SUMMARY

Heterogeneous microstructures, comprised of grains, dislocation cells, precipitates, and solute atoms, typically determine the macroscopic mechanical properties of engineering alloys. However, traditional models are usually phenomenological, overlooking the underlying interactions between dislocations and heterogeneous microstructures. Moreover, the contributions to strengthening from various sources in heterogeneous microstructures often become intertwined. Therefore, a reasonable partitioning of the strengthening effect from each microstructure is valuable for material design. To overcome these challenges, physically based models that arcuately capture the mechanics effects of heterogeneous microstructures are necessary.
In the proposed thesis research, we transition from purely phenomenological models to physically based ones to represent the evolution of internal stresses within a heterogeneous microstructure under monotonic and cyclic loading conditions. We develop a crystal plasticity model to capture the evolution of internal stresses, focusing on its back stress component in the early stage of plastic deformation for structural alloys such as stainless steel and nickel base alloys processed by conventional routes. This model incorporates deformation induced back stresses associated with dislocation cells and precipitates. Furthermore, we develop a crystal plasticity model for additively manufactured stainless steel, accounting for back stresses arising from both printing and deformation effects. Crystal plasticity finite element simulations based on these models agree closely with experimentally measured stress-strain responses for the respective types of alloys.
This Ph.D. thesis research will provide new microstructure sensitive crystal plasticity models for engineering alloys. These models can not only simulate the strengthening effects from various sources but also offer guidance for tailoring mechanical properties in future material design.