Transfert d'échelle dans la modélisation thermodynamique et cinétique des alliages

La prédiction des microstructures représente un enjeu majeur pour l'étude des processus de vieillissement des alliages métalliques, en particulier sous irradiation. Les résultats des calculs ab initio de structure électronique peuvent être utilisés pour paramétrer les méthodes cinétiques de Monte Carlo Atomique et permettent ainsi de simuler quantitativement la diffusion des atomes et l'évolution de la microstructure qui en résulte. Cette méthode est cependant limitée par le temps de calcul qu'elle exige. Les simulations mésoscopiques évitent cet écueil, mais souffrent généralement de ne pouvoir être paramétrées sur les résultats obtenus aux échelles inférieures, limitant ainsi leur pouvoir de prédiction. Dans ce travail, une méthode de simulation appelée Monte Carlo cellulaire cinétique a été développée pour relier les échelles atomiques et mésoscopiques tout en conservant la nature discrète des atomes. Une procédure de paramétrisation basée sur les simulations Monte Carlo à l'échelle atomique a été établie. Elle permet de reproduire quantitativement les propriétés macroscopiques d'équilibre des alliages indépendamment de la taille des cellules utilisées. Une application à l'alliage fer-cuivre est présentée. Afin de décrire les propriétés cinétiques à ces échelles, un outil générique de calcul de la matrice d'Onsager dans les alliages a été mis en place. Il est fondé sur la theorie du Champ Moyen Auto-Cohérent. Les résultats obtenus sur des cinétiques de diffusion et de précipitation dans un alliage modèle sont présentés et validés par une comparaison systématique avec des simulations Monte Carlo à l'échelle atomique.Predicting microstructural evolution is a decisive step in the study of aging processes in alloys, especially under irradiation. The results of ab initio calculations of electronic structures can be used to parameterize kinetic methods such as Atomic Kinetic Monte Carlo simulations that allow reproducing quantitatively atomic diffusion and the resulting microstructure. Their use is however limited by their computational cost. Mesoscopic simulations are less concerned by such limitation, but suffer from the lack of reliable parameterization method to use data from simulations at lower scales that leads to a limited prediction capacity. A simulation method called Cellular Kinetic Monte Carlo is developed in this work to bridge the gap scales between atomic and mesoscopic scale simulation of diffusion. A crystal is there modeled as a. This method is based on a description of the crystal as a lattice of cells described by the discrete number of solute atoms they represents. The properties are then obtained by a controlled coarse-graining procedure based on Atomic Kinetic Monte Carlo simulations. It allows reproducing quantitatively macroscopic equilibrium for all cell sizes and has been applied to the Iron-Copper alloy. In order to describe kinetic properties at these scales, a generic computational tool has been developed to compute the Onsager matrix of alloys, based on the Self Consistent Mean field method. Diffusion and precipitation simulations have been done and the results are presented and assessed by a systematic comparison with Atomic Kinetic Monte Carlo simulations.PARIS11-SCD-Bib. électronique (914719901) / SudocSudocFranceF

The role of phase compatibility in martensite

Shape memory alloys inherit their macroscopic properties from their mesoscale microstructure originated from the martensitic phase transformation. In a cubic to orthorhombic transition, a single variant of marten- site can have a compatible (exact) interface with the austenite for some special lattice parameters in contrast to conventional austenite/twinned martensite interface with a transition layer. Experimentally, the phase compat- ibility results in a dramatic drop in thermal hysteresis and gives rise to very stable functional properties over cycling. Here, we investigate the microstructures observed in Ti50Ni50-xPdx alloys that undergo a cubic to orthorhombic martensitic transformation using a three dimensional phase field approach. We will show that the simulation results are in very good agreement with transmission electron microscopy observations. However, the understanding of the drop in thermal hysteresis requires the coupling of phase transformation with plastic activity. We will discuss this point within the framework of thermoelasticity, which is a generic feature of the martensitic transformation.Comment: Accepted for publication in in Journal of Applied Physic

Kinetics of coherent order-disorder transition in Al3ZrAl_3 Zr

Within a phase field approach which takes the strain-induced elasticity into account, the kinetics of the coherent order-disorder transition is investigated for the specific case of $Al_3 Zr$ alloy. It is shown that a microstructure with cubic $L1_2$ precipitates appears as a transient state during the decomposition of a homogeneous disordered solid solution into a microstructure with tetragonal $DO_{23}$ precipitates embedded into a disordered matrix. At low enough temperature, favored by a weak internal stress, only $L1_2$ precipitates grow in the transient microstructure preceding nucleation of the $DO_{23}$ precipitates that occurs exclusively at the interface of the solid solution with the $L1_2$ precipitates. Analysis of microstructures at nanoscopic scale shows a characteristic rod shape for the $DO_{23}$ precipitates due to the combination of their tetragonal symmetry and their large internal stress.Comment: 2 postscript figures and 1 JPG pag

A Vacancy Model of Pore Annihilation During Hot Isostatic Pressing of Single Crystals of Nickel-Base Superalloys

An improved diffusion model of pore annihilation during hot isostatic pressing of single crystals of nickel-base superalloys is proposed. The model considers dissolution of pores by emission of vacancies and their diffusion sink to low-angle boundaries. The calculation, which takes into account pore size distribution,predicts the kinetics of pore annihilation similar to experimental one

Creep of Single Crystals of Nickel-Based Superalloys at Ultra-High Homologous Temperature

The creep behavior of single crystals of the nickel-based superalloy CMSX-4 was investigated at 1288°C, which is the temperature of the hot isostatic pressing treatment applied to this superalloy in the industry. It was found that at this super-solvus temperature, where no gamma' strengthening occurs, the superalloy is very soft and rapidly deforms under stresses between 4 and 16 MPa. The creep resistance was found to be very anisotropic, e.g., the creep rate of [001] crystals was about 11 times higher than that of a [111] crystal. The specimens of different orientations also showed a very different necking behavior. The reduction of the cross-sectional area Psi of [001] crystals reached nearly 100 pct, while for a [111] crystal Psi=62 pct. The EBSD analysis of deformed specimens showed that despite such a large local strain the [001] crystals did not recrystallize, while a less deformed [111] crystal totally recrystallized within the necking zone. The recrystallization degree was found to be correlated with deformation behavior as well as with dwell time at high temperature. From the analysis of the obtained results (creep anisotropy, stress dependence of the creep rate, traces of shear deformation, and TEM observations), it was concluded that the main strain contribution resulted from (011){111} octahedral slip

Etude du vieillissement des superalliages à base nickel par la méthode de champs de phase

PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Kinetics of coherent order-disorder transition in Al3ZrAl_3 Zr

2 postscript figures and 1 JPG pageWithin a phase field approach which takes the strain-induced elasticity into account, the kinetics of the coherent order-disorder transition is investigated for the specific case of $Al_3 Zr$ alloy. It is shown that a microstructure with cubic $L1_2$ precipitates appears as a transient state during the decomposition of a homogeneous disordered solid solution into a microstructure with tetragonal $DO_{23}$ precipitates embedded into a disordered matrix. At low enough temperature, favored by a weak internal stress, only $L1_2$ precipitates grow in the transient microstructure preceding nucleation of the $DO_{23}$ precipitates that occurs exclusively at the interface of the solid solution with the $L1_2$ precipitates. Analysis of microstructures at nanoscopic scale shows a characteristic rod shape for the $DO_{23}$ precipitates due to the combination of their tetragonal symmetry and their large internal stress

Etude des effets de taille atomiques sur les diagrammes de phases et les microstructures

PARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF