Microstructure evolution and constitutive modeling for mesoscaled tension of pure titanium thin sheet

Abstract

The design and fabrication of micro components of pure titanium thin sheets face a challenging problem with size effects. Size effects will affect the plastic deformation mechanism of thin sheets in addition to causing unpredictable flow stresses. Although the impact of size on strength is obvious, further research is still needed, particularly for metal thin sheet with HCP lattice, to thoroughly understand the impact on deformation mechanisms. To assess the size effects on flow stress, uniaxial tensile experiments were carried out on specimens with varying thicknesses and grain sizes in this work. Additionally, Electron Backscattered Diffraction (EBSD) technology was used to analyze the dislocation evolution and twinning behavior in surface and inner grains for specimens with various thicknesses and grain sizes employing electrolytically polished specimens under different strain conditions. The results demonstrate that the feature size (t/d) has a substantial impact on the flow stress of pure titanium sheet, as seen by a rapid reduction in flow stress when t/d < 7. Microstructure analysis showed that the feature size effect has a significant impact on the deformation mechanism. Inhomogeneous strain distribution and geometrically necessary dislocation (GND) density distribution are produced by the reduction in thickness and the increase in grain size. There is a change from slip dominated to slip-twin deformation mechanism as the t/d decrease. Decreased twin nucleation stress and worsened inhomogeneous deformation are the inherent causes of twin activation as t/d decreases. When the t/d is less than 1, the dislocation starving mechanism causes a paradoxical increase in the flow stress. Furthermore, there are similarities and variances between the surface and the inner grains. Regarding GND density, there is not much of a difference between the two. However, there is a significant difference between the two in terms of twin volume fraction. The twinning volume fraction in the surface grains is much lower than that in the inner grains. To evaluate the contribution of size effect to flow stress, the variation Equation of twinning volume fraction and dislocations in surface and inner grains are quantified. These results were incorporated into a novel mesoscaled constitutive model that includes lattice friction and size-dependent grain boundaries, dislocations and twinning strengthening. The reduction of flow stress in relation to t/d is correctly predicted by the novel model, which also agrees with the experimental results. This study deepens the understanding of deformation mechanism of pure titanium thin sheets in microforming and provides numerical model for accurate forming.