SUMMARY

Fabrication of surface gradient microstructure by surface severe plastic deformation has been shown as an effective process to enhance functional performance and extend the service life of engineering components. The imposed gradient in strain results in a graded nanostructured material with an average grain size of < 100 nm in the topmost surface layer, while the interior region remains in a state similar to the undeformed bulk. Materials with these graded structures exhibit desirable properties including increased resistance to scratching, fatigue, and corrosion, without sacrificing overall ductility. Various mechanical surface modification (MSM) processes have been studied to fabricate components with gradient microstructure over the past decade. Main classes of MSM-based processes include indentation-type methods which involve impressions made on a work surface and sliding-type methods which involve frictional interaction by a hard asperity on a surface. Although microstructural characteristics and mechanical properties of the processed surface have been investigated, a clear correlation between the spatial and temporal heterogeneity of the mechanics inherited in these processes and the resultant microstructure state has yet to be elucidated. This work aims to quantify fundamental process-deformation-microstructure linkages during MSM processing so to realize model-driven control of manufacturing configurations for engineering of specific microstructure surface designs. The framework employed in this work involves representation of complex multi-stage processes using process unit models, so to examine the evolution of surface/subsurface deformation parameters given geometrical constraints of tool and boundary conditions. In this regard, simultaneous characterization of deformation history and microstructure in the surface/subsurface will provide a valuable knowledge foundation to interpret evolution of gradient microstructure in this important class of processes.