Influence of pre-strain and size effect on Bauschinger effect of superalloy ultrathin sheet during microscale cyclic deformation

During the fabrication of thin-walled superalloy structures, materials often undergo complex loading paths and exhibit different mechanical properties compared to those under simple loading paths. For example, when subjected to cyclic loading, superalloy ultrathin sheet exhibits pronounced Bauschinger effect. Furthermore, the achievement of precise forming in thin-walled superalloy components is hindered by size effect. To enhance the forming accuracy of superalloy components, this study systematically investigated the interplay between pre- strain, size effect, and Bauschinger effect, along with their underlying mechanisms. Cyclic deformation mechanical response of GH4169 ultrathin sheets with different thicknesses and grain sizes were thoroughly examined under diverse pre-strain conditions through cyclic shearing test. The evolution laws of Bauschinger parameters, specifically as they relate to varying pre-strains, thicknesses and grain sizes, were further determined. To rationalize these Bauschinger parameter evolution laws, electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were used to characterize the microstructure of GH4169 ultrathin sheets. Results indicated that the primary mechanism driving Bauschinger effect in GH4169 ultrathin sheets is the arising of back stress from dislocation slip and accumulation. As the level of pre-strain intensifies, Bauschinger effect becomes more pronounced, which is attributed to the intricate interplay between the evolving densities of moving and forest dislocations. Moreover, as grain size decreases, Bauschinger effect undergoes an enhancement due to changes in moving dislocation density and grain boundary strengthening effect. Conversely, size effect inherent to GH4169 ultrathin sheets exerts a dampening influence on Bauschinger effect. This weakening is intimately tied to surface layer effect, which modulates the dislocation density within GH4169 ultrathin sheets. These intricate interactions underscore the complexity of Bauschinger effect in thin-walled superalloy structures and highlight the need for nuanced approaches in material design and processing.