Secondary precipitation of Cr-rich carbides in heat resistant austenitic stainless steels has been investigated both experimentally and using finite element simulations. The microstructural evolutions in two commercial grades were characterized using electron microscopy. A special emphasis was given on the peculiar spatial distribution of M23C6 secondary carbides exhibiting precipitate free zones surrounding primary carbides, and high density precipitate zones extending with longer aging times. Solidification induced chemical composition gradients were clearly exhibited in the as-cast alloys with significant chromium depletion in the vicinity of primary M7C3 carbides. It is then proposed that these gradients play a major role in the secondary precipitation mechanism. Classical nucleation and growth theories have been adapted to account for (i) the flux of solutes with large difference in diffusion coefficients, (ii) the initial composition gradients in the matrix and, (iii) the chemical driving force for nucleation and growth of M23C6 carbides. Within this framework, the whole kinetics has been reproduced. It clearly shows that the spatial distribution of secondary carbides that play a key role in the creep resistance of these alloys is the result of a complex interaction between initial composition gradients in as-cast alloys and solute flux resulting from phase transformation during aging.

The high temperature oxidation behavior of two highly alloyed refractory austenitic steels is studied at 1000 degrees C under air. Both short-term oxidation kinetics and long-term oxidation products are investigated. Structural, elemental and morphological analyses are coupled to characterize the oxide phases' formation. Chromia and spinel are the main phases for both samples although they suffer important descaling after 4 weeks oxidation, but alloying elements are responsible for the formation of many other oxides. Microstructures are compared and discussed. New Raman spectra are reported for manganese chromite spinels. Their assignments are provided and discussed at the light of the proposed cationic distribution.

A comparison has been made between the phase equilibrium calculated within the Calphad method with different thermodynamic databases (TCFE7, TCNI5, TTNI7) and experimental data obtained on a Nb–Ti-modified industrial HP grade. The experimental measurements include phase equilibrium data at different temperatures and liquidus/solidus determination. The obtained agreement between experiments and calculations is generally good. Conclusions are made concerning the most suitable thermodynamic database to describe highly alloyed austenitic refractory steels.

A Fe–14Cr oxide dispersion strengthened (ODS) ferritic steel is studied as a potential material for cladding tube application for the next generation of fast-breeder nuclear reactors. Tensile specimens machined out from a hot extruded round bar in three different orientations are used to evaluate the mechanical anisotropy of this steel for temperatures in the range 20–750°C. Its anisotropy is discussed both in terms of mechanical strength and fracture mode. At high temperatures (HTs), above 500°C, the longitudinal direction appears to be the most ductile and most resistant direction. Longitudinal creep tests between 650°C and 900°C were also carried out. They show this ODS steel has a high HT creep lifetime and a low creep failure strain. Intergranular cracks aligned along the loading axis were observed on fractured creep specimens. They reveal a particular weakness of prior particle boundaries and suggest to modify the elaboration process through mechanical alloying and hot extrusion.

Ferritic 14%Cr and 18%Cr ODS steels produced at CEA in round bars or plates were tested mechanically. The present paper reports results obtained in tension, impact, fatigue, creep and toughness tests. These tests were carried out at various temperatures and in different directions. These materials show a pronounced anisotropy at all tested temperatures. No matter the loading, the transversal direction is always found to be far less resistant than the longitudinal one. This anisotropy is mainly observed in terms of damage mechanisms, with intergranular fracture preferentially occurring along the extrusion direction. This intergranular fracture mode leads to very low and anisotropic toughness values and to the absence of tertiairy creep stage, pointing out the unstable nature of fracture, even at high temperature. The unrealistically high values of the Norton exponent measured in creep suggests the existence of a threshold stress, which is consistent with the mainly kinematic nature of the stress as revealed by fatigue tests. (C) 2012 Elsevier B.V. All rights reserved.

In this present work, the plasticity of a rod bar of a 14% Cr ferritic ODS steel is examined through a multiscale approach based on both macroscopic and microscopic results. This bar was elaborated at CEA by powder metallurgy and consolidated by hot extrusion. The microstructure of the material has been characterized.First, the tensile behavior of this material is studied in a wide range of temperatures. Thereafter, through in situ Transmission Electron Microscopy (TEM) straining experiments, dislocation/dislocation and dislocation/precipitates interactions are observed. The collapse of the tensile properties noticed from 400 degrees C can be explained by a change in the deformation mechanism. At lower temperatures, the hardening seems to be due to the precipitates, dislocations are pinned on oxides. At higher temperatures, the hardening role of the precipitates is still observed, but the dislocations seem to move in a more steady way, thermal activation of dislocations sources is observed and leads to formation of cavities at the grain boundaries. (C) 2011 Elsevier B.V. All rights reserved.

