Multi-scale Modeling and design of heterogeneous nano-microstructure for improved mechanical properties

Mehdi Hamid Vishkasougheh

The effect of grain boundary back stress field, slip transfer across grain boundaries and grain boundary sliding is analyzed for gradient microstructures under loading conditions like tensile and compression. One of the objectives is to address the existing trade-off between the strength and ductility for metallic materials. A multiscale crystal plasticity based framework is developed and gradient microstructures based on electron back scatter diffraction microscopy results are designed and implemented to the framework. The gradient microstructures are manufactured with severe plastic deformation techniques like high pressure torsion and shot pinning for pure Cu and Mg AZ31. The modeling and experimental mechanical performance of these samples are analyzed. The microstructures are designed by controlling the spatial grain size distribution in the range of a few hundreds of nanometers to tens of micrometers. Several layers with various thicknesses and grain sizes are combined to design these structures. A control voronoi tessellation method is applied for this purpose. The effect of different mechanisms like stress/strain gradient, slip transfer across grain boundaries, back stress field of grain boundaries and grain boundary sliding is studied for different metallic materials like IF (Interstitial Free) steel, Cu, Mg AZ31, and silver with gradient and bi-modal microstructures. The multiscale modeling span is further developed by coupling the stress/strain gradient formulation along with dislocation density evaluation relations to finite element solver as a user materials model to analyze the compression and buckling behavior of 3D printed micropillars as an application for this multiscale modeling framework. The main objective of this part of research is to add the grain size effect, strain rate sensitivity, statistically stored dislocation density effect, and geometrical necessary dislocation density to a continuum mechanics modeling environment. The generic tailor made crystal plasticity based user material model is written in Fortran environment. A parametric study is added to explain the effect of geometrical imperfection/eccentricity along with dislocation density content, grain size and strain rate sensitivity besides the predicted force vs. displacement results are compared with experimental results. The void size, porosity percentage and grain size effect of these AM micropillars are also analyzed and compared with experiments.

Multi-scale Modeling and design of heterogeneous nano-microstructure for improved mechanical properties

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