ON DEFORMATION STABILITY AND DUCTILE DAMAGE IN NANOTWINNED METALS

Abstract

The focus of this thesis is to understand the role of twin boundaries (TBs) in the deformation and failure characteristics of nanotwinned (NT) materials. To that end, we implement a crystal plasticity finite element framework for nanotwinned FCC metals, which accounts for slip gradients that are intrinsically coupled to TB migration. Using this framework, we extract intricate coupling between rates of TB migration and twin size, load orientation and the energy barrier of dislocation-TB interactions. The resulting kinetic relation is then adopted in developing a coarse grained model for probing the size-dependent stress-strain responses and microstructural instabilities in polycrystalline NT materials, without explicitly modeling twins. Finally, we investigate the role of twinning mediated microstructure evolution on the damage in NT materials due to the growth and coalescence of nanovoids over a range of biaxial stress states and strength anisotropies. This work addresses the following two broad questions: (i) What are the rates of TB migration as a function of microstructural length scales, loading orientation and key material properties?, and, (ii) How do microstructural length scales and TB migration affect the micromechanical stability of deformation and ductile damage in nanotwinned materials?