Being able to predict the evolution of plastic deformation at the microstructural scale is of paramount importance in the engineering of materials for advanced applications. However, this is not straightforward because of the multiscale nature of deformation heterogeneity, both in space and time . The present thesis combines four related studies in a coherent work, which is aimed to develop experimental methods for studying crystal plasticity at the micro and mesoscale. A novel methodology for gold remodelling is initially proposed and used to apply high-density speckle patterns on the surface of stainless steel specimens. The unique proprieties of the speckle pattern enabled plastic deformation mapping with submicron resolution using digital image correlation (HDIC). It was therefore possible to study the concomitant evolution of microbands and transgranular deformation bands in such alloy. High-resolution deformation mapping also enabled comparison with high-resolution electron backscatter diffraction (EBSD) observations. The only partial correspondence of results proved the limits of EBSD in characterizing plastic deformation. The cause of such limitation is later identified in the reduced sensitivity to lattice slip of the EBSD technique. Hence, a novel method of HDIC data analysis is proposed to separate the contributions of lattice slip and lattice rotation from the deformation mapping. The method is adopted to characterize plasticity in austenitic stainless steel and at the plastic deformation zone (PDZ) around a silicon particle embedded in a softer aluminum matrix. Results show that the proposed experimental methodology has the unique capability of providing a complete description of the micro and mesoscale mechanics of crystal plasticity. HDIC therefore emerges as a key technique in the development of accurate physical-based multiscale crystal plasticity models.

There are a number of experiments showing that the ductility of aluminum alloys decreases during age-hardening heat treatment. Observing the grains of age-hardened aluminum alloys at the micron scale, one can notice that there are precipitate-free zones (PFZs) along the grain boundaries. PFZ has yield stress three times lower than the grain interior (bulk) due to absence of alloying elements. As a result, PFZ is suspected to be the reason for ductility reduction of alloys. On the other hand, a number of experiments performed on specimens with micron-scale dimensions have shown that the plastic deformation of crystalline materials is size-dependent. These micron-scale dimensions which can influence the mechanical behavior, such as yield stress or hardening, are not taken into account in the conventional plasticity theory, therefore another theory has been developed. That theory is Strain Gradient Plasticity (SGP). The specific SGP theory used here is a so called ‘higherorder theory’ in the sense that higher order stresses as well as additional boundary conditions are included in the theory. SGP theory also includes length scale parameters in order to be dimensionally consistent. On a recent study conducted by Fourmeau et al. (Fourmeau, 2015), transmission electron microscopy (TEM) is used to display the geometrical properties and the chemical composition of PFZ in the AA7075-T651 aluminum alloy. It is observed that the width of PFZ is about 20 to 40 nm. In the present thesis, the properties for PFZ and bulk material provided by that study are used for a micromechanical finite element model of a microstructure including the bulk, PFZ and the grain boundary (GB). A uniaxial loading condition is applied to the representative volume element (RVE) and SGP theory is hired in order to capture the plastic strain fields as well as the stress triaxiality in PFZ and bulk region. Moreover a damage criterion is employed and studied for models with PFZ and without PFZ to understand the role of PFZ in reduction of the ductility of aluminum alloys. It is found that the damage parameter is much higher in the presence of PFZ. Finally, the void growth is studied by adding voids at critical locations to the model.

