Microtextural Analyses of the Ni/NiO System after High Temperature Oxidation

Modélisation auto cohérente du comportement viscoplastique en grandes déformations et à grandes vitesses d'un métal cubique centré préalablement choqué

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Coupling between Diffusion, Stress Field and Chemical Reaction in a Metal-Gas Oxidation

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Near-coincidence lattice method for the determination of epitaxy strains during oxidation of metals

Abstract A theoretical method is proposed to evaluate the strains due to epitaxy between a metal and its oxide. Based on Bollmann’s approach of two adjoining grains, it uses the quantitative texture analysis of the two materials separated by the phase boundary. Our study of the Zr/ZrO 2 and Mo/MoO 3 systems reveals strong correlations between the criterion of best fit proposed by Bollmann and the orientation distribution function obtained by a quantitative texture analysis. The results of this study are used in a thermo-mechano-chemical simulation of the oxidation process of zirconium, which leads to this observation: two different zirconia orientations induce two different oxidation kineti…

A Mechano-Chemical Coupling for Hydrogen Diffusion in Metals Based on a Thermodynamic Approach

Hydrogen diffusion in metals is still an ongoing topic of research due to its technical relevance (hydrogen embrittlement, hydrogen storage...). In the last decades, significant progress in understanding the time evolution of the hydrogen concentration in solids was completed. This paper presents a modeling of hydrogen diffusion with a general and thermodynamically based diffusion concept coupled with mechanical and chemical aspects. This model was previously used to simulate the oxidation of a metal [1][2]. This concept has been upgraded to offer a thoroughly macroscopic behavior law used to simulate hydrogen diffusion in metal parts under mechanical loadings. The thermodynamic approach of…

An empirical model for free surface energy of strained solids at different temperature regimes.

Abstract We have developed an empirical formulation, based on the elastic theory, to calculate the variation of the surface free energy when a crystal is strained in the elastic regime. The model permits to obtain the variation of the surface energy at different strains and temperatures when are known the thermal dependence on the bulk and surface elastic constants. Molecular dynamics (MD) simulations were performed using the three low index surfaces of Al, to validate the accuracy of the model. The comparison between the empirical model and the MD simulations shows a good agreement for temperatures ranging between 0 and 900 K, and for deformation between −2% and 2%.

Improving the high temperature (HT) oxidation resistance of a Beta-Titanium alloys by mechanical treatments

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Texture Effects on Zircaloy Oxidation : Experiment and Simulation

Micromechanical Approach of the High Temperature Oxidation of Zirconium

Improving the high temperature oxidation resistance of pure titanium by shot-peening treatments

International audience; Shot-peening (SP) treatments have shown their capacity to improve the oxidation resistance of titanium and zirconium thanks to the large compressive stresses and the surface hardening induced by this mechanical process. However, shot-peening treatments can produce a surface chemical deposit, which can modify the high temperature oxidation resistance. Here, we study pure titanium samples shot-peened with different type of balls: tungsten carbide, alumina or glass. The oxidation behavior was studied at 700 °C in dry air by thermo gravimetric analysis for short isotherm oxidation periods up to 100 h. Also, long oxidation tests (3000 h) at 700 °C were performed with an i…

Influence of surface Shock-peening treatment on high temperature oxidation of Titanium

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Impact of rolling conditions on the hydriding of grade 2 titanium welded tubes

In this article, we study the impact of rolling conditions on the texture of the commercially pure titanium grade 2. In a previous work, NEOTISS in collaboration with ICB laboratory, shown that the texture highly influences the precipitation of hydrides in Titanium. In order to create different textures, Titanium sheets grade 2 are cold rolled asymmetrically and symmetrically with or without lubricant. The inverse pole figures and direct pole figures obtained allow us to deduce that symmetrical cold rolling does not change the grains orientation but generates a rotation of grains along c-axis (normal to basal plan). However, asymmetrical cold rolling leads to the formation of a new crystall…

Pre-Stressed Sub-Surface Contribution on Bulk Diffusion in Metallic Solids

Our recent modelling works and corresponding numerical simulations realized to describe the UO2 oxidation processes confirm the theory showing that an applied mechanical strain can strongly affect the local oxygen diffusion in a stressed solid. This result allows us to assume that stress field, previously applied at the surface of a metallic sample on several microns, will delay the degradation during its oxidation. Considering this hypothesis, we implemented a FEM simulation code developed in our laboratory to numerically investigate some different stress fields applied on a sample sub-surface, that might significantly modify the volume diffusion of oxygen during the oxidation process. The…

Improving the high temperature oxidation resistance of Ti-β21S by mechanical surface treatment

The improvement of the high temperature oxidation resistance of titanium alloys is currently a technological challenge. Mechanical surface treatments as shot-peening (SP) have shown their ability to improve the behaviour of pure zirconium and titanium. However, shot-peening treatments can induce a significant surface contamination. Laser shock peening (LSP) appears as a good alternative. Here, we have investigated the effect of SP and LSP treatments on the HT oxidation behavior of Ti-β21S. Samples treated by these methods have been compared to untreated ones for long exposures (3000 h) at 700 °C in dry air. The samples placed in a furnace at 700 °C were periodically extracted to be weighed.…

An empirical method to determine the free surface energy of solids at different deformations and temperatures regimes : An application to Al.

