DISLOCATION-BASED MULTI-SCALE MODELING FOR SIZE-DEPENDENT PLASTICITY OF HETEROGENEOUS MATERIALS
The objective of this research is to investigate the plastic deformation of heterogeneous material with different microstructures including grain orientations, grain size effect, and grain spatial distributions etc., and understand the mechanisms that cause specific mechanical response. Particularly, the mechanical behavior and deformation of body center cubic steel are studied. In this work, a multi-scale modeling framework based on dislocation mechanisms, including discrete dislocation dynamics, continuum dislocation dynamics, and a viscoplastic self-consistent model, has been developed. Furthermore, in order to investigate size effect, a strain gradient theory as well as a stress gradient theory are implemented into the framework. First, this framework is applied to study the size effect and deformation mechanisms of dual phase steel. Here, only the strain gradient theory is employed to predict the response of dual phase steel under constant strain rate tensile test. The predicted mechanical behavior and texture evolution are in good agreement with experimental works. Next, the size-dependent of dual phase is investigated by employing both strain gradient theory and stress gradient theory. It is found that these two theories are complementary rather than competing theories based on different dislocation interaction mechanisms. By comparing simulation results from each theory and combined theory, the results show that the combined theory can predict the size effect behavior over a wide range and is not affected by strain stage and saturation dislocation densities. We also find the dominance transition of two theories in grain size scale and strain scale. Finally, the effect of heterogeneous microstructures is investigated by introducing heterogeneous grid generated by voronoi tessellation. The effect of grain size distribution, grain size gradient and phase spatial distribution are investigated, which considers the grain size, grain locations as a stochastic internal parameter of microstructures. It is shown that different strength can be obtained by adjusting the grain size distribution and grain size gradient. This effect is further investigated by designing special gradient structures with nano region embedded micron size grain. The mechanical behavior of an interstitial free (IF) steel with such microstructures are studied and correlated with a microstructural geometry parameter-‘grain size gradient’.