Revealing mechanism of ductility improvement of titanium thin sheet under normal stress at mesoscale from perspective of microstructure evolution

Improving the formability of sheet metal is a constant challenge in microforming. In this study, applying normal stresses to the specimen surface is found to be an effective method for improving the ductility of pure titanium sheets. This case only occurs when the normal stress is higher than a critical value. By characterizing the microstructure, it is found that the normal stress induces a change in the deformation mechanism, which improves the work-hardening rate and the capacity for homogeneous deformation. The plastic deformation mechanism of pure titanium thin sheets undergoes a transformation from exclusively slip-based to a multi-mechanistic mode that couples slip, twinning, and FCC phase transformation. Normal stress exacerbate the deformation of surface grains and inhibit surface roughening. Moreover, normal stress activates deformation twins and FCC phase transformation by increasing the Schmid factor of the associated twin/slip systems. FCC phases and deformation twins contribute to enhancing the work-hardening rate through mechanisms such as the dynamic Hall-Petch effect, reorientation texture hardening, and dislocation substructure strengthening. Moreover, they enhance the material's ductility by providing additional deformation modes to accommodate strain. By virtue of the coordinated action of various deformation mechanisms, a more uniform distribution of thickness strain is achieved. It delays onset of plastic instability and enhances the formability of thin sheets. Considering the changes in dislocation density induced by different microstructures, a modified model is constructed. Based on the dislocation density and the surface layer model, this model predicts the flow stress size effect, as well as changes in flow stress and work hardening rate induced by normal stress due to microstructure transformation. This work provides a complete understanding of the mechanical property response and microstructure evolution under normal stress. It also gives a feasible solution for improving the formability of titanium thin sheet in microforming.