Advanced materials improve the quality of life and significantly impact the sustainability of society. The friction stir processing technique can produce ultrafine-grained aluminum alloys with both higher strength and higher high cycle fatigue life when compared to conventional alloys. However, how the improved mechanical properties emerge from the unique microstructure is not well understood. The microstructure in these new materials consists of grains smaller than one micrometer and a high fraction of stable high-angle grain boundaries. These microstructural features strongly influence the dislocation-based deformation mechanisms. This award supports fundamental research on advanced computational modeling of cyclic deformation mechanisms as well as experimental investigations and characterization to establish microstructure-property relationship for an ultrafine-grained aluminum alloy system. The research will generate knowledge in advancing computational materials engineering for the selected alloy system but will be broadly applicable to similar engineering materials. Results from this research will contribute to the national goal of accelerating the insertion of new materials in the engineering practice. This work will support graduate student research and also help incorporate new understanding of deformation mechanisms into undergraduate courses such as mechanical behavior of materials and advanced materials by microstructural design.
This award supports an integrated computational and experimental approach to systematically study the correlation between cyclic deformation and heterogeneous microstructures of dispersion containing ultrafine-grained alloys. Well-proven concepts of persistent slip bands and extrusion/intrusion in microcrystalline alloys do not apply directly to ultrafine-grained materials because their smaller grain sizes and more high-angle grain boundaries change dislocation micromechanisms of multiplication, recovery and substructure formation. A multi-grain modeling strategy with three-dimensional discrete dislocation dynamics will be used to predict mesoscale dislocation processes and the initiation of slip localization at early stages of cyclic deformation. Specifically, this strategy will use information generated by atomistic simulations to create dislocation-grain boundary models for the discrete dislocation dynamics method. Incorporating dislocation-grain boundary and dislocation-precipitate models, the dislocation dynamics simulations will provide insights into dislocation-based damage accumulation processes in heterogeneous alloy systems. Experimental activities involve performing interrupted mini-fatigue tests and microstructure characterizations using orientation imaging microscopy and transmission electron microscopy to obtain cyclical stress/strain curves, grain size distributions and dislocation arrangements at various stages of high cycle fatigue. These efforts will provide insights into damage accumulation and evolution, help verify computational prediction with early stage data, and build a database of the entire fatigue behavior of the ultrafine-grained alloys.
