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Titanium and its alloys undergo temperature-driven martensitic phase transformation leading to the development of complex microstructures at mesoscale. Optimizing the mechanical properties of these materials requires an understanding of the correlations between the processing parameters and the mechanisms involved in the microstructure formation and evolution. In this work, we study the temperature-induced phase transition from BCC to HCP in pure titanium by atomistic modeling and investigate the influence of local stress conditions on the final martensite morphology. We simulate the transition under different stress conditions and carry a detailed analysis of the microstructural evolution during transition using a deformation gradient map that characterizes the local lattice distortion. The analysis of final martensite morphologies shows how mechanical constraints influence the number of selected variants and the number/type of defects in the final microstructure. We give insight on the origin and structure of different interfaces experimentally observed, such as inter-variant boundaries and antiphase defects. In particular, we show how antiphase defects originate from the twofold degeneracy shuffling displacement arriving during the transition and how the triple junction formation drives the texture evolution when local stresses prevent a free shape change of the matrix surrounding the growing martensite nuclei.

International audience; The behavior of materials under irradiation crucially depends on the microstructure evolution, which is driven by the absorption of point defects by the sinks (dislocations, cavities and loops). Cluster dynamics is a noteworthy theoretical tool of material science that can predict the microstructure evolution over long physical time. In this approach, the absorption kinetics of point defects depends on the values of sink strengths. An accurate evaluation of sink strengths is therefore necessary to obtain a reliable prediction of the materials evolution under irradiation.This work aims at computing accurate sink strength values by explicitly simulating the diffusion of point defects using Object Kinetic Monte-Carlo simulations. This study emphasizes the importance of elasticity on the diffusion of point defects. More precisely, elastic interactions between sinks and point defects are taken into account, and defects are described by their elastic dipoles and polarizabilities at stable and saddle points, which are computed by ab initio calculations.Our study of point defect migration to single sinks reveals that the anisotropy of point defects at saddle points is a key parameter in sink strength calculations, as it strongly affects the migration paths of point defects. We have also analyzed the migration of point defects in microstructures containing several sinks to investigate the effect of the neighboring clusters on sink strengths.

International audience; Plutonium based MOX fuels (U,Pu)O2 are one of the most commonly used fuel in actual nuclear plants in France. However, little is known on its microscopic behaviour during irradiation. This is mainly resulting from the fact that experiments are difficult and expensive to set up with such compounds at such scales and conditions. Hence, atomistic simulations can bring valuable information and increase the scope of knowledge in this field. Molecular dynamics simulations have proved to be an excellent tool to study point defects creation, because of its intrinsic length and time scales. We present in this paper the insight brought by molecular dynamics simulations on primary damages in (U,Pu)O2 solid solution. The efficiency of four of the most recent rigid ions pair interactions for (U,Pu)O2 solid solution will be exhibited by comparing their thermo-mechanical and thermophysical properties at different temperatures and plutonium contents. Then, the primary damage state under irradiation will be discussed based on the analysis of the results of displacement cascades initiated with energies up to 500 keV and for different plutonium contents.

International audience; Plutonium based MOX fuels (U,Pu)O$_2$ are one of the most commonly used fuel in actual nuclear plants in France. However, little is known on its microscopic behaviour during irradiation. This is mainly resulting from the fact that experiments are difficult and expensive to set up with such compounds at such scales and conditions. Hence, atomistic simulations can bring valuable information and increase the scope of knowledge in this field. Molecular dynamics simulations have proved to be an excellent tool to study point defects creation, because of its intrinsic length and time scales. Therefore, we will present in this paper the first, to our knowledge, molecular dynamics simulations of displacement cascades in (U,Pu)O$_2$ solid solution. We will first present the assessment of four of the most recent rigid ions pair interactions for (U,Pu)O$_2$ solid solution by comparing their thermo-mechanical and thermophysical properties at different temperatures and plutonium contents. Then, the primary damage state under irradiation will be discussed by analysing the results of displacement cascades initiated with energies up to 500 keV and for different plutonium contents.

International audience; Two topological parameters derived from pattern formation theory are introduced, and their relevance for the description of the elasticity-induced coarsening in Ni-based single-crystal superalloys is investigated. For that purpose, and given the crucial role of the spatial arrangement of the precipitates on microstructure evolution, two-dimensional phase field calculations of ideal microstructures undergoing slight perturbations have been carried out. The response of the system against Eckhaus and zigzag perturbations is discussed and quantitatively described by means of the developedtopological parameters.; Deux paramètres topologiques dérivés de la théorie des structures dissipatives (pattern formation) sont introduits, et leur potentiel à décrire l'influence de l'élasticité sur la coalescence dans les superalliages monocristallins à base de nickel est investigué. Pour cela, étant donné le rôle crucial de l'arrangement spatial des précipités sur les évolutions microstructurales, nous avons réalisé des simulations de type champ de phase avec des microstructures 2D périodiques, soumises à de légères perturbations topologiques. La réponse du système vis-à-vis de perturbations de type Eckhaus et zigzag est discutée, et analysée quantitativement au moyen des paramètres topologiques développés.

International audience; Microstructure evolution during tensile creep of 011-oriented samples of first-generation Ni-based single-crystal superalloys was investigated both experimentally and numerically. Based on scanning electron microscopy and 3-D volume reconstruction using a dual-beam system, it was shown that the initially cuboidal microstructure first elongates along the cubic axis perpendicular to the tensile axis, and then coalesces along the two other cubic directions. We found that the normal to the platelets, initially close to a cubic axis, slightly rotates towards the tensile axis direction and the destabilization of the platelet microstructure appears to control the onset of the tertiary creep stage. In a second part, microstructure evolutions are analyzed using 3-D phase-field simulations. The inhomogeneous and anisotropic elastic and plastic driving forces are discussed, as well as the importance of a small misorientation of the tensile axis from the [011] direction. Surprisingly, contrary to the case of h100i-oriented sample, complete rafting is not obtained in the simulations, suggesting that an additional mechanism at the level of individual dislocations could be missing. Finally, we have developed an energetic model for a perfectly rafted microstructure, which proves that plasticity in the gamma phase is at the origin of the gamma/gamma' interface rotation during creep.