Modeling of Nanoindentation and Microstructural Ductile Behavior in Metallic Material Systems

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Title: Modeling of Nanoindentation and Microstructural Ductile Behavior in Metallic Material Systems
Author: Ma, Jeong Beom
Advisors: Mohammed Zikry, Committee Chair
Abstract: A new hierarchical computational method has been developed and used with a dislocation-density multiple-slip crystalline formulation, and a specialized grain-boundary (GB) kinematic scheme to understand and predict how nanoindentation affects ductile behavior in face centered cubic crystalline (f.c.c.) aggregates at scales that span the molecular to the continuum level in ductile crystalline systems. The hierarchical approach is based on using displacement profiles from molecular dynamics (MD) simulations of nanoindentation to obtain scaling relations, which are then related to the strains pertaining to indented depths. These strains, which are scale invariant, and the scaling factors related to different indented depths, grain-sizes, and grain aggregate sizes are then used in a microstructural finite-element method (FEM) formulation that accounts for dislocation-density evolution, dislocation-density interactions with different GB orientations and distributions, crystalline structures, and grain-sizes. The accuracy of this hierarchical computational scheme was validated by comparing the predicted hardness values of crystalline gold with a number of experimental measurements, and the predictions accurately matched these measurements for different indented depths. This approach provides a new methodology to link molecular level simulations, with a microstructurally-based FEM formulation that can be used to ascertain the effects of dislocation-density evolution, grain-sizes, GB orientations, and crystalline structure on nanoindented surfaces at temporal and spatial scales that are commensurate with microstructural behavior. In this study, different aggregates with different low and high angle GBs and grain-sizes were investigated. The interrelated effects of GB interfaces and orientations, dislocation-density impedances and transmissions through GB regions, and the loading and unloading of the nanoindenter were characterized as a function of how local hydrostatic pressures, accumulated plastic strains, dislocation-densities, and slip-system orienations evolved in critical indented regions. The activation and switching of different slip-systems due to local stress unloading were also predicted and characterized for all aggregates and GB orientations in terms of dislocation-densities and pressures at specific orientations and spatial positions within different grains and GB regions. This validated hierarchical approach provides a predictive framework that can be used to design new experiments related to nanoindenation. But more importantly, it provides a general framework that can relate the transition of material behavior from the nano to the micro level, such that different phenomena and physical mechanisms can be accurately predicted from a nucleation threshold stage to a desired evolving microstructural mode. Hence, this can be used to potentially tailor crystalline aggregates over a broad spectrum of scales for desired applications at scales that span the nano to the micro levels.
Date: 2006-09-01
Degree: PhD
Discipline: Mechanical Engineering

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