Mechanics of Heterogeneous Metallic Materials

Overcoming the strength-ductility tradeoff is a widely pursued goal in the materials community. In recent years, design, fabrication, and optimization of heterogeneous microstructures have been extensively explored to achieve exceptional combinations of strength and ductility. However, there is currently a critical lack of the mechanics understanding of heterogeneous microstructures. In general, structural heterogeneities generate mechanical heterogeneities that are manifested as spatially non-uniform back stresses and forward stresses. These long-range, directional internal stresses can result in enhanced yield strength, work hardening, and tensile ductility. To understand the effects of heterogeneous microstructures and associated internal stresses on mechanical properties, this thesis is focused on development of novel constitutive and atomistic models for several emergent heterogeneous material systems, including additively manufactured metal alloys, gradient nanotwinned metals, nanocrystalline thin films, and nanodispersion-strengthened composites. Overall, the thesis research provides a new framework to bridge the structural heterogeneities and mechanical heterogeneities in several heterogeneous material systems through new constitutive models of strain gradient plasticity, internal-stress-dependent crystal plasticity, and dual-phase crystal plasticity. Atomistic simulations uncover the critical deformation processes that are strength/rate-controlling. Coupled with novel material processing, characterization, and testing, the modeling and simulation results offer quantitative predictions and mechanistic insights toward the design of heterogeneous metallic materials with improved combinations of strength and ductility.Ph.D