Modeling of Nanoindentation and Microstructural Ductile Behavior in Metallic Material Systems
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MODELING OF NANOINDENTATION AND MICROSTRUCTURAL DUCTILE BEHAVIOR IN METALLIC MATERIAL SYSTEMS Jeong Beom Ma1, W. Ashmawi1, M.A. Zikry1, D. Schall2, D.W. Brenner2 1 Department of Mechanical and Aerospace Engineering, 2 Department of Materials Science and Engineering North Carolina State University, Raleigh, NC 27695-7910 ABSTRACT Specialized large-scale computational finite-element and molecular dynamic models have been used to understand and predict how dislocation density emission and contact stress fields due to nanoindentation affect inelastic deformation evolution at scales that span the molecular to the continuum level in ductile crystalline systems. Dislocation density distributions and local stress fields have been obtained for different crystalline slip-system and grain-boundary orientations. The interrelated effects of grain-boundary interfaces and orientations, dislocation density evolution and crystalline structure on indentation inelastic regions have been investigated. INTRODUCTION Physically based descriptions are needed that can account for dominant physical mechanisms that may occur at different physical scales pertaining to nanoindentation. Grain-boundary (GB) structure, orientation, and distribution are essential microstructural features that characterize the initiation and evolution of deformation and failure modes in crystalline metals, alloys, and intermetallics. Nanoindentation offers a controlled process that can be used to predict material properties, such as hardness and ductility. However, as noted by [1-3], one of the major challenges is determining how dislocation density nucleation, emission, transmission, and absorption within GB regions affect overall inelastic behavior for crystalline materials subjected to nanoindentation. The physical scale on which to investigate different dislocation mechanisms involves different spatial and temporal levels that span the atomistic to the continuum levels. Atomistic methods are best suited for predictions related to defect nucleation, while microstructural and continuum-based methods are best suited for how defects evolve beyond the nucleation threshold stage and how several dislocations evolve to a population density . The primary purpose of this study is to be able to understand and predict how dislocationdensities and inelastic deformation modes evolve beyond the defect nucleation stage. We used displacement profiles, which were obtained from molecular dynamic (MD) simulations of nanoindentation, and these displacement profiles were then used in specialized microstructurally based finite-element formulations to track the evolution of dislocation-densities and inhomogeneous deformation modes in crystalline aggregates. An inelastic dislocation densitybased multiple-slip crystalline constitutive formulation was used to obtain a detailed understanding and accurate prediction of interrelated local material mechanisms that control and affect inhomogeneous deformation modes in f.c.c. polycrystalline aggregates with random GB orientations and d
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