Abstract We have performed molecular dynamics (MD) simulations using the three low index surfaces of Al to determine the variation of the surface energy as a function of deformation and temperature. We have also developed an empirical formulation for the surface free energy as a function of deformation. The observed difference between the numerical and analytical results has led us to divide the deformation into a mechanical and a thermal contribution. From this observation, we have obtained an expression for the surface free energy placing the temperature dependence on the bulk and surface elastic constants. Our simulations permitted us to analyze the multilayer relaxation for the particul…

Determination of the stress distribution at the interface metal-oxide: Numerical and theoretical considerations

In this paper we give a brief presentation of the approaches we have recently developed on the oxidation of metals. Firstly, we present an analytical model based on non-equilibrium thermodynamics to describe the reaction kinetics present during the oxidation of a metal. Secondly, we present the molecular dynamics results obtained with a code specially tailored to study the oxidation and growth of an oxide film of aluminium. Our simulations present an excellent agreement with experimental results.

Influence of the U3O7 domain structure on cracking during the oxidation of UO2

Abstract Cracking is observed when a UO 2 single crystal is oxidised in air. Previous studies led to the hypothesis that cracking occurs once a critical depth of U 3 O 7 oxidised layer is reached. We present some μ-Laue X-ray diffraction results, which evidence that the U 3 O 7 layer, grown by topotaxy on UO 2 , is made of domains with different crystalline orientations. This observation was used to perform a modelling of oxidation coupling chemical and mechanical parameters, which showed that the domain patterning induces stress localisation. This result is discussed in comparison with stress localisation observed in thin layer deposited on a substrate and used to propose an interpretation…

Study of the Texture of Steelcord during the Wet Drawing Process - Influence of the Patenting and the Friction on the Dies

An Interfacial Thermodynamic Model for the Oxidation Kinetics of a Metal: Epitaxial Stress Effects

Laser Ultrasonic Testing for Estimation of Adhesion of Al2O3Plasma Sprayed Coatings

Abstract The aim of the present study was to investigate the possibility of using a laser ultrasonic technique for the estimation of the adhesion of ceramic coatings, deposited onto metallic substrates by thermal spraying techniques. For this purpose, a pulsed Nd : YAG laser (λ = 1064 nm, 14 ns) was used to irradiate Al2O3 coatings, of different thickness (30–350 µm), deposited onto stainless steel substrates by atmospheric plasma spraying. The laser generated ultrasonic waves were in situ recorded at the epicentre using a laser heterodyne interferometer. The acoustic waveforms were correlated with the interaction between the pulsed laser radiation and the coated metal, taking into account …

Microchemical Model for High Temperature Oxidation of Zirconium

Retarding the high temperature oxidation of Titanium by shot-peening surface treatment

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Towards a Study of Effects on Hydrogen Diffusion into T40 Titanium Alloys

In a global study of titanium alloys behavior in specific aqueous solution (embrittlement, corrosion and corrosion under stresses), the present work focuses on hydrogen diffusion into the metal and the consequences on its microstructure. Two ways of hydrogen charging were used to investigate this issue (gaseous and cathodic charging). The final aim is to determine a fitted method to create an identified microstructure and then to perform accelerated aging tests of titanium U-Bend samples into an autoclave with a specific environment. Hydrogen absorption and formation of titanium hydride have been studied by SEM analyses and by X-ray diffraction methods.

Circular Texture in Thin Wires

Self-Induced Stress and Matter Transport Interactions during Metal Oxidation

Mechanical Stresses: Inhibitor of Catalyst of High Temperature Oxidation?

Oxidation of metals is a complex reaction in which mechanical and chemical phenomena occur. A dynamic and macroscopic model is developed in order to simulate oxidation kinetics of a metal. It includes the stress/diffusion coupling in the bulk and the interfacial phenomena at metal/oxide interface. Its application to the Zr/ZrO 2 system shows the important role of stress field distribution in oxide on kinetic behavior. According to the sign of stress gradient in the oxide scale, the oxidation rate can speed up or slow down. The calculated kinetic curves could he fitted using a k p .t 1/n law where n and kp vary all over the process, like for the experimental kinetic curves.

Influence of the Residual Stresses Induced by Shot-Peening on the Oxidation of Zr Plates

The present study evaluates the influence of the residual stress on the oxidation rate of Zr at 650°C. It is shown that the oxidation rate can be substantially decreased by shot-peening. A systematic instigation of the various factors (such as the residual stress, the surface pollution and the chemical composition of the oxidation atmosphere) is realized to separate their influence on the oxidation. The influence of an annealing made after the shot-peening is also considered. One show that the pollution of the surface during the shot-peening process is not responsible for the oxidation rate decrease.

Kinetics study of mechanically treated pure titanium with enhancement of the high temperature oxidation resistance

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Micromechanical approach of the high temperature oxidation of zirconium: study of the Zr/ZrO2 interfacial epitaxy with Bollmann's method

The method of Bollmann has been used to determine the role of epitaxy in the formation of the mechanical stress field near the metal/oxide interface during the high temperature oxidation of zirconium. The strains due to epitaxy combine with those due to both oxygen diffusion in metal and thermal expansion of the metal/oxide composite in a model based on a micromechanical formalism. The calculated values are compared to experimental results and the applicability of Bollmann's method to this problem is discussed.

Role of nitrogen in the high temperature oxidation of mechanically treated pure titanium

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