Size effect on nonlinear unloading behavior and Bauschinger effect of Ni-based superalloy ultrathin sheet

Superalloy ultrathin sheet is widely utilized in thin-walled components of aero-engine because of its excellent comprehensive performance. Nevertheless, it is hard to precisely predict the springback phenomenon during the plastic deformation of superalloy ultrathin sheet due to the size effect. To explore size effect on nonlinear unloading behavior and Bauschinger effect of Ni-based superalloy sheets, the uniaxial tensile, cyclic loading and unloading, and cyclic shearing tests were conducted on superalloy ultrathin sheets with diverse grain sizes and thicknesses of 0.2 and 0.25 mm. The experimental results demonstrated that the increasing grain size is accompanied with the decrease of flow stress of superalloy ultrathin sheet and the change of density of forest dislocation and moving dislocation. The dislocation annihilation rate of superalloy ultrathin sheet increases with decreasing t/d ratio, and the nonlinear unloading behavior becomes more obvious. Meanwhile, the degradation of elastic modulus becomes more evident with the increasing grain size. Driven by back stress, Bauschinger effect of superalloy ultrathin sheet becomes more apparent with larger t/d ratio. In addition, the fine-grained superalloy ultrathin sheet shows obvious earlier re-yielding and large instantaneous softening rates and the variation of t/d ratio also affect the permanent softening and work-hardening stagnation of superalloy ultrathin sheet.