Caractérisation et modélisation des phénomènes de vieillissement dans les alliages de zirconium

International audienceDe nombreuses manifestations mécaniques de phénomènes de vieillissement (interactions dislocations-atomes de soluté) dans le domaine de températures 20°C-600°C ont été observées dans les alliages de zirconium [1]. L'influence systématique des éléments d'addition (oxygène, niobium) et la caractérisation précise du domaine de vieillissement restent toutefois mal connues. Dans cette étude, de nouveaux alliages de zirconium ont été spécialement élaborés à partir d'une barre de zirconium 2.2% hafnium à faible teneur en oxygène (80 ppm), puis caractérisés par différents essais mécaniques. Les phénomènes de vieillissement dynamique observés sont associés à des valeurs faiblement positives de sensibilité de la contrainte à la vitesse de déformation. Ces phénomènes ont ensuite été modélisés pour l'alliage de référence nommé Zr702 (sens travers) à forte teneur en oxygène (1300 ppm): creux de sensibilité de la contrainte à la vitesse de déformation autour de 300°C, arrêt de fluage et de relaxation à 200°C et 300°C et hétérogénéités de déformation plastique en traction le long du fût de l'éprouvette. Le modèle de comportement utilisé en éléments finis est fondé sur le modèle phénoménologique de vieillissement dynamique proposé par MacCormick [2], [3], validé sur des alliages d'aluminium et des aciers [4], [5], [6] et appliqué ici à l'alliage Zr702 dans le domaine de températures 20°C-400°C [7]

Short-term inhibition of acetylene reduction activity by low CO2 concentrations around the nodulated roots of various legumes

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A statistical TEM investigation of dislocation channeling mechanism in neutron irradiated zirconium alloys.

Plastic deformation in irradiated zirconium alloys occurs in a very heterogeneous manner at the grain scale by the clearing up of radiation-induced prismatic loops by gliding dislocations and the channeling of these dislocations inside narrow bands of the grain. A statistical TEM investigation of this mechanism has been performed on neutron irradiated recrystallized Zr alloys tested at 350 degreesC. Due to the strong anisotropy of plastic deformation, different loading conditions have been investigated. It is shown that for transverse tensile tests and closed end burst tests, only basal channels are observed, whereas for axial tensile tests, prismatic and pyramidal channels are observed. This phenomenon can be understood in terms of texture and interactions between dislocations and irradiation induced loops. From Schmid factor calculations, we have also been able to prove that irradiation leads to a higher increase of critical resolved shear stresses for prismatic and pyramidal slip systems than for basal slip system

A statistical TEM investigation of dislocation channeling mechanism in neutron irradiated zirconium alloys.

Plastic deformation in irradiated zirconium alloys occurs in a very heterogeneous manner at the grain scale by the clearing up of radiation-induced prismatic loops by gliding dislocations and the channeling of these dislocations inside narrow bands of the grain. A statistical TEM investigation of this mechanism has been performed on neutron irradiated recrystallized Zr alloys tested at 350 degreesC. Due to the strong anisotropy of plastic deformation, different loading conditions have been investigated. It is shown that for transverse tensile tests and closed end burst tests, only basal channels are observed, whereas for axial tensile tests, prismatic and pyramidal channels are observed. This phenomenon can be understood in terms of texture and interactions between dislocations and irradiation induced loops. From Schmid factor calculations, we have also been able to prove that irradiation leads to a higher increase of critical resolved shear stresses for prismatic and pyramidal slip systems than for basal slip system

Finite element simulations of dynamic strain ageing effects at V-notches and crack tips.

Finite element simulations of PLC effects in notched and CT (compact tensile) specimens of aluminium alloys are presented, based on a macroscopic PLC constitutive model. They predict the formation and propagation of intense strain rate localization bands. In particular, the predicted size of the plastic zone around the crack tip in a pre-cracked CT specimen is compared to the value found when using a standard elastoplastic model neglecting PLC effects

Strain localization phenomena associated with static and dynamic strain ageing in notched specimens: experiments and finite element simulations.

The aim of the present work is to use an available constitutive model for the description of the Portevin-Le Chatelier effect to simulate the deformation of notched specimens in tension (smooth and sharp U-notched specimens). This model can be used to account for dynamic strain ageing as suggested by Estrin et al. [Acta Mater. 49 (2000) 1087] but also static strain ageing as suggested by Kubin et al. [Acta Metall. Mater. 40 (1992) 1037]. The experimental material studied is an Al-Cu alloy, displaying dynamic strain ageing at room temperature. Regarding Luders band propagation, the experimental material studied is a mild steel. This work shows that the PLC serrations disappear progressively on the macroscopic curve when the notch radius decreases. However, strain localization still takes place in the deformed notched zone, and can escape from the notched zone. Regarding the Luders behavior, the computed spreading of deformation bands is in agreement with experimental observations

Sucrose phosphate synthase : an enzyme with heterotic activity correlated with maize growth

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