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This thesis develops a new quantitative multi-phase field model for polycrystalline solidification of binary alloys. We extend the thin interface formalism of Karma and co-workers to multiple order parameters. This makes it possible to model segregation and interface kinetics during equiaxed dendritic growth quantitatively, a feature presently lacking from polycrystalline or multi-phase solidification models. We study dendrite tip speed convergence as a function of interface width during free dendritic growth. We then analyze the steady state and grain coalescence properties of the model. It is shown that the model captures the correct physics of back diffusion and repulsive grain boundary coalescence as outlined by Rappaz and co-workers. Finally, the model is applied to simulate solidification and coarsening in delta-ferrite solidification. / Thesis / Master of Applied Science (MASc)

The emerging field of nanomechanics is providing a new focus in the study of the mechanics of materials, particularly in simulating fundamental atomic mechanisms involved in the initiation and evolution of damage. Simulating fundamental material processes using first principles in physics strongly motivates the formulation of computational multiscale methods to link macroscopic failure to the underlying atomic processes from which all material behavior originates. A combined concurrent and sequential multiscale methodology is developed to analyze fracture mechanisms across length scales. Unique characterizations of grain boundary fracture mechanisms in an aluminum material system are performed at the atomic level using molecular dynamics simulation and are mapped into cohesive zone models for continuum modeling within a finite element framework. Fracture along grain boundaries typically exhibit a dependence of crack tip processes (i.e. void nucleation in brittle cleavage or dislocation emission in ductile blunting) on the direction of propagation due to slip plane orientation in adjacent grains. A new method of concurrently coupling molecular dynamics and finite element analysis frameworks is formulated to minimize the overall computational requirements in simulating atomistically large material regions. A sequential multiscale approach is advanced to model microscale polycrystal domains in which atomistically-based cohesive zone parameters are incorporated into special directional decohesion finite elements that automatically apply appropriate ductile or brittle cohesive properties depending on the direction of crack propagation. The developed multiscale analysis methodology is illustrated through a parametric study of grain boundary fracture in three-dimensional aluminum microstructures. / Ph. D.

Les alliages à base Nickel polycristallins sont largement utilisés pour les pièces aéronautiques soumises à des sollicitations extrêmes en service. Des objectifs toujours plus ambitieux en termes de rendement énergétique des moteurs d’avions ont conduit les constructeurs à augmenter leur température de fonctionnement. Les nuances utilisées jusqu’alors dans les parties chaudes, tels que l’Inconel 718, n’ont pas une tenue mécanique suffisante à ces températures. Le René 65 est un nouvel alliage à microstructure γ-γ’ élaboré spécifiquement pour ces applications. Il a été retenu par Safran Aircraft Engines comme constituant des disques de turbine basse pression du nouveau turboréacteur LEAP. Pour garantir la bonne tenue des disques, une microstructure fine et homogène est requise. Le procédé de forgeage de ces pièces est une séquence d’étapes de déformation à chaud et de traitements thermiques, durant lesquelles la microstructure évolue. Si les phénomènes physiques gouvernant les évolutions microstructurales sont connus, leurs mécanismes exacts et leurs cinétiques varient d’un alliage à l’autre.Des essais de déformation à chaud ont été réalisés en laboratoire dans différentes conditions de température, vitesse et taux de déformation représentatifs des procédés industriels. L’étude précise des mécanismes de recristallisation dynamique, ainsi que de leurs cinétiques, constitue la première partie de ce travail. La caractérisation fine des microstructures déformées a permis de mettre en évidence un nouveau mécanisme de recristallisation, dit de recristallisation en hétéroépitaxie, qui se superpose aux autres mécanismes conventionnels. L’interaction entre ces différents mécanismes ainsi que leurs cinétiques relatives ont été établies dans une vaste gamme de conditions de déformation. Il est démontré que ce mécanisme de recristallisation s'applique également à d'autres alliages γ-γ’. La deuxième partie de l’étude est consacrée à la stabilité des microstructures déformées lors de leur exposition à haute température. L'alliage René 65, comme d’autres alliages à base Nickel, est sensible à un phénomène indésirable dit de croissance sélective de grains. Ses conditions de déclenchement ont été déterminées, de manière à délimiter une fenêtre de forgeage critique. Les mécanismes microstructuraux à l’origine de ce phénomène ont été discutés, ainsi que la possibilité d’une solution préventive. / Polycrystalline Nickel-based alloys are widely used as components for rotative parts of jet engines submitted to extreme conditions. Endlessly increasing objectives in terms of energy efficiency have led the engine manufacturers to increase their service temperature. As a consequence, Inconel 718 and similar alloys -that were used until now- cannot withstand such severe conditions anymore, and lack mechanical resistance at the increased temperature. René 65 is a new γ-γ’ superalloy which has been designed specifically for that purpose by General Electric. It has been selected by Safran Aircraft Engines as the material for low-pressure turbine disks in the new LEAP engine. To reach the desired mechanical properties, a fine and homogeneous microstructure is required. The forging process is a complex sequence which involves various hot deformation stages and thermal treatments, during which the microstructure evolves. Although the underlying mechanisms governing the microstructure evolutions are quite known, their specific mechanism and kinetics may vary depending on the alloy.Interrupted compression tests were conducted at laboratory scale under thermomechanical conditions (temperature, strain and strain rate) in accordance with the industrial process. In the first part, the focus is placed on the dynamic recrystallization mechanisms. Accurate characterization of the deformed microstructures has enabled to highlight a new recrystallization mechanism which superimposes with more conventional ones. It was named heteroepitaxial recrystallization. The interactions between those mechanisms as well as their relative kinetics have been established in a wide range of deformation conditions. . It is demonstrated that this mechanism occurs in other γ-γ’ Nickel-based alloys. The second part of the study is dedicated to the stability of deformed microstructures when exposed to high temperature thermal treatments. René 65, as many other Nickel-based alloys, is subjected to the undesirable phenomenon of selective grain growth, which leads to very heterogeneous microstructures containing abnormally large grains in a fine matrix. Critical deformation conditions leading to heterogeneous microstructures during subsequent annealing have been determined in an aim to identify the critical forging window which should be avoided. The microstructural mechanisms responsible of this phenomenon have been investigated, and the possibility of a preventive solution is discussed.