TEM and EXAFS study of Ar+ implanted Mo thin films

TEM observations and CEEXAFS measurements of Ar+ implanted molybdenum films were performed. The appearance of a fcc phase is clearly identified with the two characterisation techniques and the lattice parameter is shown to be a = 0.420 nm. No solid argon bubbles can be detected. An interpretation is given based on the possible interna1 stresses created by Ar+ implantation

Recombinaison des paires proches après irradiation aux neutrons à 20 K d'alliages Fe50-Co50 et Fe49-Co 49-V2

Two Fe50-Co50 and Fe49-Co49-V 2 ordered and disordered alloys were irradiated with fission neutrons at liquid hydrogen temperature. We have studied the recovery of the radiation induced resistivity during isochronal annealings. In the temperature range between 20 K and 150 K an important recovery stage is noticed at 55 K on the disordered alloys at 65 K and 90 K. This stage is attributed to the close pair recombination. A calculation is proposed taking into account the nature of the migrating interstitial atom with respect to that of the vacancy site. The energy difference between the two substages is in good agreement with the value found experimentally.Deux alliages Fe50-Co50 et Fe49-Co 49-V 2 ordonnés et désordonnés ont été irradiés aux neutrons de fission à la température de l'hydrogène liquide. Nous avons étudié la restauration de l'excès de résistivité produit en effectuant des recuits isochrones. Dans le domaine de températures comprises entre 20 K et 150 K, un stade important est enregistré à 55 K sur l'alliage désordonné et l'on assiste à son dédoublement : 65 K et 90 K lorsque le degré d'ordre de l'alliage est maximum. Ce stade est attribué à la recombinaison des paires proches de Frenkel. Un modèle est présenté tenant compte de la nature de l'interstitiel migrant vis-à-vis de celle du site lacunaire. Le décalage en énergie des deux stades ainsi calculés correspond assez bien à la valeur trouvée expérimentalement

Magnetic anisotropy in iron thin films evaporated under ultra-high vacuum

α-iron thin films with thickness ranging between 20 and 1500 nm have been evaporated using an electron gun under ultra-high vacuum conditions ( 5.10-7 Pa). The columnar structure observed in cross-sectional TEM is related to the large surface diffusion. From Mössbauer spectra the spin orientation is deduced and found to be influenced by the column axis. Spins can be obtained perpendicularly to the film plane by rotating the substrate during the deposition. The magnetization of the samples is reduced by about 30 % and the reduction attributed to the interstitial space which increases with the incident angle. The substrate rotation also decreases Ku(//) by a factor 2 and increases Ku(⊥)

Application de la méthode des moments à l'alliage équiatomique Fe-Co

The moment expansion technique, valid for transition metals, is applied to the equiatomic Fe-Co ordering alloy. This study shows that the antiphase boundary energy may be'very well explained for this alloy by using only the différences between the overlap integrals of Fe-Fe, Co-Co and Fe-Co wave functions. The lack of numerical values concerning such integrals has led us to fit the parameters of the method with the aid of a thermodynamical theory of atomic order-disorder transformation.La méthode des moments, applicable aux métaux de transition, est utilisée pour un alliage subissant la transformation ordre-désordre : le Fe-Co équiatomique. Cette étude permet de montrer que les énergies de paroi antiphase, peuvent très bien s'expliquer, pour cet alliage, uniquement par des différences entre les diverses intégrales de recouvrement des fonctions d'onde Fe-Fe, Co-Co, Fe-Co. L'insuffisance de valeurs numériques, pour ces intégrales, nous conduit à fixer les paramètres ajustables de la méthode à l'aide d'une théorie thermodynamique de la transformation ordre-désordre

On the growth strain origin and stress evolution prediction during oxidation of metals