L'irradiation peut modifier les propriétés mécaniques des aciers inoxydables austénitiques. Une diminution de la ténacité à la rupture des aciers en fonction de la dose est observée. La rupture ductile due à la croissance et la coalescence des cavités est toujours un mécanisme dominant dans les aciers irradiés jusqu'à 10 dpa. Des cavités peuvent être crées de manière différente : nucléées à partir des inclusions ou des précipités d'irradiation, ou créées directement par irradiation. Cette thèse a pour objectif d'étudier la rupture ductile des aciers irradiés due à la croissance et la coalescence des cavités intragranulaires. Basée sur la plasticité cristalline, des simulations en éléments finis sont effectuées sur les cellules unitaires pour étudier l'effet de l'orientation cristallographique et de la triaxialité de contraintes sur la croissance et la coalescence des cavités. L'effet de l'écrouissage post-irradiation sur la croissance et la coalescence des cavités est étudié avec un modèle de la plasticité cristalline prenant compte des défauts d'irradiation. En outre, un modèle élastomère-visco-plastique en grandes transformations est proposé pour décrire la croissance des cavités dans le monocristal. Le modèle est appliqué à la simulation de l'endommagement ductile dans le monocristal et le polycristal. Des cavités peuvent avoir des tailles différentes et la taille peut avoir une influence sur la ténacité à la rupture des aciers. Afin d'étudier cet effet, un modèle micromorphe de plasticité cristalline est proposé et appliqué à la simulation de la croissance et la coalescence des cavités intragranulaires de différentes tailles ainsi qu'aux phénomènes de localisation dans les monocristaux. / Irradiation causes drastic modifications of mechanical properties of austenitic stainless steels and a decrease in the fracture toughness with irradiation has been observed. Ductile fracture due to void growth and coalescence remains one dominant fracture mechanism for doses in the range of 0-10 dupa. Voids may have different origins : nucleated at inclusions or irradiation-induced precipitates during mechanical loading, or produced directly by irradiation. The present work is to investigate ductile fracture of irradiated steels due to growth and coalescence of intragranulaire voids. Based on continuum crystal plasticity theory, FE simulations are performed on unit cells for studying effects of lattice orientation and stress triaxiality on void growth and coalescence. The influence of post-irradiation hardening/softening on void growth ans coalescence is evaluated with a physically based crystal plasticity model. Besides, an elastoviscoplastic model at finite strains is proposed to describe void growth up to coalescence in single crystals, and is assessed based unit cell simulations. The model is then applied to simulate ductile damage in single crystals ans polycrystals. As voids in irradiated steels may have different origins, they may have different sizes, which potentially have an influence on ductile fracture process and fracture toughness of irradiated steels. In order to assess the size effect, a micromorphic crystal plasticity model is proposed and applied to simulate growth and coalescence of intragranular voids of different sizes.

Contrary to elastic deformation, plastic deformation of crystalline materials, such as metals, is size-dependent. Most commonly, this phenomenon is present but unnoticed, such as the effect of microstructural length scales. The grain size in metallic materials is a length scale that affects material parameters such as yield stress and hardening moduli. In addition, several experiments performed in recent years on specimens with geometrical dimensions on the micron scale have shown that these dimensions also influence the mechanical behaviour. The work presented in this thesis involves continuum modelling and simulation of size-dependent plastic deformation, with emphasis on thin films and the formulation of interface conditions. A recently published strain gradient plasticity framework for isotropic materials [Gudmundson, P., 2004. A unified treatment of strain gradient plasticity. Journal of the Mechanics and Physics of Solids 52, 1379-1406] is used as a basis for the work. The theory is higher-order in the sense that additional boundary conditions are required and, as a consequence, higher-order stresses appear in the theory. For dimensional consistency, length scale parameters enter the theory, which is not the case for conventional plasticity theory. In Paper A and B, interface conditions are formulated in terms of a surface energy. The surface energy is assumed to depend on the plastic strain state at the interface and different functional forms are investigated. Numerical results are generated with the finite element method and it is found that this type of interface condition can capture the boundary layers that develop at the substrate interface in thin films. Size-effects are captured in the hardening behaviour as well as the yield strength. In addition, it is shown that there is an equivalence between a surface energy varying linearly in plastic strain and a viscoplastic interface law for monotonous loading. In paper C, a framework of finite element equations is formulated, of which a plane strain version is implemented in a commercial finite element program. Results are presented for an idealized problem of a metal matrix composite and several element types are examined numerically. In paper D, the implementation is used in a numerical study of wedge indentation of a thin film on an elastic substrate. Several trends that have been observed experimentally are captured in the theoretical predictions. Increased hardness at shallow depths due to gradient effects as well as increased hardness at more significant depths due to the presence of the substrate are found. It is shown that the hardening behaviour of the film has a large impact on the substrate effect and that either pile-up or sink-in deformation modes may be obtained depending on the material length scale parameter. Finally, it is qualitatively demonstrated that the substrate compliance has a significant effect on the calculated hardness of the film. / QC 20100723