Le renforcement par dispersion d'oxydes nanometriques permet, d'une maniere generale, d'ameliorer la resistance mecanique des materiaux metalliques. Il autorise donc une augmentation de leur temperature maximale d'utilisation. De nombreux travaux de recherche sont menes au Commissariat a l'Energie Atomique et aux Energies Alternatives sur les aciers ODS, pour Oxide Dispersion Strengthened steels. S'inscrivant dans le cadre du developpement du nucleaire civil de quatrieme generation, ces travaux ont pour but de qualifier un materiau pouvant etre utilise en tant que materiau de gainage combustible a une temperature de 650℃. Ce travail de these a pour objectif d'ameliorer la comprehension des proprietes mecaniques des aciers ODS, en cherchant d'une part a caracteriser et a modeliser leur comportement en fluage, et d'autre part a caracteriser leur nisotropie de comportement et a en identifier l'origine. Pour cela, de nombreux essais mecaniques ont ete effectues entre 20℃ et 900℃ sur une nuance d'acier ODS ferritique de composition nominale Fe-14Cr1W0,26Ti + 0,3Y2O3 . Cette nuance a ete elaboree au CEA, par mecanosynthese puis extrusion a chaud, sous la forme d'une barre ronde. Les resultats obtenus demontrent la grande resistance mecanique a haute temperature de l'acier ODS etudie. Ils mettent egalement en evidence une forte dependance de la ductilite et de la resistance du materiau vis-a-vis de la vitesse de sollicitation. Sur la base des differentes caracterisations experimentales realisees, un modele de comportement macroscopique uniaxial a ete developpe. S'appuyant sur la description de trois ecrouissages cinematiques et d'un terme de restauration statique, ce modele demontre une capacite remarquable a reproduire le comportement mecanique du materiau en traction, en fatigue, en fluage et en relaxation. Par ailleurs, la caracterisation de l'anisotropie mecanique de la nuance d'acier ODS etudiee s'avere dependre de la temperature. Deux modelisations polycristallines differentes ont ete mises en place afin de reproduire cette anisotropie de comportement a partir des textures cristallographique et morphologique du materiau. Le desaccord observe entre les previsions de ces modeles et les resultats experimentaux conduit a formuler d'autres hypotheses sur la deformation des aciers ODS.

A crystalline elastoplasticity model is proposed to describe the cyclic behaviour of face-centred cubic crystals. It is based on many experimental observations correlating the observed dislocation structures with the orientations of corresponding crystals. The model distinguishes between two families of crystals. The first family gathers crystals for which the tension–compression loading axis is located in the centre of the standard stereographic triangle. These crystals, in which bundle and/or slip band dislocation structures are usually observed, are subjected to single slip deformation. The second family gathers crystals in which labyrinths or wall dislocation structures develop. These crystals are subjected to multiple slip deformation. Crystalline plasticity parameters are adjusted using only the single crystal cyclic stress strain curves measured for one orientation of each of the two families. The relevance of the model is evaluated through finite elements calculations of the uniaxial cyclic deformation of texture-free nickel polycrystals at room temperature. The macroscopic predictions are in reasonable agreement with experimental data concerning both the cyclic stress–strain curve and the hysteresis loops provided either large grain sizes or intermediate to high plastic strains are considered. By construction, the modelling is unable to predict grain size effect observed at low plastic strain. The distributions of the mean grain plastic strains become narrower as the macroscopic plastic strain amplitude increases, which appears consistent with the large scattering in high-cycle fatigue lifetimes usually observed. On the contrary, the distributions of mean grain axial stresses get broader, in agreement with neutron and X-ray diffraction measurement values published in the literature. The influence of the material parameters is then discussed. Finally, the cumulative probability curves of the number of cycles to fatigue microcrack nucleation are deduced and discussed with respect to scattering mentioned previously.

