SUMMARY

The main objective of this thesis is to obtain critical insight on the role of crystalline incompatibilities, induced in presence of line defects i.e. dislocations and disclinations, on the energy and geometry of specific features of the local microstructure, and on the bulk mechanical response of nanocrystalline/ultra-fine grained materials. To this end, studies are performed at two length scales: 1) At fine scale, the energetic interactions between incompatibilities associated with grain boundaries and their influence on energy of triple lines is studied, 2) at mesoscale, deformation of initially defect-free as well as defected polycrystalline aggregates is studied to identify favorable conditions and potential sites for nucleation of incompatibilities. These studies have led to the development of a new multi-scale elasto-viscoplastic constitutive model of defect incompatibilities. The desired scale transitioning is achieved via novel a) phenomenological defect density transport equations and b) higher order/grade incompatible constitutive laws.