grain boundary has an influence on interface modification velocity. Nevertheless they postulate the relation where L is the grain length, c is a constant, s is grain size and the fractal dimension. The aims of this paper is to experimentally analyze by image analysis the fractal dimension of A5 aluminum sheet grain boundaries during heating and to simulate their evolution by a Monte Carlo method to validate experimental data.. It is shown by Monte-Carlo simulation and confirmed experimentally that the grain growth process decreases the fractal dimension of grain border. It can be concluded that it is very hazardous to build a model of grain growth without including the effect of grains morphology. The macroscopic fractal morphology of the grain structure could then be used to validate microscopic relation between Monte Carlo Steps time and real time.

Abstract: Fractal functions are used to model a metallic interface. An analytical model described by three partial differential equations is built to model time evolution of the surface during heating including three different mechanisms of diffusion: superficial diffusion (SD), volume diffusion (VD) and diffusion by evaporation-condensation (DEC). Initial topographies are modeled by Stochastic Weierstraβ functions because of their ability to reproduce experimental roughness profiles. Applied to an aluminum alloy at 550°C, a high number of roughness parameters and their variance are calculated. A classification method shows that the best geometrical approach that discriminates heat effect is the fractal dimension. The most popular parameter, R_a, badly discriminates processes (classification number = 58). The four order spectral moments of the roughness profile are correlated with the evolution of profile. It is shown theoretically that the superficial diffusion depends directly to the fourth spectral moment of the roughness profile.

Abstract: This contribution presents results which show the possibility to develop protective coatings from perhydropolysilazane (PHPS) on low carbon steel. In order to have an effective protection, the pyrolysis conditions should be carefully controlled. Nanoindentation tests allow characterizing the mechanical resistance of the coating after different pyrolysis conditions. The pyrolysis of coated samples has also been monitored by acoustic emission; the results show that the structural conversion of the coating from polymer to ceramics can be detected by acoustic emission.

Abstract: A generalization of the Bollmann’s method is used to evaluate the epitaxial strains at the metal/oxide interface. The numerical approach is described and applied to the Ni/NiO system in order to study the result sensitivity to some numerical parameters.

Abstract: The SiMo cast iron is a spheroidal graphite cast iron which major alloying elements are silicon and molybdenum. This alloy is currently used in diesel engines as part of exhaust gas system like exhaust manifold or turbocharger housing components. It is then designed to work at high temperature under corrosive atmosphere. However, this alloy presents sometimes unusual high corrosion rates, and it is thus advisable to define the critical conditions leading to this type of problem. In the present study, the SiMo cast iron is tested in complex atmospheres containing N2, O2, SO2, CO2 and H2O in a temperature range lying between 300°C and 600°C. SEM observations and EDX and XRD analyses allow to qualify the nature of the corrosion products. Some thermodynamic calculations complete the experimental results. Three experimental parameters are especially studied in this work: the temperature of the isothermal corrosion tests, the presence of water vapour and the SO2 content in the gas mixture. The main results show that the corrosion products are metallic sulphides, sulfates and oxides. When sulphides and / or sulfates are present, the corrosion scale does not ensure a sufficient protection of the substrate and it is necessary to find conditions where only oxides are present. Such conditions are obtained for the highest temperature used in this study (600°C), or with highly oxidant atmospheres.

Abstract: The behavior of three different materials with respect to their Fe, Cr, Ni contents have been studied between 450°C and 700°C, in waste incinerators combustion conditions, where it occurs a main influence of chlorine, sulfur and water vapor. This works analyzes the behavior of these materials which depends on their capability to form melted compounds and on the nature of these melted phases. Also as the corrosion mechanism is influenced by the presence or missing of any corrosive species, the study focalizes on the specific influence of SO2 and alkaline chorine. Effect of additive ashes on the corrosion behavior of tested materials is also studied. The mechanism of corrosion in waste incinerators is supported by thermodynamic calculations performed with the “Thermocalc” software.

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 mechanical behavior of oxidized low alloyed steel has been studied at temperatures between 700°C and 900°C. Such a work is carried out to obtain information about the damages which occur in the oxide scales developed on steels during hot rolling process. Actually the mechanical behavior is tested with a four point bending apparatus at high temperature under controlled atmosphere. Some information about mechanical damages have been collected during the mechanical loading thanks to recording of the sample acoustic emission. During this study, two procedures have been used which differ in the thermal scheme used to build the oxide scales. The results obviously show that the procedure has a great influence on the mechanical behavior of the oxidized low alloyed steel. For instance a marked decrease of the stress is observed at constant displacement between the 900°C and 800°C tests of one procedure, as the opposite behavior is observed with the other procedure. Different points are considered to explain such results, as the thermal strains which are only present in one of the two procedures, the phase transformations in the metal and in the oxide.