In the current thesis constitutive models are proposed for dominant mechanisms in hot forming and nano-crystalline materials including: inter-granular deformation, grain boundary sliding, grain boundary diffusion and grain growth. Besides a Taylor type polycrystalline constitutive model, two visco-plastic models are used to predict inter-granular deformation. Previous works on grain boundary mechanisms are extended to drive three dimensional macro stress-strain rate relationships for these mechanisms. The proposed models are first used to simulate hot forming processes. It is shown that the two visco-plastic models can be used to predict material behavior for various grain sizes, strain, stress state and strain rates with reasonable computational costs. Then a model is added to the proposed constitutive models to predict void initiation in superplastic forming. It is shown that this model’s predictions are in excellent agreement with experimental data. Finally the proposed constitutive models are used to predict nano-crystalline materials behavior. The stresses obtained from these constitutive equations match well with the experimental data for nano-crystalline copper for different strains and strain rates. Variations of strain rate sensitivity, inverse Hall-Petch effect, tension compression asymmetry and yield locus shape are also well predicted by the model. Key Words Hot forming, Nanocrystalline materials, Constitutive equations, Crystal plasticity, Grain boundary diffusion, Grain boundary sliding.
