Metallic materials stand for the standard materials for structural applications in engineering. However, the strength of pure metals is rather limited due to the development of plastic deformation by dislocation slip. Different strengthening mechanisms have been employed to create obstacles that hinder dislocation motion through the crystalline lattice. Grain boundaries stand among the strongest barriers to dislocation slip in polycrystals, playing a critical role in their mechanical properties. However, grain boundaries can trigger damage, induce size effects such as the Hall-Petch effect, or weaken the material due to irradiation and hydrogen embrittlement. Designing materials with grain boundaries resistant to such effects is essential for increasing the lifespan and safety of critical components and requires understanding the interaction of dislocations with grain boundaries. The experimental evidence has revealed dislocations can transfer slip to the neighbor grain through the grain boundary in some cases, while dislocation pile-ups are formed in other grain boundaries. Nevertheless, there is still no general agreement on the criteria to predict slip transfer in polycrystalline materials.

This thesis aims to analyze the mechanisms of dislocation/grain boundary interaction in metallic polycrystals from the experimental and simulation viewpoints. The occurrence of slip transfer and blocking has been analyzed in thin pure nickel (face-centered cubic) and pure titanium (hexagonal close-packed) polycrystals. State-of-the-art experimental
techniques, such as electron backscatter diffraction-based slip trace analysis, diffraction contrast tomography, and high-resolution digital image correlation, have been used to characterize the microstructure and determine the conditions for slip transfer. In particular, diffraction contrast tomography was employed to determine the effect of the threedimensional grain boundary geometry on slip transfer, and high-resolution digital image correlation was used to account for the effect of the local stress state around grain boundaries. A large database of slip transfer/blocking events acquired in nickel and titanium
was used to validate/disprove the accuracy of the geometrical criteria for slip transfer across grain boundaries available in the literature.

In addition, an existing crystal plasticity model was extended to include the strengthening effect of grain boundaries and the possibility of slip transfer on the mechanical behavior of face-centered cubic and hexagonal close-packed polycrystals. A variety of geometrical slip transfer criteria were successfully implemented and were used to simulate the effect of grain size on the flow strength of face-centered cubic and hexagonal close-packed polycrystals by means of computational homogenization. The simulations were validated against experimental data of the tensile response of well-annealed cubic and hexagonal polycrystals found in the literature. Subsequently, the effect of slip transfer on intergranular fracture was studied through the insertion of cohesive surfaces at the grain boundaries of polycrystalline foils to determine the effect of grain boundary character and geometry on the strength and fracture of metallic polycrystals.