Grain size seems to have only a minor influence on the cyclic strain strain curves (CSSCs) of metallic polycrystals of medium to high stacking fault energy (SFE). That is why many authors tried to deduce the macroscopic CSSCs curves from the single crystals ones. Either crystals oriented for single slip or crystals oriented for multiple slip could be considered. In addition, a scale transition law should be used (from the grain scale to the macroscopic scale). Authors generally used either the Sachs rule (homogeneous single slip) or the Taylor one (homogeneous plastic strain, multiple slip). But the predicted macroscopic CSSCs do not generally agree with the experimental data for metals and alloys, presenting various SFE values. In order to avoid the choice of a particular scale transition rule, many finite element (FE) computations have been carried out using meshes of polycrystals including more than one hundred grains without texture. This allows the study of the influence of the crystalline constitutive laws on the macroscopic CSSCs. Activation of a secondary slip system in grains oriented for single slip is either allowed or hindered (slip planarity), which affects strongly the macroscopic CSSCs. The more planar the slip, the higher the predicted macroscopic stress amplitudes. If grains oriented for single slip obey slip planarity and two crystalline CSSCs are used (one for single slip grains and one for multiple slip grains), then the predicted macroscopic CSSCs agree well with experimental data provided the SFE is not too low (316L, copper, nickel, aluminium). Finally, the incremental self-consistent Hill-Hutchinson homogeneization model is used for predicting CSS curves and partially validated with respect to the curves computed by the FE method.

Grain size seems to have only a minor influence on the cyclic strain strain curves (CSSCs) of metallic polycrystals of medium to high stacking fault energy (SFE). Many authors therefore tried to deduce the macroscopic CSSCs curves from the single crystals ones. Either crystals oriented for single slip or multiple slip were considered. In addition, a scale transition law should be used (from the grain scale to the macroscopic scale). The Sachs rule (homogeneous stress, single slip) or the Taylor one (homogeneous plastic strain, multiple slip) were usually used. But the predicted macroscopic CSSCs do not generally agree with the experimental data for metals and alloys, presenting various SFE values. In order to avoid the choice of a particular scale transition rule, many finite element (FE) computations are carried out using meshes of polycrystals including more than one hundred grains without texture. This allows the study of the influence of the crystalline constitutive laws on the macroscopic CSSCs. Activation of a secondary slip system in grains oriented for single slip is either allowed or hindered (slip planarity), which affects strongly the macroscopic CSSCs. The more planar the slip, the higher the predicted macroscopic stress amplitudes. If grains oriented for single slip obey slip planarity and two crystalline CSSCs are used (one for single slip grains and one for multiple slip grains), then the predicted macroscopic CSSCs agree well with experimental data provided the SFE is not too low (austenitic steel 316L, copper, nickel, aluminium).

A ferritic oxide dispersion strengthened steel is under study for fuel cladding applications in future nuclear systems. Tensile tests and creep tests are carried out at various temperatures to determine its mechanical properties along the extrusion direction. For these two types of loading, the material exhibits a high mechanical resistance. Its ductility appears to be strongly influenced by the strain rate and the temperature. Deformation mechanisms linked to diffusion phenomena are suspected and intergranular damage is observed on fractured specimens.

Un modele de comportement elastoplastique cyclique cristallin est propose pour les metaux et alliages cubiques a face centree, et en particulier pour le nickel. Il est utilise lors de simulations par elements finis de la deformation cyclique uniaxiale d’un polycristal non-texture, represente par l’assemblage de grains a geometrie cubique et a orientation cristalline aleatoire. Ce modele distingue les grains pour lesquels l’axe de chargement se situe dans la partie centrale du triangle stereographique standard (auxquels un mode de deformation en glissement planaire est impose) et les grains pour lesquels il est situe pres des bords du triangle standard (auxquels un comportement en glissement multiple est attribue). Les resultats des simulations obtenus sont en accord avec les donnees experimentales disponibles, autant au niveau de la courbe d’ecrouissage cyclique, qu’au niveau des boucles d’hysteresis decrivant le comportement stabilise. Ce modele sera ensuite employe pour l’etude de la localisation de la deformation sous forme de bandes de glissement persistantes, comme observe lors du chargement cyclique de polycristaux de nickel.

The search for a new cladding material is part of the research studies carried out at CEA to develop a sodium-cooled fast reactor meeting the expectations of the Generation IV International Forum. In this study, the tensile properties of a ferritic oxide dispersion strengthened steel produced by hot extrusion at CEA have been evaluated. They prove the studied alloy to be as resistant as and more ductile than the other nano-reinforced alloys of literature. The effects of the strain rate and temperature on the total plastic strain of the material remind of diffusion phenomena. Intergranular damage and intergranular decohesion are clearly highlighted.