Enhancing elongation and trading off strength versus ductility of commercially pure titanium sheets using cyclic bending under tension and annealing

Abstract

This paper describes results acquired in an investigation into determining the influence of cyclic-bending-under-tension (CBT) and annealing on improving elongation-to-fracture (ETF) and optimizing strength and ductility of commercially pure titanium (cp-Ti) sheets. The space of process parameter involving crosshead velocity and bending depth along with sheet thickness was explored to establish a set of optimal parameters providing the greatest ETF for cp-Ti. Enhancements in ETF of about 3× were achieved using CBT relative to simple tension. Given the uniform elongation facilitated by CBT to very large strains, tradeoffs in strength and ductility of the material were examined by subjecting a set of sheets to a certain number of CBT cycles under the optimized parameters and annealing. In doing so, strength of the material increased by a factor of 1.6 along the sheet softest direction, while by a factor of 1.3 along the sheet strongest direction reducing the anisotropy. Microstructural evolution was characterized using electron-backscattered diffraction, while texture evolution was measured using neutron diffraction. These results revealed slip dominated deformation with minor activity of twinning. The role of CBT in preserving integrity of the sheets to large plastic strains is discussed by comparing measured and simulated geometries and mechanical fields.