International audienceHigh temperature oxidation of metals leads to residual stresses in the metal and in the oxide. In this work, we try to predict the evolution of the residual stresses in the growing oxides layers, during isothermal oxidation. The origin of these stresses is based on the microstructural model of Clarke, however, another justification is proposed, assuming a proportional dependence of the growth strain with the oxide layer thickness. Using the mechanics of thin layers, as well as the analysis proposed to describe the growth strain, a system of equations are deduced that predict the stresses evolution with oxidation time. Numerical analysis is performed, leading to a set of theoretical curves. © 2005 Elsevier B.V. All rights reserved

General approach on the growth strain versus viscoplastic relaxation during oxidation of metals

High temperature oxidation of metals induces residual stresses in metals and growing oxides. In this work, the evolution of the residual stresses in those oxides layers, during isothermal oxidation of metals is studied. A new justification is proposed in order to explain the origin of those stresses leading to a proportional dependence between the growth strain and the oxide layer thickness. Moreover, we emphasize the relation between viscoplastic strain and growth strain. Using the mechanics of thin layers, as well as the analysis proposed to describe the growth strain, a system of equations is deduced that predicts the stresses evolution with oxidation time. Comparison with previous experimental results is also made. © 2007 Elsevier B.V. All rights reserved

Role of a chromium ion implantation on the corrosion behaviour of nickel in artificial sea water

Ni bulk specimens have been implanted with Cr ions (60 keV, 4.1016 ions/cm2), and classical corrosion experiments performed in artificial sea water by means of potentiodynamics i(E) and potentiostatics i(t) studies. Both experiments show that an important decrease of the current density is obtained in comparison with pure nickel. Moreover, a significant extension of the passivation region can be observed, with mainly an increase of the pitting potential. This behaviour is respectively related to an X-ray photoelectron spectroscopy (XPS) analysis of the passive film formed on Ni (Ni, Cr and O environments), and to previous results obtained on similar alloys

Competition between stress generation and relaxation in iron oxide films. Experiments and modelling

International audienceIn the present work, the calculated growth stresses arising from iron oxide layers are compared to measurements of the macroscopic residual stresses. High temperature x-ray diffraction was used for determining the stress created in thin iron oxide layers formed on phosphated α-Fe, as a function of the oxidation time. Experiments are conducted both in situ during oxidation and after cooling, allowing us to derive respectively, growth and thermal stresses. A numerical model was developed to calculate the growth stress and the relaxation phenomena occurring during isothermal oxidation. This model takes into account the mechanical behaviour of both the substrate and the oxide, and parameters such as oxide growth kinetics, substrate thickness, etc. The elasto-viscoplastic model correlates the experimental results with the various parameters of the study. © 2004 Taylor & Francis Ltd

Raman microprobe spectroscopy measurements of residual stress distribution along blisters in Cr 2 O 3 thin films

Raman spectroscopy has been used to characterise the buckling phenomenon of Cr 2 O 3 films obtained by oxidation in air at 900 °C of Ni33 at%Cr. The observed circular blisters are described by measuring the radius from the optical top view, the profile via an autofocus device and the residual stress in each point of the chromia film: far away from the centre of the blister, in the vicinity and across the blister. The subsequent spalls are related to the morphology of the blisters and to the stress. © 2009 Elsevier B.V. All rights reserved

Substrate grain boundary effect on residual stress distribution at micrometer scale in chromia oxide thin films

Residual stresses in chromia oxide films formed at 700 °C on Ni Cr30 have been investigated with confocal Raman spectroscopy. The high lateral resolution allows us to determine stress distribution in the oxide growing above both substrate grains and grain boundaries. It is demonstrated that the magnitude of the equibiaxial compressive stress is related to local oxide microstructure evolution: during the initial oxidation, stress is smaller in the chromia located above substrate grain boundaries as compared to that above substrate grains. Subsequently, this gap in stress magnitude progressively vanishes due to the occurrence of oxide grains forming bridges across depressions in the oxide scale above substrate grain boundaries. © 2008 American Institute of Physics