Polycrystal model of the mechanical behavior of a Mo-TiC30vol.% metal-ceramic composite using a 3D microstructure map obtained by a dual beam FIB-SEM

The mechanical behavior of a Mo-TiC30 vol.% ceramic-metal composite was investigated over a large temperature range (25^{\circ}C to 700^{\circ}C). High-energy X-ray tomography was used to reveal the percolation of the hard titanium carbide phase through the composite. Using a polycrystal approach for a two-phase material, finite element simulations were performed on a real 3D aggregate of the material. The 3D microstructure, used as starting configuration for the predictions, was obtained by serial-sectioning in a dual beam Focused Ion Beam (FIB)-Scanning Electron Microscope (SEM) coupled to an Electron Back Scattering Diffraction system (3D EBSD, EBSD tomography). The 3D aggregate consists of a molybdenum matrix and a percolating TiC skeleton. As most BCC metals, the molybdenum matrix phase is characterized by a change in the plasticity mechanisms with temperature. We used a polycrystal model for the BCC material, which was extended to two phases (TiC and Mo). The model parameters of the matrix were determined from experiments on pure molydenum. For all temperatures investigated, the TiC particles were considered as brittle. Gradual damage of the TiC particles was treated, based on an accumulative failure law that is approximated by an evolution of the apparent particle elastic stiffness. The model enabled us to determine the evolution of the local mechanical fields with deformation and temperature. We showed that a 3D aggregate representing the actual microstructure of the composite is required to understand the local and global mechanical properties of the studied composite

Modelling of the compaction phase during Hot Isostatic Pressing process at the mesoscopic scale

International audienceDuring Hot isostatic pressing (HIP) of metal powder, power-law creep is the dominant mechanism during the densification process. However, the understanding of the global impact of the thermo-mechanical boundary conditions and of the powder granulometry on the microstructure obtained after this first mechanism is not straightforward. A finite element methodology based on the use of a level set framework coupled with a remeshing technique is proposed in order to model the viscoplastic deformation of powder particles during HIP at the mesoscopic scale thanks to a Representative Elementary Volume. The methodology consists in generating, in a finite element mesh, a sphere packing of particles by representing implicitly all particles by means of a limited set of level-set functions. Mesh adaptation is also performed at particle boundaries to describe properly the particles and to manage the discontinuity of the physical properties. Such 2D scale mesoscopic densification simulations are presented and discusse

Experimental characterization and mechanical behaviour modelling of Molybdenum -Titanium Carbide composite for high temperature applications.

International audienceSimulations of the elastic-viscoplastic behaviour of ceramic-metal composite, over the temperature range 298-993K, are performed on realistic aggregates built up from Electron Back Scatter Diffraction methods. Physical based constitutive models are developed in order to characterize the deformation behaviour of body centered cubic (bcc) metal and face centered cubic (fcc) ceramic under various temperatures. While the ceramic keeps elastic, the viscoplastic behaviour of the metal part is described with a dislocation - based model, implemented in the finite element code ABAQUS, in order to compute local strain and stress fields during compressive tests. It is shown that the adopted constitutive laws are able to give back local complex experimental evidence on weak points of the microstructure

Etudes expérimentale et numérique du comportement mécanique d'un composite métal – céramique : MoTiC30%

In the scope of refractory materials development for structural applications in the core of the future nuclear reactors, several studies have been developed. The aim of this work is to increase the knowledge of the mechanical behaviour and the damage of the ceramic-metal composite Mo(TiC)x% under the temperature range [25 – 700 °C].The identification of the third phase, formed by diffusion during the sintering step was identified by microstructutal characterization. Experimental study also revealed the percolation of the ceramic particles through the structure.Mechanical tests highlight the main characteristics of the material: the macroscopic behaviour depends on the strain rate on the first hand and the temperature on the other hand. These mechanisms are attributed to the thermally activated behaviour of molybdenum.Simulations have been made on several microstructures considering elastic-brittle inclusion in a viscoelastic matrix. A polycrystalline model was used to simulate the evolution of the mechanical behaviour of the composite. The numerical aggregate, used for the simulation, was built from a 3D reconstruction technique thanks to acquisition of FIB/EBSD/SEM data. The results of the model are in accordance with the experimental results and allow to describe under different temperature: - the plasticity mechanisms of molybdenum, taking into account of the low/high temperature transition;- the damage of titanium carbide and the percolation effects.Dans le cadre du développement des réacteurs de génération IV, de nouvelles études sont menées dans le domaine des matériaux. L'objectif de ce travail est d'améliorer la compréhension du comportement mécanique et de l'endommagement de composites à matrice métallique Mo(TiC)x% contenant une forte fraction volumique de particules, et cela, dans le domaine de températures [25 – 700 °C].La caractérisation microstructurale a permis de comprendre l'histoire du matériau et d'identifier la nature d'une troisième phase (Mo,Ti)C fomée par diffusion du molybdène dans le carbure de titane. L'étude expérimentale a aussi révélé la percolation des particules céramiques au sein de la structure.Les essais mécaniques ont mis en évidence les principales caractéristiques du matériau : le comportement macroscopique dépend à la fois de la vitesse de déformation et de la température. Ces mécanismes sont attribués au comportement thermiquement activé du molybdène.Nous proposons alors diverses simulations de microstructures comprenant des inclusions élastiques-fragiles dans une matrice viscoélastique. L'évolution du comportement mécanique du composite a été modélisée à l'aide d'une approche cristalline sur un agrégat 3D réel. L'agrégat numérique utilisé pour modéliser le comportement mécanique fait appel à une technique de reconstruction 3D via une acquisition par FIB/SEM/EBSD.Ainsi, la réponse du modèle est en bon accord avec les résultats expérimentaux et permet de décrire en fonction de la température :- les mécanismes de plasticité du molybdène, en tenant compte de la transition basse/haute température ;- l'endommagement du carbure de titane et les effets de la percolation.    

- Experimental characterization and mechanical behaviour modelling of MoTic composite for high temperature applications;

International audienc

Microstructure and Thermal Conductivity of Mo-TiC processed by hot isostatic pressing . Accepté pour publication dans Le Flem M. , Allemand A., Urvoy S., Cédat D., Rey C..-

International audienc